Post on 09-Jul-2020
Ocular surface changes with short-term contact lens wear
Garima Tyagi B.S. (Optom), PGDHRM
A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy
Professor Michael Collins (Principal supervisor)
Dr Scott Read (Associate supervisor)
Mr Brett Davis (Associate supervisor)
Institute of Health and Biomedical Innovation
School of Optometry
Queensland University of Technology
Brisbane, Australia
2011
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Keywords
Contact lens
Cornea
Corneal topography
Corneal thickness
Corneal swelling
Pentacam
Medmont
Videokeratoscope
Eyelids
Blepharoptosis
Lid-wiper epitheliopathy
Tarsal conjunctiva
Tear film
Tear film surface quality
Non-invasive
High-speed videokeratoscopy
ii
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Abstract Contact lenses are a common method for the correction of refractive errors of
the eye. While there have been significant advancements in contact lens
designs and materials over the past few decades, the lenses still represent a
foreign object in the ocular environment and may lead to physiological as well
as mechanical effects on the eye. When contact lenses are placed in the eye,
the ocular anatomical structures behind and in front of the lenses are directly
affected. This thesis presents a series of experiments that investigate the
mechanical and physiological effects of the short-term use of contact lenses on
anterior and posterior corneal topography, corneal thickness, the eyelids, tarsal
conjunctiva and tear film surface quality.
The experimental paradigm used in these studies was a repeated
measures, cross-over study design where subjects wore various types of
contact lenses on different days and the lenses were varied in one or more key
parameters (e.g. material or design). Both, old and newer lens materials were
investigated, soft and rigid lenses were used, high and low oxygen permeability
materials were tested, toric and spherical lens designs were examined, high
and low powers and small and large diameter lenses were used in the studies.
To establish the natural variability in the ocular measurements used in the
studies, each experiment also contained at least one “baseline” day where an
identical measurement protocol was followed, with no contact lenses worn. In
this way, changes associated with contact lens wear were considered in
relation to those changes that occurred naturally during the 8 hour period of the
experiment.
In the first study, the regional distribution and magnitude of change in
corneal thickness and topography was investigated in the anterior and posterior
cornea after short-term use of soft contact lenses in 12 young adults using the
Pentacam. Four different types of contact lenses (Silicone hydrogel/
Spherical/–3D, Silicone Hydrogel/Spherical/–7D, Silicone Hydrogel/Toric/–3D
and HEMA/Toric/–3D) of different materials, designs and powers were worn for
8 hours each, on 4 different days. The natural diurnal changes in corneal
thickness and curvature were measured on two separate days before any
contact lens wear. Significant diurnal changes in corneal thickness and
curvature within the duration of the study were observed and these were taken
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into consideration for calculating the contact lens induced corneal changes.
Corneal thickness changed significantly with lens wear and the greatest corneal
swelling was seen with the hydrogel (HEMA) toric lens with a noticeable
regional swelling of the cornea beneath the stabilization zones, the thickest
regions of the lenses. The anterior corneal surface generally showed a slight
flattening with lens wear. All contact lenses resulted in central posterior corneal
steepening, which correlated with the relative degree of corneal swelling. The
corneal swelling induced by the silicone hydrogel contact lenses was typically
less than the natural diurnal thinning of the cornea over this same period (i.e.
net thinning). This highlights why it is important to consider the natural diurnal
variations in corneal thickness observed from morning to afternoon to
accurately interpret contact lens induced corneal swelling.
In the second experiment, the relative influence of lenses of different
rigidity (polymethyl methacrylate – PMMA, rigid gas permeable – RGP and
silicone hydrogel – SiHy) and diameters (9.5, 10.5 and 14.0) on corneal
thickness, topography, refractive power and wavefront error were investigated.
Four different types of contact lenses (PMMA/9.5, RGP/9.5, RGP/10.5,
SiHy/14.0), were worn by 14 young healthy adults for a period of 8 hours on 4
different days. There was a clear association between fluorescein fitting pattern
characteristics (i.e. regions of minimum clearance in the fluorescein pattern)
and the resulting corneal shape changes. PMMA lenses resulted in significant
corneal swelling (more in the centre than periphery) along with anterior corneal
steepening and posterior flattening. RGP lenses, on the other hand, caused
less corneal swelling (more in the periphery than centre) along with opposite
effects on corneal curvature, anterior corneal flattening and posterior
steepening. RGP lenses also resulted in a clinically and statistically significant
decrease in corneal refractive power (ranging from 0.99 to 0.01 D), large
enough to affect vision and require adjustment in the lens power. Wavefront
analysis also showed a significant increase in higher order aberrations after
PMMA lens wear, which may partly explain previous reports of “spectacle blur”
following PMMA lens wear.
We further explored corneal curvature, thickness and refractive changes
with back surface toric and spherical RGP lenses in a group of 6 subjects with
toric corneas. The lenses were worn for 8 hours and measurements were taken
before and after lens wear, as in previous experiments. Both lens types caused
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anterior corneal flattening and a decrease in corneal refractive power but the
changes were greater with the spherical lens. The spherical lens also caused a
significant decrease in WTR astigmatism (WRT astigmatism defined as major
axis within 30 degrees of horizontal). Both the lenses caused slight posterior
corneal steepening and corneal swelling, with a greater effect in the periphery
compared to the central cornea.
Eyelid position, lid-wiper and tarsal conjunctival staining were also
measured in Experiment 2 after short-term use of the rigid and SiHy contact
lenses. Digital photos of the external eyes were captured for lid position
analysis. The lid-wiper region of the marginal conjunctiva was stained using
fluorescein and lissamine green dyes and digital photos were graded by an
independent masked observer. A grading scale was developed in order to
describe the tarsal conjunctival staining. A significant decrease in the palpebral
aperture height (blepharoptosis) was found after wearing of PMMA/9.5 and
RGP/10.5 lenses. All three rigid contact lenses caused a significant increase in
lid-wiper and tarsal staining after 8 hours of lens wear. There was also a
significant diurnal increase in tarsal staining, even without contact lens wear.
These findings highlight the need for better contact lens edge design to
minimise the interactions between the lid and contact lens edge during blinking
and more lubricious contact lens surfaces to reduce ocular surface micro-
trauma due to friction and for.
Tear film surface quality (TFSQ) was measured using a high-speed
videokeratoscopy technique in Experiment 2. TFSQ was worse with all the
lenses compared to baseline (PMMA/9.5, RGP/9.5, RGP/10.5, and SiHy/14) in
the afternoon (after 8 hours) during normal and suppressed blinking conditions.
The reduction in TFSQ was similar with all the contact lenses used, irrespective
of their material and diameter. An unusual pattern of change in TFSQ in
suppressed blinking conditions was also found. The TFSQ with contact lens
was found to decrease until a certain time after which it improved to a value
even better than the bare eye. This is likely to be due to the tear film drying
completely over the surface of the contact lenses. The findings of this study
also show that there is still a scope for improvement in contact lens materials in
terms of better wettability and hydrophilicity in order to improve TFSQ and
patient comfort.
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These experiments showed that a variety of changes can occur in the
anterior eye as a result of the short-term use of a range of commonly used
contact lens types. The greatest corneal changes occurred with lenses
manufactured from older HEMA and PMMA lens materials, whereas modern
SiHy and rigid gas permeable materials caused more subtle changes in corneal
shape and thickness. All lenses caused signs of micro-trauma to the eyelid
wiper and palpebral conjunctiva, although rigid lenses appeared to cause more
significant changes. Tear film surface quality was also significantly reduced
with all types of contact lenses. These short-term changes in the anterior eye
are potential markers for further long term changes and the relative differences
between lens types that we have identified provide an indication of areas of
contact lens design and manufacture that warrant further development.
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Table of contents
Keywords .................................................................................................. i Abstract .................................................................................................. iii Table of contents ................................................................................... vii List of Figures ......................................................................................... xi List of Tables ........................................................................................ xix
Abbreviations ....................................................................................... xxiii Statement of original authorship .......................................................... xxv Acknowledgements ............................................................................ xxvii Chapter 1 ................................................................................................ 1 Literature Review ................................................................................... 1
1.1 Eyelid: structure and functions .................................................. 1
1.2 Tarsal conjunctiva ..................................................................... 2 1.3 Tear film: structure and functions .............................................. 3
1.4 Cornea ...................................................................................... 3 1.4.1 Corneal anatomy ................................................................ 3 1.4.2 Corneal physiology............................................................. 4
1.5 Shape of the cornea .................................................................. 5
1.5.1 Current methods of measuring corneal topography ........... 7 1.5.2 Corneal topographic reference points .............................. 11
1.5.3 Classification of corneal topography ................................ 12 1.5.4 Corneal topography in normal population ........................ 15 1.5.5 Variations in corneal topography ...................................... 18
1.6 Corneal thickness .................................................................... 20 1.7 Contact lenses ........................................................................ 21
1.7.1 Properties of contact lens materials ................................. 22 1.8 Contact lenses and the eyelids ............................................... 25
1.9 Contact lenses and the tarsal conjunctiva ............................... 26 1.10 Contact lenses and the tear film .............................................. 26
1.11 Contact lenses and anterior corneal topography ..................... 28 1.11.1 PMMA contact lens wear and corneal topography ........... 28
1.11.2 RGP contact lens wear and corneal topography .............. 29 1.11.3 Soft hydrogel contact lenses and corneal topography...... 30 1.11.4 Silicone hydrogel contact lenses and corneal topography 31
1.11.5 Extended wear contact lenses and corneal topography ... 31 1.11.6 Toric soft contact lenses and corneal topography ............ 32
1.11.7 Time of recovery of corneal changes caused by contact lenses …………………………………………………………………33
1.12 Contact lenses and posterior corneal topography ................... 33 1.13 Contact lenses and corneal thickness ..................................... 34
1.13.1 Effect of contact lens wear on central corneal thickness .. 34 1.13.2 Effect of contact lens wear on peripheral corneal thickness 35
1.13.3 Mechanism of corneal thinning ........................................ 36 1.14 Contact lenses and orthokeratology ........................................ 36 1.15 Rationale ................................................................................. 37
Chapter 2 .............................................................................................. 41 Corneal changes following short-term soft contact lens wear........ 41
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2.1 Introduction ............................................................................. 41
2.2 Methodology ........................................................................... 43 2.2.1 Subjects ........................................................................... 43 2.2.2 Instrumentation ................................................................ 43
2.2.3 Contact lenses ................................................................. 44 2.2.4 Measurements and Protocol ............................................ 46
2.3 Data analysis .......................................................................... 48 2.3.1 Curvature and thickness difference maps ....................... 48 2.3.2 Regional analysis ............................................................ 49
2.3.3 Corneal best fit sphero-cylindrical power ......................... 49 2.3.4 Statistical analysis ........................................................... 50 2.3.5 Contact lens centration and rotation ................................ 50 2.3.6 Baseline day diurnal changes .......................................... 51
2.4 Results .................................................................................... 54
2.4.1 Diurnal Changes .............................................................. 54 2.4.2 Corneal thickness ............................................................ 54 2.4.3 Anterior corneal curvature ............................................... 56
2.4.4 Posterior corneal curvature ............................................. 57 2.4.5 Association between changes in thickness and curvature58 2.4.6 Corneal best fit sphero-cylindrical power ......................... 59
2.4.7 Contact lens centration and rotation ................................ 62 2.5 Discussion .............................................................................. 63 2.6 Conclusion .............................................................................. 67
Chapter 3 ............................................................................................. 69 Corneal changes following short-term rigid contact lens wear ...... 69
3.1 Introduction ............................................................................. 69 3.2 Methodology ........................................................................... 71
3.2.1 Subjects ........................................................................... 72 3.2.2 Contact Lenses................................................................ 72
3.2.3 Measurements and Instruments ...................................... 73 3.3 Data Analysis .......................................................................... 77
3.3.1 Corneal topography and thickness data .......................... 77 3.3.2 Pentacam data: Corneal curvature and thickness ........... 77
3.3.3 Medmont data: Correlation between the rigid lens fluorescein pattern and corneal topography changes .................... 78 3.3.4 Medmont data: Corneal refractive power ......................... 82 3.3.5 COAS data: Ocular wavefront error ................................. 82 3.3.6 Lens movement videos: Position of contact lens with respect to limbus centre ................................................................. 82
3.4 Results .................................................................................... 83
3.4.1 Anterior corneal axial curvature ....................................... 83 3.4.2 Posterior corneal axial curvature ..................................... 84 3.4.3 Corneal thickness ............................................................ 85 3.4.4 Correlation between corneal curvature and thickness ..... 87 3.4.5 Correlation between rigid lens fluorescein pattern and corneal topography changes .......................................................... 89 3.4.6 Refractive power .............................................................. 89 3.4.7 Ocular wavefront error ..................................................... 90 3.4.8 Position of contact lens .................................................... 91
3.5 Discussion .............................................................................. 92
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3.6 Conclusion .............................................................................. 96
Chapter 4 .............................................................................................. 99 Corneal changes with spherical versus back surface toric rigid contact lens wear ................................................................................ 99
4.1 Introduction ............................................................................. 99 4.2 Methodology .......................................................................... 100
4.2.1 Subjects ......................................................................... 100 4.2.2 Contact lenses ............................................................... 101 4.2.3 Measurements and Instruments ..................................... 103
4.3 Data Analysis ........................................................................ 104 4.3.1 Corneal topography and thickness data ......................... 104 4.3.2 Pentacam data: Corneal curvature and thickness .......... 104 4.3.3 Medmont data: Corneal refractive power ....................... 105 4.3.4 COAS data: Ocular wavefront error ............................... 105
4.3.5 Lens movement videos: Position of contact lens on cornea (with respect to limbus centre) ...................................................... 106
4.4 Results .................................................................................. 106
4.4.1 Anterior corneal axial curvature ..................................... 106 4.4.2 Posterior corneal axial curvature .................................... 107 4.4.3 Corneal thickness........................................................... 108
4.4.4 Refractive power ............................................................ 109 4.4.5 Ocular wavefront error ................................................... 109 4.4.6 Position of contact lenses .............................................. 110
4.5 Discussion ............................................................................. 111 4.6 Conclusion ............................................................................ 113
Chapter 5 ............................................................................................ 115 Eyelid changes following short-term rigid and soft contact lens wear .................................................................................................... 115
5.1 Introduction ........................................................................... 115
5.2 Methodology .......................................................................... 117 5.2.1 Subjects ......................................................................... 118 5.2.2 Contact lenses ............................................................... 118 5.2.3 Measurements and Instruments ..................................... 118
5.3 Data Analysis ........................................................................ 121 5.3.1 Eyelid position (Blepharoptosis) ..................................... 121 5.3.2 Tarsal staining ................................................................ 122 5.3.3 Lid-wiper epitheliopathy ................................................. 123
5.4 Results .................................................................................. 125
5.4.1 Eyelid position (Blepharoptosis) ..................................... 125 5.4.2 Tarsal conjunctival staining ............................................ 126
5.4.3 Lid-wiper epitheliopathy ................................................. 127 5.4.4 Association between blepharoptosis and tarsal conjunctival staining ……………………………………………………………….128
5.5 Discussion ............................................................................. 128 5.6 Conclusion ............................................................................ 134
Chapter 6 ............................................................................................ 135 Tear film surface quality with rigid and soft contact lenses .......... 135
6.1 Introduction ........................................................................... 135 6.2 Methodology .......................................................................... 137
6.2.1 Subjects ......................................................................... 137
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6.2.2 Instrument ..................................................................... 138
6.2.3 Contact lenses ............................................................... 138 6.2.4 Protocol ......................................................................... 139
6.3 Data Analysis ........................................................................ 140
6.4 Results .................................................................................. 143 6.4.1 TFSQ in natural blinking conditions ............................... 143 6.4.2 Blink frequency in natural blinking conditions ................ 144 6.4.3 TFSQ in suppressed blinking conditions ....................... 145 6.4.4 Trend of TFSQ with time in suppressed blinking conditions ……………………………………………………………….147 6.4.5 Association between TFSQ value and blink rate ........... 150 6.4.6 Association between TFSQ value and tarsal conjunctival and lid-wiper staining ................................................................... 150
6.5 Discussion ............................................................................ 151
6.6 Conclusion ............................................................................ 153 Chapter 7 ........................................................................................... 155 Conclusions ...................................................................................... 155
7.1 Changes in ocular structures posterior to the contact lens ... 156 7.1.1 Corneal thickness changes and contact lenses ............. 156 7.1.2 Anterior corneal curvature changes and contact lenses 158
7.1.3 Posterior corneal curvature changes and contact lenses ………………………………………………………………..161 7.1.4 Wavefront aberrations and rigid contact lenses ............. 161
7.2 Changes in ocular structures anterior to the contact lens ..... 162 7.2.1 Lid related changes and contact lenses ........................ 162
7.2.2 Tear film surface quality (TFSQ) and contact lenses ..... 165 7.3 Conclusion and clinical implications...................................... 166
References......................................................................................... 169 Appendices........................................................................................ 199
Appendix A: Ethics and Consent form ............................................. 199 Appendix B: Conference abstracts arising from this thesis ............. 199 Appendix C: Publications arising from this thesis ............................ 199
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List of Figures Figure 1-1: Structure of the eyelid. Redrawn from Bergmanson (2009). ........... 2
Figure 1-2: Layers of the cornea. Thicknesses as described by Gipson (1994). 4
Figure 1-3: (a) Medmont E300 videokeratoscope (b) Reflection of Placido disc
image from cornea (c) Medmont Placido disc (d) Subject‟s eye in position for
measurement. .................................................................................................. 9
Figure 1-4 (a) Oculus Pentacam system (b) Rotating Scheimpflug camera
system (c) Anterior segment image with the Pentacam .................................. 10
Figure 1-5: Various corneal topographic reference points. Note the
misalignment of the videokeratoscope axis from line of sight. Adapted from
Mandell (1996). .............................................................................................. 12
Figure 1-6: Anatomical classification of the corneal surface. Adapted from
Mountford et al. (2004). .................................................................................. 13
Figure 1-7: Explanation of axial and tangential curvatures at a point on the
cornea. Adapted from Mejía-Barbosa and Malacara-Hernández (2001). ........ 14
Figure 1-8: Illustration of the types of topography maps for a representative
subject, as captured by the Medmont E300 videokeratoscope: (a) Axial power
(b) Tangential power (c) Refractive power and (d) Elevation .......................... 14
Figure 1-9: Pre- and post-lens tear films and a contact lens on the cornea. Pre-
and post-lens thickness values by King-Smith et al. (2004). ........................... 27
Figure 2-1: The powers, designs and materials of the contact lenses used. The
comparisons to investigate the effect of lens characteristics on corneal
thickness and curvature are represented by curved arrows. The materials were
silicone hydrogel (SiHy) and hydroxyethyl methacrylate (HEMA). .................. 45
Figure 2-2: Contact lens thickness profiles for the SiHy/Sph/–3 (a), SiHy/Sph/–7
(b) and SiHy and HEMA/Toric/–3 (c). The color scale represents lens thickness
in mm. The thickness profile for the SiHy and HEMA toric contact lens is
identical (c). .................................................................................................... 46
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Figure 2-3: Axial curvature difference maps for a subject after 60 minutes of
downgaze task in (a) baseline (no contact lens wear) and (b) with a soft contact
lens in eye ...................................................................................................... 48
Figure 2-4: Cornea divided into central (4 mm diameter) and peripheral (4 mm
annulus) regions. ............................................................................................ 49
Figure 2-5: Digital image of a soft contact lens (SiHy/Toric/–3) on a subject‟s
eye. The lens centration for this subject was recorded as 0 (optimal), with less
than 0.5 mm decentration. The lens rotation for this lens was calculated using
the Imetrics software to be 16 degrees nasal. ................................................ 51
Figure 2-6: Diurnal variation in corneal pachymetry analysis. This figure shows
thickness difference maps for subject 2, SiHy/Toric/–3 lens. .......................... 53
Figure 2-7: Group mean changes in corneal thickness (mm) relative to baseline
days for the four different types of contact lenses. The lenses included different
combinations of lens material [hydrogel (HEMA) and silicone hydrogel (SiHy)],
design [spherical (Sph), toric] and power (–3.00, –7.00 D). ............................ 55
Figure 2-8: Group mean changes in anterior axial curvature (mm) relative to
baseline days for the four different types of contact lenses. The lenses included
different combinations of lens material [hydrogel (HEMA) and silicone hydrogel
(SiHy)], design [spherical (Sph), toric] and power (–3.00, –7.00 D). ............... 57
Figure 2-9: Group mean changes in posterior axial curvature (mm) relative to
baseline days for the four different types of contact lenses. The lenses included
different combinations of lens material [hydrogel (HEMA) and silicone hydrogel
(SiHy)], design [spherical (Sph), toric] and power (–3.00, –7.00 D). ............... 58
Figure 2-10: (a) Correlation between changes in posterior (central) corneal
curvature with (a) central corneal thickness (b) peripheral corneal thickness. P-
values in are shown in red. ............................................................................. 59
Figure 2-11: Changes in best fit sphere (M), with-the-rule and against-the-rule
astigmatism (J0), oblique astigmatism (J45) and sphero-cylinder RMSE (from
baseline) for the anterior cornea. Significant change indicated by * p<0.05 and
# p<0.01. Error bar represents one standard error of the mean. Negative
change in M represents a decrease in corneal axial power (hypermetropic
shift). Negative change in J0 represents a decrease in WTR astigmatism.
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Positive change in J0 represents an increase in WTR astigmatism. Positive J45
represents a negative cylinder axis closer to 45° and negative J45 represents a
negative cylinder axis closer to 135°. ............................................................. 60
Figure 3-1: Photo of the set up with digital camera to record movement of the
contact lens. Illumination of the eye is provided by a fluorescent ring light,
mounted behind a diffuser. ............................................................................. 75
Figure 3-2: Sequence of measurements taken in the morning before and
following insertion of contact lens in eye ........................................................ 76
Figure 3-3: Sequence of measurements taken in the afternoon after 8 hours of
lens wear. ....................................................................................................... 77
Figure 3-4: Steps involved in correlating rigid lens fluorescein pattern and
corneal topographic changes. (b) White cross showing LC (c) small white cross
showing LC and bigger white cross showing VK centre. HVID: horizontal visible
iris diameter, VK: videokeratoscope centre, LC: limbus centre. ...................... 81
Figure 3-5: Image showing the position of a rigid contact lens on cornea (light
blue ring), limbus (yellow ring) and upper (red arc) and lower eyelid (blue arc).
....................................................................................................................... 83
Figure 3-6: Group mean changes in anterior axial corneal curvature (mm)
relative to baseline day for the four different types of contact lenses. The lenses
included different materials (PMMA, RGP and SiHy) and diameters (9.5, 10.5
and 14.0 mm). Details of the lenses are shown in Table 3-1. Positive change
represents flattening and negative change represents steepening. ................ 84
Figure 3-7: Group mean changes in posterior axial corneal curvature (mm)
relative to baseline day for the four different types of contact lenses. The lenses
included different materials (PMMA, RGP and SiHy) and diameters (9.5, 10.5
and 14.0 mm). Details of the lenses are shown in Table 3-1. Positive change
represents flattening and negative change represents steepening. ................ 85
Figure 3-8: Group mean changes in corneal thickness (mm) relative to baseline
day for the four different types of contact lenses. The lenses included different
materials (PMMA, RGP and SiHy) and diameters (9.5, 10.5 and 14.0 mm).
Details of the lenses are shown in Table 3-1. Positive change represents
swelling and negative change represents thinning. ........................................ 86
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Figure 3-9: Correlation between changes in central corneal thickness and
central (a) and peripheral (b) back curvature. Correlation between changes in
peripheral corneal thickness and central (c) and peripheral (d) back curvature.
....................................................................................................................... 88
Figure 3-10: Correlation between distance of points of minimum clearance
(between cornea and contact lenses, in fluorescein pattern) and points of
maximum corneal flattening, from the videokeratoscope (VK) centre. Data is
shown for inferior (V2) points along vertical meridian and nasal (H1) points
along the horizontal meridian. ........................................................................ 89
Figure 3-11: Schematic demonstration of anterior and posterior curvatures and
thickness of the cornea, before and after PMMA and RGP contact lens wear for
8 hours based on the experimental data. The solid lines represent the baseline
anterior and posterior surfaces of cornea. The dotted line represents the
anterior and posterior surfaces of the cornea after contact lens wear for 8
hours. (a) PMMA contact lens showing greater central corneal swelling
compared to peripheral resulting in anterior corneal steepening and posterior
corneal flattening. (b) RGP contact lens showing greater peripheral corneal
swelling resulting in anterior corneal flattening and posterior corneal
steepening. Note that the diagram is not to scale. .......................................... 93
Figure 4-1: Axial corneal curvature maps of all subjects showing pattern of
corneal astigmatism and difference in curvature of the two principal meridians.
Note that all subjects had central astigmatism except for subject 04 who
showed limbus-to-limbus astigmatism. ......................................................... 101
Figure 4-2: Fluorescein patterns with a spherical (a) and back surface toric (b)
lens (same eye) on a subject (04) with high astigmatism (∆K = 3.3 D), limbus-
to-limbus. In the lower panels a spherical (c) and back surface toric (d) lens
(same eye) on a subject (06) with a lower amount of corneal astigmatism (∆K =
1.4 D), central. Note axis markings/scribe marks of the toric lens on the flatter
corneal meridian in both subjects (panels b and d). ...................................... 103
Figure 4-3: Group mean changes in anterior axial corneal curvature (mm)
relative to baseline day for the spherical and back toric RGP lenses. Details of
the lenses are shown in Table 4-1. Positive change represents flattening and
negative change represents steepening. ...................................................... 106
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Figure 4-4: Mean axial radii of curvature (mm) in the vertical meridians for
baseline, spherical lens and back surface toric lens. VK: Videokeratoscope. 107
Figure 4-5: Group mean changes in posterior axial corneal curvature (mm)
relative to baseline day for the spherical and back surface toric RGP lenses.
Positive change represents flattening and negative change represents
steepening. .................................................................................................. 108
Figure 4-6: Group mean change in corneal thickness (mm) relative to baseline
day for the spherical and back toric RGP lenses. Details of the lenses are
shown in Table 4-1. Positive change represents swelling and negative change
represents thinning. ...................................................................................... 108
Figure 5-1: (a) Set up of digital camera to take the photo of external eyes (b)
Ruler next to the eye to allow calibration. ..................................................... 119
Figure 5-2: (a) Position of eyelids on the baseline day afternoon (no contact
lens in eyes) (b) Position of eyelids with contact lens in left eye in the afternoon
after 8 hours of lens wear. Palpebral aperture (PA) height is shown in mm.
Yellow rings indicate the limbus outline, upper eyelid margin is shown in red
and lower eyelid margin is shown with blue. ................................................. 120
Figure 5-3: External photo of the eye showing the palpebral aperture (PA)
height with respect to limbus centre. Markings in yellow indicate the limbus
margins, Markings in red indicate the position of upper lid and markings in blue
indicate the position of the lower lid. ............................................................. 122
Figure 5-4: Digital images showing upper tarsal conjunctival staining with
fluorescein on a grade of 0 (None) to 4 (Severe). ......................................... 123
Figure 5-5: Examples showing grading of lid-wiper epitheliopathy from three
representative subjects. ............................................................................... 124
Figure 5-6: Changes in height of palpebral aperture (mm) of the right and left
eye relative to baseline afternoon. Negative values mean that palpebral
aperture height is less compared to baseline afternoon. Each error bar
represents one standard error of the mean. # represents statistically significant
p-values (<0.03), * represents p-value approaching significance (p=0.06). Right
eye is control eye (no lens) and left eye is the contact lens wearing eye. ..... 125
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Figure 5-7: Mean lid-wiper epitheliopathy grades with different types of contact
lenses and on the baseline (BL) day (no contact lens), in the morning and
afternoon. p-values indicated for change in lid-wiper compared to baseline. #
indicates p-value <0.05. Each error bar indicates one standard deviation. .. 127
Figure 5-8: Lens edge profiles of the RGP/9.5 and soft lenses used in the
study. PMMA and RGP/10.5 lenses had similar edge profiles to the RGP/9.5
lens. ............................................................................................................. 133
Figure 6-1: Steps involved in estimation of TFSQ value on the corneal or
contact lens surface. ROI: region of interest. AOA: area of analysis ............. 142
Figure 6-2: Image frames from high speed videokeratoscopy with an RGP lens
on the cornea. Reflections of the Placido disc pattern (a) Immediately after blink
(TFSQ value = 0.85) and (b) few seconds after blink showing tear break up
(TFSQ value = 0.68). Yellow lines enclose the area of analysis. .................. 142
Figure 6-3: Mean TFSQ values in 30 seconds with the four contact lenses and
on baseline day (no contact lens), in the morning and afternoon, in natural
blinking conditions. The TFSQ is calculated on a scale of 0 to 1 where 0 is very
poor and 1 is very good quality. * indicates significant difference compared to
baseline. Error bars represent standard error of the mean. ......................... 143
Figure 6-4: Mean blink frequencies (number of blinks per minute) in natural
blinking conditions with and without contact lenses. Error bars represent
standard error of the mean. .......................................................................... 145
Figure 6-5: The group mean TFSQ values in suppressed blinking conditions
with time for the 6 seconds after a blink, for the baseline day and with the four
different contact lenses in the morning (a) and afternoon (b). ....................... 146
Figure 6-6: The four different types of representative patterns of TFSQ with
time over 30 seconds. .................................................................................. 148
Figure 6-7: TFSQ over time for a representative subject, showing an increase
during the first second post-blink (build-up time) and then a constant reduction
in TFSQ over time till the end of the measurement. Corresponding Placido disc
maps can be seen at the beginning (clear rings), middle (breaks in the ring
pattern) and end (severe distortion of the ring pattern) of the measurement.
Yellow lines enclose the area of analysis. .................................................... 149
xvii
Figure 6-8: TFSQ over time for a representative subject, showing an increase
during first second post-blink, then a reduction is seen with time till a certain
point after which it shows an improvement and reaches a value more than the
baseline. Corresponding Placido disc maps can be seen at the beginning (clear
rings), middle (few breaks in the ring pattern) and end (very clear and regular
ring pattern) of the measurement. This later period seems to correspond to
complete drying of the lens surface which now acts like a mirror to produce a
high TFSQ value. Yellow lines enclose the area of analysis. ........................ 149
Figure 6-9: Correlation between mean TFSQ values and blink rates (number of
blinks per minute) in the morning and afternoon, for all the lenses combined.
(Afternoon measurements only) ................................................................... 150
Figure 6-10: Correlation between mean TFSQ values and (a) Tarsal staining
(b) Lid-wiper staining for all the lenses combined for morning and afternoon.151
Figure 7-1: Schematic representation of ocular structures and parameters
affected by short-term use of contact lenses presented in this thesis. .......... 155
Figure 7-2: Changes in ocular structures and parameters (posterior to contact
lenses) affected by short-term use of contact lenses, in comparison to baseline
day changes. ................................................................................................ 156
Figure 7-3: Schematic diagram showing difference between central and uniform
anterior swelling. .......................................................................................... 160
Figure 7-4: Changes in ocular structures and parameters (anterior to contact
lenses) affected by short-term use of contact lenses, in comparison to baseline
day changes. PA: Palpebral aperture. .......................................................... 162
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List of Tables Table 1-1: Mean anterior corneal radius of curvature R and anterior corneal
asphericity Q as reported by various authors. ................................................ 16
Table 1-2: Distribution of qualitative topographic patterns as reported in various
studies with videokeratoscopes ...................................................................... 16
Table 1-3: Mean posterior corneal radius of curvature R (mm) and asphericity
Q as reported by various authors. V and H represent vertical and horizontal
meridians, respectively. .................................................................................. 18
Table 2-1: Details of the lenses used in the study. ......................................... 45
Table 2-2: Methods to study the diurnal changes in corneal curvature and
thickness. ....................................................................................................... 52
Table 2-3: Relationship studied to check for diurnal changes. ........................ 52
Table 2-4: Mean corneal thickness changes relative to baseline days, with the
four contact lenses in central and peripheral corneal regions. Values where
pair-wise comparison revealed a significant change from baseline are
highlighted with asterisks (p-value ≤ 0.001 is ***). Positive change represents
swelling and a negative change represents thinning. ..................................... 56
Table 2-5: Mean changes in anterior and posterior axial corneal curvatures
relative to baseline days, with the four contact lenses in the central and
peripheral corneal regions. Values where pair-wise comparison revealed a
significant change from baseline are highlighted with asterisks (p-value ≤ 0.05
is *, ≤ 0.01 is ** and ≤ 0.001 is ***). ................................................................ 56
Table 2-6: Mean lens centrations calculated using custom-written software and
digital images of lenses on the corneas .......................................................... 62
Table 3-1: Details of the four lenses used in the study ................................... 73
Table 3-2: Mean changes in anterior and posterior axial corneal curvatures
relative to baseline days with the four contact lenses in the central and
peripheral regions. ......................................................................................... 85
xx
Table 3-3: Mean corneal thickness changes relative to baseline days with the
four contact lenses in central and peripheral corneal regions. ........................ 86
Table 3-4: Correlation between corneal thickness with anterior and posterior
curvatures for the four different types of contact lenses. ................................. 88
Table 3-5: Mean changes in best fit sphere(M), with/against the rule
astigmatism (J0) and oblique astigmatism (J45) in Dioptres, relative to baseline
day with the four contact lenses for the 4 and 6 mm corneal diameter............ 90
Table 3-6: Mean changes in HO RMS, 2nd, 3rd and 4th order RMS, relative to
baseline day with the four contact lenses for 4 mm (n=14) and 5.5 mm (n=10)
pupil diameters. „n‟ is the number of subjects included in the analysis. ........... 91
Table 3-7: Mean distances of contact lens centre to limbus centre (mm) and
ranges (mm) in the horizontal and vertical directions for the three types of rigid
contact lenses. ............................................................................................... 92
Table 4-1: Details of the lenses used in the study. ....................................... 102
Table 4-2: Mean changes in anterior and posterior axial corneal curvatures
relative to baseline days with the two lens types in the central and peripheral
regions. ........................................................................................................ 107
Table 4-3: Mean corneal thickness changes relative to baseline days with the
two contact lens types in central and peripheral corneal regions. ................. 109
Table 4-4: Mean changes in best fit sphere (M), with/against the rule
astigmatism (J0) and oblique astigmatism (J45) in dioptres, relative to baseline
day with the two lens types for the 4 and 6 mm corneal diameters. .............. 110
Table 4-5: Mean changes in HO RMS, 3rd and 4th order RMS, relative to
baseline day with the two lens types for 4 and 5.5 mm pupil diameters. ....... 110
Table 4-6: Mean distance of contact lens centre to limbus centre (mm) and
range in the horizontal and vertical directions for the two lens types. ........... 111
Table 5-1: Grades of horizontal length and sagittal width staining of lid-wiper.
Grading was done using both fluorescein and lissamine green. ................... 124
xxi
Table 5-2 Lid-wiper epitheliopathy classification system for final score as
described by Korb et al. (2005). ................................................................... 124
Table 5-3: Changes in upper tarsal conjunctival staining (relative to baseline) in
morning and afternoon. Positive values mean that tarsal staining has increased
compared to baseline. Note: The increase in staining in the mornings following
approximately 45 minutes of lens wear. ....................................................... 126
Table 5-4: Changes in upper tarsal conjunctival staining in the afternoon
(relative to morning). Positive values mean that tarsal staining has increased
compared to morning. .................................................................................. 126
Table 5-5: Changes in lid-wiper staining grade in the afternoon (relative to
morning). Positive values indicate increased staining compared to morning. 128
Table 6-1: Dry eye screening tests, screening criterion and mean scores of the
study subjects. Subjects who failed 2 or more dry eye tests were not included in
the study. ..................................................................................................... 138
Table 6-2: Description of the lenses used in the study. ................................. 140
Table 6-3: Mean change in TFSQ values relative to baseline, with the four
contact lenses in the morning and afternoon, in natural blinking conditions over
a period of 30 seconds. Negative values of TFSQ indicate that TFSQ is worse
with contact lenses. ...................................................................................... 144
Table 6-4: Mean change in TFSQ values in the afternoon relative to morning,
with the four contact lenses in natural blinking conditions. Negative values of
TFSQ indicate that TFSQ is worse in the afternoon. .................................... 144
Table 6-5: Mean changes in TFSQ values in suppressed blinking conditions
relative to baseline. ...................................................................................... 146
Table 6-6: Analysis of recordings showing an increase in TFSQ value with time.
..................................................................................................................... 148
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xxiii
Abbreviations
PMMA – Polymethyl methacrylate
RGP – Rigid gas permeable
HEMA – Hydroxyethyl methacrylate
SiHy – Silicone hydrogel
TFSQ – Tear film surface quality
PLTF– Pre-lens tear film
PCTF – Pre-corneal tear film
Dk – Oxygen permeability
Dk/t – Oxygen transmissibility
BOZR – Back optic zone radius
FOZD – Front optic zone diameter
BOZD – Back optic zone diameter
WTR – With-the-rule
ATR – Against-the-rule
M – Best fit sphere
J0 – With/against the rule astigmatism
J45 – Oblique astigmatism
RMS – Root mean square
HORMS – Higher order root mean square
Ortho-k – Orthokeratology
LC – Limbus centre
PA – Palpebral aperture
VK – Videokeratoscope
TBUT – Tear break-up time
NITBUT – Non-invasive tear break-up time
ROI – Region of interest
AOA – Area of analysis
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Statement of original authorship
The work contained in this thesis has not been previously submitted to
meet requirements of a degree or diploma or for any other higher
education institution. To the best of my knowledge and belief, the thesis
contains no material previously published or written by another person
except where due reference is made.
Signature:
Date:
xxvi
xxvii
Acknowledgements
It gives me great joy and satisfaction to submit this work as a doctorate thesis
and I would like to thank the people who have been instrumental in making this
dream possible.
This work has been possible due the support, guidance, motivation and
constant encouragement provided by my supervisor, Professor Michael Collins.
I have been very fortunate to have him as my supervisor and I sincerely thank
him for being the best supervisor.
I am grateful to Dr Scott Read, my associate supervisor, for his time,
help and involvement which have been invaluable to me. I am also very
thankful to my associate supervisor Mr Brett Davis, for his especially useful
ideas and friendly discussions.
I would like to acknowledge the financial support during my PhD
candidature: International Postgraduate Research Scholarship, Queensland
University of Technology and Australian Government, Cornea and Contact
Lens Society of Australia awards for two years and the Faculty of Health
research scholarship, Queensland University of Technology.
I acknowledge Dr Robert Iskander for the analysis software used in my
studies. Thanks to Ms Shila Roshani for help with analysis of digital images and
Dr David Alonso-Caneiro for help with analysis of videokeratoscopy recordings.
Special thanks to each and every participant of my studies for their help
and patience and for completing the studies with a smile. Thanks to everyone
in our lab for their company and for making the last three years special and
memorable for me.
Finally, I wish to thank my husband, Ankit, for the encouragement and
understanding. A big thank you to everyone in my family, who mean a lot to
me, for always being there for me. This thesis is dedicated to my parents for
their unconditional love, support and encouragement.
- 1 -
Chapter 1
Literature Review Contact lenses are commonly used for the correction of refractive errors of the
eye. These lenses are placed on the anterior surface of the eye and are
therefore in direct contact with the ocular structures in front of and behind the
lenses. In the following Sections these ocular structures and the effects of
contact lenses on these structures, are discussed.
1.1 Eyelid: structure and functions
The eyelids protect the eyes from injury and excessive light. The upper and
lower lids join at the nasal and temporal canthi and are separated by an
elliptical opening called the palpebral aperture. They are also responsible for
the even distribution of tears over the ocular surface during blinking.
The eyelid margins play an important role both in the formation and
distribution of the tear film across the ocular surface (Lemp et al. 1970; Guillon
and Guillon 1994). They contain the openings of meibomian glands (Figure 1-
1), and secrete the outermost lipid layer of the tear film, preventing the inner
aqueous layer of tears from evaporating quickly (Craig and Tomlinson 1997).
Additionally, the portion of the palpebral conjunctiva adjacent to the lid margin,
referred to as the “lid-wiper”, sweeps and spreads the tears over the ocular
surface (Korb et al. 2002; Ruskell and Bergmanson 2007). Disruption of this
portion of the conjunctiva observed by staining with fluorescein or rose bengal
dye is termed “lid-wiper epitheliopathy” (Korb et al. 2002; Korb et al. 2005).
During a blink, the upper eyelid does most of the movement whereas
the lower eyelid moves minimally (McCulley 1988). Eyelid closure during a blink
occurs as a result of relaxation of the levator palpebral superioris muscle
(innervated by the oculomotor nerve III) (McCulley 1988), followed by
contraction of the palpebral portion of orbicularis oculi (innervated by the facial
nerve VII) (Figure 1-1). The opening of the lids occurs as a result of the
contraction of levator palpebral superioris muscle, and the Müller‟s muscle
(innervated by the sympathetic nerves) assists in widening of palpebral fissure
during fear or excitement (Forester et al. 2008). The average spontaneous blink
rate is approximately 12-15 blinks per minute (Carney and Hill 1982; Moses
Chapter 1: Literature Review
2
1987) and a normal blink takes about 250 ms to complete (Hung et al. 1977;
Doane 1980).
Figure 1-1: Structure of the eyelid. Redrawn from Bergmanson (2009).
1.2 Tarsal conjunctiva
The conjunctiva is a thin translucent mucous membrane which attaches the
eyeball to the tarsal surface of the lids and also forms the superior and inferior
fornices or conjunctival sacs (McCulley 1988). The conjunctival sacs behave as
tear reservoirs. The conjunctiva is responsible for producing the mucous
component of the tear film from its goblet cells, which makes the otherwise
hydrophobic corneal surface hydrophilic for the formation of the tear film
(McCulley 1988). The conjunctiva is also immunologically active and has a
variety of defence mechanisms against infections (Knop and Knop 2005).
The conjunctiva is divided into 3 parts: tarsal or palpebral, fornicial and
bulbar. The fornicial conjunctiva forms the conjunctival sac and the bulbar
conjunctiva lines the anterior portion of the eye ball from limbus to fornix
(Ruskell and Bergmanson 2007; Forester et al. 2008). The tarsal conjunctiva
lines the tarsal surface of the lids and is firmly attached to the tarsal plate. The
Chapter 1 Literature Review
3
vessels of the tarsal conjunctiva are thought to provide an oxygen supply to the
cornea when the lids are closed (Brandell et al. 1988; Friend and Hassell
1994).
1.3 Tear film: structure and functions
The pre-ocular tear film covering the cornea and conjunctiva, is typically
described as a three layered structure with an estimated thickness of 3 microns
(King-Smith et al. 2004). The superficial lipid layer secreted by the meibomian
glands is important to prevent evaporation of the underlying aqueous layer
(Mishima and Maurice 1961) and contamination of tear film by skin lipids
(McDonald 1968). The middle aqueous layer, secreted by the main lacrimal
gland and the accessory glands of Krause and Wolfring (Figure 1-1) contains
dissolved ions and proteins. The inner mucous layer of the tear film secreted by
the goblet cells of the conjunctiva and crypts of Henle, helps in lubrication and
protects the epithelial surfaces. Thus the tear film plays a vital role in
maintaining a healthy and functional ocular surface and visual system.
A healthy tear film regulates the small irregularities in the corneal
surface to provide a smooth and regular optical surface, moistens the ocular
surface and minimizes friction for movement of the eyelids and therefore
prevents the desiccation of ocular surface cells. It flushes the cellular debris
and foreign matter towards the caruncle for removal. It also contains
antibacterial components and antibodies (e.g. lysozyme, secretory IgA) and
acts as the first line of defense against the ocular infections (McClellan et al.
1973; Farris 1985).
1.4 Cornea
This section discusses the anatomy and physiology of the cornea.
1.4.1 Corneal anatomy
The cornea is an avascular, transparent, richly innervated surface tissue of the
eye which encloses (together with sclera) the delicate internal ocular structures.
The surface area of the cornea is estimated to be 1.1 cm2, which is about 7% of
the surface area of the globe (Maurice 1984).
The cornea consists of five distinct layers: the epithelium, Bowman‟s
layer, stroma, Descemet‟s membrane and endothelium (Figure 1-2). The
Chapter 1: Literature Review
4
epithelium represents about 10% of the corneal thickness and forms a cellular
barrier to minimize fluid loss and entry of substances and pathogens into the
eye (Klyce and Beuerman 1998). Bowman‟s layer, also known as the anterior
limiting membrane, is composed of randomly oriented groups of fine collagen
fibrils that merge into the more organized anterior stroma. The stroma
represents 90% of the corneal thickness. It mainly consists of collagen fibrils
embedded in a matrix of proteoglycan which are more regularly arranged in
posterior stroma than in the anterior stroma (Klyce and Beuerman 1998).
Descemet‟s membrane is the basement membrane of the corneal endothelium
and is secreted by the endothelium. The endothelium is a single layer of
squamous cells with a density range of 1,400 to 3,400 cells/mm2 in adults
(Sturrock et al. 1978; Klyce and Beuerman 1998). It plays an important role in
maintaining corneal deturgescence that is the relative state of dehydration
required for maintaining corneal transparency (Edelhauser et al. 1994).
Figure 1-2: Layers of the cornea. Thicknesses as described by Gipson (1994).
1.4.2 Corneal physiology
The cornea maintains its transparency, cell reproduction, temperature, and
transport of tissue materials through constant metabolic activities. It requires a
constant supply of oxygen, glucose and amino acids for these functions. When
the eyes are open, oxygen is mainly supplied to the cornea by the pre-corneal
Chapter 1 Literature Review
5
tear film from the atmosphere by the process of diffusion. In the closed eye
condition, the oxygen level in the tears is in balance with the palpebral
vasculature (Brandell et al. 1988; Friend and Hassell 1994). Glucose, amino
acids and other nutrients are provided by the aqueous humour (Friend and
Hassell 1994).
Normally, the corneal uptake of glucose from the aqueous humor is
balanced by the loss of corneal lactate (O'Neal and Bonanno 1994). However,
during some forms of corneal metabolic stress or when the oxygen supply to
the cornea is reduced, for example during sleep, the rate of lactate production
increases due to the cornea being more reliant upon anaerobic metabolic
processes. This may result in epithelial edema which can alter the cellular
refractive index and produce Sattler‟s veil phenomenon which is often reported
as increased glare and halos surrounding bright lights (Dallos 1946). Stromal
edema that occurs during hypoxia is also a result of excess osmotic solute load
caused by lactate accumulation. Finally, increased metabolic lactate production
can cause localized stromal acidosis which can alter endothelial morphology
and function. All of these changes can compromise the optical properties of the
cornea (Kwok 1994).
1.5 Shape of the cornea
The cornea is the most powerful optical surface of the eye, and provides two-
thirds of the total refractive power of an unaccommodated eye (approximately
42 D), which is mostly due to the change in refractive index at the air-tear
interface present over the cornea (Guillon and Guillon 1994). Therefore, even
subtle changes in corneal shape can considerably affect vision. Measurement
of corneal shape forms an important clinical technique for procedures such as
contact lens fitting and refractive surgery. In addition, evaluating the sequential
changes in corneal topography with time has an important role in monitoring
corneal pathologies, contact lens-induced changes (Wilson et al. 1990), corneal
refractive treatments (e.g. orthokeratology) (Lui and Edwards 2000), diagnosis
and management of keratoconus (Schwiegerling and Greivenkamp 1996) and
the management of surgically induced astigmatism (Holck et al. 1998).
Given its importance and accessibility, corneal shape and other
parameters have been extensively investigated. The average corneal diameter
horizontally is 12.6 mm and vertically is about 11.7 mm (Klyce and Beuerman
Chapter 1: Literature Review
6
1998). The radius of curvature of the anterior surface of the central cornea is
about 7.8 mm whereas for the posterior surface it is around 6.5 mm (Atchison
and Smith 2000). Due to this difference in the curvatures of the two surfaces,
the cornea shows a variation in thickness from centre to periphery. The central
corneal thickness is approximately 520 µm which increases to about 650 µm or
more in the periphery (Klyce and Beuerman 1998). The mean refractive index
of the cornea is 1.376 (Gullstrand 1909; Atchison and Smith 2000).
The anterior corneal surface often exhibits toricity, which may result in
overall ocular astigmatism. The anterior corneal surface is generally steeper
along the vertical meridian than along the horizontal meridian in young eyes
(Hayashi et al. 1995; Goto et al. 2001). This is often referred to as “with-the-rule
(WTR)” astigmatism. With age, the cornea changes its shape such that the
horizontal meridian becomes steeper. This is referred to as “against-the-rule
(ATR)” astigmatism (Atchison and Smith 2000). Grosvenor (1978) suggested
that the band-like pressure on the cornea due to tightness of the lids (especially
upper lid) in young adults may result in corneal steepening along the vertical
meridian, causing WTR astigmatism and as the lid tension decreases with age
there is a tendency towards ATR astigmatism.
The corneal surface is also aspheric, that is, it flattens progressively
from centre to periphery. The surface can be mathematically expressed as a
conicoid using the equation: h2 + (1 + Q) Z2 – 2 ZR = 0, where the Z axis is the
optical axis, R is the apical radius, Q is the asphericity constant and h2 = X2 +
Y2, where X and Y are horizontal and vertical Cartesian coordinates. Q
determines the way the shape of a surface will change away from the apex and
is a sphere when Q = 0, oblate (aspheric surface that steepens away from the
apex) when Q > 0 is and, prolate (aspheric surface that flattens away from the
apex such as cornea) when 1 < Q < 0 (Kiely et al. 1982). The asphericity
constant Q varies from 0.30 to 0.18 (Atchison and Smith 2000). An aspheric
corneal surface helps in reducing the spherical aberration of the eye (Kiely et
al. 1982).
Another common method to describe corneal shape mathematically, is
to use Zernike polynomials (Schwiegerling et al. 1995). An advantage of using
these functions is that they are orthogonal over a unit circle, that is, the
coefficients are independent of each other and are not affected by addition or
Chapter 1 Literature Review
7
deletion of terms used for describing the surface. Zernike polynomials are also
commonly used to describe corneal wavefront aberrations.
1.5.1 Current methods of measuring corneal topography
This section briefly describes the various techniques that have been used to
measure corneal shape.
1.5.1.1 Keratometry
Keratometry is one of the oldest and most common methods for measuring
corneal curvature. A keratometer measures the anterior curvature of the central
cornea. The optical principle of keratometry involves a relationship between the
object size and the image size reflected from the cornea (Purkinje image I)
which acts as a convex mirror (Wilson and Klyce 1991). The radius of curvature
of the cornea is then calculated based upon the object size and the distance
between the image and the object. It uses a standard keratometric refractive
index of 1.3375 to convert these values to dioptric power.
The keratometer assumes the cornea to be sphero-cylindrical and
measures anterior corneal radius of curvature at two locations 2.5 - 4.0 mm
apart in the central cornea (depending on the corneal power) along each of the
two orthogonal principal meridians with maximum and minimum power
(Mandell 1988; Klyce and Wilson 1989). It provides a good estimate of central
curvature for a normal cornea, which is nearly spherical. However, it is not
accurate for measuring aspheric or irregular surfaces and provides no
information about corneal shape in the periphery. This is a major drawback of
keratometry which has led to development of more advanced instruments
providing detailed information of peripheral corneal topography.
1.5.1.2 Photokeratoscopy and videokeratoscopy
Photokeratoscopes are instruments to assess the curvature and topography of
the anterior surface of the cornea. They work on a principle similar to the
keratometer in which the cornea behaves as a convex mirror (Wilson and Klyce
1991). However, unlike keratometers, photokeratoscopes normally consist of
an alternating pattern of black and white illuminated concentric rings (called a
Placido Disc pattern), which is reflected from the cornea (Figure 1-3b), and is
imaged by the camera. The image of these concentric rings reflected from the
cornea is then compared with the rings of the instruments‟ Placido disc pattern
Chapter 1: Literature Review
8
to determine the slope of corneal surface at various locations across the cornea
(Schwiegerling et al. 1995; Seitz et al. 1997). The optical power of the cornea is
computed from the derivatives of the corneal surface slopes (Schwiegerling et
al. 1995). This instrument gives the corneal topography of both the central and
peripheral cornea unlike a keratometer.
Videokeratoscopes are an advanced version of photokeratoscopes and
are equipped with a video camera and image processing computer software to
record and analyze the keratoscope images and display the topographic data
on the monitor (Morrow and Stein 1992). These instruments provide corneal
surface information from a large number of corneal locations using a Placido
disc pattern. The number of data points for determining corneal power can be
as much as 15120 with 32 rings in the Placido disc pattern (Medmont E300
corneal topographer, Figure 1-3) and 300 data points in each semi meridians.
The exact number of corneal data points depends on the target/instrument
design and some features of the individual eye being measured. The target can
be large or small. Larger targets reduce the chances of alignment errors as
they allow larger working distances. Smaller targets (Figure 1-3c) allow a larger
corneal coverage by minimizing the corneal area obscured by eyelashes, nose
and eye brow (Courville et al. 2004). The videokeratoscopes made by different
manufacturers may also differ in the focusing and alignment method and
computer features.
The Placido disc based videokeratoscopes have been shown to be
accurate and repeatable (Cho et al. 2002) with the accuracy reported to vary
from 0.1 D to 0.25 D in terms of corneal power (or 0.018 mm to 0.045 mm of
axial radius of curvature). However, inaccuracies in Placido disc based
instruments may arise due to focusing and centration errors (Seitz et al. 1997)
and corneas with sudden change in slopes (Belin and Ratliff 1996). Modern
videokeratoscopes are equipped with various mechanisms to minimize these
errors, for example range finding devices that are used determine the distance
from corneal apex to the instrument‟s camera and captures the image
automatically when the eye is in good focus and well aligned (Mattioli and
Tripoli 1997).
Chapter 1 Literature Review
9
Figure 1-3: (a) Medmont E300 videokeratoscope (b) Reflection of Placido disc image from cornea (c) Medmont Placido disc (d) Subject’s eye in position for measurement.
1.5.1.3 Corneal profile topography
Corneal profile photography or an optical slit-scanning mechanism is used by
the Orbscan topographer. This technique allows acquisition of topographic
measurements of the anterior and posterior corneal surfaces, as well as the
anterior lens surface. The Orbscan II is a more recent computerized
topographer which uses the slit-scanning technology in combination with a
Placido disc to measure the corneal curvature (Cairns and McGhee 2005). The
instrument scans across the anterior corneal surface, obtaining 40 slit images
(20 from the right and 20 from the left) in a sequence in 2.1 seconds from an
angle 45 degrees to the instrument axis. It also records eye movements and
reflection data from a Placido disc device at the same time. The data are then
combined to form a three-dimensional anterior and posterior corneal surface.
The instrument provides information in the form of curvature topography maps.
The repeatability of anterior elevation as measured by Orbscan II is reported to
be approximately 2 µm, which is said to decrease towards the periphery of the
cornea (Cairns and McGhee 2005).
1.5.1.4 Scheimpflug imaging
Instruments that use Scheimpflug photography have recently been introduced
for imaging the anterior eye. The Pentacam HR system (Oculus Inc, Wetzlar
Chapter 1: Literature Review
10
Germany) (Figure 1-4) is an instrument based on this principle that allows
measurement of the anterior and posterior corneal topography, corneal
thickness and anterior chamber depth. It consists of a 180-degree rotating
Scheimpflug camera and a monochromatic, slit light source (blue LED at 475
nm) that rotate together around the optical axis of the eye. It works as a
topographer in that its software creates three-dimensional models of the cornea
based upon cross-sectional images. This instrument has been shown to have
good repeatability for central and peripheral corneal thickness (Barkana et al.
2005; Lackner et al. 2005; O'Donnell and Maldonado-Codina 2005; Amano et
al. 2006; Uçakhan et al. 2006; Khoramnia et al. 2007; Shankar and Pesudovs
2008), anterior and posterior corneal curvature (Chen and Lam 2007; Shankar
et al. 2008) and anterior chamber depth measurements (Lackner et al. 2005;
Meinhardt et al. 2006; Savant et al. 2008; Shankar et al. 2008).
Figure 1-4 (a) Oculus Pentacam system (b) Rotating Scheimpflug camera system (c) Anterior segment image with the Pentacam
1.5.1.5 Other methods of measuring corneal shape
Corneal shape has been evaluated using many other techniques such as raster
photogrammetry (for example – PAR corneal topography system, PAR vision
systems, USA) in which a grid pattern is projected on the cornea and
distortions in the pattern based on the grid projection and camera angles are
analysed to determine corneal elevation data (Belin et al. 1995). Although
these instruments have certain advantages such as they are not dependent on
reflection from cornea (therefore can be used for scarred or irregular corneas)
and can measure larger area of the cornea, they are not as accurate as Placido
disc videokeratoscopes (Tang et al. 2000).
Another method that has been used to measure corneal shape is laser
holographic interferometry (e.g. CLAS 1000, Kerametrics Corp., USA) in which
Chapter 1 Literature Review
11
the optical path difference between the laser beam illuminated and reflected
from the cornea is used to calculate corneal elevation (Kasprzak et al. 1995;
Naufal et al. 1997). These instruments have not been adopted for widespread
clinical use because it is hard to describe a variety of corneal shapes using a
single interference reference (Courville et al. 2004).
1.5.2 Corneal topographic reference points
Corneal topography can be referenced to a number of points on the cornea
such as the corneal geometric centre and the corneal sighting centre. The ideal
reference point depends on the purpose of corneal topography. If corneal
topography measurement was done for contact lens fitting then corneal
geometric centre is appropriate. Corneal sighting centre (Figure 1-5), which is
where the line joining the object and centre of entrance pupil (line of sight)
intersects the cornea (Mandell 1994), is a more appropriate reference point
when one is interested in the optical effects of the corneal shape. The corneal
sighting centre, on average, has been found to be located 0.21 ± 0.16 mm
nasally relative to the corneal geometric centre (Pande and Hillman 1993).
However, the videokeratoscopic images are usually referenced to a point
where the instrument axis intersects the cornea called the “vertex normal”
(Figure 1-5) which is different from both corneal sighting and corneal geometric
centre. The vertex normal is, on average, 0.38 ± 0.1 mm away from the corneal
geometric centre (Mandell et al. 1995). The relative distances between these
reference points are usually small and therefore will typically have little effect
on corneal topography measurements (Mandell et al. 1995). However, large
misalignments between vertex centre and line of sight may result in
considerable errors when comparing the corneal topographic measurements
with other measures of ocular optics which are referenced to the line of sight,
such as total wavefront aberration measurements (Salmon and Thibos 2002).
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Figure 1-5: Various corneal topographic reference points. Note the misalignment of the videokeratoscope axis from line of sight. Adapted from Mandell (1996).
1.5.3 Classification of corneal topography
The corneal surface is most commonly described as a sphero-cylinder as
measured by a keratometer, but this description is valid only for the central
cornea and provides no information about the peripheral cornea. With the
development of videokeratoscopes, profile topographers and Scheimpflug
topographers, detailed measurement of the corneal surface is possible.
Corneal topography can be described qualitatively as well as quantitatively.
This section discusses different ways of describing the corneal topography.
1.5.3.1 Qualitative descriptors of corneal topography
Modern videokeratoscopes measure corneal shape at a large number of data
points. Therefore, the videokeratoscopes generate various maps to summarize
information such as axial curvature or power, tangential curvature or power,
elevation maps and refractive power maps. The maps can be further classified
into regions (Figure 1-6) such as central, paracentral, peripheral and limbal
zones (Mountford et al. 2004).
A detailed description of the various corneal topographic maps (Figure 1-8) is
as follows:
Elevation map (Corneal height map)
The videokeratoscope uses corneal slope data to derive the height or sag of
the cornea relative to a reference plane (Schwiegerling et al. 1995). Therefore,
this method obscures any localized changes in the corneal height due to the
marked spherical shape of the cornea (Young and Siegel 1995; Schwiegerling
and Greivenkamp 1997). However, the localized changes in height can be
Chapter 1 Literature Review
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more easily visualized by subtracting the corneal best fit sphere from the
corneal height data (Young and Siegel 1995; Schwiegerling and Greivenkamp
1997).
Figure 1-6: Anatomical classification of the corneal surface. Adapted from Mountford et al. (2004).
Axial and tangential curvatures
Corneal surface slope relative to the vertex normal can be expressed using an
axial radius or a tangential radius (Figure 1-7). Axial or sagittal radius is defined
as the perpendicular distance from a point on the cornea to the vertex normal
of the instrument. These maps are a reasonable representation of corneal
topography for normal corneas, but they do not highlight localized corneal
changes similar to those seen in conditions such as keratoconus (Bafna et al.
1998; Elsheikh et al. 2007).
The tangential or instantaneous radius of curvature is defined as the
mathematical radius of curvature for the surface. It is only dependent on the
local curvature and its radius is independent of any axis. Therefore, it is more
sensitive to show localized changes in radius such as those in keratoconus. A
tangential map shows the position of the corneal apex more accurately than an
axial map.
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Refractive power map
The refractive power maps are generated from the focal length of the cornea
using Snell‟s law applied to paraxial rays intersecting the cornea (Atchison and
Smith 2000). These maps represent the effect of the shape of the cornea on
the optics of the cornea (Bafna et al. 1998).
Figure 1-7: Explanation of axial and tangential curvatures at a point on the cornea. Adapted from Mejía-Barbosa and Malacara-Hernández (2001).
Figure 1-8: Illustration of the types of topography maps for a representative subject, as captured by the Medmont E300 videokeratoscope: (a) Axial power (b) Tangential power (c) Refractive power and (d) Elevation
Chapter 1 Literature Review
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1.5.3.2 Quantitative descriptors of corneal regularity
Some authors have attempted to represent the regularity of the central corneal
shape using topographic indices (Wilson and Klyce 1991; Smolek et al. 1998;
Chastang et al. 1999). Two such indices are surface regularity index (SRI) and
surface asymmetry index (SAI). SRI is a representation of localized variation in
corneal power as measured over 10 central Placido disc rings of the
videokeratoscope. The lower the value of SRI the smoother is the surface. SAI
is a measure of the central corneal surface power asymmetry. SAI is a centrally
weighted summation of differences in corneal power at points 180° apart over
128 equally spaced meridians (Wilson and Klyce 1991). Again, the lower the
value of SAI, the more symmetric is the surface.
1.5.4 Corneal topography in normal population
Advances in corneal topographers have allowed detailed topography
measurement of not only anterior cornea but also the posterior cornea. This
section discusses the topography of the anterior and posterior cornea as seen
in the normal population.
1.5.4.1 Anterior corneal topography
Knowledge of corneal topography is important for contact lens fitting (Szczotka
1997; Reddy et al. 2000; Szczotka et al. 2002), conventional and customized
laser refractive surgery (Alessio et al. 2000; Knorz and Jendritza 2000;
Ambrosio et al. 2003; Kanjani et al. 2004; Kymionis et al. 2004; Varssano et al.
2004), diagnosis of corneal ecstatic disorders such as keratoconus
(Schwiegerling and Greivenkamp 1996), and to understand the effects of these
abnormalities on vision (Dingeldein and Klyce 1989). Generally, the cornea
flattens in the periphery or is prolate in shape as described in Section 1.5.
However, the topography of the normal cornea exhibits variations across the
normal population. Corneal asphericity also shows variation with the nasal and
superior cornea typically more prolate than the inferior and temporal cornea
(Clark 1974).
A common way of describing the corneal shape is in terms of the best
fitting conic section with radius of curvature R and asphericity Q, as described
earlier in Section 1.5. The mean corneal anterior radius of curvature varies from
7.67 mm to 7.86 mm among various studies (Kiely et al. 1982; Guillon et al.
1986; Eghbali et al. 1995; Atchison 2006; Read et al. 2006; Atchison et al.
2008). Read et al. (2006), using a videokeratoscope, measured corneal
Chapter 1: Literature Review
16
topography in 92 subjects and found mean corneal radius of curvature of 7.77 ±
0.2 mm and corneal asphericity of 0.19 ± 0.1 when a conic fit was applied to
data from the cornea‟s central 6 mm. Table 1-1 shows the mean values of
anterior corneal radius and asphericity as reported by various studies.
Table 1-1: Mean anterior corneal radius of curvature R and anterior corneal asphericity Q as reported by various authors.
Author (year) Method N Age
range (years)
Conic fit diameter
(mm)
Mean R (mm)
Mean Q
Kiely et al. (1982) Photokeratoscope 88 16 - 80 6 7.72 ± 0.3 0.26 ± 0.2
Guillon et al. (1986) Photokeratoscope and keratometer
110 17 - 60 9 # 7.78 ± 0.3 0.15 ± 0.2
Eghbali et al. (1995) Videokeratoscope 41 23 - 61 6 7.67 ± 0.2 0.18 ± 0.2
Douthwaite et al. (1999)
Videokeratoscope 98 20 - 59 6 7.86 ± 0.2 0.21 ± 0.1
Franklin et al. (2006) Videokeratoscope 15 19 - 36 7 7.72 ± 0.2 0.24 ± 0.01
Read et al. (2006) Videokeratoscope 92 18 - 35 6 7.77 ± 0.2 0.19 ± 0.1
Atchison et al. (2008) Videokeratoscope 106 18 - 69 6 7.75 ± 0.24
0.13 ± 0.14
# The topographer used by Guillon et al. (1986) was capable of measuring up to 9 mm, however the actual conic fit corneal diameter was smaller but not defined. N is the number of subjects.
Some authors have also qualitatively described corneal shape based on
the patterns seen in color coded axial topographic maps generated by the
videokeratoscope. The results of these studies are shown in Table 1-2.
Table 1-2: Distribution of qualitative topographic patterns as reported in various studies with videokeratoscopes
Topographic pattern Bogan et al.
(1990), N = 216 Rabinowitz et al. (1996), N = 195
Kanpolat et al. (1997), N = 114
Round 22.6% 25.1% 14%
Oval 20.8% 37% 11%
Symmetric bowtie 17.5% 21.8% 29%
Asymmetric bowtie 32.1% 10.2% 33%
Irregular 7.1% 5.9% 12%
To make the results consistent between studies, the distribution for superior steep and inferior steep patterns were combined with oval pattern, symmetric bowtie-skewed radial axis pattern was combined with symmetric bowtie pattern and asymmetric bowtie-superior steep, asymmetric bowtie-inferior steep and asymmetric bowtie-skewed radial axis patterns were combined with asymmetric bowtie pattern. N is the number of subjects.
1.5.4.2 Posterior corneal topography
The posterior corneal surface is optically less powerful than the anterior surface
due to the small difference in the corneal and aqueous humour‟s refractive
indices. It exhibits toricity (Dunne et al. 1991) and is thought to change in shape
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before the anterior corneal surface in disorders such as keratoconus (Mannis et
al. 1992; Tomidokoro et al. 2000) and therefore its topography can aid early
detection. Dunne et al. (1992) reported that the posterior corneal surface is
more toric than the anterior corneal surface. Accurate in-vivo measurement of
the posterior corneal shape is complicated due to the magnification and
distortion produced by the anterior cornea. Various methods have been
employed to measure posterior corneal shape such as Purkinje imaging
(Royston et al. 1990; Dunne et al. 1992; Garner et al. 1997; Lam and
Douthwaite 1997), combination of videokeratoscopy and pachymetry (Patel et
al. 1993; Edmund 1994; Lam and Douthwaite 1997), combination of slit
scanning and Placido disc (Liu et al. 1999; Módis et al. 2004; Patel et al. 2008),
and Scheimpflug photography (Dubbelman et al. 2002; Dubbelman et al. 2006;
Read et al. 2007; Uçakhan et al. 2008).
The Purkinje imaging technique is applicable only to perfectly spherical
surfaces, so was incapable of measuring corneal asphericity, which was then
estimated by combining the videokeratoscopic and pachymetry data (Patel et
al. 1993; Edmund 1994; Lam and Douthwaite 1997). The method of combining
videokeratoscopic and pachymetry data is susceptible to errors due to
misalignment and is rather time consuming.
Scheimpflug imaging on the other hand, is a newer technique that has
been applied to the measurement of posterior corneal topography (Shankar et
al. 2008). It eliminates any alignment errors as the anterior corneal curvature,
corneal thickness and posterior corneal curvature measurements are taken
simultaneously, accelerating the measurement procedure (Brown 1973).
Standard Scheimpflug imaging however, requires correction of the distortion
due to the geometry of the system and the refraction of anterior corneal
surface, which can otherwise lead to erroneous measurements of posterior
corneal shape (Dubbelman and Van der Heijde 2001; Dubbelman et al. 2005).
Posterior corneal curvature measurements reported by various authors are
summarized in Table 1-3.
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Table 1-3: Mean posterior corneal radius of curvature R (mm) and asphericity Q as reported by various authors. V and H represent vertical and horizontal meridians, respectively.
Author (year) Method N Age (yrs)
(years)
Mean R (mm) Mean Q
Royston et al. (1990) Purkinje imaging 5 - V = 6.42 -
Dunne et al. (1991) Purkinje imaging - V = 6.4 -
Patel et al. (1993) Videokeratoscopy and pachymetry
20 19 - 23 V = 5.80 ± 0.42 H = 5.82 ± 0.40
V = 0.36 ± 0.37
H = 0.48 ± 0.30
Garner et al. (1997) Purkinje imaging 120 16 - 17 V = 6.42 -
Edmund (1994) Videokeratoscopy and pachymetry
- V = 6.71 ± 0.23 V = 0.35
Lam and Douthwaite (1997)
Videokeratoscopy and pachymetry
60 20 # 6.51 ± 0.40 0.66 ± 0.38
Dubbelman et al. (2002)
Scheimpflug imaging 83 16 - 62 6.40 ± 0.28 0.52 ± 0.27
# median, N is the number of subjects.
1.5.5 Variations in corneal topography
Corneal topography has been shown to vary with age and gender and also
show some diurnal variations. It can also be affected by eyelid forces, refractive
error and ocular rubbing. This section discusses how the corneal topography is
affected by some of the above factors.
1.5.5.1 Diurnal variations in corneal topography and thickness
The shape and thickness of the cornea show slight diurnal variation. The
knowledge of these variations in corneal parameters is of importance for any
clinical or research related purposes requiring accurate measurements of these
corneal parameters. The diurnal changes in anterior corneal shape have been
documented by many authors with some differences amongst the studies.
Typically, the anterior cornea is flattest immediately after waking and then
gradually steepens as the day progresses (Reynolds and Poynter 1970;
Rengstorff 1972; Kiely et al. 1982; Read et al. 2005). The rate of corneal
steepening after waking is higher during the first half of the day compared to
the second half (Kiely et al. 1982; Read et al. 2005). Also, the increase in
corneal curvature along the horizontal meridian is more than the increase in
vertical meridian (Reynolds and Poynter 1970; Kiely et al. 1982). On the other
hand, the posterior cornea is found to be steepest immediately after waking,
followed by a gradual flattening during the day. Posterior corneal shape also
shows a slight increase in astigmatism in the morning with steepening of the
vertical meridian (Read and Collins 2009).
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1.5.5.2 Eyelid forces and corneal topography
Eyelid pressure due to reading (Buehren et al. 2003) and other near work
(Collins et al. 2006; Vasudevan et al. 2007) has been reported to cause a
range of corneal topographic (Buehren et al. 2001; Buehren et al. 2003; Collins
et al. 2005; Read et al. 2005; Collins et al. 2006; Collins et al. 2006; Shaw et al.
2009) and astigmatic changes (Read et al. 2007). Read et al. (2007) reported
that their subjects with larger palpebral apertures also had slightly flatter
corneas. They also reported that the angles of the upper and lower lid were
associated with corneal oblique astigmatism (J45).
Monocular diplopia after reading has also been recorded in a few
studies due to changes in corneal topography because of lid positions while
reading (Mandell 1966; Knoll 1975; Ford et al. 1997). Certain eyelid
abnormalities are also associated with changes in corneal topography, for
example ptosis (Holck et al. 1998; Brown et al. 1999; Ugurbas and Zilelioglu
1999), chalazia (Asseman et al. 1965; Rathschuler 1970; Nisted and Hofstetter
1974; Santa Cruz et al. 1997) and lagophthalmos (Goldhahn et al. 1999).
Narrowing and retraction of palpebral fissure have been shown to alter corneal
astigmatism (Wilson et al. 1982; Grey and Yap 1986; Lieberman and Grierson
2000). The narrower palpebral fissure during reading has been found to result
in corneal topographic changes causing slightly increased corneal power
(Shaw et al. 2008) and increased higher order aberrations (Buehren et al.
2005).
Authors studying the corneal topographic changes due to lid forces
have reported wave-like corneal distortions, especially in the areas of lid
contact on the cornea (Buehren et al. 2001; Buehren et al. 2003; Collins et al.
2005; Read et al. 2005; Collins et al. 2006; Collins et al. 2006; Shaw et al.
2009). The topographic changes are greater with longer durations (Collins et al.
2005; Shaw et al. 2009) and for larger angles of down gaze (Collins et al.
2006). Also, tasks involving eye movements affect greater change in corneal
topography than those requiring static fixation (Collins et al. 2006).
1.5.5.3 Refractive errors and corneal topography
Since the cornea is responsible for two-thirds of ocular refractive power, it is
logical to think that it may also have a contribution to the eye‟s refractive status.
Many studies have reported that the cornea becomes steeper with increasing
myopia (Stenstrom 1948; Sheridan and Douthwaite 1989; Goh and Lam 1994;
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20
Grosvenor and Scott 1994; Carney et al. 1997; Goss et al. 1997; Budak et al.
1999; Atchison 2006) and flattens with hypermetropia (Sheridan and
Douthwaite 1989; Mainstone et al. 1998; Strang et al. 1998; Llorente et al.
2004)
Most studies have reported no change in corneal asphericity with the
degree of myopia (Sheridan and Douthwaite 1989; Pärssinen 1991; Carkeet et
al. 2002; Atchison 2006) or hypermetropia (Sheridan and Douthwaite 1989;
Mainstone et al. 1998; Budak et al. 1999; Carkeet et al. 2002). However,
Carney et al. (1997) and Horner et al. (2000) found an increase in corneal
asphericity with myopia, whereas Davis et al. (2005) noticed a decrease in
corneal asphericity with myopia. Llorente et al. (2004) found that
hypermetropes had more positive corneal asphericity than myopes.
In spite of the inconsistencies among studies reporting changes in
corneal topography with myopia, it appears that refractive error does influence
the shape of the cornea (i.e. myopes have steeper corneas whereas
hypermetropes have flatter corneas than emmetropes). Corneal asphericity
does not seem to be affected by refractive status of the eye. However, it is still
unclear whether the change in corneal radius is the cause or consequence of
altered refractive status of the eye.
1.6 Corneal thickness
Corneal thickness provides an index of corneal hydration and indicates the
metabolic status of the cornea (Hedbys and Mishima 1966). Changes in
thickness indicate the physiological status of the cornea during hypoxia, trauma
and disease (Klyce 1981; Johnson et al. 1985; Mandell et al. 1989; Huff 1991).
There have been many studies reporting the diurnal changes in corneal
thickness. The central corneal thickness is maximum immediately after waking
and gradually decreases throughout the day (Mertz 1980; Kiely et al. 1982;
Feng et al. 2001; du Toit et al. 2003; Read et al. 2005; Read and Collins 2009).
The overnight increase in corneal thickness is about 5.5% (Harper et al. 1996).
The time to reach minimum corneal thickness varies from 5 to 10 hours after
waking (Kiely et al. 1982; Harper et al. 1996) although the largest changes are
noted in the first 1 to 2 hours after waking. The overnight increase in corneal
thickness results because the closure of the lids blocks the atmospheric oxygen
supply to the cornea, thereby inducing anaerobic metabolism and subsequent
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accumulation of lactate. This triggers osmotic influx of water into the cornea,
and increasing its thickness (Efron and Carney 1979; Klyce 1981; Holden et al.
1983). There is a strong correlation in diurnal changes seen in the anterior
(Kiely et al. 1982; Read and Collins 2009) and posterior (Read and Collins
2009) corneal curvatures and corneal thickness, suggesting that a substantial
proportion of the diurnal change in corneal curvature can be explained by the
diurnal change in corneal thickness.
1.7 Contact lenses
Contact lenses are optical devices placed on the cornea, most commonly used
to correct refractive errors such as myopia, hypermetropia, astigmatism and
presbyopia; but they can also be used for cosmetic and therapeutic purposes
(Lazarus 2007). Contact lenses are currently estimated to be used by 125
million people worldwide (Barr 2004). They are the second most common
option to correct refractive errors after spectacle lenses.
Contact lenses can be broadly classified into three main types: hard or
rigid, soft hydrogel and silicone hydrogel contact lenses depending on the
rigidity of the material from which they are made. Today the term “rigid lens”
refers to rigid gas permeable (RGP) lenses, unlike a couple of decades ago
when it was used for lenses made of polymethyl methacrylate (PMMA). PMMA
contact lenses were used in the past but their use is now limited to trial lenses.
These lenses are not gas permeable and therefore, interfere with the normal
metabolic activity of the cornea and can result in hypoxia and corneal edema.
RGP lenses were developed in the 1970s as an alternative to PMMA
contact lenses (DeRubeis and Shily 1985). These lenses permit increased
oxygen to the cornea compared to the PMMA contact lenses by direct
transmission through the lens material and by tear exchange due to lens
movement. RGP contact lenses are more flexible compared to the PMMA
lenses which aids in the tear exchange behind these lenses. Though soft
contact lenses are used more commonly today, RGPs are still widely used for
irregular (Griffiths et al. 1998) and post-surgical corneas (Beekhuis et al. 1991;
Steele and Davidson 2007) when soft contact lenses are not able to sufficiently
improve vision quality, for orthokeratology (Swarbrick 2006), and for general
refractive power correction (Johnson and Schnider 1991; Fonn et al. 1995).
Chapter 1: Literature Review
22
Soft hydrogel contact lenses are the most popular type of contact
lenses. Due to the lack of rigidity they conform to the shape of cornea providing
an increased level of comfort compared to RGP lenses. One of the earliest soft
contact lens materials available is hydroxyethyl methacrylate (HEMA). These
are stable, hydrophilic materials with good wettability but low oxygen
permeability. Soft hydrogel contact lenses are available in materials of varying
oxygen permeability. These lenses are more comfortable and require very little
adaptation compared to RGP lenses.
Silicone hydrogel (SiHy) contact lenses introduced in the late 1990‟s are
the latest development in soft contact lens materials (Tighe 2002). These
lenses have high oxygen permeability and are more comfortable compared to
the rigid lenses. Being hydrophobic by nature, SiHy lenses (first and second
generation) are treated by application of hydrophilic coatings or the addition of
certain wetting agents to make them hydrophilic. These lenses have very high
oxygen transmissibility and are better suited for extended and continuous wear.
Some of the third generation SiHy lenses do not require any hydrophilic
coatings or internal wetting agent.
1.7.1 Properties of contact lens materials
It is important to have an understanding of contact lens material properties
before we can investigate how they affect the corneal topography and
thickness. This section discusses some of the important contact lens material
properties.
In an open eye without a contact lens, the cornea receives most of its
oxygen from the atmosphere through the tear film, in addition to contributions
from the aqueous humour in the anterior chamber and limbal blood vessels
(Benjamin 1994). However, each of these routes caters to different parts of the
cornea (i.e. posterior corneal layers receive their oxygen through diffusion from
aqueous humour, about 1 mm of peripheral cornea receives it from limbal
vessels and the rest of the anterior cornea receives it from the tear film). When
the eye is closed the atmospheric oxygen is partly replaced by oxygen diffusion
from the palpebral conjunctival vessels (Benjamin 1994).
When a contact lens is placed on the cornea, it acts as a barrier
between the cornea and atmosphere and impedes oxygen supply to the cornea
resulting in hypoxic changes in the cornea (Smelser 1952). Therefore, to
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maintain normal corneal physiology, contact lenses need to be designed in
such a way that they allow constant exchange of lacrimal fluid between the lens
and the cornea, and also allow transmission of oxygen through the lens. The
ability of a contact lens to transmit oxygen to the cornea is described using two
parameters: oxygen permeability and oxygen transmissibility.
1.7.1.1 Oxygen permeability
Oxygen permeability (Dk) is the oxygen transmitted through a unit area of
contact lens material of unit thickness under unit pressure difference (Benjamin
1994). It is the product of the diffusion constant (D) and solubility (k) of oxygen
in the lens (Hwang et al. 1971; Brennan et al. 1987; Brennan et al. 1987;
Weissman and Fatt 1991; Tighe 2004). Dk is measured in the units of (cm/s)
(ml O2/ [ml x mmHg]). Dk is a physical property of contact lens material and is
not affected by contact lens design.
1.7.1.2 Oxygen transmissibility
Oxygen transmissibility (Dk/t) is oxygen permeability per unit thickness (in cm)
of a contact lens under specific conditions (Benjamin 1994). The units of Dk/t
are (cm2/s) (ml O2/ [ml x mmHg]). Dk/t is a physical property of the material and
the thickness and design of the lens. Dk/t is dependent on thickness of the
contact lens, and so varies with the power and design of the lens. Therefore,
Dk/t is more representative of the on-eye performance of the contact lens than
Dk alone.
1.7.1.3 Water content
Water content is a material property. The oxygen permeability of a contact lens
material is influenced by the water content of the material for the hydrogel
lenses. The logarithm of oxygen permeability shows a linear relationship with
the water content of the material (Sarver et al. 1981; Fatt and Chaston 1982).
In hydrogel materials, most of the oxygen diffuses through the water within the
hydrogel material and not through its polymer structure (Benjamin 1994).
However, this linear relationship does not present the true picture of the on-eye
performance of the lens because with increase in water content of the lens
material, there is an associated increase in lens thickness required to fabricate
a lens of a given optical power. The increase in thickness in turn results in a
decrease in Dk/t. For a given lens material, a critical minimum thickness is
required for durability and stability of lens parameters. The critical thickness is
about 0.03 mm for a 38% water hydroxyethyl methacrylate (HEMA) lens, while
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24
for a 70% water content material it is about 0.12 mm (Brennan and Carney
1987).
Oxygen transmissibility of rigid and silicone hydrogel lenses are not
limited by the water content of the material. The oxygen permeable component
in these materials is usually either silicone (siloxane) and/or a fluoro-polymer.
Although, the incorporation of these components in the rigid lens material is
limited by their inherent flexibility and non-wettability (Benjamin and Bourassa
1989), the rigid and silicone hydrogel lenses provide more oxygen to cornea
than hydrogel lenses. The Dk of rigid lens materials varies from 9 to 63 units
whereas that of silicone elastomer is as high as 104 units. The reason behind
the high Dk of silicone-based materials is that oxygen is more soluble in
silicone than it is in water. Therefore, unlike hydrogels, in silicone-based
contact lenses, oxygen is transmitted through the silicone polymer and the
water content of the material has little effect on the Dk of the material (Tighe
2004). The silicone contact lenses are usually coated to improve their
wettability, but this has negligible effect on their oxygen transmissibility (Refojo
et al. 1982).
1.7.1.4 Modulus of elasticity
This is a mechanical property of the contact lens material. Young‟s modulus of
a contact lens material is a measure of its ability to retain its shape against
forces such as the wrapping associated with the difference in contact lens base
curve and corneal curvature and lid pressure. A hard (or rigid contact lens)
material has high modulus (PMMA ≈ 2000 MPa, RGP (Boston XO) = 1500
MPa) whereas a soft material has lower modulus (Silicone elastomer = 8 MPa,
pHEMA = 0.5 MPa) and therefore, conforms to the shape of the cornea
(Stevenson 1994). In hydrogel lenses, the modulus is largely dependent on
their water content such that the modulus of a hydrogel lens (38% water
content is about 0.64 MPa whereas with 73% water content it is about 0.35
MPa) so that modulus exhibits an inverse relationship with the water content of
hydrogel lenses (Stevenson 1994).
1.7.1.5 Lubricity/coefficient of friction
Friction or coefficient of friction is a measure of the „lubricity‟ of a material, a
term first used by Isaac Newton (Jacobson 1991). Tribology is the study of
friction and lubrication on living tissues (Cher 2003). Lubricity of a contact lens
material is the amount of friction experienced by the eyelid during a blink when
Chapter 1 Literature Review
25
moving across the lens surface (Cher 2003; Tighe 2004). Plasma treatment is
used for the RGP (Yin et al. 2008; Yin et al. 2009) and silicone hydrogel contact
lenses (Valint Jr et al. 2001; Valint Jr et al. 2001) to increase its wettability or
hydrophilicity thus reducing the friction and improving comfort. The silicone-
based contact lenses which are hydrophobic, are treated to make them
hydrophilic by one of two processes. This is done by either surface treatment
with plasma gas to form an ultra-thin coat or by plasma oxidation to convert the
silicone to silicate. Another way to increase wettability and reduce friction is to
add a wetting agent to the material, for example polyvinylpyrrolidone (PVP)
(Carney et al. 2008).
1.8 Contact lenses and the eyelids
The eyelids constantly interact with the contact lens edges and surface during
each blink during the waking hours. If the contact lens surface is not smooth or
the edges are rough, it can lead to trauma of the eyelid surface or structures. A
range of changes in the eyelids and related structures have been linked with
contact lens wear.
The lid-wiper is a part of the marginal conjunctiva of the upper eyelid
that spreads the tears across the ocular surface or contact lens surface during
blinking (Korb et al. 2002). The lid-wiper consists of conjunctival tissue as well
as stratified squamous epithelium (similar to the cornea) and has thus been
shown to stain with both fluorescein as well as rose bengal/lissamine green
dyes. Lid-wiper epitheliopathy is often seen in contact lens wearers with dry
eye symptoms and in some asymptomatic soft contact lens wearers. It is
identified by an increase in fluorescein and rose bengal staining or lissamine
green staining (Korb et al. 2002; Yeniad et al. 2010). The influence of shorter
periods of contact lens wear upon the presence and severity of lid-wiper
staining and eyelid position is not known.
Blepharoptosis or ptosis is drooping of the upper eyelids which can be
noticed in early adaptive period of rigid lenses and has been documented after
long-term use of PMMA and RGP contact lens wear (Fonn and Holden 1988;
Van den Bosch and Lemij 1992; Fonn et al. 1995). Ptosis in the early adaptive
period is due to lid edema because of mechanical trauma due to lens edge and
usually dissipates as the eye gets adapted to the lens but the changes due to
long-term lens wear are usually persistent (Phillips 2007).
Chapter 1: Literature Review
26
1.9 Contact lenses and the tarsal conjunctiva
The tarsal conjunctiva is in close contact with the contact lens surface during
lens wear, and therefore can be affected directly by constant friction during
blinks. Redness and roughness of the tarsal conjunctiva secondary to use of
contact lenses have been widely reported in the past (Efron et al. 2001;
Skotnitsky et al. 2002; Maldonado-Codina et al. 2004). Differences in severity
of tarsal conjunctival redness was noted in soft and hard contact lens wearers,
with soft lenses showing greater redness (Korb et al. 1981; Korb et al. 1983).
No differences in palpebral roughness or redness between long-term high-Dk
silicone hydrogel contact lens wearers and non-lens wearers have been
reported (Covey et al. 2001).
Long term contact lens has also been reported to lead to a condition
called “contact lens papillary conjunctivitis (CLPC)” which manifests as
inflammatory changes in the tarsal conjunctiva (Allansmith et al. 1977;
Allansmith et al. 1978; Korb et al. 1980; Efron 1999). Silicone hydrogel lenses
are commonly associated with CLPC (Skotnitsky et al. 2002). However, CLPC
can occur with both soft and hard contact lenses especially in long-term contact
lens wearers (Allansmith et al. 1977; Maldonado-Codina et al. 2004).
1.10 Contact lenses and the tear film
As described in Section 1.3, the tear film plays a vital role in maintaining a
healthy, normal, functioning cornea. Dry eye symptoms are reported in about
50% of contact lens wearers (Doughty et al. 1997; Begley et al. 2000; Nichols
et al. 2002) and contact lens-related dry eye is frequently seen in the clinical
setting (Lemp 1995; Begley et al. 2000; Begley et al. 2001). When a contact
lens is placed on the cornea it divides the tear film into pre- and post-lens tear
films (Figure 1-9). The pre-lens tear film is thinner than the pre-corneal tear film
(Wang et al. 2003; Nichols and King-Smith 2004).
Normally, the oily lipid layer is spread over the aqueous layer by the
blinking action of the eyelids. But in the presence of a contact lens the lipid
layer rests on a considerably thinner aqueous layer and there is also a
disturbance in the smooth ocular surface over which the lids must move to
spread the tears (Guillon 1986). Contact lenses also result in changes in the
composition of the tear film due to altered rate of tear flow (Tomlinson 1992).
The tear film osmolarity is initially reduced (due to reflex tearing) followed by an
Chapter 1 Literature Review
27
increase in long term contact lens wearers either due to increased evaporation
as a consequence of disturbed lipid layer (Tomlinson and Cedarstaff 1982;
Iskeleli et al. 2002), or due to a decreased tear production as a consequence of
reduction in corneal sensitivity (Gilbard et al. 1986). Changes in mucin
production has also been reported in contact lens users (Greiner and
Allansmith 1981).
The lipid layer is an effective evaporation barrier and is reported to be
altered over the hydrogel contact lens (Guillon 1986; Young and Efron 1991)
and absent in the case of rigid lenses (Guillon 1986). Thinning of the lipid layer
is associated with an instability of the tear film in contact lens wear as indicated
by reduction in tear break-up time (TBUT) (Young and Efron 1991; Guillon and
Guillon 1994) and development of corneal staining (Guillon et al. 1990). The
average thinning rate of the pre-lens tear film has been shown to be less
compared to the pre-corneal tear film (Nichols et al. 2005).
Figure 1-9: Pre- and post-lens tear films and a contact lens on the cornea. Pre- and post-lens thickness values by King-Smith et al. (2004).
Studies have reported tear film surface quality using invasive and non-
invasive techniques. Bhatia et al. (1993) found mean pre-corneal fluorescein
TBUT in RGP contact lens wearers reduced significantly if the lenses were
worn for more than 8 hours per day. The pre-lens non-invasive tear break-up
time (NIBUT) in studies has been shown to be as low as 2-3 seconds on RGP
Chapter 1: Literature Review
28
contact lenses and 5-6 seconds in soft contact lenses (Guillon and Guillon
1993; Morris et al. 1998). The mean NIBUT of the pre-lens tear film of hydrogel
contact lens wearers using a custom instrument was found to be 6.1 seconds
(Faber et al. 1991). Pre-lens NIBUT was found to be 13.7 seconds after 6
hours of hydrogel contact lens use compared to 21.3 seconds at baseline in
tolerant contact lens users (Glasson et al. 2006). Whereas the pre-corneal
NIBUT in symptomatic contact lens wearers is reported to be as low as 3-10
seconds (Guillon and Guillon 1993). Tear film surface quality (TFSQ) has also
been described using high speed dynamic videokeratoscopy. A significant
reduction in TFSQ has been shown with hydrogel and silicone hydrogel contact
lenses compared to bare eye (Kopf et al. 2008; Alonso-Caneiro et al. 2009).
1.11 Contact lenses and anterior corneal topography
Contact lens wear is known to alter anterior corneal topography. The
magnitude of contact lens-induced topographic changes has been found to be
largely dependent on the material, design and duration of wear of the contact
lens. Corneal topography changes have been recorded with PMMA, RGP as
well as soft contact lenses (Miller 1968; Phillips 1990; Wilson et al. 1990; Ruiz-
Montenegro et al. 1993).
Many researchers have studied the effect of different types of contact
lenses on corneal topography and these studies are summarized below. The
studies are classified on the basis of instrument used (i.e. keratometer,
photokeratoscope and videokeratoscope).
1.11.1 PMMA contact lens wear and corneal topography
PMMA contact lens wear can lead to changes in corneal curvature due to the
mechanical forces associated with the wear of a hard lens and due to edema
caused by decreased oxygen supply to the cornea (DeRubeis and Shily 1985).
Studies using a keratometer
Central corneal steepening in 53% of his subjects was recorded by Miller
(1968) with the use of PMMA contact lenses and he correlated the steepening
with increased central corneal thickening due to corneal edema after 1 to 3
months of lens wear. On the other hand, studies measuring central corneal
Chapter 1 Literature Review
29
curvature after long-term use of PMMA lenses have found corneal flattening
(Rengstorff 1969; Levenson 1983), corneal steepening followed by flattening
(Hovding 1983), deformation (Bonnet and el-Hage 1968) or irregularity
(Levenson 1983).
Studies using a photokeratoscope
An increase in corneal asymmetry and irregularity in PMMA contact lens
wearers was reported by Ruiz-Montenegro et al. (1993) using a topographic
modelling system. They also noticed relative flattening of corneal curvature
under the contact lens and a relative steepening in other parts of the cornea.
These topographic changes correlated with the resting position of the contact
lenses (Wilson et al. 1990). Wilson et al. (1990) suggested that the cornea
begins to return to its original shape after cessation of lens wear.
To summarize the above studies, the central corneal curvature changes
after the use of PMMA contact lenses, showing corneal steepening with short-
term use of these lenses and flattening with long-term use of more than a year.
Local corneal edema in the central cornea could be the cause of corneal
steepening occurring in the early stages of the lens use.
1.11.2 RGP contact lens wear and corneal topography
Studies using a keratometer
Previous studies have found central corneal steepening with long term (3
months to few years) RGP contact lens wear (Bailey and Carney 1970;
Rengstroff 1973). Others have reported central corneal distortion after long
term use of many years (Calossi et al. 1996) or little changes after short-term
wear (DeRubeis and Shily 1985).
Studies using a photokeratoscope or videokeratoscope
An increase in corneal asymmetry and irregularity (Ruiz-Montenegro et al.
1993), altered corneal topographic pattern (Wilson et al. 1990) and a reversal
of normal topographic pattern from prolate to oblate (Maeda et al. 1994) have
been reported with the use of RGP contact lenses. On the other hand, no
significant corneal curvature changes after 21 days of high Dk RGP contact
lens wear using a videokeratoscope has also been observed (Yebra-Pimentel
et al. 2001). A relative flattening of the cornea under a decentred contact lens
and a relative steepening of the rest of the cornea has also been reported with
Chapter 1: Literature Review
30
RGP contact lens wear (Ruiz-Montenegro et al. 1993; Maeda et al. 1994;
Calossi et al. 1996).
In summary, most of the investigations on RGP contact lenses users
seem to suggest central corneal steepening occurs with these lenses. An
important observation of relative flattening of corneal curvature under a
decentred contact lens and a relative steepening of the rest of cornea have
been made in both PMMA and RGP contact lenses wearers, using the
photokeratoscope.
1.11.3 Soft hydrogel contact lenses and corneal topography
There have been many studies reporting the effect of soft hydrogel contact lens
on corneal topography but there is some inconsistency in the reports.
Studies using a keratometer
Most investigators who have investigated corneal curvature changes with the
use of soft contact lenses for a few months to many years have found central
corneal steepening (Harris et al. 1975; Montés-Micó et al. 2002; Schornack
2003). Montés-Micó et al. (2002) found a mean corneal steepening of up to
0.07 mm after 1 day of lens wear. On the contrary, Hovding (1983) reported
corneal flattening with the use of these lenses for a period of 1 year. Some
authors have noticed initial central flattening followed by steepening (Grosvenor
1975; Barnett and Rengstorff 1977; Hovding 1983) whereas others found no
change (Carney 1972; Sanaty and Temel 1996) after a period of a few months
to years of lens wear.
Studies using a photokeratoscope or videokeratoscope
Carney (1972) and Bailey et al. (1972) have reported corneal thickness
changes but no change in corneal curvature with short and long term use of
soft contact lenses. Mid-peripheral corneal steepening of about 0.5 D (Collins
and Bruce 1993) and steepening of most regions from the central to about 6.6
mm diameter (Yeniad et al. 2003) has been observed in soft contact lens
wearers after the lenses were worn for a period of 3 months to 12 months,
respectively. Ruiz-Montenegro et al. (1993) have reported corneal topography
alterations (change in asymmetry and irregularity) whereas Tomlinson (1976)
Chapter 1 Literature Review
31
observed inconsistent changes (steepening, flattening or no change) in central
and peripheral corneal topography in a small group of subjects.
In summary, most studies indicate corneal steepening after short and
long term use of soft contact lenses but reports suggesting corneal flattening
after long-term wear are also in the literature. The differences in the results of
changes in corneal curvature with contact lenses are likely to be due to
differences in lens designs, materials and duration of lens wear or time of
measurements. The mechanism of the contact lens induced corneal curvature
changes is unclear. The physical molding of the cornea by the back surface of
the contact lens and contact lens induced corneal edema have most commonly
been suggested as the causes of these changes (Carney 1975; Hovding 1983;
Ruiz-Montenegro et al. 1993).
1.11.4 Silicone hydrogel contact lenses and corneal topography
No studies describing corneal topography changes after short-term (hours or
days) use of silicone hydrogel lenses in open eye conditions are available.
Alba-Bueno et al. (2009) reported no significant changes in corneal topography
in a group of subjects using two different types of silicone hydrogel contact
lenses (first and second generation) worn for a period of 3 months on a daily
wear basis.
Corneal flattening during the first 3 months (which returned to baseline
at the end of 12 months) was observed in subjects fitted with silicone hydrogel
contact lenses on a continuous wear basis (Gonzalez-Meijome et al. 2003).
Santodomingo-Rubido et al. (2005) reported no change in corneal topography
after 6 months use of silicone hydrogel lenses in daily or continuous wear
basis.
1.11.5 Extended wear contact lenses and corneal topography
Extended wear of contact lenses can be defined as “the wearing of lenses
without removal during eye closure, for periods ranging from occasional
overnight wear to 30 days or more of continuous wear” (Phillips and Speedwell
1997). During the 1980‟s continuous use of contact lenses was considered
unsafe and thus this modality was not popular. But it was reintroduced in the
Chapter 1: Literature Review
32
late 1990‟s after the introduction of improved contact lens materials (Tighe
2002).
Studies using a keratometer
Central corneal steepening has been reported by many authors with the use of
hydrogel contact lenses on an extended wear basis (Binder 1979; Montés-Micó
and Ferrer-Blasco 2002). Iskeleli (1996) reported central corneal flattening with
the use of RGP contact lenses on an extended wear basis for a period of 6
months. Jalbert et al. (2004) found central corneal steepening in low Dk
hydrogel lens wearers and flattening in high Dk silicone hydrogel contact
lenses over a period of 12 months.
Studies using a videokeratoscope
Gonzalez-Meijome (2003) found initial corneal flattening returning to baseline in
subjects using silicone hydrogel lenses for 12 months. Significant corneal
curvature changes (Masnick 1971) and an altered corneal topographic pattern
(Ruiz-Montenegro et al. 1993) was reported in two different groups of subjects
using soft hydrogel contact lenses.
In summary, not many reports are available on corneal topography with
extended wear of contact lenses. Central corneal steepening with the use of
hydrogel contact lenses and central flattening with RGP contact lenses have
been shown. Studies using silicone hydrogel lenses suggested overall corneal
flattening. Jalbert et al. (2004) proposed redistribution of corneal tissue due to
pressure exerted by silicone hydrogel lenses as the cause of corneal flattening
with these lenses. The studies of contact lens induced corneal changes in
open-eye cannot be directly compared to those in closed-eye conditions due to
differences in hypoxia and eyelid pressure under closed-eye conditions.
1.11.6 Toric soft contact lenses and corneal topography
Corneal topography changes with toric soft contact lenses have not been
reported by many authors and no controlled studies are available in the
literature. Some case reports of patients presenting with corneal changes after
use of toric lenses have been presented. Hagan (1998) reported inferior
corneal steepening after soft toric lens wear using a videokeratoscope and
Schornack (2003) reported infero-nasal steepening using a slit-scanning
Chapter 1 Literature Review
33
topographer in toric soft contact lens wearing subjects. These reports of inferior
corneal steepening could be due to mechanical pressure by the thickened
region of the toric lenses, but the reports were based on just a few subjects.
1.11.7 Time of recovery of corneal changes caused by contact lenses
The time taken by the cornea to return to normal or baseline topographic
values has been investigated by many researchers. This has an important
application in contact lens patients opting for refractive surgery. Corneal and
refractive stability is vital before any such procedure can be performed.
Wang et al. (2002) found that the mean corneal topography recovery
rates in long term contact lens wearers (mean duration of 21.2 years) differed
for different types of contact lenses (soft daily wear 2.5 weeks, soft extended-
wear contact lens 11.6 weeks, soft toric 5.5 weeks and RGP lens 8.8 weeks).
Wilson et al. (1990) suggested that the time to reach corneal stability was up to
6 weeks for soft, 21 weeks for RGP and 6 months for PMMA lenses. Machat
(1996) reported the recovery time for soft daily wear (2-7 days), soft extended
wear (1-2 weeks), and PMMA (4 to 6 weeks) contact lens wearers. Budak et al.
(1999) found no differences in corneal topographic patterns between non-
contact and contact lens wearers after discontinuing soft contact lens wear for
2 weeks and RGP contact lens wear for 5 weeks. Ng et al. (2007) studied time
to stability in full-time soft contact lens wearers and found mean time for
stability of all subjects was about 16 days for keratometry, 28 days for
topography and 35 days for pachymetry after discontinuing the lens wear.
In summary it can be concluded that the time to stability of corneal
topography is proportional to the duration of lens wear. It also depends on the
type of the lens (corneal topography takes longer to recover from PMMA and
RGP contact lens wear compared to soft contact lenses wear), design of the
lens (corneas with toric contact lenses take longer to recover compared to
spherical contact lenses), modality of lens wear (eyes with extended wear
modality take longer to recover compared to daily wear).
1.12 Contact lenses and posterior corneal
topography
The posterior cornea is not affected mechanically by contact lenses. However,
posterior corneal curvature changes have been associated with corneal edema
Chapter 1: Literature Review
34
(Kikkawa and Hirayama 1970; Lee and Wilson 1981; Erickson et al. 1999).
Previous studies have recorded posterior corneal flattening along with central
corneal swelling associated with low Dk soft (Martin et al. 2009) and low Dk soft
and PMMA lens wear (Moezzi et al. 2004). These studies were conducted in
closed eye or extended wear conditions. The posterior corneal curvature
changes related to daily wear (open eyes) of various contact lenses have not
been studied.
1.13 Contact lenses and corneal thickness
Corneal (central and peripheral) thickness is an important indicator of the
metabolic changes occurring in the cornea with the use of contact lenses. As
mentioned earlier, contact lenses can affect the corneal oxygen uptake, thereby
altering the cornea‟s metabolism. One of the most important indicators of
corneal hypoxia is a change in corneal thickness (Section 1.4.2). The effect of
contact lens wear on corneal thickness has been investigated in many studies
and is discussed in the sections below. Central and peripheral corneal
thickness changes are discussed in separate sections.
1.13.1 Effect of contact lens wear on central corneal thickness
A review of the literature reveals a number of studies documenting the effect of
short and long term use of contact lenses. Contact lens wear of up to 6 hours
results in an increase in central corneal thickness. Fonn et al. (1984) reported
corneal thickening of 5.5% and 2.2% with PMMA and RGP lenses,
respectively. They also observed that thinner RGP lenses cause less swelling
than thicker lenses, and flatter fitting lenses caused less swelling than steeper
RGP lenses. Carney (1974) found an increase in thickness of about 30 to 37
µm subsequent to PMMA contact lens wear for 3 months. Harris et al. (1981)
reported a 4% increase in corneal thickness after 6 hours of hydrogel soft lens
wear under closed eye conditions.
Previous studies reporting the longer term effects of more than 2 years
of contact lens wear have generally found central corneal thinning ranging from
13 to 37 µm with RGP lenses and 3 to 22 µm with soft lenses (Myrowitz et al.
2002; Braun and Anderson Penno 2003; Iskeleli et al. 2006). Liu and
Pflugfelder (2000) found a reduction in corneal thickness of 40 µm in subjects
who had worn soft and RGP contact lenses for more than 5 years. Braun and
Chapter 1 Literature Review
35
Anderson Penno (2003) and Iskeleli et al. (2006) found a similar magnitude
(difference of 2 µm) of corneal thinning in subjects who had worn RGP and soft
contact lens wearers for 2 to 5 years. Myrowitz et al. (2002), on the other hand,
reported corneal thinning of 3.2 µm with soft and 37 µm with RGP in contact
lens wearers of about 16 years.
In summary, corneal thickness changes appear to relate to the duration
of lens wear. Generally, short-term contact lens wear seems to increase the
central corneal thickness whereas long-term contact lens wear leads to
decrease in central corneal thickness. With long-term contact lens wear, central
corneal thinning is greater with RGP contact lenses compared to soft contact
lenses.
1.13.2 Effect of contact lens wear on peripheral corneal thickness
Carney (1974) reported peripheral corneal swelling of up to 20 µm at the two
peripheral corneal points after 3 months of PMMA contact lens wear. Liu and
Pflugfelder (2000) found corneal thinning of 40 µm in long term (more than 5
years) contact lens wearers at 8 peripheral corneal locations. Yeniad et al.
(2003) studied the corneal thickness at the same 8 peripheral corneal locations
and found that both RGP and soft contact lens wear caused significant corneal
thinning after 6 and 12 months.
Wang et al. (2003) found 3.8% and 3.0% increase in peripheral corneal
thickness with soft (HEMA) and PMMA contact lenses respectively after 3
hours of wear under closed eye conditions. Martin et al. (2008) measured
thickness at 4 peripheral corneal locations and found that high Dk lenses
induced significantly less corneal swelling than low Dk lenses after 1 week of
extended wear.
To conclude, as in the central cornea, short-term lens wear leads to
peripheral corneal swelling whereas long-term lens wear leads to peripheral
corneal thinning. Corneal thickness changes associated with contact lens wear
depend on many factors, including the type and material of lens (such as
PMMA, RGP or soft contact lens), the duration of lens wear (such as short-term
- few hours and long term - many years), the modality of lens wear (such as
daily wear and extended wear) and also on the thickness and fit of the lenses.
Chapter 1: Literature Review
36
1.13.3 Mechanism of corneal thinning
Long-term contact lens wear has been shown to cause corneal thinning. One of
the mechanisms proposed is a loss of keratocyte cells found in the stroma (Liu
and Pflugfelder 2000). Keratocytes are responsible for the synthesis of
collagen, glycoproteins and proteoglycans which form the mass of the corneal
stroma (Holden et al. 1985). Apoptosis of the keratocytes occurs due to
secretion of mediators such as interleukin-1 from the damaged corneal
epithelial cells (Bonanno and Polse 1985). Loss of stromal tissue could also be
caused by accumulation of lactic acid in the cornea secondary to chronic
hypoxia, resulting in loss of mucopolysaccharide ground substance (Braun and
Anderson Penno 2003).
1.14 Contact lenses and orthokeratology
Orthokeratology is a clinical procedure to temporarily reduce or eliminate
refractive error by using specially designed contact lenses to alter corneal
topography. Orthokeratology gained acceptance and popularity in the mid-
1990s with the advent of the computerized corneal topography, development of
high Dk RGP materials and reverse-geometry lens designs. The typical design
of a reverse-geometry contact lens for myopia correction consists of a central
BOZR that is flatter than the central corneal curvature which flattens the central
corneal curvature by pressure. This technique is most popular for correction of
low to moderate levels of myopia of up to 4 D but has also been attempted for
correction of hyperopia (Gifford and Swarbrick 2008; Gifford and Swarbrick
2009) and with-the-rule astigmatism (Mountford and Pesudovs 2002).
Changes in anterior corneal curvature with reverse-geometry ortho-k
contact lenses have been well studied (Swarbrick et al. 1998; Owens et al.
2004). Significant central corneal flattening most prominent in the central 5-6
mm diameter with ortho-k lenses within one night of wear has been reported
using videokeratoscopy (Swarbrick et al. 1998; Nichols et al. 2000; Swarbrick
and Alharbi 2001; Soni et al. 2003; Owens et al. 2004). These changes have
been shown to occur within minutes after open eye orthokeratology (Sridharan
and Swarbrick 2003; Jayakumar and Swarbrick 2005). Researchers have
presented varying results on changes in posterior corneal curvature with ortho-
k. Tsukiyama et al. (2008) showed evidence that no changes occur in posterior
cornea using a Pentacam and that ortho-k changes are mainly anterior
Chapter 1 Literature Review
37
whereas posterior corneal flattening in both central and peripheral regions has
been reported by others (Owens et al. 2004).
Corneal curvature and thickness changes have been suggested to
occur due to a redistribution of corneal tissue rather than overall bending of the
cornea (Swarbrick et al. 1998; Choo et al. 2004; Tsukiyama et al. 2008). This
results in central corneal thinning, with the reported thinning from 12 to 24 µm
(Swarbrick et al. 1998; Nichols et al. 2000; Swarbrick and Alharbi 2001; Soni et
al. 2003; Owens et al. 2004) along with mid-peripheral corneal thickening
(Swarbrick et al. 1998; Wang et al. 2003). Most of the corneal thickness change
occurs after one day of ortho-k contact lens wear (Nichols et al. 2000; Soni et
al. 2003). Further evidence for this is provided by authors who have reported
histological changes in animal corneas (Matsubara et al. 2004; Cheah et al.
2008) showing central epithelial thinning with mid-peripheral thickening. These
findings were supported by Choo et al. (2008) who showed that changes in the
cornea are dependent on the design of the lens back surface and duration of
the contact lens wear.
The corneal changes that occur with orthokeratology show a quick
recovery to the baseline values. The recovery is as rapid as the development of
the anterior corneal curvature changes (Gonzalez-Meijome et al. 2008). Haque
et al. (2004) observed a recovery of corneal curvature changes within 3 days of
lens discontinuation whereas Soni et al. (2004) reported full recovery after 1
week of lens discontinuation. The corneal thickness returns to baseline values
even faster. The thickness recovers almost fully just after one night of lens
discontinuation (Soni et al. 2004).
1.15 Rationale
The influence of soft, hard and rigid contact lenses on corneal topography has
typically been limited to the measurement of central anterior corneal curvature
with keratometers. However, the advent of new technologies such as
computer-based videokeratoscopes and Scheimpflug imaging allow
assessment of corneal topography of large corneal areas with many thousands
of data points. Additionally, the magnitude and nature of changes in the
posterior cornea after daily wear (open eyes) of various contact lenses is not
known.
Chapter 1: Literature Review
38
Similarly, the effects of contact lenses on corneal thickness have been
measured either at the centre of the cornea or at a few locations in the
peripheral cornea along the horizontal meridian. Regional changes in corneal
periphery with soft toric lenses (which have their thickest portions in the
periphery) have never been investigated. Again the use of Scheimpflug imaging
technology allows the thickness of the cornea to be measured across most of
the cornea and also provides information about the shape of the posterior
cornea, a surface which has received little research attention. In the following
series of experiments we plan to use Scheimpflug imaging and
videokeratoscopy to accurately assess the changes in corneal topography and
thickness associated with the short-term wear of contact lenses, considering
both the central and peripheral corneal surface.
Many of the previous studies of the effects of contact lens wear on the
cornea were conducted with only a few subjects and often the lens type,
design, material or duration of lens wear were not strictly controlled. Therefore,
one aim of this project was to provide better control of wearing time and lens
parameters, so as to allow more meaningful comparisons of the influence of a
range of contact lens variables (materials, designs, powers and diameters) on
corneal curvature and thickness. This was achieved by using a randomised
controlled cross-over study design. A range of older (HEMA, PMMA) and newer
(SiHy, Boston XO) lens materials were selected in order to compare the best
available lens materials with high oxygen permeability against some older
materials with poor oxygen permeability. The experiments were conducted over
a period of 8 hours in order to study a range of lenses on each subject, with
care taken to ensure a sufficient wash-out period after each lens wearing day,
before the next lens was worn. In addition to corneal changes, contact lenses
can also affect the eyelids and tarsal conjunctival surface which are in constant
friction with the lens edge and surface. This micro-trauma has been shown to
result in ptosis, lid-wiper epitheliopathy or contact lens papillary conjunctivitis
with long-term lens wear. However, there is a lack of understanding of the
effect of short-term wear of contact lenses of different types on eyelid
structures and this will be investigated by grading the staining of tarsal and lid-
wiper conjunctiva and evaluating the lid position.
The tear film forms an important optical surface of the eye and also
lubricates the ocular surface. Contact lenses are known to disrupt and cause a
Chapter 1 Literature Review
39
variety of changes to the tear film. New method has been developed to quantify
TFSQ using non-invasive technique which is based on properties of Placido
disc images. This technique has been used to quantity TFSQ with and without
soft contact lenses. But the TFSQ with RGP contact lenses using a non-
invasive technique is not known.
We hypothesized that there are no significant ocular surface changes
with short-term contact lens wear. The following series of experiments therefore
aimed to study the short-term (8 hours) influence of contact lens wear on the
surface of the eye and the eyelids. By carefully controlling the contact lens
variables and the lens wearing times and accounting for natural diurnal
variations, we could directly compare the changes that these contact lens
parameters caused to the anterior eye.
Chapter 1: Literature Review
40
- 41 -
Chapter 2
Corneal changes following short-term soft contact lens wear
2.1 Introduction
Contact lens wear is known to alter corneal shape and thickness. The anterior
corneal topography can be altered by changes in the mechanical forces (e.g.
orthokeratology) and/or normal metabolism (e.g. induced swelling). Changes in
anterior corneal topography are reported with different types of contact lenses
including poly methyl methacrylate (PMMA) (Hartstein 1965; Miller 1968;
Wilson et al. 1990), rigid gas permeable (RGP) (Bailey and Carney 1970; Ruiz-
Montenegro et al. 1993; Schwallie et al. 1995; Yebra-Pimentel et al. 2001), soft
hydrogel (Carney 1975; Grosvenor 1975; Harris et al. 1975; Collins and Bruce
1993; Montés-Micó et al. 2002; Arranz et al. 2003; Schornack 2003) and
silicone hydrogel (Gonzalez-Meijome et al. 2003) contact lenses. Signs of
corneal topography changes may include central corneal flattening or
steepening, changes in regular or irregular astigmatism, changes in the axis of
astigmatism and loss of radial symmetry (Wilson et al. 1990; Ruiz-Montenegro
et al. 1993) or changes in optical higher order aberrations (Lu et al. 2003).
While these contact lens-induced topography changes are well reported with a
range of different lens materials and designs, no controlled studies
investigating the effect of soft toric contact lens designs on corneal shape and
thickness are available.
Additionally, little is known about the effects of contact lens wear on
posterior corneal shape, with some evidence of posterior corneal flattening
found to be associated with one week of extended wear (Martin et al. 2009)
and 3 hours of closed eye soft and PMMA lens wear (Moezzi et al. 2004). The
magnitude and nature of posterior corneal change associated with daily wear of
various contact lenses is not known. Whilst the posterior cornea makes a
smaller contribution to the eyes total refractive power than the anterior surface,
knowledge of how contact lens wear influences its shape is important for any
research and clinical application requiring accurate and precise posterior
corneal measures, particularly for monitoring corneal changes over time.
Knowledge of the impact of contact lens wear on the posterior cornea may also
be important given recent suggestions that posterior corneal measures may be
Chapter 2: Corneal changes following short-term soft contact lens wear
42
useful in the early detection of corneal ectatic disorders (Rao et al. 2002;
Bessho et al. 2006).
Corneal thickness is an important indicator of the metabolic status of the
cornea (Smelser and Ozanics 1952; Hedbys and Mishima 1966) and hypoxic
stress induced by the contact lenses determines the changes in corneal
thickness with contact lens wear (Bergmanson and Chu 1982). Corneal
swelling has been reported after short periods of wear of soft contact lenses in
open (Bailey and Carney 1973) and closed eye conditions (Harris et al. 1975;
Harris et al. 1981) in the central cornea as well as in closed eye conditions in
the peripheral cornea (Wang et al. 2003). The results of majority of these
studies are based on corneal swelling measurement of a single point in the
centre (Harris et al. 1977) or single points in the periphery (Kaluzny et al. 2003)
to give the peripheral swelling. This does not give an accurate measure of
regional corneal swelling. An important aim of this study was to investigate
regional swelling in corneal periphery since soft toric lenses have their thickest
portions in the periphery of these lenses.
The diurnal changes in corneal thickness are well studied. These
changes are small but significant with the largest changes being obvious in the
morning on awakening (du Toit et al. 2003; Read and Collins 2009) with
significant corneal swelling along with slight flattening of anterior corneal
curvature and slight steepening of the posterior corneal surface (Read and
Collins 2009). Slight thinning of the cornea throughout the day (morning to
afternoon) has also been reported (Feng et al. 2001; Read and Collins 2009).
These natural corneal diurnal variations can potentially confound studies
investigating contact lens induced corneal changes (for both daily and
extended wear conditions) if they are not taken into consideration.
Thus the main aims of this study were to investigate the short-term
effect of different types of contact lenses on the regional distribution and
magnitude of change in corneal thickness and topography in the anterior and
posterior cornea. We also measured the natural diurnal changes in corneal
thickness and curvature on separate days to account for these variations and
provide an accurate measure of the corneal changes related to the contact
lenses.
Chapter 2: Corneal changes following short-term soft contact lens wear
43
2.2 Methodology
2.2.1 Subjects
Twelve healthy young adult (mean age = 26.8 ± 2.9 years; range 21 – 32
years) subjects (5 females and 7 males) with mean spherical equivalent
refractive error of –1.6 ± 2.6 D (range: –6.25 D to 0.75 D) were recruited for the
study. All subjects had a refractive or corneal astigmatism of ≤ 1.50 D and a
best corrected visual acuity of 6/6 or better. Normal tear film, anterior segment
and central corneal thickness [in the range of 475 to 596 microns, based on the
normative values reported by Doughty and Zaman (2000)] were ensured by a
series of preliminary tests. None of the subjects were regularly using any ocular
or systemic medication and none reported any history of ocular injury, infection
or surgery.
One of the subjects was a regular soft contact lens wearer but had
discontinued lens wear at least one month before the start of the study. None of
the subjects were RGP contact lens users. It was calculated based upon pilot
studies conducted before the start of this study, that the sample size of 12
subjects would give 80% power to detect 2.7 microns change in corneal
thickness and 0.01 mm change in anterior corneal curvature at the 0.05 level of
significance. The study was approved by Queensland University of Technology
(QUT) Human Research Ethics Committee and followed the tenets of
Declaration of Helsinki. All subjects signed a written informed consent before
the start of the study (Appendix A).
2.2.2 Instrumentation
The Pentacam HR system (Oculus, Wetzlar, Germany) which uses a rotating
Scheimpflug camera (a digital camera with a slit illumination system) to assess
the anterior segment of the eye was used to measure regional corneal
thickness and anterior and posterior corneal topography. The instrument has a
central fixation target and a monochromatic (blue light emitting diode at 475
nm) slit of light which illuminates the anterior eye of the subject. The “25 picture
3D scan” mode which gives 25 cross-sectional images of the anterior eye was
used for the measurements. It automatically captures images when correct
alignment is attained and provides a measurement of reliability as a „quality
specification‟ (QS) for each 3D scan which checks for alignment, eye
movements and any missing or invalid data. Any unreliable measurements
Chapter 2: Corneal changes following short-term soft contact lens wear
44
were repeated and a total of 5 reliable corneal scans were captured during
each measurement session. Studies have reported the Pentacam to be highly
repeatable for measurements of central and peripheral corneal thickness
(Shankar et al. 2008; Miranda et al. 2009). The Pentacam also shows excellent
repeatability and reasonable accuracy for measuring the anterior corneal axial
curvature (Read et al. 2009), as well as reasonable repeatability for posterior
corneal curvature, especially when the average of a few readings are taken
(Chen and Lam 2007).
2.2.3 Contact lenses
The lenses were empirically ordered according to manufacturer‟s
recommendations (Gelflex Laboratories, Perth, Western Australia) but any
unacceptable fits were reordered to obtain optimal movement and centration.
The lenses used were specifically ordered for the study and were of 4 different
types and were chosen to compare the effects of different lens materials
(hydrogel and silicone hydrogel), designs (spherical and toric) and powers (–
3.00 and –7.00 D) on corneal thickness and curvature (Figure 2-1 and Table 2-
1). For example SiHy/Sph/–3 (lens 1) and SiHy/Sph/–7 (lens 2) are of same
design with different powers whereas SiHy/Sph/–3 (lens 1) and SiHy/Toric/–3
(lens 3) are of different designs (sphere vs. toric) but with the same power. The
toric lenses had no cylindrical power but a spherical power in the optic zone
and a toric stabilized design. The thickest portion of the toric lenses was 0.35
mm which was at the location of the stabilization zones (i.e. at 4 and 8 o‟clock
positions or about 30 degrees below horizontal) (Figure 2-2c). This allowed the
effect of the lens stabilizing design to be studied in the comparison between
SiHy/Sph/–3 (lens 1) and SiHy/Toric/–3 (lens 3), with all other parameters
remaining the same. SiHy/Toric/–3 (lens 3) and HEMA/Toric/–3 (lens 4) had the
same design and power but different materials [i.e. silicone hydrogel (SiHy) and
hydroxyethyl methacrylate (HEMA)] (Figure 2-1).
The lenses used in the study were selected based on a number of pilot
studies in which a variety of contact lenses were worn for varying durations. All
the contact lenses were tested for base curve, power and lens diameter by the
manufacturer. The lenses passed the acceptance tolerances of the
manufacturer, which were centre thickness ± 0.01 mm (when measured dry),
base curve: ± 0.20 mm, lens diameter: ± 0.20 mm, and lens power: ± 0.50 D.
The back vertex power, total diameter and BOZR were also rechecked and
Chapter 2: Corneal changes following short-term soft contact lens wear
45
found to conform to the manufacturers stated tolerances. The contact lens
parameters are described in Table 2-1.
Figure 2-1: The powers, designs and materials of the contact lenses used. The comparisons to investigate the effect of lens characteristics on corneal thickness and curvature are represented by curved arrows. The materials were silicone hydrogel (SiHy) and hydroxyethyl methacrylate (HEMA).
Table 2-1: Details of the lenses used in the study.
Parameter Lens 1 Lens 2 Lens 3 Lens 4
Acronym SiHy/Sph/–3 SiHy/Sph/–7 SiHy/Toric/–3 HEMA/Toric/–3
Design Spherical Spherical Toric Toric
Material SiHy SiHy SiHy HEMA
Power (Dioptres)
–3.00
(±0.50) *
–7.00
(±0.50) *
–3.00
(±0.50) *
–3.00
(±0.50) *
BOZR (mm) 7.7 – 8.9
(± 0.20) *
7.7 – 8.9
(± 0.20) *
7.7 – 8.9 (± 0.20) *
7.8 – 9.0 (± 0.20) *
Total diameter (mm)
14.8 (± 0.20) *
14.8 (± 0.20) *
14.8 (± 0.20) *
14.0 (± 0.20) *
FOZD (mm) 8.00 8.00 8.00 8.00
BOZD (mm) 13.8 13.8 13.8 13.0
Water content 54% 54% 54% 38%
Dk 53 53 53 8-10
Modulus of elasticity (MPa)
0.35 0.35 0.35 0.50
Manufacturing method
Lathe Lathe Lathe Lathe
Surface treatment
Plasma Plasma Plasma None
Centre thickness (mm)
0.11 (± 0.01) *
0.11 (± 0.01) *
0.11 (± 0.01) *
0.11 (± 0.01) *
Edge thickness (mm)
0.10 0.10 0.08 0.09
Manufacturer lens parameter tolerances are given in brackets *. Lens centre thickness was checked by the manufacturer in its dry state. SiHy: silicone hydrogel, HEMA: hydroxyethyl methacrylate (hydrogel), BOZR: back optic zone radius, mm: millimetre, FOZR: front optic zone radius, BOZR: back optic zone radius, Dk: oxygen permeability, MPa: megapascal, unit of modulus of elasticity. The lenses supplied by the manufacturer were checked for back vertex power, lens diameter, BOZR and edge and surface quality after use in the study.
Chapter 2: Corneal changes following short-term soft contact lens wear
46
The contact lens thickness profile was calculated for the soft contact
lenses using the design parameters provided by the manufacturer (Figure 2-2).
The edge of the front optic of higher power –7.00 D spherical lens (Figure 2-2
b) is thicker compared to the –3.00 D lens (Figure 2-2 a). The thickness profile
of the two toric lenses (Figure 2-2 c) shows the thickest portion of the lens at
the location of the stabilization zones i.e. at 4 and 8 O clock or about 30
degrees below horizontal.
Figure 2-2: Contact lens thickness profiles for the SiHy/Sph/–3 (a), SiHy/Sph/–7 (b) and SiHy and HEMA/Toric/–3 (c). The color scale represents lens thickness in mm. The thickness profile for the SiHy and HEMA toric contact lens is identical (c).
2.2.4 Measurements and Protocol
Subjects wore a different type of contact lens in the left eye on four separate
days, for 8 hours each day. All 4 types of contact lenses were worn by each
study participant and the order of wear of the contact lens type was
randomized. New lenses were used for each subject and for each trial. The
lens wearing eye (left eye) was occluded during the 8 hours of lens wear with a
frosted spectacle lens, to avoid any visual discomfort due to aniseikonia. Eight
hours lens wear duration was chosen because it was practical (most students
and staff spent about 8 hours at work/university), we found statistically
significant corneal changes within 8 hours of lens wear in our pilot experiments,
and this short duration allowed us to study a variety of lenses on each of the
subjects along with a sufficient wash-out period between the wearing of
different lenses.
Measurements of the cornea were taken twice daily. Morning
measurements were taken between 8 and 11 am and at least 2 hours after
waking. This was done to avoid the influence of overnight corneal swelling on
the morning measurements. It has been observed that the cornea is the
Chapter 2: Corneal changes following short-term soft contact lens wear
47
thickest immediately after waking in the morning and typically returns to normal
thickness within 2 hours (Kiely et al. 1982; Harper et al. 1996; Read and Collins
2009). A second set of measurements were taken in the afternoon (between 4
and 7 pm). Pentacam measurements were completed within 5 - 10 minutes of
lens removal in the afternoon to avoid any substantial recovery of corneal
changes caused by contact lens wear.
In order to record the individual‟s natural diurnal and between day
variations in corneal thickness and topography two days of baseline
measurements were taken on the left eye without any contact lenses being
worn, both in the morning between 8 and 11 am) and repeated in the afternoon
(between 4 and 7 pm ) after 8 hours.
A two day wash-out period was used after each lens wearing day to
allow for the full recovery of corneal curvature and thickness changes due to
contact lens wear. This wash-out period was chosen based on pilot studies
performed using soft and rigid contact lenses worn for a period of 8 hours. We
found that regression of corneal changes was rapid, with the majority of
changes recovered by the first day after lens wear, while on the second follow-
up day the corneal curvature and thickness had returned to baseline values
(within the limits of measurement error). As a further precaution, for the first two
subjects in the study, measurements were taken on two consecutive days
following each lens wearing day to monitor the regression of the contact lens
induced curvature and thickness changes. We again found that the regression
of corneal changes was rapid and by the second follow-up day the corneal
curvature and thickness returned to baseline values. Based on these findings, a
two day recovery period was scheduled after each day of lens wear and before
the wearing of the next lens, to ensure no persistence of lens related ocular
changes into subsequent lens wearing days.
Given that the subjects were typically at work or university during the
day, there could be some topography changes associated with reading and
downward gaze (Buehren et al. 2003; Collins et al. 2006; Vasudevan et al.
2007). However, a pilot study using contact lenses revealed that these changes
in corneal curvature caused by the eyelids were relatively small in comparison
with the effects of the contact lenses on corneal shape and thickness (See
Figure 2.3). Subjects were also instructed to avoid any significant reading work
(or any other activity involving down gaze), at least one hour before the
Chapter 2: Corneal changes following short-term soft contact lens wear
48
measurement sessions, in order to avoid substantial corneal topography
changes due to reading and down gaze. A questionnaire was completed at the
end of the day to record the type of visual tasks performed during the 8 hours
of lens wear and the subjects were mostly found to be involved in computer
work. The use of computers typically involves about 10 degrees down gaze and
should cause minimal corneal changes compared with reading which involves
20 degrees or more down gaze and leads to eyelid locations closer to the
corneal centre (Shaw et al. 2008; Shaw et al. 2009).
In order to determine the centration of lenses and rotation of the toric
lens in relation to the cornea, digital images of the eye with contact lens in vivo
were captured in the morning on the lens wearing days with a high resolution
digital camera attached to a slit lamp and sufficient time (10 to 20 minutes) was
given for the contact lens to settle in the eye. The subject was positioned on the
head rest of the slit lamp with eyes in primary position and an external white
ring light was used for illumination.
Figure 2-3: Axial curvature difference maps for a subject after 60 minutes of downgaze task in (a) baseline (no contact lens wear) and (b) with a soft contact lens in eye
2.3 Data analysis
2.3.1 Curvature and thickness difference maps
Corneal thickness and axial curvature maps, in the form of a square grid with a
point spacing of 0.1 mm, were exported from the Pentacam. An average of the
5 maps (taken during each measurement session) was calculated using
custom software developed in the Contact Lens and Visual Optics Laboratory,
QUT. Thickness and curvature difference maps were generated to compare the
baseline maps to post-lens removal maps from all the 12 subjects, for each of
Chapter 2: Corneal changes following short-term soft contact lens wear
49
the 4 lens types. „Thickness difference maps‟ were generated by subtracting
the „average baseline thickness‟ map from the „average thickness map after 8
hours of lens wear‟. Similarly, „curvature difference maps‟ were generated by
subtracting the „average baseline curvature map‟ from the „average curvature
map after 8 hours of lens wear‟.
2.3.2 Regional analysis
To study the regional changes in corneal thickness and curvature after contact
lens wear, the data from all the subjects was averaged. The average corneal
thickness and curvature was then calculated for each subject within two corneal
regions (i.e. central and peripheral) as shown in the Figure 2-4. A diameter of 8
mm was selected for this analysis, since data was available for all the subjects
out to this diameter and there were no gaps (e.g. upper lid interference) in the
data.
Figure 2-4: Cornea divided into central (4 mm diameter) and peripheral (4 mm annulus) regions.
2.3.3 Corneal best fit sphero-cylindrical power
Corneal axial power data was used to calculate the best fit corneal sphero-
cylinder in the form of power vectors M (best fit sphere), J0 (with and against-
the-rule astigmatism) and J45 (oblique astigmatism) using the method
described by (Thibos et al. 1997) for the central (0-4 mm) and peripheral (4-8
mm) corneal annulus regions. The root mean square error (RMSE) between
the axial corneal power and best fit corneal sphero-cylinder were also
calculated for each map. The RMSE from the best fit corneal sphero-cylinder
represents the higher order aberrations of the corneal surface height. This is
similar to the optical aberrations of the anterior corneal surface, but represents
Chapter 2: Corneal changes following short-term soft contact lens wear
50
the surface height deviations not the optical path deviations. The best fit
corneal sphero-cylinder was calculated both for anterior and posterior axial
corneal power maps.
2.3.4 Statistical analysis
The data was found to be normally distributed. A repeated measures analysis
of variance (ANOVA) was used to calculate statistical significance of changes
in corneal curvature and thickness due to contact lens wear, with lens type,
region and segment as within-subject factors. Degrees of freedom were
adjusted using the Greenhouse-Geisser correction to prevent any type 1 errors,
where violation of the sphericity assumption occurred. Bonferroni adjusted pair-
wise comparisons were carried out for individual comparisons.
2.3.5 Contact lens centration and rotation
Soft contact lens fit was assessed in terms of corneal coverage, lens centration
(in the horizontal and vertical meridian), lens movement (with blink) and lens
rotation (only for the toric lenses). The grading scale described by Guillon
(1994) was used to assess the lens fit (including corneal coverage, lens
centration and lens movement).
Digital images of contact lens on the eye were also analysed to
calculate the centration of the contact lens in relation to the corneal limbus,
using custom written software (Iskander et al. 2004). A common scale for each
of the digital images was calculated using the Medmont E300
videokeratoscope images to calculate the horizontal visible iris diameter (HVID)
for each subject‟s eye. The distance between the contact lens centre and
corneal geometric centre (centroid of the limbus) was used to calculate the lens
centration on the eye. Lens rotation for the toric contact lenses was calculated
in degrees using the software with average rotation values from two digital
images for each lens for every subject. Figure 2-5 shows a digital image of one
of the subject‟s eye with a toric lens showing the toric lens markings.
Chapter 2: Corneal changes following short-term soft contact lens wear
51
Figure 2-5: Digital image of a soft contact lens (SiHy/Toric/–3) on a subject’s eye. The lens centration for this subject was recorded as 0 (optimal), with less than 0.5 mm decentration. The lens rotation for this lens was calculated using the Imetrics software to be 16 degrees nasal.
2.3.6 Baseline day diurnal changes
To study the diurnal changes in corneal thickness occurring during the 8 hours
of the contact lens wearing period, measurements at the beginning and end of
the 8 hours period on the two baseline days (without contact lenses wear) were
examined. The diurnal difference was analysed by applying repeated measures
ANOVA on the baseline day data.
There were two possible analysis methods that could be used to assess
the changes in corneal thickness and topography associated with contact lens
wear for 8 hours. In the first method, curvature difference maps are generated
by subtracting the curvature map before lens wear (morning) from the curvature
map after lens wear (afternoon). In the second method, curvature difference
maps are generated by subtracting the curvature map of the baseline day
(afternoon) from curvature map after lens wear (also afternoon) (Table 2-2).
Chapter 2: Corneal changes following short-term soft contact lens wear
52
Table 2-2: Methods to study the diurnal changes in corneal curvature and thickness.
Method 1: Difference between afternoon & morning (lens wear day)
Curvature difference map = Curvature map after lens wear (pm) – Curvature map before lens wear (am)
Similarly,
Thickness difference map = Thickness map after lens wear (pm) – Thickness map before lens wear (am)
Method 2: Difference between afternoon (lens wear) and afternoon (baseline)
Curvature difference map = Curvature map after lens wear (pm) – Curvature map of baseline day (pm)
Similarly,
Thickness difference map = Thickness map after lens wear (pm) – Thickness map of baseline day (pm)
It was concluded that method 2 (afternoon of lens wear minus afternoon
of baseline) was a more accurate representation of the corneal changes
associated with the contact lenses because it takes into consideration the
diurnal changes occurring in the curvature as well as thickness of the cornea.
In order to confirm that the diurnal change was appropriately compensated in
the analysis, we studied the following relationship (Table 2-3). This relationship
is also illustrated in the Figure 2-6.
Table 2-3: Relationship studied to check for diurnal changes.
[Thickness map after lens wear (pm) – Thickness map before lens wear (am)]
should equal
[Thickness map after lens wear (pm) – Thickness map of baseline day (pm)] +
[Thickness map baseline day (pm) – (am)]
It is clear from the results in Figure 2-6 that some diurnal variation does
occur in corneal thickness and curvature and that the method we used was
providing an accurate representation of the corneal changes associated with
Chapter 2: Corneal changes following short-term soft contact lens wear
53
contact lens wear, without the influence of diurnal changes in the cornea. Thus
all difference maps in this study were calculated using the second method,
where thickness difference map was generated by subtracting the thickness
map of baseline day (afternoon) from thickness map after lens wear
(afternoon).
Figure 2-6: Diurnal variation in corneal pachymetry analysis. This figure shows thickness difference maps for subject 2, SiHy/Toric/–3 lens.
(a) Thickness difference map = Thickness map in afternoon – Thickness map in morning
(b) Thickness difference map – Thickness map in afternoon – Thickness map in afternoon of baseline day
(c) Normal diurnal change in thickness or Thickness difference map = Thickness map in afternoon – morning of baseline day
(d) Thickness difference map from (b) + Thickness difference map from (c) = Thickness difference map from (a)
Chapter 2: Corneal changes following short-term soft contact lens wear
54
2.4 Results
2.4.1 Diurnal Changes
There was a significant difference between the morning and afternoon
measurements on the baseline days (no contact lens wear). The group mean
diurnal change in corneal thickness showed a significant thinning of 7.9 ± 1.1
microns (p=0.001) in the central corneal region and 9.3 ± 1.7 microns (p=0.001)
in the peripheral corneal annular region. The anterior corneal curvature
exhibited a slight steepening of 0.01 ± 0.02 mm (p=0.49) centrally, which was
not significant and significant steepening of 0.02 ± 0.02 mm (p=0.005)
peripherally. The posterior corneal curvature exhibited a slight flattening of 0.01
± 0.02 mm (p=0.31) centrally, which was not significant and significant
flattening of 0.01 ± 0.01 mm (p=0.007) peripherally. Given the magnitude of
these diurnal changes occurring during the day with no lens wear, especially in
corneal thickness, the corneal changes associated with contact lens wear have
been analysed by taking this diurnal thinning into consideration.
To study the variability in morning measurements (prior to lens wear) on
the six study days (2 baseline days + 4 contact lens wear days), the curvature
and pachymetry measurements on the 6 days for each subject were compared.
A repeated measures ANOVA was applied with „day‟ as the within subject
factor. No significant differences (all p>0.05) were found between the days
suggesting that the morning measurements were not significantly different
across the study days, for all subjects.
2.4.2 Corneal thickness
The type of contact lens had a significant effect on corneal thickness change
after 8 hours of lens wear (p<0.001). There was also a significant interaction
between the type of lens thickness change in corneal annular regions
(p<0.001). Group mean changes in corneal thickness relative to the baseline
days, with the four contact lenses are shown in Table 2-4 and Figure 2-7. The
HEMA/Toric/–3 contact lens caused the greatest level of corneal thickening in
the central (20.3 ± 10.0 microns or 3.5 ± 1.7%, p<0.001) as well as peripheral
cornea (24.1 ± 9.1 microns or 3.7 ± 1.4%, p<0.001) after 8 hours of lens wear.
In post-hoc testing, this change in corneal thickness with the HEMA/Toric/–3
lens was significantly greater than all the other lenses (all p<0.001) used in the
study. The average thickness difference map (lens wear pm minus baseline
Chapter 2: Corneal changes following short-term soft contact lens wear
55
pm) for the HEMA/Toric/–3 shows an obvious regional thickening in the nasal
and temporal edges of the cornea, in the areas beneath the stabilizing zones of
the lens (Figure 2-7 d). However, these regional changes were not obvious in
the average thickness difference map for the other toric lens, the SiHy/Toric/–3
lens (Figure 2-7 c).
The three silicone hydrogel lenses produced similar patterns of change
in corneal thickness (Figure 2-7 a, b, c) The SiHy/Toric/–3, SiHy/Sph/–7 and
SiHy/Sph/–3 lenses on average caused slight central corneal thinning and
minor peripheral corneal thickening, but these changes were not statistically
significant compared with the baseline days (p>0.05).
Figure 2-7: Group mean changes in corneal thickness (mm) relative to baseline days for the four different types of contact lenses. The lenses included different combinations of lens material [hydrogel (HEMA) and silicone hydrogel (SiHy)], design [spherical (Sph), toric] and power (–3.00, –7.00 D).
Chapter 2: Corneal changes following short-term soft contact lens wear
56
Table 2-4: Mean corneal thickness changes relative to baseline days, with the four contact lenses in central and peripheral corneal regions. Values where pair-wise comparison revealed a significant change from baseline are highlighted with asterisks (p-value ≤ 0.001 is ***). Positive change represents swelling and a negative change represents thinning.
Central Peripheral
Mean change
Lens type µm ± SD % µm ± SD %
SiHy/Sph/–3 (Lens 1) –1.4 ± 6.6 -0.3 2.3 ± 7.0 0.4
SiHy/Sph/–7 (Lens 2) –0.3 ± 6.2 -0.1 3.9 ± 6.2 0.6
SiHy/Toric/–3 (Lens 3) –0.6 ± 5.2 -0.1 4.5 ± 5.5 0.7
HEMA/Toric/–3(Lens 4) 20.3 ± 10.5 *** 3.5 24.1 ± 9.6 *** 3.7
Table 2-5: Mean changes in anterior and posterior axial corneal curvatures relative to baseline days, with the four contact lenses in the central and peripheral corneal regions. Values where pair-wise comparison revealed a significant change from baseline are highlighted with asterisks (p-value ≤ 0.05 is *, ≤ 0.01 is ** and ≤ 0.001 is ***).
Anterior axial curvature Posterior axial curvature
Central Peripheral Central Peripheral
Lens type Mean change ± SD (mm) Mean change ± SD (mm)
SiHy/Sph/–3 (Lens 1) 0.01 ± 0.03 0.03 ± 0.02 * –0.02 ± 0.03 –0.02 ± 0.01 **
SiHy/Sph/–7 (Lens 2) 0.03 ± 0.02 * 0.02 ± 0.02 * –0.03 ± 0.03 –0.02 ± 0.01 **
SiHy/Toric/–3 (Lens 3) 0.01 ± 0.03 0.03 ± 0.02 ** –0.03 ± 0.03 –0.03 ± 0.02 **
HEMA/Toric/–3 (Lens 4) 0.02 ± 0.03 0.02 ± 0.02 –0.07 ± 0.04 *** –0.02 ± 0.02
Positive change represents flattening and a negative change represents steepening.
2.4.3 Anterior corneal curvature
The type of lens had a significant effect on the change in anterior corneal
curvature following lens wear (p<0.001). Table 2-5 and Figure 2-8 show the
group mean change in anterior axial curvature relative to the baseline days,
with the four contact lenses. The anterior corneal surface generally showed
slight flattening after 8 hours of contact lens wear (Table 2-5, Figure 2-8),
except for SiHy/Sph/–3 which caused some localized areas of steepening
(Figure 2-8 a). The three silicone hydrogel contact lenses (i.e. SiHy/Sph/–3,
SiHy/Sph/–7 and SiHy/Toric/–3) caused slight flattening of 0.03 ± 0.02, 0.02 ±
0.02, 0.03 ± 0.02 mm (respectively) in the peripheral annular region of the
anterior cornea which was statistically significant (all p<0.05) compared with
the baseline days.
Chapter 2: Corneal changes following short-term soft contact lens wear
57
Figure 2-8: Group mean changes in anterior axial curvature (mm) relative to baseline days for the four different types of contact lenses. The lenses included different combinations of lens material [hydrogel (HEMA) and silicone hydrogel (SiHy)], design [spherical (Sph), toric] and power (–3.00, –7.00 D).
2.4.4 Posterior corneal curvature
The type of lens had a significant effect on posterior corneal curvature change
(p<0.001). There was also a significant interaction between the type of lens and
corneal annular region (p<0.001). Group mean changes in posterior axial
curvature, relative to the baseline days, with the four contact lenses are shown
in Table 2-5 and Figure 2-9.
The wear of all contact lenses resulted in posterior corneal steepening
compared with the baseline days, which was more prominent in the inferio-
nasal cornea (Figure 2-9) for the three silicone hydrogel lens types (all p≤0.01
for the peripheral annulus), and was greater in the central region after wear of
the HEMA/Toric/–3 lenses (p≤0.001) (Figure 2-9 d).
Chapter 2: Corneal changes following short-term soft contact lens wear
58
Significant corneal steepening occurred in the posterior corneal
periphery for the 3 silicone hydrogel lenses SiHy/Sph/–3 (0.02 ± 0.01 mm),
SiHy/Sph/–7 (0.02 ± 0.02 mm) and SiHy/Toric/–3 (0.04 ± 0.02 mm) (Figure 2-
9). The HEMA/Toric/–3 lens, on the other hand showed the greatest steepening
in the posterior central cornea of –0.07 ± 0.04 mm (p<0.01) (Figure 2-9).
Figure 2-9: Group mean changes in posterior axial curvature (mm) relative to baseline days for the four different types of contact lenses. The lenses included different combinations of lens material [hydrogel (HEMA) and silicone hydrogel (SiHy)], design [spherical (Sph), toric] and power (–3.00, –7.00 D).
2.4.5 Association between changes in thickness and curvature
To study the association between changes in anterior and posterior corneal
curvatures and corneal thickness, we calculated the linear regression and
significance of the association between each of the study variables. There was
a negative correlation between the change in posterior central corneal
curvature and change in central corneal thickness (R2 = 0.302, p=0.06, F =
Chapter 2: Corneal changes following short-term soft contact lens wear
59
4.468, B = -2.39) which approached significance (Figure 2-10 a). The change
in posterior central corneal curvature and change in peripheral corneal
thickness was significantly negatively correlated (R2 = 0.446, p=0.02, F = 7.44,
B = - 2.86) (Figure 2-10 b). The change in anterior corneal curvature and
posterior peripheral curvature did not show any significant correlation with
change in corneal thickness (all p>0.05).
Figure 2-10: (a) Correlation between changes in posterior (central) corneal curvature with (a) central corneal thickness (b) peripheral corneal thickness. P-values in are shown in red.
2.4.6 Corneal best fit sphero-cylindrical power
Changes in best fit sphero-cylinder for the anterior and posterior corneal axial
powers were analysed for the central and peripheral annular regions (i.e. 0 – 4
mm diameter and 4 – 8 mm diameter). The difference in refractive index at the
anterior (air to cornea) and posterior (cornea to aqueous) corneal surfaces
means that a flattening of the anterior corneal radius leads to a decrease in
anterior corneal power, whereas a steepening of the posterior corneal radius
also leads to a decrease of posterior corneal power. The decrease in axial
power (M) of the posterior cornea was smaller in magnitude than the anterior
cornea due to the refractive index difference at this surface, even though the
Chapter 2: Corneal changes following short-term soft contact lens wear
60
change in radius was greater in the posterior surface (Figures 2-11 & 2-12
respectively).
The change in anterior corneal best-fit sphere (M) was significantly
affected by the type of lens and there was a significant interaction with corneal
annular region (repeated measures ANOVA, both p<0.001). The change in
anterior corneal best fit sphere (M), with/against the rule astigmatism (J0),
oblique astigmatism (J45), and sphero-cyl RMS error is shown in Figure 2-11.
There were significant hyperopic shifts (decrease in refractive power of the
cornea) in best fit sphere (M) for SiHy/Sph/–7 lens in the central cornea (0-4
mm) and for SiHy/Sph/–3, SiHy/Sph/–7, SiHy/Toric/–3 and HEMA/Toric/–3
lenses in the peripheral cornea (4-8 mm). The changes in J45 and sphero-cyl
RMS were not significant for any of the lenses.
Figure 2-11: Changes in best fit sphere (M), with-the-rule and against-the-rule astigmatism (J0), oblique astigmatism (J45) and sphero-cylinder RMSE (from baseline) for the anterior cornea. Significant change indicated by * p<0.05 and # p<0.01. Error bar represents one standard error of the mean. Negative change in M represents a decrease in corneal axial power (hypermetropic shift). Negative change in J0 represents a decrease in WTR astigmatism. Positive change in J0 represents an increase in WTR astigmatism. Positive J45 represents a negative cylinder axis closer to 45° and negative J45 represents a negative cylinder axis closer to 135°.
Chapter 2: Corneal changes following short-term soft contact lens wear
61
The posterior corneal best-fit sphere (M) was also significantly affected
by the type of lens (repeated measures ANOVA, p<0.001). There was also a
significant interaction (p<0.001) between the type of contact lens and corneal
annular region. Changes in posterior corneal best fit sphere (M), with/against
the rule astigmatism (J0), oblique astigmatism (J45), and sphero-cyl RMS error
are shown in Figure 2-12. There was a significant hyperopic shift in best fit
sphere (M) for the HEMA/Toric/–3 lens in the central posterior cornea (0-4 mm)
and for the SiHy lenses (i.e. SiHy/Sph/–3, SiHy/Sph/–7 and SiHy/Toric/–3) in
the peripheral posterior cornea (4-8 mm). There was a significantly greater
hyperopic shift in M in the central corneal region with the HEMA/Toric/–3 lens
(–0.07 ± 0.03 D) compared to the SiHy/Sph/–3 (–0.02 ± 0.02 D) and
SiHy/Sph/–7 (–0.03 ± 0.03 D).
Posterior corneal J0, J45 and sphero-cyl RMS error did not show any
significant changes from baseline with any of the lenses. But there was a
significantly greater increase in sphero-cyl RMS error in the peripheral region
with the HEMA/Toric/–3 lens (–0.03 ± 0.02 D) compared to the SiHy/Sph/–3 (–
0.02 ± 0.02 D) and SiHy/Toric/–3 lenses (–0.02 ± 0.02 D).
Chapter 2: Corneal changes following short-term soft contact lens wear
62
Figure 2-12: Changes in best fit sphere (M), with-the-rule and against-the-rule astigmatism (J0), oblique astigmatism (J45) and sphero-cylinder RMSE (from baseline) for the posterior cornea. Significant change indicated by * p<0.05 and # p<0.01. Each error bar represents one standard error of the mean. Negative change in M represents a decrease in corneal axial power (hypermetropic shift). Negative change in J0 represents a decrease in WTR astigmatism. Positive change in J0 represents an increase in WTR astigmatism. Positive J45 represents a negative cylinder axis closer to 45° and negative J45 represents a negative cylinder axis closer to 135°.
2.4.7 Contact lens centration and rotation
Mean lens centrations for all subjects in the horizontal and vertical direction are
shown in Table 2-6. The mean lens rotations (in degrees) for the two toric
lenses used in this study were 8.0 ± 13.8 degrees nasal for SiHy/Toric/–3 and
5.0 ± 10.1 degrees nasal for the HEMA/Toric/–3 lenses but the difference
between the two lenses was not statistically different (p>0.05).
Table 2-6: Mean lens centrations calculated using custom-written software and digital images of lenses on the corneas
Lens Horizontal centration (x) - mm
Direction Vertical centration (y) - mm
Direction
SiHy/Sph/–3 –0.02 ± 0.16 Nasal –0.02 ± 0.33 Inferior
SiHy/Sph/–7 –0.05 ± 0.1 Nasal –0.08 ± 0.25 Inferior
SiHy/Toric/–3 –0.02 ± 0.16 Nasal +0.05 ± 0.29 Superior
HEMA/Toric/–3 +0.03 ± 0.19 Temporal +0.01 ± 0.23 Superior
Chapter 2: Corneal changes following short-term soft contact lens wear
63
2.5 Discussion
This is the first systematic investigation of the effect of soft toric contact lens
design on corneal thickness and topography. Previous anecdotal reports
discuss cases of patients presenting with blurred vision and inferior corneal
steepening after long term soft hydrogel toric contact lens wear (Hagan et al.
1998; Schornack 2003). In this study, the effect of the thicker lens stabilization
zones on corneal swelling can be clearly observed in the thickness difference
map, where maximum edema can be observed in the corneal periphery at the
locations corresponding to the thickest regions of the soft toric contact lenses
(approximately 4 and 8 o‟clock positions). This suggests that these regions of
the cornea would be most likely to suffer from the negative consequences of
chronic hypoxia. The magnitude of corneal thickness and curvature changes
that we have observed following a short period of soft toric lens wear are
unlikely to influence clinical measures of vision or refraction, however it is likely
that corneal changes associated with longer term toric lens wear may be larger
(Hagan et al. 1998; Schornack 2003). Future research involving controlled
clinical studies of longer term soft toric lens wear is required to improve our
understanding of the nature and magnitude of the longer term corneal effects.
The HEMA/Toric/–3 contact lens caused significant corneal thickening
in the central as well as peripheral cornea (3.5% centrally and 3.7%
peripherally). This corneal swelling was significantly greater than those
observed with all other lenses including the other toric lens (SiHy/Toric/–3). The
difference in corneal swelling between the HEMA/Toric/–3 and SiHy/Toric/–3
lenses is most likely due to the difference in oxygen permeability of the two lens
materials (SiHy, Dk = 53 and HEMA, Dk = 8 to 10), since the lenses were
identical in design. Previous studies have shown a smaller increase in central
corneal thickness of 0.8% (Polse et al. 1976) and 0.5% (Harris et al. 1977) after
8 hours wear but with spherical ultra-thin hydrogel contact lenses. The
increased average thickness of the toric design of HEMA/Toric/–3 lens has
presumably led to a slightly greater amount of corneal swelling in this study
(3.5% centrally and 3.7% peripherally).
We found the magnitude of corneal curvature change associated with
contact lens wear to be greater in the central posterior cornea. This is
consistent with various reports that have shown that the anterior corneal
curvature shows little changes in response to corneal hypoxia, contact lens
Chapter 2: Corneal changes following short-term soft contact lens wear
64
induced or otherwise, and it is the posterior cornea that shows the greatest
changes in curvature in response to corneal edema (Kikkawa and Hirayama
1970; Lee and Wilson 1981; Erickson et al. 1999). This has been attributed to
the differences in the structure (Kikkawa and Hirayama 1970; Komai and Ushiki
1991; Muller et al. 2001; Bergmanson et al. 2005) and composition of the
stroma (Bettelheim and Plessy 1975; Castoro et al. 1988). Reports indicate that
the posterior stroma is capable of swelling more than the anterior stroma at a
given swelling pressure (Kikkawa and Hirayama 1970; Lee and Wilson 1981;
Erickson et al. 1999). The swelling of the posterior cornea was also shown to
be significantly greater in the central region compared to the peripheral region
in rabbit, cat and bovine corneas (Kikkawa and Hirayama 1970). These
differences in the physiological properties of the cornea relate to stromal
structure and composition. The anterior lamellae in the stroma are reported to
be tightly interwoven compared to the posterior stroma in human and animal
corneas (Kikkawa and Hirayama 1970; Komai and Ushiki 1991; Muller et al.
2001). The density of the lamellae in the anterior portion of the stroma is about
50% greater than the posterior stroma in human eye bank corneas
(Bergmanson et al. 2005). There are also differences in the anterior and
posterior stroma in terms of composition of the proteoglycan, causing the
differences in swelling properties of the stroma. For example, keratin sulphate
(a more hydrophilic proteoglycan) is commonly present in the posterior stroma
whereas dermatan sulphate (a much less hydrophilic proteoglycan) is
commonly present in the anterior stroma in bovine corneas (Bettelheim and
Plessy 1975; Castoro et al. 1988).
We also studied the correlation between the corneal curvature and
thickness changes and found that the posterior (central) corneal curvature
showed a negative correlation with the central and peripheral corneal
thicknesses. The change in anterior corneal curvature did not show any
significant correlation with change in corneal thickness in this study, in
agreement with previous studies that have reported substantial changes in
corneal thickness without any change in anterior corneal topography with the
use of contact lenses (Bailey and Carney 1972; Carney 1972; Carney 1975).
The difference in power between the two spherical lenses (SiHy/Sph/–3 and
SiHy/Sph/–7), resulted in greater front optic zone edge thickness in the – 7.00
D lens and hence lower regional and average Dk/t. This difference in thickness
profile did not lead to any significant difference in the amount of change in
Chapter 2: Corneal changes following short-term soft contact lens wear
65
anterior corneal curvature or corneal thickness with these lenses. The mean
peripheral thickening with SiHy/Sph/–7 lens (3.9 ± 2.0 microns) was slightly
greater than the SiHy/Sph/–3 lens (2.3 ± 7.0 microns), but this difference was
not statistically significant. The changes in anterior and posterior corneal
curvatures were also minimal with both of these lenses. This suggests that at
least for short-term silicone hydrogel lens wear, these differences in lens
thickness do not lead to significantly different corneal changes.
To gain better understanding of why the posterior corneal steepening
was greatest in the inferior-nasal corneal region with all the lens types, we
estimated the mean distance of the centre of the topography map from the
geometric centre of the cornea (limbus centroid). A mean offset of 0.36 ± 0.19
mm temporally and 0.05 ± 0.09 mm superiorly was calculated using
videokeratoscope maps of all subjects. It has been shown that the centre of the
topography map from the Medmont E300 videokeratoscope coincides closely
with the centre of the topography map from the Pentacam system (Read et al.
2009). Thus the nasal and inferior corneal steepening can partly be explained
by the offset of the topography map in the same direction.
There were some variations in the amount of swelling induced by the
same lens in different subjects. For example, the average corneal swelling with
HEMA/Toric/–3 lens ranged from 5 to 40 microns (0.8 to 6.5%). This is
consistent with reports in which the amount of corneal edema in response to
hypoxia has been shown to vary from 3.6 to 12.2% between subjects (Sarver et
al. 1983; Efron 1986). Bonanno et al. (2003) demonstrated that inter-subject
variability in corneal swelling is affected by corneal metabolic activity and that
there is an association between corneal swelling and endothelial function.
Therefore the amount of oxygen required to maintain normal corneal
metabolism and avoid edema varies from subject to subject and these inter-
subject differences in corneal physiology are the likely reason for the variability
we observed in corneal edema.
This is the first study to investigate the influence of „open eye‟ daily
contact lens wear upon posterior corneal shape. Whilst statistically significant
changes were observed with a number of lenses, the majority of changes were
small (–0.03 mm and smaller) and unlikely to be of clinical significance. The
largest magnitude of change was observed with the HEMA/Toric/–3 lens, with a
central steepening of the posterior cornea observed (average change of –0.07
Chapter 2: Corneal changes following short-term soft contact lens wear
66
mm). The larger magnitude of posterior corneal change observed with this lens
is likely due to the greater amount of corneal swelling also observed with this
lens. In contrast to our findings, previous studies investigating posterior corneal
change associated with extended wear of soft spherical lenses have noted a
significant flattening of the posterior cornea (Martin et al. 2009). The central
steepening of the posterior cornea that we have observed with the hydrogel
toric lens appears to be related to the regional pattern of swelling with this lens,
where the thickness changes within the central corneal region (4 mm) have led
to central corneal steepening (Figure 2.7d). This difference in corneal swelling
within the central 4 mm zone of the cornea is likely to be due to the reduced
oxygen transmissibility of the thicker peripheral stabilization zones in this lens
causing more peripheral corneal regions to have a higher degree of swelling
than the centre.
We were careful in this study to measure the natural diurnal variation in
corneal thickness and curvature in each of the subjects and then use these
data to measure the true change in the cornea associated with contact lens
wear (factoring out the diurnal changes). This was proven to be important in
arriving at reliable results, since the magnitude of natural diurnal changes in
corneal thickness from morning to afternoon were typically much larger (8 to 9
microns of thinning) than the changes associated with the contact lenses (0.4
to 4.5 microns of thickening), with the exception of the HEMA toric lenses (20 to
24 microns of thickening). The magnitude of corneal thinning that we observed
in the subjects from morning to afternoon without contact lens wear was similar
to that reported by Read et al. (2009).
We found slightly greater swelling in the corneal periphery (3.7 ± 1.7%)
compared to the centre (3.5 ± 1.7%) with HEMA/Toric/–3 lens. This observation
is similar to that of Kaluzny et al. (2003) who also found greater corneal
swelling in the corneal periphery (3.26%) than in the centre (1.54%) after 2
weeks of soft spherical contact lenses used on a daily wear basis. Martin at al.
(2008) found no difference between central and peripheral corneal swelling with
low Dk soft contact lens after 1 week of extended wear. The measurements in
Martin at al. (2008) study were done between 4 and 8 pm to ensure that the
corneal edema induced by overnight eye closure had resolved and that
changes were mostly due to contact lens wear. However many earlier studies
performed under closed eye conditions have reported greater swelling in the
Chapter 2: Corneal changes following short-term soft contact lens wear
67
centre of the cornea compared to the peripheral cornea (Bonanno and Polse
1985; Holden et al. 1985; Herse et al. 1993; Moezzi et al. 2004).
In our study, the greater corneal swelling in the peripheral cornea could
be attributed to thicker peripheral thickness of the negative power lenses and
thicker stabilization zones in the corneal periphery of the toric lenses. It is well
known that decrease in Dk/t (with increase in contact lens thickness) leads to a
greater swelling in the peripheral cornea (Bonanno and Polse 1985; Bonanno
et al. 1986). The greater central corneal swelling observed after eye closure
could be because of lack of atmospheric oxygen, which affects the central
cornea more, while the peripheral cornea is still supplied by the limbal
vasculature. Other explanations for greater central corneal swelling compared
to the periphery, reported in studies under closed eye conditions, is reduced
tear mixing, thereby averaging the oxygen tension under the lens (Bonanno
and Polse 1985; Bonanno et al. 1986) and the physical clamping by the limbus
which limits swelling of the peripheral cornea so that further hypoxia causes
increase only in the centre of the cornea (Maurice and Giardini 1951).
In contrast, Martin et al. (2009) reported a significant amount of central
corneal swelling (2.41%) accompanied by posterior corneal flattening with the
low Dk lenses after 1 week of extended wear. The anterior corneal curvature
steepened slightly but not significantly. In another study Moezzi et al. (2004)
also found posterior corneal flattening but no changes in anterior corneal
curvature in response to corneal swelling (more in the centre than the
periphery) with low Dk soft contact lenses, after 3 hours under closed eyelid
conditions. In the above studies the central corneal thickening may have
caused a backward movement of the central posterior surface resulting in
corneal flattening.
2.6 Conclusion
To conclude, there was an obvious regional corneal swelling apparent
after wearing the low Dk soft toric lenses, due to the location of the thicker
stabilization zones of the toric lenses. The corneal swelling and curvature
changes seen in this study after 8 hours of lens wear are comparable to those
seen after overnight sleep and are not likely to affect the wearer‟s fit, comfort or
vision. The natural diurnal variations in corneal thickness that we measured
from mid-morning to afternoon, were typically larger than the changes caused
Chapter 2: Corneal changes following short-term soft contact lens wear
68
by the silicone hydrogel contact lenses and this factor should be considered in
short-term studies of contact lens induced corneal swelling.
- 69 -
Chapter 3
Corneal changes following short-term rigid contact lens wear
3.1 Introduction
In Chapter 2, corneal thickness and curvature changes were investigated with a
variety of soft contact lens materials (silicone hydrogel and hydrogel), designs
(spherical and toric) and powers (–3.00 and –7.00 D). The largest changes
were observed following wear of the hydrogel toric lens which caused
significant corneal swelling and these changes correlated with the corneal
curvature changes. We also found significant diurnal changes in these corneal
parameters over the 8 hour duration of the study.
Rigid gas permeable (RGP) lenses, although currently less popular than
soft contact lenses as a refractive correction option, may still offer some
advantages for the wearer. Apart from a superior quality of vision (Johnson and
Schnider 1991; Fonn et al. 1995), these lenses have been reported to cause
fewer complications compared to any other available contact lens type or
modality. RGP lenses have been shown to have the lowest incidence of
microbial keratitis (Stapleton et al. 2008), severe and non-severe keratitis in
daily and extended wear (Morgan et al. 2005), and corneal infiltrative events
(Efron et al. 2005) compared to other types of contact lenses. Dart et al. (2008)
found that these lenses reduce the risk of microbial keratitis by 84% compared
to programmed replacement soft lenses. The incidence of toxic or allergic
reactions to lens care solutions is also reported to be lower with RGP lenses
(Stapleton et al. 1992).
RGP lenses are an important option for conditions such as keratoconus
(Mandell 1997; Griffiths et al. 1998), post laser refractive surgery (Steele and
Davidson 2007), post keratoplasty (Beekhuis et al. 1991) and post trauma
(Kanpolat and Ciftci 1995) where the cornea is irregular and vision is not
satisfactorily corrected with spherical or toric soft lenses. RGP lenses provide
better visual performance compared to soft lenses when fitted to irregular
corneas because the anterior surface of these lenses in-eye is not affected by
the corneal shape. The rigidity of these lenses forms a post-lens tear lens in
Chapter 3: Corneal changes following short-term rigid contact lens wear
70
between the cornea and the contact lens which neutralizes the optical
aberrations of the anterior cornea. The refractive index of the tear lens (n =
1.336) being very similar to that of the cornea (n = 1.376), helps in neutralising
the majority of aberrations of the anterior corneal surface. Thus, these lenses
help in providing better visual acuity compared to soft lenses.
Reports of long term (1 month to few years) corneal topographic
changes induced by daily wear of RGP contact lenses are inconsistent, with
some studies reporting corneal steepening (Rengstroff 1973; Sanaty and
Temel 1996), some reporting flattening under decentred lens (Maeda et al.
1994; Calossi et al. 1996), and others reporting no significant changes
(DeRubeis and Shily 1985; Yebra-Pimentel et al. 2001). Yeniad et al. (2003)
noted corneal flattening in the first month of wear, with steepening at 6 months
in a group of subjects using RGP contact lenses on a daily wear basis. There
are no controlled studies in the literature reporting the effect of rigid contact
lenses on corneal curvature after short-term use. These changes are important
to study as RGP lenses can be worn for short-term occasional wear by people
with irregular corneas, who can mostly manage with glasses, but require better
vision for certain activities such as sports. A better understanding of the short-
term variations in the cornea with RGP lenses may also provide insights into
the potential longer term effects of these lenses.
During RGP contact lens fitting, the assessment of the post-lens tear
film using sodium fluorescein (NaFl) is an important component of determining
lens fitting characteristics. It is also a basis for achieving the desired curvature
and refractive results in myopic and hyperopic orthokeratology (Soni et al.
2003; Lu et al. 2007). Some studies in the past have found correlation between
corneal topography and the resting position of the contact lens on the cornea
(Wilson et al. 1990; Ruiz-Montenegro et al. 1993) but no statistical analysis was
performed in these studies. Since the fluorescein pattern is an integral part of
assessing the fit of standard and specialized RGP contact lenses, studying the
association between fluorescein fitting characteristics (i.e. regions of minimum
clearance in the fluorescein pattern) and the resulting corneal shape changes
may provide important insights into the underlying causes of the corneal
changes associated with short-term wear of RGP contact lenses.
The reports of corneal thickness changes due to short-term RGP
contact lens wear are also inconsistent. Fonn et al. (1984) noticed an increase
Chapter 3: Corneal changes following short-term rigid contact lens wear
71
in corneal thickness of 1.2 to 4.4% after 6 hours wear of RGP contact lenses of
different centre thickness and fits. Sarver et al. (1977) did not find any
significant changes in mean corneal thickness with RGP lenses after 8 hours of
use. Yeniad et al. (2003) reported an increase in corneal thickness in the first
month, but thinning was seen after 6 months of RGP contact lens use on a
daily wear basis. There are no studies available that have systematically looked
at the influence of factors such as contact lens material (Dk) and diameter on
corneal thickness after short-term use of RGP contact lenses.
Therefore, the aim of this controlled cross-over study is to investigate
the changes in corneal thickness and anterior and posterior corneal topography
with the wear of different types of rigid contact lenses for eight hours. We also
studied the relative influence of different contact lens materials (PMMA, RGP-
Boston XO) and diameters (9.5, 10.5) on the measurements of corneal
topography and thickness, as an extension of Experiment 1. A commercially
available silicone hydrogel lens was also included as a control, to provide a link
to Experiment 1 and to compare the results with a lens used commonly in
current clinical practice.
3.2 Methodology
This study was approved by the QUT university human research ethics
committee (see Appendix A) and followed the tenets of declaration of Helsinki.
All subjects were asked to read the study information sheet and were given an
opportunity to ask any questions before signing an informed consent. A
required sample size of 12 subjects was calculated based upon pilot studies, to
provide 80% power to detect 0.01 mm change in anterior corneal curvature and
2.7 microns change in corneal thickness at the 0.05 level of significance.
The protocol followed was similar to that in Experiment 1 (Chapter 2).
This study was conducted over a period of 5 days (one baseline and 4 lens
wearing days). On each day, measurements were taken in the morning and
then again in the afternoon 8 hours later. On day one, baseline measurements
were taken without any contact lens in the eye, in the morning (0 hours) and
repeated in the afternoon after 8 hours. On days 2, 3, 4 and 5 of the study the
subjects wore different types of contact lenses in the left eye only, with
measurements collected in the morning before the lens was inserted and again
after 8 hours of wear. A 2-3 day recovery period was observed after each lens
Chapter 3: Corneal changes following short-term rigid contact lens wear
72
wear day before commencing wear of another lens, based on pilot studies
(described in Methods section of Chapter 2). Since for most people the right
eye is the dominant eye (Eyre and Schmeeckle 1933), it was decided that the
contact lens will cause less visual disturbance for the majority of subjects if
used in the left eye. Lens wear typically commenced between 8 and 11 am and
at least 2 hours after waking, to limit the potential influence of the corneal
changes that are typically evident immediately after sleep (Read and Collins
2009). The lenses were removed in the afternoon between 4 and 7 pm, after 8
hours of lens wear.
3.2.1 Subjects
The study included 14 young, healthy adult subjects aged between 20 to 33
years (mean age 27.8 ± 4.0 years) with visual acuity of 6/6 or better and
corneal astigmatism of ≤ 1.5 D, as determined by the Medmont E300
videokeratoscope (Medmont Pty. Ltd., Victoria, Australia). Five of the subjects
were females. The mean spherical equivalent refractive error was –0.6 ± 1.3 D.
Prior to commencement of the study, all subjects were screened for any tear
film abnormalities and anterior segment pathology using slit-lamp
biomicroscopy. None of the subjects had a history of corneal injury, infection or
surgery. Two of the subjects were habitual soft contact lens wearers but they
were asked to discontinue lens wear one month prior to the start of the study,
to allow any effects of soft lens wear to largely resolve. None of the subjects
were previous rigid contact lens wearers.
3.2.2 Contact Lenses
A contact lens trial fitting was performed for each subject with the rigid lenses
before ordering, to determine the optimum back optic zone radius (BOZR) for
each lens in the left eye only. Similar fitting characteristics of central alignment,
midperipheral touch and moderate edge lift was achieved with all the rigid
lenses for all subjects. Three different types of custom made rigid contact
lenses and one soft contact lens were ordered of each subject. The rigid lenses
were 9.5 or 10.5 mm in diameter with a spherical BOZR and an aspheric
periphery, and made from either PMMA or Boston XO material. The soft
contact lens was a Bausch & Lomb PureVision (Balafilcon A) silicone hydrogel
lens with 14.0 mm diameter. Other details of the lenses are shown in Table 3-1.
The type of lens to be worn on each study day was randomised.
Chapter 3: Corneal changes following short-term rigid contact lens wear
73
Table 3-1: Details of the four lenses used in the study
Lens PMMA/9.5 RGP/9.5 RGP/10.5 SiHy/14.0
Design (BOZ) Spherical Spherical Spherical Spherical
Design (BPZ) Aspheric Aspheric Aspheric B&L PureVision
Material PMMA RGP (Boston
XO) RGP (Boston
XO) Silicone Hydrogel
Power (Dioptre) –0.50 –0.50 –0.50 –0.50
Mean BOZR (mm)
7.77 ± 0.32 7.77 ± 0.32 7.85 ± 0.30 8.6
Total diameter (mm)
9.5 9.5 10.5 14.0
BOZD (mm) 8.1 8.1 8.8 8.9
Water content (%)
0 0 0 36
Dk 0.1 100 100 99
Modulus (MPa) ≈ 2000 1500 1500 1.1
Manufacturing method
Lathe Lathe Lathe Cast moulding
Surface treatment
None None None Performa
Centre thickness (mm)
0.18 ± 0.02 0.19 ± 0.01 0.20 ± 0.01 0.12 *
Mean Dk/t 0.06 52.6 50 82.5
Asterisk (*) based on assumption from other lens design. PMMA: polymethyl methacrylate, RGP: rigid gas permeable, SiHy: silicone hydrogel, BOZ: back optic zone, BPZ: back peripheral zone, B&L: Bausch and Lomb, BOZR: back optic zone radius, BOZD: back optic zone diameter, Dk: oxygen permeability, MPa: megapascal, unit of modulus of elasticity, mm: millimetres. Unit of Dk/t = (cm/sec) (mLO2/mL X mmHg). The lenses supplied by the manufacturer were checked for back vertex power, lens diameter, BOZR and edge and surface quality before use in the study.
3.2.3 Measurements and Instruments
A range of ocular measurements were collected at each measurement session
in order to quantify corneal shape and thickness, ocular optics and contact lens
fitting and centration characteristics over the course of the study.
Anterior and posterior corneal topography and regional corneal
thickness were measured using the Pentacam HR system (Oculus, Wetzlar,
Germany) which uses a rotating Scheimpflug camera (a digital camera with a
slit illumination system) to evaluate the anterior segment of the eye. A total of 5
measurements were completed using the “25 picture 3D scan” mode, which
gives 25 cross-sectional images of the anterior eye.
Anterior corneal topography was also measured using the Medmont
E300 videokeratoscope (Medmont Pty. Ltd., Victoria, Australia) which is based
on the Placido disc principle. A total of 4 Medmont images, with a quality score
Chapter 3: Corneal changes following short-term rigid contact lens wear
74
of 95 or greater were taken at each measurement session and saved for
analysis.
Ocular monochromatic aberrations were measured using the Complete
Ophthalmic Analysis System (COAS, Wavefront Sciences Ltd, USA).
Measurements were performed without the use of any eye drops in natural
pupil conditions and in dim room illumination. A total of 4 measurements were
taken during each measurement session, with 20 frames per measurement.
Subjects were instructed to keep away from any significant reading
work or any other activities involving long hours of downward gaze (Shaw et al.
2008), before taking the measurements. A questionnaire was completed by
each subject to monitor the visual tasks performed during the period of lens
wear. Subjects were engaged in similar tasks (e.g. computer work) during the
study period each day. The morning and afternoon measurements on contact
lens wearing days were conducted at around same time of day as on day 1
(baseline day) to allow comparison without confounding effects due to diurnal
variations, as discussed in Chapter 2.
Digital photos of the rigid contact lens on eye were taken to record the
fluorescein pattern using a Canon Digital Rebel EOS 300 D 6.3 mega pixels
Digital SLR (Canon Inc Tokyo, Japan) camera attached to a slit lamp. The slit
aperture was maximized and the slit lamp magnification was kept constant at
10X for all images. The same colour balance setting of the camera was used
for all the images in auto-mode. The images were taken at approximately the
same time of day every day, in the same room with the same temperature (24.9
± 1.0 ºC), humidity (58.0 ± 5.2%), and ambient lighting conditions (slit lamp
illumination: approximately 990 lux, plane of subject‟s eye). Fluorescein strip
(Fluorets, fluorescein sodium sterile ophthalmic strips) moistened with a drop of
unpreserved sterile unit dose saline was lightly touched on the upper bulbar
conjunctiva. The photos were taken in white light and with cobalt blue light (and
a Wratten filter # 12), 4-5 sec after fluorescein instillation.
A 30-second video recording was also taken for each subject with each
lens in order to analyse the most frequent position of the lens on the cornea (in-
between blinks). A Casio Digital SLR EX-F1 (Casio computer Co., Tokyo,
Japan) camera in auto mode, attached to a custom made adjustable camera
mount and illumination system was used for this (Figure 3-1). The illumination
Chapter 3: Corneal changes following short-term rigid contact lens wear
75
was kept at approximately 390 lux at the plane of subject‟s eye using an
external fluorescent ring light mounted behind a diffuser. The standard movie
mode with frame rate of 30 frames per second was used which gave images
with an aspect ratio of 4:3 and resolution of 640 x 480 pixels. The videos were
recorded in same room with approximately same humidity (59.6 ± 7.5%) and
temperature (24.8 ± 1.08 ºC), in dark room conditions at approximately the
same time of day every day (morning and afternoon). The subject was
positioned in the head rest with eyes in primary position and was instructed to
fixate on the middle of camera lens. The subject was instructed to make gentle,
complete blinks during the recording, and two video recordings were captured
for 30 seconds each. Measurements were performed in the following order at
each session: Medmont, Pentacam, COAS and then digital photography and
video recording.
Figure 3-1: Photo of the set up with digital camera to record movement of the contact lens. Illumination of the eye is provided by a fluorescent ring light, mounted behind a diffuser.
Figures 3-2 and 3-3 describe the sequence of measurements taken in
the morning and afternoon on contact lens wearing days. On the baseline days
when no contact lenses were worn, the same measurements and in same
order were taken. The measurements for Experiments described in chapters 5
and 6 were also collected together with measurements for this experiment
using the same set of contact lenses.
Chapter 3: Corneal changes following short-term rigid contact lens wear
76
Figure 3-2: Sequence of measurements taken in the morning before and following insertion of contact lens in eye
Chapter 3: Corneal changes following short-term rigid contact lens wear
77
Figure 3-3: Sequence of measurements taken in the afternoon after 8 hours of lens wear.
3.3 Data Analysis
3.3.1 Corneal topography and thickness data
Pentacam corneal thickness and axial (anterior and posterior) curvature data
and Medmont anterior tangential curvature and corneal height data from each
measurement session were exported from the two instruments. An average of
the 4 maps (Medmont) and 5 maps (Pentacam), taken for each subject during
each measurement session, was calculated using custom-written software
developed at the Contact Lens and Visual Optics Laboratory, QUT.
3.3.2 Pentacam data: Corneal curvature and thickness
The thickness and curvature difference maps from Pentacam data were
generated to compare the baseline maps to post-lens wear maps. „Thickness
Chapter 3: Corneal changes following short-term rigid contact lens wear
78
difference maps‟ and „curvature difference maps‟ were generated as described
in Chapter 2 (Section 2.3.1). Group average difference and significance maps
were also generated by averaging the data from all the subjects for each of the
4 lens types. From the results of Experiment 1, it was concluded some small
but significant diurnal variations in corneal thickness and curvature occur within
the 8 hours duration of the study. Thus all difference maps in this study were
generated by subtracting the thickness map of the baseline day (afternoon)
from the thickness map after lens wear (afternoon).
The Pentacam data from all the subjects were averaged using a
custom-written software (Topoview, developed at Contact Lens and Visual
Optics Laboratory) in order to study the regional changes in corneal thickness
and curvature following lens wear. The average corneal thickness and
curvature was calculated for each subject within two corneal regions i.e. central
(0 - 4 mm) and peripheral (4 - 8 mm) as illustrated in Figure 2-4 (Chapter 2).
To study the statistical significance of corneal changes due to contact
lens wear, a repeated measures analysis of variance (ANOVA) was used with
lens type and region as within-subject factors. Degrees of freedom were
adjusted using Greenhouse-Geisser correction to prevent any type 1 errors,
where violation of the sphericity assumption occurred. Bonferroni adjusted pair-
wise comparisons were carried out for individual comparisons. Pearson‟s
correlation was calculated to study the association between the changes in
corneal thickness and anterior and posterior curvature changes in the central
and peripheral corneal regions, using SPSS statistical software. The correlation
was calculated for the mean of the results from all four lenses and then for the
results of each of the lenses individually.
3.3.3 Medmont data: Correlation between the rigid lens fluorescein pattern and corneal topography changes
Tangential curvature difference maps were calculated for each subject with the
smaller diameter rigid lenses (PMMA/9.5, RGP/9.5) as described above using
the Medmont data, and the location of the points of maximum corneal flattening
were determined along the vertical and horizontal meridians along the map
centre. The tangential curvature map was used for this because it describes
localised changes in corneal topography. Additionally each digital slit lamp
image of the fluorescein fitting pattern was analysed to determine the region of
minimal clearance along the vertical and horizontal meridians. Pearson‟s
Chapter 3: Corneal changes following short-term rigid contact lens wear
79
correlation was then used to investigate the association between the spatial
location of the points of minimum clearance in the fluorescein image and the
points of corneal flattening in the topographic map.
Figure 3-4 shows the steps involved in calculating the points of
minimum clearance (in the fluorescein images) and points of maximum
flattening (topographic maps). First, the horizontal visible iris diameter (HVID)
and the distance from the videokeratoscope (VK) centre to limbus centre (LC)
was calculated for each subject using the Placido disc image from the
Medmont E300, using custom written software (Iskander et al. 2004). The
location of the limbus and its centroid was then located in the fluorescein
image, and the image was scaled using each subject‟s HVID measure from the
Placido disc image. The coordinates of the location of the VK centre in the
fluorescein image was then determined using the VK centre to LC offset from
the Placido disc image. The fluorescein pattern image was then analysed
using a Matlab-based algorithm which quantifies fluorescence along a
meridian. Fluorescence was estimated for the vertical and horizontal corneal
meridians along the coordinates of the VK centre, and the point of least
fluorescence was taken as the point of minimum clearance. The locations of
four points of minimum clearance were therefore estimated: superior and
inferior points of clearance along the vertical meridian and nasal and temporal
points of clearance along the horizontal meridian. The points of maximum
flattening in the midperipheral corneal region of the tangential curvature
topographic map, along the vertical and horizontal corneal meridians (centred
on VK axis), were also calculated using custom-written software (Topoview,
developed at Contact Lens and Visual Optics Laboratory). Pearson correlation
and significance was then calculated for these points.
Due to the constraints of size of the tangential curvature map from the
VK, the points of flattening (topographic map) corresponding to the
midperipheral points of minimum clearance in the fluorescein map were not
available in the superior and temporal regions for a substantial number of
subjects. Hence, these points (superior and temporal) were not included in the
analysis. Sufficient data was available for the inferior (19 lenses) and nasal (26
lenses) points, thus data for only these points (inferior and nasal) of minimum
clearance and flattening data for the smaller diameter lenses (PMMA/9.5 and
RGP/9.5) were included in the analysis. The data from the large diameter lens
Chapter 3: Corneal changes following short-term rigid contact lens wear
80
(RGP/10.5) were also excluded due to the same reasons (limited number of
data points of flattening in the topographic map).
- 81 -
Figure 3-4: Steps involved in correlating rigid lens fluorescein pattern and corneal topographic changes. (b) White cross showing LC (c) small white cross showing LC and bigger white cross showing VK centre. HVID: horizontal visible iris diameter, VK: videokeratoscope centre, LC: limbus centre.
- 82 -
3.3.4 Medmont data: Corneal refractive power
Corneal refractive power was estimated based upon each subject‟s average
corneal height maps, from the Medmont VK, assuming a corneal refractive
index of 1.376. Least squares fitting of a sphero-cylindrical surface to the
refractive power map (Maloney et al. 1993) was performed using custom-
written software. The surface was referenced to the VK axis. The refractive
power change was described and analysed in terms of power vectors (Thibos
et al. 1997): best fit sphere (M), with/against-the-rule astigmatism (J0) and
oblique astigmatism (J45) for 4 and 6 mm corneal diameters. The refractive
power changes were tested for statistical significance using repeated measures
ANOVA with lens type and corneal diameter as within-subject factors.
3.3.5 COAS data: Ocular wavefront error
Zernike coefficients up to the 8th radial order were exported from the COAS
aberrometer and then averaged for each subject using a custom-written
software (developed at Contact Lens and Visual Optics Laboratory) for 4 mm
(photopic) and 5.5 mm (scotopic) pupil size. These coefficients were further
analysed to calculate higher-order root mean square (HO RMS), 2nd, 3rd and 4th
order RMS for the baseline and contact lens wearing days. Repeated
measures ANOVA with lens type as within-subject factor was performed to
calculate the statistical significance of the changes.
3.3.6 Lens movement videos: Position of contact lens with respect to limbus centre
The videos were exported as image frames for further analysis. A 30 seconds
video gave 30 frames per second, i.e. approximately 900 image frames. In
order to calculate the most frequent position of contact lens (in between blinks)
in relation to the limbus centre, 3 image frames were analysed after 3 different
blinks. The image frames 1 second after each blink were selected for analysis
to give the contact lens time to settle.
The image analysis was performed by the same independent masked
observer using custom-written software (Topoview, developed at Contact Lens
and Visual Optics Laboratory). It involves the operator manually locating the
position of the coordinates of the limbus (8 points), contact lens (8 points), the
upper lid margin (8 points) and the lower lid margin (8 points) Figure 3-5. The
software then determines the best fitting ellipse to the limbus and contact lens
Chapter 3: Corneal changes following short-term rigid contact lens wear
83
co-ordinates and the best fitting quadratic function to the upper and lower
eyelid coordinates. The position of contact lens centre with respect to the LC
(x-horizontal and y-vertical) was then calculated for the 3 images after 3
different blinks. A mean, SD and range of the horizontal and vertical
coordinates of the lens position were then calculated for the 3 images with each
lens for all the subjects.
Figure 3-5: Image showing the position of a rigid contact lens on cornea (light blue ring), limbus (yellow ring) and upper (red arc) and lower eyelid (blue arc).
3.4 Results
3.4.1 Anterior corneal axial curvature
Figure 3-6 and Table 3-2 show the group mean change in anterior axial corneal
curvature (relative to the baseline day) for the four types of lenses. The type of
lens had a significant effect on the changes in anterior axial corneal curvature
(p<0.001, repeated measures ANOVA). Overall, the PMMA/9.5 lens showed
primarily steepening (significant in the centre) whereas RGP/9.5, RGP/10.5
lenses caused flattening in both the central and peripheral anterior corneal
surface (Table 3-2). RGP/10.5 lens led to significant flattening in the central
(0.05 ± 0.04 mm, p=0.007) and peripheral (0.03 ± 0.02 mm, p<0.001) corneal
regions. Significant flattening in the peripheral corneal region was observed
following wear of the RGP/9.5 lens (Table 3-2). Only small magnitude changes
in corneal curvature were observed with the SiHy/14.0 lens, and these changes
did not reach statistical significance.
Chapter 3: Corneal changes following short-term rigid contact lens wear
84
Figure 3-6: Group mean changes in anterior axial corneal curvature (mm) relative to baseline day for the four different types of contact lenses. The lenses included different materials (PMMA, RGP and SiHy) and diameters (9.5, 10.5 and 14.0 mm). Details of the lenses are shown in Table 3-1. Positive change represents flattening and negative change represents steepening.
3.4.2 Posterior corneal axial curvature
The type of lens also had a significant effect on the changes in posterior
corneal axial curvature (p<0.001, repeated measures ANOVA). Figure 3-7 and
Table 3-2 show the group mean changes in posterior corneal curvature
(compared to the baseline day) for the four lenses. PMMA/9.5 lens showed
flattening in both the central (0.09 ± 0.05 mm, p<0.001) and peripheral (0.04 ±
0.03 mm, p=0.006) cornea, whereas the RGP/9.5, RGP/10.5 and SiHy/14.0
lenses showed no significant changes.
Chapter 3: Corneal changes following short-term rigid contact lens wear
85
Figure 3-7: Group mean changes in posterior axial corneal curvature (mm) relative to baseline day for the four different types of contact lenses. The lenses included different materials (PMMA, RGP and SiHy) and diameters (9.5, 10.5 and 14.0 mm). Details of the lenses are shown in Table 3-1. Positive change represents flattening and negative change represents steepening.
Table 3-2: Mean changes in anterior and posterior axial corneal curvatures relative to baseline days with the four contact lenses in the central and peripheral regions.
Anterior axial curvature Posterior axial curvature
Lens Central Mean
change ± SD
(mm)
Peripheral mean
change ± SD
(mm)
Central Mean
change ± SD
(mm)
Peripheral mean
change ± SD
(mm)
PMMA/9.5 –0.05 ± 0.05
(p=0.03 )
–0.01 ± 0.02
(p=1.0)
0.09 ± 0.05
(p<0.001)
0.04 ± 0.03
(p=0.006)
RGP/9.5 0.01 ± 0.04
(p=1.0)
0.03 ± 0.02
(p<0.001)
–0.01 ± 0.03
(p=1.0)
–0.01 ± 0.02
(p=0.59)
RGP/10.5 0.05 ± 0.04
(p=0.007)
0.03 ± 0.02
(p<0.001)
–0.02 ± 0.03
(p=0.41)
–0.02 ± 0.02
(p=0.10)
SiHy/14.0 0.004 ± 0.03
(p=1.0)
0.004 ± 0.01
(p=1.0)
–0.003 ± 0.04
(p=1.0)
–0.002 ± 0.02
(p=1.0)
Positive change represents flattening and negative change represents steepening.
3.4.3 Corneal thickness
Corneal thickness was significantly affected by the type of lens and corneal
region (both p<0.001, repeated measures ANOVA).The group mean changes
in corneal thickness (relative to the baseline day) with the four lenses is shown
in Figure 3-8 and Table 3-3. PMMA/9.5 lens showed the greatest level of
Chapter 3: Corneal changes following short-term rigid contact lens wear
86
corneal swelling in both the central (27.92 ± 15.49 µm, p<0.001) and peripheral
(17.78 ± 12.11 µm, p=0.001) corneal regions. RGP/9.5 and RGP/10.5 lenses
showed lesser amounts of corneal swelling whereas the SiHy/14.0 lens showed
smaller changes again. The corneal swelling seen with PMMA/9.5 lens was
significantly greater than the swelling seen with the RGP/9.5, RGP/10.5 and
SiHy/14.0 lenses in the central region and the SiHy/14.0 lens in the peripheral
annular region. There were no significant differences in corneal swelling
between the RGP and SiHy lenses.
Table 3-3: Mean corneal thickness changes relative to baseline days with the four contact lenses in central and peripheral corneal regions.
Lens Mean change in central corneal thickness (relative to baseline)
Mean change in peripheral corneal thickness (relative to baseline)
(µm) ± SD (%) (µm) ± SD (%)
PMMA/9.5 27.92 ± 15.49 (p<0.001) 4.77 17.78 ± 12.11 (p=0.001) 2.71
RGP/9.5 2.30 ± 12.46 (p=1.0) 0.41 6.26 ± 13.30 (p=1.0) 0.97
RGP/10.5 4.47 ± 12.56 (p=1.0) 0.80 9.42 ± 12.15 (p=0.12) 1.46
SiHy/14.0 –1.88 ± 13.07 (p=1.0) –0.34 –1.03 ± 14.21 (p=1.0) –0.16
Positive change represents swelling and negative change represents thinning.
Figure 3-8: Group mean changes in corneal thickness (mm) relative to baseline day for the four different types of contact lenses. The lenses included different materials (PMMA, RGP and SiHy) and diameters (9.5, 10.5 and 14.0 mm). Details of the lenses are shown in Table 3-1. Positive change represents swelling and negative change represents thinning.
Chapter 3: Corneal changes following short-term rigid contact lens wear
87
3.4.4 Correlation between corneal curvature and thickness
A linear regression was performed on changes in corneal curvature and
thickness with all the lenses. Significant negative correlation between the
changes in central corneal thickness (swelling) and central (R2 = 0.63, p=0.001,
F = 20.03, B = –2.183) and peripheral (R2 = 0.57, p=0.002, F = 16.22, B = –
1.386) posterior corneal curvatures change (Figures 3-9 a & b) were found.
Similarly, there was a significant negative correlation between changes in
peripheral corneal thickness and central (R2 = 0.74, p<0.001, F = 34.54, B = –
2.328) and peripheral (R2 = 0.69, p<0.001, F = 26.78, B = –1.487) posterior
corneal curvatures change (Figure 3-9 c & d). The change in anterior corneal
curvature did not show a significant correlation with the change in corneal
thickness (all p>0.05).
Relationships between corneal thickness and front and back curvatures
for the individual lenses are shown in Table 3-4. Generally, RGP/9.5, RGP/10.5
and SiHy/14.0 lenses showed a highly significant negative correlation (all
p≤0.003) between corneal thickness and posterior corneal curvature whereas
PMMA/9.5 lens did not show any significant correlations between these
parameters.
Chapter 3: Corneal changes following short-term rigid contact lens wear
88
Figure 3-9: Correlation between changes in central corneal thickness and central (a) and peripheral (b) back curvature. Correlation between changes in peripheral corneal thickness and central (c) and peripheral (d) back curvature.
Table 3-4: Correlation between corneal thickness with anterior and posterior curvatures for the four different types of contact lenses.
Correlations Lens
PMMA/9.5
Lens
RGP/9.5
Lens
RGP/10.5
Lens
SiHy/14.0
Central thickness &
posterior central curvature NS
R2 = 0.832
p<0.001
R2 = 0.610
p=0.001
R2= 0.641
p<0.001
Central thickness &
posterior peripheral curvature NS
R2 = 0.834
p<0.001
R2 = 0.534
p=0.003
R2 =0.776
p<0.001
Peripheral thickness &
posterior central curvature NS
R2 = 0.828
p<0.001
R2 = 0.671
p<0.001
R2 = 0.841
p<0.001
Peripheral thickness &
posterior peripheral curvature NS
R2 = 0.857
p<0.001
R2 = 0.64
p=0.001
R2 = 0.821
p<0.001
Central thickness &
anterior central curvature NS NS
R2 = 0.303
p=0.04 NS
Central thickness &
anterior peripheral curvature
R2=0.354
p=0.03 NS
R2 = 0.462
p=0.007 NS
Peripheral thickness &
anterior central curvature NS NS NS
R2 = 0.313
p=0.04
Peripheral thickness &
anterior peripheral curvature NS NS
R2 = 0.386
p=0.02 NS
NS: not statistically significant, all correlations are in negative direction
Chapter 3: Corneal changes following short-term rigid contact lens wear
89
3.4.5 Correlation between rigid lens fluorescein pattern and corneal topography changes
To study the association between the fluorescein pattern‟s regions of minimal
clearance and changes in corneal curvature (in the topographic map), a
Pearson‟s correlation was calculated for the spatial location of these regions.
There was a significant positive correlation between the location of points of
minimum clearance (in fluorescein pattern) and maximum corneal flattening (in
the topographic maps) for inferior (R2 = 0.599, p<0.001) points along vertical
meridian and nasal (R2 = 0.528, p<0.001) points along the horizontal meridian
(Figure 3-10).
Figure 3-10: Correlation between distance of points of minimum clearance (between cornea and contact lenses, in fluorescein pattern) and points of maximum corneal flattening, from the videokeratoscope (VK) centre. Data is shown for inferior (V2) points along vertical meridian and nasal (H1) points along the horizontal meridian.
3.4.6 Refractive power
The change in anterior corneal best fit sphere (M) was significantly affected by
lens type and the size of corneal diameter analysed (both p<0.001, repeated
measures ANOVA). The group mean changes in M relative to the baseline day
for the four types of lenses, for 4 and 6 mm corneal diameter are shown in
Table 3-5. PMMA/9.5 lens (0.11 ± 0.07 D, p=1.00) showed an increase in
corneal power (which was not significant) whereas the RGP/9.5 (–0.34 ± 0.05
D, p<0.001), RGP/10.5 (–0.44 ± 0.07 D, p<0.001) and SiHy/14.0 (–0.11 ± 0.03
Chapter 3: Corneal changes following short-term rigid contact lens wear
90
D, p=0.01) lenses showed a significant decrease in refractive power for a 6 mm
corneal diameter.
With/against-the-rule astigmatism (J0) generally showed a decrease
with all the lenses, but the change only approached significance with RGP/9.5
lens (–0.08 ± 0.03 D, p=0.06). Oblique astigmatism (J45) showed an increase
with lenses PMMA/9.5, RGP/9.5 and RGP/10.5 whereas there was a decrease
with SiHy/14.0 lens. The increase in oblique astigmatism (J45) was significant
only with the RGP/9.5 lens (0.06 ± 0.01 D, p=0.01).
Table 3-5: Mean changes in best fit sphere(M), with/against the rule astigmatism (J0) and oblique astigmatism (J45) in Dioptres, relative to baseline day with the four contact lenses for the 4 and 6 mm corneal diameter.
Lens
Mean change in
M ± SD
(Dioptres)
p-value
Mean change in
J0 ± SD
(Dioptres)
p-value
Mean change in
J45 ± SD
(Dioptres)
p-value
4 mm corneal diameter
PMMA/9.5 0.19 ± 0.32 0.47 –0.04 ± 0.18 1.0 0.02 ± 0.12 1.0
RGP/9.5 –0.31 ± 0.22 0.002 –0.09 ± 0.13 0.28 0.01 ± 0.08 1.0
RGP/10.5 –0.49 ± 0.32 0.001 –0.06 ± 0.16 1.0 0.02 ± 0.07 1.0
SiHy/14.0 –0.13 ± 0.15 0.09 –0.02 ± 0.07 1.0 –0.01 ± 0.06 1.0
6 mm corneal diameter
PMMA/9.5 0.11 ± 0.24 1.0 –0.02 ± 0.10 1.0 0.02 ± 0.06 1.0
RGP/9.5 –0.34 ± 0.17 < 0.001 –0.08 ± 0.10 0.06 0.06 ± 0.06 0.01
RGP/10.5 –0.44 ± 0.26 < 0.001 –0.05 ± 0.13 1.0 0.04 ± 0.07 0.54
SiHy/14.0 –0.11 ± 0.10 0.01 –0.01 ± 0.05 1.0 –0.01 ± 0.04 1.0
Positive change represents increase and negative change represents decrease in corneal refractive power. Negative change in M represents decrease in corneal refractive power (hypermetropic shift). Positive change in M represents increase in corneal refractive power (myopic shift). Negative change in J0 represents decrease in WTR astigmatism. Positive J45 represents negative cylinder axis closer to 45° and negative J45 represents negative cylinder axis closer to 135°.
3.4.7 Ocular wavefront error
HO RMS, 2nd, 3rd and 4th order RMS wavefront errors were calculated for all 14
subjects for 4 mm pupil and for 10 subjects for 5.5 mm pupil (as the pupil size
for other subjects was less than 5.5 mm). The type of lens had a significant
effect on HO RMS, 2nd, 3rd and 4th order RMS wavefront errors for both 4 (all
p≤0.002) and 5.5 (all p≤0.02) mm pupil diameters. The group mean changes in
HO RMS, 2nd, 3rd and 4th order RMS relative to the baseline day for the four
types of lenses, for 4 and 5.5 mm pupil diameters are shown in Table 3-6.
Overall, PMMA/9.5, RGP/9.5 and RGP/10.5 lenses showed an increase in HO
RMS, 3rd and 4th order RMS.
Chapter 3: Corneal changes following short-term rigid contact lens wear
91
PMMA/9.5 lens showed a significant increase in HO RMS (0.09 ± 0.07
µm, p=0.005), 2nd (0.22 ± 0.19 µm, p=0.009), 3rd (0.08 ± 0.07 µ, p=0.007) and
4th (0.03 ± 0.04 µm, p=0.05) order RMS and RGP/9.5 lens showed significant
increase in 4th (0.02 ± 0.02 µm, p=0.01) order RMS, for the 4 mm pupil
diameter (Table 3-6). There was also a significant increase in HO RMS (0.29 ±
0.10 µm, p=0.001), 3rd (0.09 ± 0.06 µm, p=0.01) and 4th (0.06 ± 0.05 µm,
p=0.05) order RMS with PMMA/9.5 lens for 5.5 mm pupil diameter (Table 3-6).
The soft contact lens caused no significant changes in any of the terms.
Table 3-6: Mean changes in HO RMS, 2nd, 3rd and 4th order RMS, relative to baseline day with the four contact lenses for 4 mm (n=14) and 5.5 mm (n=10) pupil diameters. ‘n’ is the number of subjects included in the analysis.
Lens Mean HO RMS
change ± SD
(µm)
Mean 2nd
order
RMS change ± SD
(µm)
Mean 3nd
order
RMS change ± SD
(µm)
Mean 4nd
order
RMS change ± SD
(µm)
4 mm pupil
PMMA/9.5 0.09 ± 0.07 (p=0.005)
0.22 ± 0.19 (p=0.009)
0.08 ± 0.07 (p=0.007)
0.03 ± 0.04 (p=0.05)
RGP/9.5 0.13 ± 0.13
(p=0.38) –0.02 ± 0.15
(p=1.00) 0.03 ± 0.08
(p=1.00) 0.02 ± 0.02
(p=0.01)
RGP/10.5 0.01 ± 0.02
(p=0.80) –0.10 ± 0.15
(p=0.27) 0.01 ± 0.03
(p=1.00) 0.01 ± 0.02
(p=1.00)
SiHy/14.0 –0.001 ± 0.01
(p=1.00) –0.03 ± 0.10
(p=1.00) 0.002 ± 0.01
(p=1.00) –0.004 ± 0.01
(p=1.00)
5.5 mm pupil
PMMA/9.5 0.29 ± 0.10 (p=0.001)
0.26 ± 0.39 (p=0.62)
0.09 ± 0.06 (p=0.01)
0.06 ± 0.05 (p=0.05)
RGP/9.5 0.21 ± 0.09
(p=1.00) –0.01 ± 0.31
(p=1.00) 0.12 ± 0.09
(p=1.00) 0.03 ± 0.04
(p=1.00)
RGP/10.5 0.17 ± 0.07
(p=1.00) –0.10 ± 0.26
(p=1.00) –0.02 ± 0.06
(p=1.00) 0.01 ± 0.05
(p=1.00)
SiHy/14.0 0.10 ± 0.08
(p=1.00) –0.09 ± 0.24
(p=1.00) –.02 ± 0.05
(p=1.00) –0.01 ± 0.03
(p=1.00)
Positive change represents increase and negative change represents decrease.
3.4.8 Position of contact lens
The mean distance of rigid contact lens centre from the limbus centre one
second after 3 different blinks is shown Table 3-7. This also represents the
most frequent on-eye resting position of the contact lens in between the blinks.
The decentration in the vertical direction was greater than in the horizontal
direction for all the lenses and PMMA/9.5 lens showed maximum decentration.
The relatively large standard deviations associated with each of the mean lens
decentration and relatively large mean range indicates a high degree of
variability in lens position both within and between subjects.
Chapter 3: Corneal changes following short-term rigid contact lens wear
92
Table 3-7: Mean distances of contact lens centre to limbus centre (mm) and ranges (mm) in the horizontal and vertical directions for the three types of rigid contact lenses.
Lens Mean horizontal distance ± SD
Range Mean vertical distance ± SD
Range
PMMA/9.5 0.24 ± 0.23 –0.33 to 0.56 1.05 ± 0.65 0.02 to 2.44
RGP/9.5 0.28 ± 0.32 –0.34 to 0.91 0.78 ± 0.58 –0.51 to 2.04
RGP/10.5 0.10 ± 0.28 –0.50 to 0.73 0.43 ± 0.42 –0.38 to 1.38
Positive sign represents temporal direction and negative sign represents nasal (horizontally) and positive sign represents superior direction and negative sign represents inferior (vertically).
3.5 Discussion
We investigated the effect of short-term (8 hours) wear of 3 different types of
contact lenses (RGP, PMMA and SiHy) of different Dk and diameters on
corneal thickness and curvature. Corneal swelling with the wear of PMMA
contact lenses has been widely documented in the literature (Carney 1974;
Fonn et al. 1984; Wang et al. 2003; Moezzi et al. 2004), and our results are
consistent with these findings. We found significantly more corneal swelling
with the PMMA contact lens compared to the RGP lenses as reported earlier by
Fonn et al. (1984) which was of greater magnitude in the central compared to
peripheral corneal region. The corneal swelling with the RGP lenses did not
reach statistical significance, however on average a greater magnitude of
swelling was observed in peripheral corneal regions compared to central
regions (both p<0.01) with both of the RGP lenses (small and larger diameter)
worn. PMMA and RGP lenses were also observed to have opposite effects on
corneal curvature. Overall, PMMA lenses caused anterior corneal steepening
(significant centrally) and significant posterior corneal flattening whereas RGP
lenses resulted in anterior corneal flattening and posterior steepening (although
not significant posteriorly) (Table 3-2). Silicone hydrogel lens wear had little
effect on corneal thickness or curvature (Table 3-2 and 3-3). The changes we
observed in corneal curvatures were well correlated with corneal thickness
changes (Figure 3-9, Table 3-4). The changes in thickness and curvatures with
the different rigid contact lenses are illustrated schematically in Figure 3-11.
With PMMA lenses the anterior steepening and posterior flattening was
associated with central corneal thickening (Figure 3-11 a). For RGP lenses,
anterior flattening and posterior steepening was accompanied by peripheral
corneal thickening (Figure 3-11 b).
Chapter 3: Corneal changes following short-term rigid contact lens wear
93
Figure 3-11: Schematic demonstration of anterior and posterior curvatures and thickness of the cornea, before and after PMMA and RGP contact lens wear for 8 hours based on the experimental data. The solid lines represent the baseline anterior and posterior surfaces of cornea. The dotted line represents the anterior and posterior surfaces of the cornea after contact lens wear for 8 hours. (a) PMMA contact lens showing greater central corneal swelling compared to peripheral resulting in anterior corneal steepening and posterior corneal flattening. (b) RGP contact lens showing greater peripheral corneal swelling resulting in anterior corneal flattening and posterior corneal steepening. Note that the diagram is not to scale.
The corneal swelling due to the PMMA lenses was of much higher
magnitude than that observed for the RGP lenses, and was confined largely to
the central cornea resulting in overall anterior corneal steepening and posterior
flattening. The difference in the effects of PMMA and RGP on corneal thickness
was expected due to the difference in Dk of the two lens materials. The fitting
characteristics of the RGP and PMMA lens were similar for these subjects, so
the curvature changes due to mechanical forces should also be similar. For the
RGP lenses, the changes in curvature are likely to be at least in part driven by
mechanical forces on anterior cornea resulting in slight flattening. More
prominent corneal flattening due to mechanical forces of RGP lenses, that
involves the migration of epithelial cells is observed with orthokeratology (Choo
et al. 2008). For PMMA lenses, any changes in curvature due to mechanical
forces (which might have been similar to those due to RGP lenses), were
probably masked by the thickness and curvature changes because of hypoxia.
Chapter 3: Corneal changes following short-term rigid contact lens wear
94
We did notice small changes in the posterior corneal curvature
(steepening) with RGP lenses, presumably related to slight peripheral corneal
swelling, but they were not significant. The larger diameter RGP lens showed
slightly more pronounced changes for thickness and central anterior curvatures
compared to the smaller diameter. The changes in curvatures and thickness
following SiHy lens wear were very small and not statistically significant. The
minimal corneal changes noted with the SiHy lens most likely relate to the lens
being thinner and having a substantially lower modulus of elasticity, which
would be expected to result in less metabolic and mechanical related corneal
changes.
The mechanical forces across the cornea due to a contact lens are
distributed based on areas of touch and clearance. For rigid lenses these areas
are identified using the corneal fluorescein pattern during the contact lens
fitting. Fluorescein patterns are used to examine whether the lens will fit loose
and ride low on the cornea or fit tight and prevent any tear exchange. Usually,
rigid lens fitting procedure aims for an optimal fit which allows enough tear
exchange and also ensures lens centration on the cornea.
The profile of the tear layer thickness should also influence the changes
in corneal curvature due to mechanical forces. For example, the areas of
minimal clearance would be expected to induce flattening, whereas areas of
increased clearance may result in steepening. Although, a rigid lens moves
with every blink, it will most likely result in corneal curvature changes based
primarily on its final resting position after the completion of the blink (the inter-
blink location). We found that regions of minimal fluorescein clearance
correlated spatially to the areas of anterior corneal flattening. We could find no
previous reports which quantitatively analyse the fluorescein pattern for rigid
contact lens fitting. Information provided by this analysis could be useful in
predicting the changes in corneal curvature that the rigid contact lens will
produce.
We investigated the change in the best fit sphero-cylinder of the anterior
corneal surface with short-term wear of different types of contact lenses. The
results were similar for both 4 mm and 6 mm corneal diameters. The PMMA
lens showed a small (but statistically insignificant) increase in best fit sphere
(M). However there was a clinically and statistically significant decrease (mean
approximately 0.50 D) in best fit sphere (M) with the RGP lenses. This
Chapter 3: Corneal changes following short-term rigid contact lens wear
95
significant refractive power change may be due to mechanical forces on the
anterior surface of the RGP lens (which was likely to be masked in case of
PMMA lens due to significant corneal swelling). This significant refractive power
change could also be a short-term consequence of peripheral corneal swelling
but if it were to persist it would require alteration in the lens power to
compensate. There are no reports of this phenomenon in the clinical literature,
which suggests that it may be a transient occurrence. Further research
investigating corneal changes with longer periods of RGP lens wear are
required to clarify the time course of these corneal refractive power changes.
For the 6 mm corneal diameter, the RGP/9.5 lens also induced a clinically
insignificant change in with/against-the-rule (J0) and oblique (J45) astigmatism.
The large diameter RGP contact lens caused greater central anterior
corneal flattening compared to the small diameter RGP lens. This could be
because of greater interaction of larger diameter lens with the upper lid
compared to smaller diameter lens, leading to differences in pressure
distribution by the two lenses. The corneal swelling caused by the large
diameter lens was also slightly more compared to small diameter lens, both in
central and peripheral cornea but these changes were not significant.
The soft SiHy contact lens resulted in a clinically insignificant change in
corneal best fit sphere (M). These results support the view that the soft contact
lenses predominantly conform to the shape of the cornea and therefore
produce little refractive power changes due to lens pressure, whereas rigid
contact lenses can alter the shape of the cornea and can produce considerable
changes in corneal refractive power.
There are many studies reporting the effects of contact lenses on-eye
on the ocular wavefront aberrations (Hong and Himebaugh 2001; Dorronsoro et
al. 2003; Lu et al. 2003). The changes in aberrations associated with contact
lens wear depend on both the optical properties of the lens and the nature of
the tear optics (Hong and Himebaugh 2001). Therefore, RGP lens wear has
been shown to result in a reduction in total ocular aberrations whereas soft
contact lens wear has usually been found to result in increases in aberrations
(Griffiths et al. 1998; Lu et al. 2003). To our knowledge there are no
experimental studies reporting residual effects of contact lens wear on ocular
aberrations measured after removal of the lens. We investigated the effect of
short-term wear of various contact lenses on wavefront aberrations of the eye
Chapter 3: Corneal changes following short-term rigid contact lens wear
96
for 4 mm and 5.5 mm pupil size. We found a significant increase in HO RMS,
2nd, 3rd and 4th order RMS wavefront error after PMMA contact lens wear for
a 4 mm pupil diameter. For a 5.5 mm pupil, there was also a significant
increase in 3rd, 4th and HO RMS but not in 2nd order RMS. This suggests that
the anterior corneal steepening and posterior flattening associated with corneal
thickening as seen with the PMMA lens, not only affects the lower order
wavefront aberrations but also results in increased higher order terms. The
increase in most aberration terms including the non-symmetric aberrations
probably depends on the final on-eye resting position of the lens which induced
curvature changes and corneal thickening asymmetrically relative to the pupil
centre (Table 3-6). It is evident from Figures 3-6 to 3-8 that the corneal
changes associated with the PMMA lens are often decentred away from the
topographic map centre. We found the mean decentration of PMMA lens centre
with respect to the limbus centre to be 0.24 ± 0.23 mm temporally and 1.05 ±
0.65 mm superiorly. These changes may result in significant reduction in ocular
image quality after lens removal (during lens wear the post-lens tear layer
would neutralize most of the higher order aberrations) and may be at least in
part be responsible for some reports of “spectacle blur” associated with PMMA
lens wear (Levenson 1983; Wilson et al. 1990).
3.6 Conclusion
To conclude, PMMA contact lens wear resulted in corneal swelling (more in the
centre compared to periphery) consistent with previous reports, and RGP
lenses caused more corneal swelling in the periphery compared to the centre.
The difference in the pattern of regional corneal swelling seen with PMMA and
RGP lenses, due to hypoxia induced by the PMMA lens material, led to
opposite effects on corneal curvature for these lenses. Overall, PMMA lenses
caused anterior corneal steepening and posterior corneal flattening whereas
RGP lenses resulted in anterior corneal flattening and posterior steepening. We
also found a significant correlation between the locations of minimum clearance
in the fluorescein pattern and the resultant corneal flattening in the topographic
maps, with the rigid lenses. This highlights the importance of lens fitting
characteristics and the resultant corneal curvature changes associated with
lens wear. This may also aid in anticipating the changes in corneal curvature
due to lens wear which can be up to 0.50 D, enough to affect vision and require
an adjustment in the lens power. Evidence of “spectacle blur” following PMMA
Chapter 3: Corneal changes following short-term rigid contact lens wear
97
lens wear has also been shown in the form of an increase in higher order
aberrations.
- 99 -
Chapter 4
Corneal changes with spherical versus back surface toric rigid contact lens wear
4.1 Introduction
Astigmatism is a commonly occurring refractive error and is found in about 13%
of the ametropic population (Porter et al. 2001). The origin of astigmatism can
be corneal or lenticular, with either one or both (anterior and posterior) corneal
and/or lenticular surfaces having different powers along the two principal
meridians. In a study by Fledelius and Stubgaard (1986) it was shown that 46%
of the population exhibits corneal astigmatism of >0.5 D but only 4.7% have
astigmatism of > 1.5 D, which was later supported by McKendrick and Brennan
(1996).
A spherical back surface RGP lens may correct up to 2 to 2.5 D corneal
astigmatism (through neutralisation of astigmatism by the tear lens) whereas
higher levels of astigmatism require that the back surface of the lens is toric in
order to provide a stable fit. However, stability of the fit also depends on a
range of factors including lens centration, lid forces and aspects of the lens
material and design characteristics (e.g. thickness, specific gravity of the
material).
In Experiments 1 (Chapter 2) and 2 (Chapter 3) we investigated
changes in corneal curvature and thickness with short-term use of soft and
RGP contact lenses respectively, in subjects with only small amounts of
astigmatism. These contact lenses caused changes in corneal thickness and
curvature, although the results varied with different materials and lens designs.
Back surface toric RGP lenses are usually fitted on or near alignment (Lindsay
2007) to the two principal meridians of astigmatic corneas, in order to ensure
rotational stability. We hypothesise that the regional mechanical pressure on
the cornea and tear exchange due to back surface toric RGP lenses will vary
from that of spherical RGP lenses and that this will result in different corneal
thickness and curvature changes with the two lens designs. In this chapter we
aim to investigate corneal curvature, thickness and refractive changes after
short-term use of back surface toric RGP lenses compared with spherical RGP
lenses on subjects with astigmatic corneas.
Chapter 4: Corneal changes with spherical versus back surface toric rigid contact lens wear
100
4.2 Methodology
The study was conducted over a period of 3 days and the protocol was similar
to that in Chapters 2 and 3. Baseline measurements were taken without any
contact lens on day one, in the morning and the afternoon after 8 hours. On
days 2 and 3 of the study the subjects wore two different types of contact
lenses in the left eye only, and measurements were taken in the morning
before the lens was inserted and repeated after 8 hours of wear. Lens wear
typically started between 8 and 11 am and at least 2 hours after waking, to limit
the potential influence of the corneal changes that are typically evident
immediately after sleep (Read and Collins 2009). The lenses were removed in
the afternoon between 4 and 7 pm, after 8 hours of lens wear. The type of lens
to be worn on each study day was randomized and a 2-3 day recovery period
was allowed after each lens wear day before commencing wear of the second
lens.
All subjects were asked to read the study information sheet before
signing an informed consent. The study followed the tenets of the declaration of
Helsinki and was approved by the QUT university human research ethics
committee (see Appendix A).
4.2.1 Subjects
The study included 6 young, healthy adult subjects, aged between 19 to 31
years (mean age ± SD = 24.8 ± 4.1 years) with best-corrected visual acuity of
at least 6/6 and a difference in central corneal curvature of at least 0.25 mm
(1.4 D) between the principal meridians. Subjects were selected to have with-
the-rule astigmatism (i.e. major axis within 30 degrees of horizontal). The mean
central corneal curvatures (simulated K reading) were 7.9 ± 0.3 mm (42.8 ± 1.6
D) in the flatter meridian and 7.6 ± 0.3 mm (44.7 ± 2.1 D) in the steeper
meridian, as determined by the Medmont E300 videokeratoscope. One of the 6
subjects showed limbus-to-limbus astigmatism and the rest showed central
corneal astigmatism (Figure 4-1). The mean spherical refractive error was –3.0
± 1.9 D and mean refractive astigmatism was –0.7 ± 0.8 D.
All subjects were screened for any anterior segment pathology or tear
film abnormalities using slit-lamp biomicroscopy. No history of corneal injury,
infection or surgery was reported by any of the subjects. One of the subjects
was an habitual soft lens wearer and three others were occasional soft contact
Chapter 4: Corneal changes with spherical versus back surface toric rigid contact lens wear
101
lens wearers. These subjects discontinued lens wear one month prior to the
commencement of the study to allow any effects of soft lens wear to largely
resolve (Wilson et al. 1990; Wang et al. 2002). None of the subjects were
previous rigid contact lens wearers.
Figure 4-1: Axial corneal curvature maps of all subjects showing pattern of corneal astigmatism and difference in curvature of the two principal meridians. Note that all subjects had central astigmatism except for subject 04 who showed limbus-to-limbus astigmatism.
4.2.2 Contact lenses
Two different types of custom made rigid contact lenses were ordered for the
left eye of each subject. The rigid lenses were both 9.5 mm in diameter and
one had a spherical BOZR and the second had a toric back surface. A contact
Chapter 4: Corneal changes with spherical versus back surface toric rigid contact lens wear
102
lens fitting trial with fluorescein assessment was performed for each subject in
order to determine the optimum back optic zone radius (BOZR) for the
spherical lens. The back surface toric lens was ordered based on the corneal
curvature derived from videokeratoscopy. Examples of the fluorescein pattern
fitting with a spherical and back surface toric RGP lens on subjects with high
and low corneal astigmatism are shown in Figure 4-2. Details of the spherical
and toric lenses are shown in Table 4-1.
Table 4-1: Details of the lenses used in the study.
Lens Sph Toric
Design (BOZ) Spherical Toric
Design (BPZ) Aspheric Aspheric
Material RGP (Boston XO) RGP (Boston XO)
Power (Dioptre) –0.5 –0.5
Mean BOZR (mm) 7.8 ± 0.3 K1 = 7.6 ± 0.3, K2 = 7.9 ± 0.3
Total diameter (mm) 9.5 9.5
BOZD (mm) 8.1 8.1
Water content (%) 0 0
Dk 100 100
Modulus (MPa) 1500 1500
Manufacturing method Lathe Lathe
Surface treatment Plasma Plasma
Centre thickness (mm) 0.18 0.19
RGP: rigid gas permeable, BOZ: back optic zone, BPZ: back peripheral zone, BOZR: back optic zone radius, BOZD: back optic zone diameter, Dk: oxygen permeability, MPa: megapascal, unit of modulus of elasticity, mm: millimetres. The lenses supplied by the manufacturer were checked for back vertex power, lens diameter, BOZR (spherical lens) and edge and surface quality before use in the study. K1 = steeper corneal curvature, K2 = flatter corneal curvature.
Chapter 4: Corneal changes with spherical versus back surface toric rigid contact lens wear
103
Figure 4-2: Fluorescein patterns with a spherical (a) and back surface toric (b) lens (same eye) on a subject (04) with high astigmatism (∆K = 3.3 D), limbus-to-limbus. In the lower panels a spherical (c) and back surface toric (d) lens (same eye) on a subject (06) with a lower amount of corneal astigmatism (∆K = 1.4 D), central. Note axis markings/scribe marks of the toric lens on the flatter corneal meridian in both subjects (panels b and d).
4.2.3 Measurements and Instruments
Corneal topography and thickness, ocular wavefront aberrations and contact
lens fitting and centration characteristics were assessed at each measurement
session. The protocol followed was similar to that in Chapters 2 and 3.
Anterior and posterior corneal topography and regional corneal
thickness were measured using the Pentacam HR system (Oculus, Wetzlar,
Germany). A total of 5 measurements were completed using the “25 picture 3D
scan” mode, which gives 25 cross-sectional images of the anterior eye. Corneal
refractive power was measured using the Medmont E300 videokeratoscope
(Medmont Pty. Ltd., Victoria, Australia). A total of 4 videokeratoscope images,
with a quality score of 95 or greater were taken at each measurement session
and saved for analysis. Ocular monochromatic aberrations were measured
using the Complete Ophthalmic Analysis System (COAS, Wavefront Sciences
Ltd, USA). Measurements were performed without the use of any eye drops
under natural pupil conditions with dim room illumination. A total of 4
measurements were taken during each measurement session, with 20 frames
per measurement.
Chapter 4: Corneal changes with spherical versus back surface toric rigid contact lens wear
104
Digital photos of the rigid contact lens on eye were taken to record the
fluorescein pattern using a digital camera. The protocol was similar to that used
in Chapter 3 (Section 3.2.3). In order to analyse the most frequent position of
the contact lens on the cornea (in between blinks) a 30 second video was also
recorded the details of which are described in Section 3.2.3.
4.3 Data Analysis
The analysis carried out on the data was similar to that in Chapter 3, Section
3.3.
4.3.1 Corneal topography and thickness data
Corneal thickness and anterior and posterior axial curvature data from the
Pentacam and Medmont corneal height data from each measurement session
were exported. An average of the 4 maps (Medmont) and 5 maps (Pentacam),
taken for each subject during each measurement session, was calculated using
custom-written software (Topoview, developed at Contact Lens and Visual
Optics Laboratory).
4.3.2 Pentacam data: Corneal curvature and thickness
All difference maps in this study were generated by subtracting the curvature or
thickness map of the baseline day (afternoon) from the curvature or thickness
map after lens wear (afternoon). This was considering small but significant
diurnal variations in corneal thickness and curvature that occurred within the 8
hours duration of the study, as seen in Chapter 2. The thickness and curvature
difference maps were generated using Pentacam data to compare the baseline
to post-lens wear maps. Thus, „Thickness difference maps‟ were generated by
subtracting the average baseline thickness map from the average thickness
maps after 8 hours of lens wear. Similarly, „curvature difference maps‟ were
generated by subtracting the average baseline curvature map from the average
curvature map after 8 hours of lens wear. Group average difference maps were
generated by averaging the data from all subjects for each of the two lens
types.
The average Pentacam data for each subject were further analysed
using the custom-written Topoview software to investigate the regional changes
in corneal thickness and curvature after contact lens wear. The average
corneal thickness and curvature was calculated for each subject within two
Chapter 4: Corneal changes with spherical versus back surface toric rigid contact lens wear
105
corneal regions, the central area (0 – 4 mm) and peripheral area (4 – 8 mm) as
shown in Figure 2-4. Complete data for each subject were available for 8 mm
corneal diameter, thus analyses were performed in this region. The mean radii
of curvature along the vertical and horizontal meridians with the spherical and
back surface toric lens and on the baseline day were calculated using the
custom-written Topoview software.
Statistical significance of changes in corneal curvature and thickness
due to contact lens wear was tested using repeated measures analysis of
variance (ANOVA), using SPSS 17.0. Lens type and region were considered as
within-subject factors. Bonferroni adjusted pair-wise comparisons were
conducted for individual comparisons. To avoid any type 1 errors, degrees of
freedom were adjusted using Greenhouse-Geisser correction, where violation
of the sphericity assumption occurred.
4.3.3 Medmont data: Corneal refractive power
Average corneal height maps for each subject from the Medmont
videokeratoscope were used to calculate corneal refractive power, assuming a
corneal refractive index of 1.376. Custom-written software (Topoview) was
used to perform a least squares fitting of a sphero-cylindrical surface to the
refractive power map as described by Maloney et al. (1993). The
videokeratoscope axis was used as a reference for the surface fit. The
refractive power sphero-cylinder was converted into power vectors (Thibos et
al. 1997): best fit sphere (M), with/against-the-rule astigmatism (J0) and oblique
astigmatism (J45) for 4 and 6 mm corneal diameters. The refractive power
changes were tested for statistical significance using repeated measures
ANOVA with lens type and corneal diameter as within-subject factors.
4.3.4 COAS data: Ocular wavefront error
Zernike coefficients (measured using the COAS aberrometer) were averaged
for each subject using custom-written software (WFM, developed at Contact
Lens and Visual Optics Laboratory) for 4 mm (photopic) and 5.5 mm (scotopic)
pupil sizes up to the 8th radial order. Higher-order root mean square (HO RMS)
wavefront error, 2nd, 3rd and 4th order RMS for the baseline and contact lens
wearing days were further calculated. A repeated measures ANOVA with lens
type as within-subject factor was performed to test the statistical significance of
these changes.
Chapter 4: Corneal changes with spherical versus back surface toric rigid contact lens wear
106
4.3.5 Lens movement videos: Position of contact lens on cornea (with respect to limbus centre)
Each 30 second video was exported into image frames (with 30 frames per
second i.e. a total of approximately 900 image frames). Three image frames
were analysed after 3 different blinks to calculate the most frequent position of
contact lens in between blinks. The contact lens was allowed to settle after the
blink by selecting the image frame 1 second after the completion of each blink
for the analysis. The mean positions of contact lens centre with respect to the
limbus centre (x- horizontal and y- vertical) were calculated for the 3 images
using a custom written software, details of which have been described earlier
(Chapter 3, Section 3.3.6).
4.4 Results
4.4.1 Anterior corneal axial curvature
The group mean changes in anterior axial corneal curvature relative to the
baseline day for the two lenses are shown in Table 4-2 and Figure 4-3. The
type of lens had a significant effect on the changes in anterior axial corneal
curvature (p<0.05, repeated measures ANOVA). Generally, both lenses caused
a small magnitude of flattening in both central and peripheral corneal regions,
but the changes were significant only in the peripheral cornea (p<0.05, pairwise
comparison). The rate of flattening in the inferior cornea from centre to
periphery appeared higher with the toric lens than with the spherical lens, but
this was not statistically significant (p>0.05) (Figure 4-4).
Figure 4-3: Group mean changes in anterior axial corneal curvature (mm) relative to baseline day for the spherical and back toric RGP lenses. Details of the lenses are shown in Table 4-1. Positive change represents flattening and negative change represents steepening.
Chapter 4: Corneal changes with spherical versus back surface toric rigid contact lens wear
107
Table 4-2: Mean changes in anterior and posterior axial corneal curvatures relative to baseline days with the two lens types in the central and peripheral regions.
Anterior axial curvature Posterior axial curvature
Lens Central mean
change ± SD
(mm)
Peripheral mean
change ± SD
(mm)
Central mean
change ± SD
(mm)
Peripheral mean
change ± SD
(mm)
Sphere 0.04 ± 0.03
(p=0.11 )
0.03 ± 0.02
(p=0.05) –0.02 ± 0.03
(p=0.32 )
–0.02 ± 0.01
(p=0.08)
Back
toric 0.02 ± 0.02
(p=0.54 )
0.03 ± 0.02
(p=0.05)
–0.02 ± 0.03
(p=0.34 )
–0.03 ± 0.02
(p=0.03)
Positive change represents flattening and negative change represents steepening. P-value of <0.05 statistically significant.
Figure 4-4: Mean axial radii of curvature (mm) in the vertical meridians for baseline, spherical lens and back surface toric lens. VK: Videokeratoscope.
4.4.2 Posterior corneal axial curvature
The type of lens and corneal region had a significant effect on the changes in
posterior axial corneal curvature (both p<0.05, repeated measures ANOVA).
Figure 4-5 and Table 4-2 show the group mean changes in posterior axial
corneal curvature (relative to the baseline day) for the two lens types. Overall,
both lenses caused steepening in both the central and peripheral corneal
regions, but the changes were significant only in the peripheral cornea with the
toric lens (p<0.05, pairwise comparison).
Chapter 4: Corneal changes with spherical versus back surface toric rigid contact lens wear
108
Figure 4-5: Group mean changes in posterior axial corneal curvature (mm) relative to baseline day for the spherical and back surface toric RGP lenses. Positive change represents flattening and negative change represents steepening.
4.4.3 Corneal thickness
The group mean changes in corneal thickness relative to the baseline day for
the two lens types are shown in Table 4-3 and Figure 4-6. Both spherical and
toric lenses resulted in corneal swelling which was greater in the periphery
compared to the central corneal region, but these changes were not significant
with either lens. The degree of corneal swelling did not differ between the
spherical and the toric lenses (p>0.05, pairwise comparison).
Figure 4-6: Group mean change in corneal thickness (mm) relative to baseline day for the spherical and back toric RGP lenses. Details of the lenses are shown in Table 4-1. Positive change represents swelling and negative change represents thinning.
Chapter 4: Corneal changes with spherical versus back surface toric rigid contact lens wear
109
Table 4-3: Mean corneal thickness changes relative to baseline days with the two contact lens types in central and peripheral corneal regions.
Lens Mean change in central corneal thickness (relative to baseline)
Mean change in peripheral corneal thickness (relative to baseline)
(µm) ± SD (%) (µm) ± SD (%)
Sphere 0.32 ± 3.75 (p=1.0 ) 0.1 5.05 ± 5.73 (p=0.25 ) 0.8
Back
toric 2.19 ± 8.27 (p=1.0 ) 0.4 6.95 ± 9.31 (p=0.35 ) 1.1
Positive change represents swelling and negative change represents thinning.
4.4.4 Refractive power
The change in anterior corneal best fit sphere (M) and with/against-the-rule
astigmatism (J0) were significantly affected by lens type (both p<0.001,
repeated measures ANOVA). The group mean changes in M, J0 and J45
compared to the baseline day for 4 and 6 mm corneal diameters with the two
lens types are shown in Table 4-4. The spherical lens caused a significant
decrease in M for both 4 and 6 mm corneal diameters (both p<0.05). The toric
lens caused similar changes in M that bordered on significance for 6 mm
corneal diameter (p=0.06). There was a decrease in WTR astigmatism
(negative change in J0) with both lens types, but the change was only
significant with the spherical lens for 6 mm corneal diameter (p<0.001). Oblique
astigmatism (J45) showed only small changes with both lenses that were not
statistically different for either lens type.
4.4.5 Ocular wavefront error
The group mean changes in HO RMS, 3rd and 4th order RMS relative to the
baseline day for the two lens types, for the 4 and 5.5 mm pupil diameters are
shown in Table 4-5. Both spherical and back surface toric lenses showed small
increases in HO RMS, but the changes were not significant for any of the
lenses. There were no significant differences between the two lens types.
Chapter 4: Corneal changes with spherical versus back surface toric rigid contact lens wear
110
Table 4-4: Mean changes in best fit sphere (M), with/against the rule astigmatism (J0) and oblique astigmatism (J45) in dioptres, relative to baseline day with the two lens types for the 4 and 6 mm corneal diameters.
Lens Mean change
in M ± SD (Dioptres)
p-value
Mean change in J0 ± SD (Dioptres)
p-value
Mean change in J45 ± SD (Dioptres)
p-value
4 mm corneal diameter
Sphere –0.26 ± 0.12 0.01 –0.13 ± 0.12 0.14 0.02 ± 0.16 1.0
Back toric
–0.15 ± 0.13 0.11 –0.10 ± 0.12 0.30 –0.04 ± 0.13 1.0
6 mm corneal diameter
Sphere –0.23 ± 0.12 0.02 –0.13 ± 0.03 <0.001 0.03 ± 0.12 1.0
Back toric
–0.17 ± 0.13 0.06 –0.09 ± 0.08 0.11 0.01 ± 0.11 1.0
Negative change in M represents decrease in corneal refractive power (hypermetropic shift). Negative change in J0 represents decrease in WTR astigmatism. Positive J45 represents negative cylinder axis closer to 45° and negative J45 represents negative cylinder axis closer to 135°.
Table 4-5: Mean changes in HO RMS, 3rd and 4th order RMS, relative to baseline day with the two lens types for 4 and 5.5 mm pupil diameters.
Lens Mean HO RMS
change ± SD (µm) Mean 3
nd order RMS
change ± SD (µm) Mean 4
nd order RMS
change ± SD (µm)
4 mm
Sphere 0.12 ± 0.07
(p=1.0 ) –0.01± 0.03
(p=1.0)
0.01 ± 0.02 (p=1.0)
Back toric
0.09± 0.06 (p=1.0 )
–0.002 ± 0.03
(p=1.0)
0.01 ± 0.02 (p=1.0 )
5.5 mm
Sphere 0.22 ± 0.09
(p=1.0) 0.01 ± 0.09
(p=1.0 ) 0.02 ± 0.06
(p=1.0 )
Back toric
0.17 ± 0.06 (p=1.0)
0.002 ± 0.06 (p=1.0 )
0.01 ± 0.04 (p=1.0)
Positive change represents increase and negative change represents decrease.
4.4.6 Position of contact lenses
Table 4-6 shows the mean decentration of the spherical and back
surface toric contact lens centre from the limbus centre at approximately one
second after 3 different blinks. The ranges of the horizontal and vertical
coordinates of the lens position are also described in the table. The most
frequent on-eye resting position of the contact lenses between the blinks for
both lenses was found to be superior-temporal. Both lenses show high degree
of variability in lens position as indicated by the range. The toric lens showed
Chapter 4: Corneal changes with spherical versus back surface toric rigid contact lens wear
111
slightly more decentration in the superior direction but this difference was not
significant (p>0.05).
Table 4-6: Mean distance of contact lens centre to limbus centre (mm) and range in the horizontal and vertical directions for the two lens types.
Lens Mean horizontal distance ± SD
Range Mean vertical distance ± SD
Range
Sphere 0.24 ± 0.38 –0.24 to 0.82 0.38 ± 1.01 –0.99 to 1.78
Back toric
0.27 ± 0.45 –0.81 to 0.78 0.68 ± 0.77 –0.64 to 1.36
Positive sign represents temporal direction (horizontally) and positive sign represents superior direction (vertically).
4.5 Discussion
We have investigated the effect of back surface toric and spherical RGP lenses
on corneal curvature and found slight flattening of the vertical anterior corneal
meridian with both the lenses in subjects with WTR corneal astigmatism, which
was significant in the periphery. The flattening of the vertical meridian resulted
in a significant decrease in WTR astigmatism (decrease in J0), for the spherical
lens for 6 mm diameter. This reduction in J0 was also seen in Chapter 3 in
subjects wearing spherical lenses on spherical and low astigmatism corneas,
but it was not significant. The toric lens caused slightly less change in J0, most
likely because it was fitted in alignment to the cornea and therefore caused less
flattening along the vertical meridian compared to the spherical lens.
Our results are similar to Mountford et al. (1997) and Cheung et al.
(2009) who found decreases in WTR astigmatism in a group of subjects with
central corneal astigmatism after the use of spherical ortho-k lenses. It has
previously been noted that patients with limbus-to-limbus astigmatism exhibit
less reduction in astigmatism with ortho-k lenses (Mountford et al. 2004). One
of our subjects had limbus-to-limbus astigmatism and this may have affected
the results because of differences in the pressure profiles beneath the lens. We
investigated the changes in WTR astigmatism in individual subjects and found
that there was a similar decrease in WTR astigmatism for all subjects for the 4
mm corneal diameter. But in the 6 mm corneal diameter, the subject with
limbus-to-limbus astigmatism showed the least change (decrease in WTR
astigmatism). Further research with a greater number of subjects with limbus-
to-limbus astigmatism is required to confirm this trend. We analysed the contact
lens centration on the cornea with respect to the limbus centre and found that
Chapter 4: Corneal changes with spherical versus back surface toric rigid contact lens wear
112
both spherical and back surface toric contact lenses were decentred in the
superior temporal direction at the end of the 8 hour wearing time. This
decentration may have occurred due to the reflex tearing and the subject‟s lack
of adaptation to the lenses. None of the subjects included in the study were
given any adaptation time and topical anaesthesia was also not administered.
Topical anaesthesia was not used in the study in order to avoid any
confounding effects of these drops on corneal thickness or curvature (Herse
and Siu 1992). The contact lens decentration of both spherical and toric lenses
(in the superior temporal direction) does appear to correlate with the anterior
curvature changes in the topographic maps (lens being used on the left eye)
which shows corneal flattening (in the inferior nasal direction) (Figure 4-3).
These corneal changes could be a result of bearing in the mid-peripheral zone
of the contact lens, causing pressure and subsequent corneal flattening in the
inferior nasal corneal quadrant.
The change in the spherical component of the refractive error (M) was
significant compared to baseline with the spherical lens. For both a 4 and 6 mm
analysis diameter, the change in M was 0.26 and 0.23 D respectively with the
spherical lens (after 8 hours of lens wear) and this change is clinically
significant. The change in M associated with the back surface toric lenses were
smaller (0.15 and 0.17 D for 4 and 6 mm analysis diameters respectively),
however, it could be argued that this change is also of borderline clinical
significance, since any change in refraction of 0.12 D or greater could lead to a
clinical change of 0.25 D in the spherical component of refractive error (i.e.
optimal lens power). Of course, a patient wearing the lens could accommodate
to compensate for the hyperopic shift that occurs as a result of this shift in M in
the minus direction. It would be interesting to know if a change in refraction
persists after longer term RGP lens wear. The changes in best fit sphere M with
the similar spherical RGP lens in Chapter 3 caused changes in the same
direction of slightly greater magnitude (mean change of 0.31 D for 4 mm
diameter and 0.34 D in 6 mm diameter).
The higher order aberrations of the eye tended to be larger after wear of
both types of RGP lens, but these differences did not reach significance. Both
lens types led to a slight posterior corneal curvature steepening and slight
corneal swelling. The corneal swelling was slightly greater in the periphery
Chapter 4: Corneal changes with spherical versus back surface toric rigid contact lens wear
113
compared with the central region of the cornea, but the difference did not reach
statistical significance.
The spherical lens caused posterior corneal steepening and corneal
swelling which were similar in magnitude and direction to that seen in Chapter
3 with the spherical RGP lens of the same diameter on more spherical corneas.
The mean peripheral corneal swelling with RGP/9.5 lens reported in Chapter 3
was 6.26 µm (0.97%) in the periphery compared to 5.05 µm (0.8%) in this study
and the mean posterior corneal steepening was –0.01 mm compared to –0.02
mm in this study. However these findings could not reach statistical
significance. The corneal swelling with the toric lens (6.95 µm or 1.1% in
periphery) was of a slightly greater magnitude compared to the spherical lens in
this study but this did not reach statistical significance. The toric lens also
showed a significant posterior peripheral steepening of about 0.03 mm, and a
weak correlation with peripheral corneal swelling, but this was not significant.
Changes in anterior and posterior corneal curvature and thickness due
to a back surface toric RGP lenses were very similar to those produced by the
spherical back surface RGP lens. The differences in forces applied by the two
lens types were obviously relatively small. The differences caused by the lens
types may have been greater if the lenses had centred more accurately, if the
degree of astigmatism was higher, if the axis of astigmatism was different, if
more of the subjects had limbus-to-limbus astigmatism or if the lenses were
worn for longer periods of time.
4.6 Conclusion
To summarize, we found a greater decrease in corneal refractive power M, and
decrease in WTR astigmatism with the spherical lens compared to the back
surface toric lens for 6 mm corneal diameter. The decrease in astigmatism
found in the subject with limbus-to-limbus astigmatism was smaller than the
other subjects with central astigmatism. These findings provide some evidence
that the pressure distribution on a toric cornea by a spherical lens can vary
from a back surface toric lens, but a study with a greater number of subjects
with astigmatic corneas would help to clarify these issues.
Chapter 4: Corneal changes with spherical versus back surface toric rigid contact lens wear
114
- 115 -
Chapter 5
Eyelid changes following short-term rigid and soft contact lens wear
5.1 Introduction
In Chapter 3, corneal thickness, curvature, refractive power and ocular
wavefront error changes after the short-term use of a range of lenses including
PMMA, RGP (2 different diameters) and SiHy were reported. In this chapter,
changes in the characteristics and biometrics of the eyelid structures that
interact with the contact lenses, such as the tarsal conjunctiva, the eyelid
margin (lid-wiper) and eyelid position are described in the same group of
subjects.
Previous research has noted that a range of changes in the eyelids and
related structures can be associated with contact lens wear. Blepharoptosis
has been reported to occur from 2 weeks (Fonn and Holden 1988) to longer
use (3 months to 10 years) of RGP and PMMA contact lenses (Van den Bosch
and Lemij 1992; Fonn et al. 1995). These changes in eyelid position can be
unilateral (Uchinuma et al. 1983; Fonn and Holden 1986) or bilateral (Fonn et
al. 1995; Fonn et al. 1996). The ptosis has been attributed to trauma during
lens removal (Van den Bosch and Lemij 1992), lid inflammation or edema from
lens edges or deposits (Fonn and Holden 1986; Levy and Stamper 1992), and
to a thinning or disinsertion of the levator aponeurosis (Van den Bosch and
Lemij 1992). Some reports suggest that long term soft contact lens wear also
increases the risk of developing aponeurogenic ptosis (Reddy et al. 2007;
Wubbels and Paridaens 2009), however other investigators suggest that there
is no evidence of ptosis in soft contact lens wearers compared to non-lens
wearers (Fonn et al. 1996). Although the longer term influence of contact lens
wear upon eyelid position has been well studied, no previous study has
investigated whether the short-term use of rigid or soft contact lenses leads to
changes in the palpebral aperture.
One of the most common and severe forms of contact lens induced
tarsal conjunctival changes is contact lens papillary conjunctivitis (CLPC)
(Allansmith et al. 1977; Allansmith et al. 1978; Korb et al. 1980; Efron 1999). It
has been reported in both soft and hard contact lens wearers mostly after long
Chapter 5: Eyelid changes following short-term rigid and soft contact lens wear
116
term wear (3 weeks to a few years) (Allansmith et al. 1977; Maldonado-Codina
et al. 2004). Although CLPC occurs with all types of contact lens wear, it is
more common with silicone hydrogel lenses (Skotnitsky et al. 2002).The
characteristic signs of CLPC are enlarged papillae with hyperemia causing
itching, discomfort, foreign body sensation, contact lens displacement and
decentration. There can also be mucoidal discharge causing blurring of vision
(Allansmith et al. 1977).
Distribution of papillae on the tarsal conjunctiva depends on the type of
lens. Hard PMMA contact lenses most commonly result in development of
papillae in zones close to the central tarsal conjunctiva and lid margin whereas
soft contact lenses cause papillae close to the tarsal fold (Korb et al. 1980;
Korb et al. 1981; Korb et al. 1983). This suggests that the site of mechanical
trauma to the conjunctival surface plays a role in development of CLPC
(Elgebaly et al. 1991). Other factors associated with CLPC aside from
mechanical trauma include meibomian gland dysfunction (Martin et al. 1992;
Mathers and Billborough 1992), type I and type IV hypersensitivity (Begley et
al. 1990; Metz et al. 1997) and deposits on the lens surface (Fowler et al.
1985).
As the tarsal conjunctiva is in direct contact with lenses during wear, the
redness and roughness of the tarsal conjunctiva has been investigated by
many researchers using various grading scales (Efron et al. 2001; Skotnitsky et
al. 2002; Maldonado-Codina et al. 2004). Greater tarsal conjunctival redness
was noted in soft contact lens wearers compared to PMMA contact lens
wearers (Korb et al. 1981; Korb et al. 1983). Covey et al. (2001) have reported
no significant difference in palpebral roughness or redness between long-term
high-Dk silicone hydrogel contact lens wearers and non-lens wearers. While
palpebral redness and roughness are well documented in contact lens wearers,
tarsal staining with fluorescein secondary to the use of different types of contact
lenses has not been systematically studied and a relevant grading scale has
not been reported.
The „lid-wiper‟, is a term introduced by Korb et al. (2002), to describe
the portion of the marginal conjunctiva of the upper eyelid that is thought to
make contact with the ocular surface (or contact lens surface) during blinking.
Shaw et al. (2010) showed using carbon paper imprinting, that a region of the
eyelid margin (of ~0.6 mm width) was applying pressure to the ocular surface.
Chapter 5: Eyelid changes following short-term rigid and soft contact lens wear
117
This region is thought to act as a wiper of the ocular/lens surface by spreading
and rejuvenating the tear film. Lid-wiper epitheliopathy (a condition
characterized by increased staining of the epithelial cells in the lid wiper region)
has been reported in the majority contact lens wearers with dry eye symptoms
and a smaller percentage of asymptomatic soft contact lens wearers using
fluorescein and rose bengal staining (Korb et al. 2002) and using fluorescein
and lissamine green (Yeniad et al. 2010). In both of these studies, the subjects
were long term daily-wear soft contact lens users (at least 1 year and 6 months
respectively). The influence of shorter periods of contact lens wear upon the
presence and severity of lid-wiper staining is not known.
Contact lenses are in contact with the eyelids and tarsal conjunctiva
during wear, creating a source of potential friction and micro-trauma. In this
study we aimed to investigate whether the short-term use (about 8 hours) of a
variety of contact lenses (PMMA, RGP and silicone hydrogel lenses), was
associated with changes in the eyelids that have their origin in a mechanical
interaction with the lens. These potential sequelae included blepharoptosis of
the eyelids, the presence and severity of eyelid wiper staining, and the
presence and severity of tarsal conjunctival staining (using a newly developed
grading scale designed as part of this study).
5.2 Methodology
The study was conducted over a period of 5 days. Measurements were taken
at the start and end of an 8 hours period on each day along with the
measurements for Chapter 3 (see Figures 3-2 & 3-3). On day one, baseline
measurements were taken without any contact lens in the eye, in the morning
(between 8 and 11 am and at least 2 hours after waking) and 8 hours later, in
the afternoon (between 4 and 7 pm). Four different types of contact lenses
were worn by the subjects on days 2, 3, 4 and 5 of the study in the left eye
only.
An approval was obtained from QUT university human research ethics
committee (Appendix A) and the tenets of declaration of Helsinki were followed.
All subjects gave a written informed consent before participation in the study.
Chapter 5: Eyelid changes following short-term rigid and soft contact lens wear
118
5.2.1 Subjects
The subject group was the same as described in Chapter 3 (fourteen young,
adult subjects aged 20 to 33 years, mean age 27.8 ± 4.0 years) with visual
acuity of 6/6 or better and corneal astigmatism of ≤ 1.5 D. The mean spherical
equivalent refractive error was –0.6 ± 1.3 D. Prior to participation in the study,
all subjects were screened for any anterior segment abnormalities using slit-
lamp biomicroscopy. Subjects were also screened for any tear film
abnormalities using a range of tests including McMonnies questionnaire
(McMonnies and Ho 1987), tear film break up test, Phenol red thread test and
fluorescein and lissamine green staining of the cornea and conjunctiva. None of
the subjects had a history of corneal injury, infection or surgery. None of the
subjects were previous rigid contact lens wearers. Two of the 14 subjects were
regular soft contact lens wearers but were asked to discontinue lens wear one
month prior to the start of the study, to allow the effects of soft lens wear to
largely resolve. It was calculated that a sample size of 14 subjects used in this
study would give 80% power to detect a change of 0.16 mm in lid position, 0.37
grade units in tarsal staining, and 0.57 grade units in lid-wiper staining at the
0.05 level of significance.
5.2.2 Contact lenses
Three different types of custom made rigid contact lenses and one soft lens
were worn by each subject. A contact lens trial fitting was performed with the
rigid lenses before ordering the lenses. The details of the lenses have been
described in Chapter 3 (Table 3-1). The rigid lenses had a spherical BOZR and
an aspheric periphery with a total diameter of 9.5 and 10.5 mm and were not
plasma treated. The soft lens used was Bausch and Lomb, PureVision silicone
hydrogel lens which had a diameter of 14.0 mm. The order of lens wear was
randomised and a recovery period of at least 2-3 days was scheduled after the
use of each lens and before the wear of another lens. Recovery of any
substantial eyelid changes (tarsal and lid-wiper staining or blepharoptosis) was
checked for all subjects before the wearing of a subsequent lens.
5.2.3 Measurements and Instruments
A questionnaire was completed by each subject to monitor the visual tasks
performed during the 8 hours of lens wear. Subjects were found to be engaged
in similar tasks (most commonly computer work) during the period of the study
each day. The morning and afternoon measurements on each of the contact
Chapter 5: Eyelid changes following short-term rigid and soft contact lens wear
119
lens wearing days were conducted at around same time of day as on day 1
(baseline day), to allow comparison without confounding effects due to diurnal
variations.
Digital photos of the external eyes in primary gaze were captured at the
start and end of each measurement day to compare the position of the upper
and lower eyelid (with respect to the centre of the limbus) of the left eye
(contact lens wearing eye) and the right eye (control eye). On contact lens
wearing days the images were captured with lens in situ, in the morning at
about 30 minutes after lens insertion and in the afternoon (8 hours later) just
before lens removal. A Fujifilm FinePix 9 mega-pixel digital camera (S9500),
with macro mode was used with the camera‟s built-in flash and automatic
aperture/shutter speed mode. The camera was positioned directly in front of the
subject, at approximately 50 cm (Figure 5-1 a), with a ruler in place next to the
eye for calibration of image size during the analysis (Figure 5-1 b). The subject
was positioned in the head rest with the eyes in primary gaze, with the
instruction to fixate the centre of the camera lens. The image was captured 1
sec after a gentle, complete blink and a total of 4 images (both eyes together,
in one image) were captured (Figure 5-2). All images were taken in the same
room with approximately the same humidity (56.3 ± 5.5%) and temperature
(24.1 ± 0.3ºC) and similar ambient lighting conditions with standard overhead
fluorescent lights (approximately 250 lux at the plane of the subject‟s eye) at
approximately the same time of day on each study day (morning and
afternoon).
Figure 5-1: (a) Set up of digital camera to take the photo of external eyes (b) Ruler next to the eye to allow calibration.
Chapter 5: Eyelid changes following short-term rigid and soft contact lens wear
120
Figure 5-2: (a) Position of eyelids on the baseline day afternoon (no contact lens in eyes) (b) Position of eyelids with contact lens in left eye in the afternoon after 8 hours of lens wear. Palpebral aperture (PA) height is shown in mm. Yellow rings indicate the limbus outline, upper eyelid margin is shown in red and lower eyelid margin is shown with blue.
Digital photos of the eyelid margins and upper and lower tarsal
conjunctiva were also taken at each measurement session to record any
corneal or conjunctival staining using a Canon Digital Rebel EOS 300 D 6.3
mega pixels Digital SLR (Canon Inc Tokyo, Japan) camera. On contact lens
wearing days these images were captured in the morning without the contact
lens in eye (after approximately 40 minutes of lens insertion) and in the
afternoon (8 hours later), immediately after the lens removal. The camera was
attached to a slit lamp biomicroscope and was used in automatic
aperture/shutter speed mode. The images were adjusted for colour balance
and the same colour balance setting was used for all the images. The slit lamp
magnification was kept constant at 10X and slit aperture was kept at maximum
length and width. The images were taken in the same room with approximately
the same humidity (58.0 ± 5.2%), temperature (24.9 ± 1.0 ºC), and ambient
lighting conditions (slit lamp illumination: approximately 990 lux at the plane of
the subject‟s eye) at approximately the same time of day every day (morning
and afternoon). The subjects were positioned in the head rest of the slit lamp
biomicroscope with the eyes in primary gaze and fixating the examiner‟s right
ear for left eye photos and vice versa. The upper eyelid was then everted and
photos were taken first under white light, and then with a cobalt blue light
through a Wratten #12 filter after instillation of sodium fluorescein dye (4-5 sec
after instillation) and then under white light after instillation of lissamine green
dye (4-5 sec after dye instillation). Two images were captured in each condition
and the best quality image of the two was used for staining grading.
Chapter 5: Eyelid changes following short-term rigid and soft contact lens wear
121
Fluorescein (Fluorets, fluorescein sodium sterile ophthalmic strips) and
lissamine green strips were moistened with a single drop of unpreserved sterile
saline and lightly touched on the upper bulbar conjunctiva with the subject
looking down.
5.3 Data Analysis
5.3.1 Eyelid position (Blepharoptosis)
The digital images of the external eyes were analysed to determine the position
and diameter of the limbus and the position of the upper and lower eyelids,
using custom-written software (Imetrics, developed at Contact Lens and Visual
Optics Laboratory). This analysis involves the operator manually locating the
position of the coordinates of the limbus (8 points), the upper lid margin (8
points) and the lower lid margin (8 points). The software then determines the
best fitting ellipse to the limbus co-ordinates and the best fitting quadratic
function to the upper and lower eyelid coordinates. The vertical palpebral
aperture (PA) height (i.e. the distance between the upper and lower lid margins
with respect to the limbus centre) is then calculated (Figure 5-3). Palpebral
aperture height was compared for the right (control eye, no lens) and left
(contact lens wearing eye) eyes and to the baseline day measurements. The
analysis of the digital images was performed by an independent masked
observer on two images and the mean of the two was taken as the PA. All the
images were calibrated for size by using a ruler next to the eye in each image
frame. Repeated measures ANOVA with lens, time of the day (am/pm) and eye
(right/left) as within-subject factors, was performed to calculate the statistical
significance of the changes. Bonferroni correction was applied to avoid any
error due to multiple comparisons.
Chapter 5: Eyelid changes following short-term rigid and soft contact lens wear
122
Figure 5-3: External photo of the eye showing the palpebral aperture (PA) height with respect to limbus centre. Markings in yellow indicate the limbus margins, Markings in red indicate the position of upper lid and markings in blue indicate the position of the lower lid.
5.3.2 Tarsal staining
The grading of the digital slit lamp images of the upper tarsal conjunctiva was
performed by an independent masked examiner. Since a grading scale for
tarsal staining using fluorescein is not available and has not been described
before, a grading scale for this purpose was developed as part of this study.
The tarsal conjunctiva staining was graded on a five-point scale of increasing
severity from 0 (none) to 4 (severe) with possible grading increments of 0.1
units, as shown in Figure 5-4. This classification approach was adapted from a
grading scale described by Efron (1998). The five-step grading scale is widely
used in the field of contact lenses (Woods 1989) and grading to nearest 0.1
scale unit allows greater sensitivity and accuracy in detecting a change in the
severity of a complication (Bailey et al. 1991). The data was not assumed to be
normally distributed, so the Wilcoxon signed rank sum test was used to test the
statistical significance of the changes. Bonferroni correction was applied to
avoid any error due to multiple comparisons.
Chapter 5: Eyelid changes following short-term rigid and soft contact lens wear
123
Figure 5-4: Digital images showing upper tarsal conjunctival staining with fluorescein on a grade of 0 (None) to 4 (Severe).
5.3.3 Lid-wiper epitheliopathy
Lid-wiper epitheliopathy was evaluated for the upper eyelid using a modification
of the grading system described by Korb et al. (2005) (Table 5-1). The grading
of the digital slit lamp images of the lid margin was performed by an
independent masked examiner using both fluorescein and lissamine green
stains. The grading of lid-wiper staining was carried out for both the horizontal
length and sagittal width of the fluorescein and lissamine green staining of the
lid-wiper as shown in Table 5-1. These gradings were then averaged to obtain
the final score for each subject. Thus the final score was the mean of the four
values (grade of horizontal length and sagittal width staining with fluorescein
and grade of horizontal length and sagittal width staining with lissamine green).
The final score determined whether the lid-wiper epitheliopathy was graded as
mild, moderate or severe (Table 5-2). Examples of grading from some
representative images are shown in Figure 5-5. The data was not assumed to
be normally distributed so the Wilcoxon signed rank sum test was used to test
the statistical significance of the changes. Bonferroni correction was applied to
avoid any error due to multiple comparisons.
Chapter 5: Eyelid changes following short-term rigid and soft contact lens wear
124
Table 5-1: Grades of horizontal length and sagittal width staining of lid-wiper. Grading was done using both fluorescein and lissamine green.
Horizontal length of staining
Grade Sagittal width of staining Grade
< 10% 0 < 2% of the width of wiper 0
10 – 20% 1 25% - < 50% of width of wiper 1
20 – 30% 2 50% - <75% of the width of wiper 2
> 40% 3 ≥ 75% of the width of wiper 3
Table 5-2 Lid-wiper epitheliopathy classification system for final score as described by Korb et al. (2005).
Lid-Wiper Epitheliopathy Classification
Grade 1 (Mild) 0.25 – 1.00
Grade 2 (Moderate) 1.25 – 2.00
Grade 3 (Severe) 2.25 – 3.00
Figure 5-5: Examples showing grading of lid-wiper epitheliopathy from three representative subjects.
Chapter 5: Eyelid changes following short-term rigid and soft contact lens wear
125
5.4 Results
5.4.1 Eyelid position (Blepharoptosis)
The type of lens (p<0.001, repeated measures ANOVA) had a significant effect
on the changes in the height of the PA. The group mean change in height of PA
relative to baseline, in the right (control) and left eye (contact lens-wearing eye)
in the afternoon is shown in Figure 5-6. Two out of three rigid lenses resulted in
a significant decrease in the PA height in the left eye after 8 hours of lens wear,
compared to the baseline day. The PMMA/9.5 (–0.78 ± 0.67 mm, p=0.008) and
RGP/10.5 (–0.73 ± 0.76 mm, p=0.03) lenses showed significant decreases in
the vertical palpebral aperture height, whereas the reduction with the RGP/9.5
(–0.46 ± 0.52 mm, p=0.06) lens approached significance (Figure 5-6). The SiHy
contact lens did not cause any significant changes in PA height. There was no
significant effect of time of the day (morning or afternoon) on the PA height
either on baseline or on contact lens wearing days. The non-lens wearing right
eye on average also showed a small decrease in PA height in the afternoon
compared with the baseline day, but this was not significant with any of the
lenses. The decrease in PA following RGP/9.5 lens wear in the nonlens-
wearing right eye was greater than in the contact lens-wearing left eye, but this
difference was not significant.
Figure 5-6: Changes in height of palpebral aperture (mm) of the right and left eye relative to baseline afternoon. Negative values mean that palpebral aperture height is less compared to baseline afternoon. Each error bar represents one standard error of the mean. # represents statistically significant p-values (<0.03), * represents p-value approaching significance (p=0.06). Right eye is control eye (no lens) and left eye is the contact lens wearing eye.
Chapter 5: Eyelid changes following short-term rigid and soft contact lens wear
126
5.4.2 Tarsal conjunctival staining
The group mean changes in tarsal staining relative to baseline, in the left eye
(contact lens wearing eye) in the morning and afternoon are shown in Table 5-
3. The PMMA/9.5 (1.31 ± 0.88, p=0.008), RGP/9.5 (1.44 ± 0.89, p<0.008), and
RGP/10.5 (1.65 ± 0.72, p<0.004) lenses showed significant increases in tarsal
staining in the afternoon (after 8 hours of lens wear) compared to the baseline
day in the afternoon (Table 5-3). The soft lens did not cause any significant
changes. There was also a small increase in tarsal staining in the morning after
45 minutes of contact lens wear (compared to baseline morning) which was
significant with the RGP/9.5 (0.85 ± 0.87, p=0.03) and RGP/10.5 lenses (0.71 ±
0.69, p=0.03) (Table 5-3). A significant increase in tarsal staining was observed
in the afternoon after 8 hours compared to the morning with all the three rigid
lenses and the changes approached significance with the soft lens (Table 5-4).
There was also a slight but significant increase in staining on the baseline day
without lens wear (Table 5-4).
Table 5-3: Changes in upper tarsal conjunctival staining (relative to baseline) in morning and afternoon. Positive values mean that tarsal staining has increased compared to baseline. Note: The increase in staining in the mornings following approximately 45 minutes of lens wear.
Lens Tarsal staining change (relative to baseline) ± SD (p-value)
Morning
(after 45 mins of lens wear ) Afternoon
(~8 hrs after lens wear)
Lens PMMA/9.5 0.57 ± 0.75 (p=0.07) 1.31 ± 0.88 (p=0.008)
Lens RGP/9.5 0.85 ± 0.87 (p=0.03) 1.44 ± 0.89 (p=0.008)
Lens RGP/10.5 0.71 ± 0.69 (p=0.03) 1.65 ± 0.72 (p=0.004)
Lens SiHy/14.0 0.41 ± 0.92 (p=0.49) 0.43 ± 0.81 (p=0.56)
Table 5-4: Changes in upper tarsal conjunctival staining in the afternoon (relative to morning). Positive values mean that tarsal staining has increased compared to morning.
Lens Tarsal staining change in afternoon (relative to
morning) ± SD (p-value)
Baseline 0.49 ± 0.30 (p=0.01)
Lens PMMA/9.5 1.22 ± 0.54 (p=0.01)
Lens RGP/9.5 1.07 ± 0.68 (p=0.01)
Lens RGP/10.5 1.42 ± 0.64 (p=0.01)
Lens SiHy/14.0 0.51 ± 0.67 (p=0.07)
Chapter 5: Eyelid changes following short-term rigid and soft contact lens wear
127
5.4.3 Lid-wiper epitheliopathy
The group mean lid-wiper staining relative to baseline, in the left eye (contact
lens wearing eye) in the morning and afternoon are shown in Figure 5-7.
Amongst the lenses, all the three rigid contact lenses PMMA/9.5 (0.88 ± 0.90,
p=0.01), RGP/9.5 (0.77 ± 0.91, p=0.02) and RGP/10.5 (0.73 ± 0.94, p=0.04)
caused a significant increase in lid-wiper staining after lens wear compared to
baseline afternoon. The changes with SiHy/14.0 lens (0.57 ± 0.90, p=0.07)
approached significance. The change in lid-wiper staining in the afternoon
(relative to morning) is shown in Table 5-5. All the four lenses, PMMA/9.5 (0.75
± 0.37, p<0.001), RGP/9.5 (0.68 ± 0.35, p<0.001), RGP/10.5 (0.59 ± 0.50,
p=0.001) and SiHy/14.0 (0.71 ± 0.60, p=0.001) caused a significant increase in
lid-wiper staining in the afternoon compared to morning (Table 5-5). There was
also an increase in lid-wiper staining on the baseline day (no contact lens)
(0.32 ± 0.50, p=0.18) in the afternoon compared to morning, but this was not
significant.
Figure 5-7: Mean lid-wiper epitheliopathy grades with different types of contact lenses and on the baseline (BL) day (no contact lens), in the morning and afternoon. p-values indicated for change in lid-wiper compared to baseline. # indicates p-value <0.05. Each error bar indicates one standard deviation.
Chapter 5: Eyelid changes following short-term rigid and soft contact lens wear
128
Table 5-5: Changes in lid-wiper staining grade in the afternoon (relative to morning). Positive values indicate increased staining compared to morning.
Lens Lid-wiper staining change in afternoon (relative
to morning) ± SD (p-value)
Baseline 0.32 ± 0.51 (p=0.18)
PMMA/9.5 0.75 ± 0.37 (p=0.01)
RGP/9.5 0.68 ± 0.35 (p=0.01)
RGP/10.5 0.59 ± 0.50 (p=0.01)
SiHy/14.0 0.71 ± 0.60 (p=0.01)
5.4.4 Association between blepharoptosis and tarsal conjunctival staining
To study the association between the blepharoptosis (assumed to be
secondary to eyelid swelling caused by mechanical trauma and/or
inflammation) and tarsal conjunctival staining (assumed to be secondary to
mechanical trauma), a Spearman‟s correlation was calculated for these
parameters. There was no significant correlation (R2 = 0.086, p=0.77) between
these parameters.
5.5 Discussion
We found small decreases in the vertical PA height in both right (control eye)
and left (contact lens-wearing) eyes after eight hours of contact lens wear when
compared to baseline day measurements, but only the changes with the
PMMA/9.5 and RGP/10.5 lenses were significant. The SiHy lens also caused a
slight decrease in PA height, but these changes were not significant. Our
results are consistent with previous studies which have shown a reduction in
PA height with long-term (2 weeks to 10 years) rigid contact lens wear (Fonn
and Holden 1988; Van den Bosch and Lemij 1992; Fonn et al. 1996). This is
the first study to report the effect of short-term (few hours) contact lens wear on
PA height.
Decrease in the PA of the contact lens-wearing eye has been well
established but the causes are still unclear. The suggested causes of changes
in PA size due to contact lenses include trauma to the levator muscle due to
squeezing of the eyelids (during lens removal), forceful rubbing (during blinking
or otherwise) (Van den Bosch and Lemij 1992), due to the lens edge (Sobara et
al. 1993) and irritation of the lid, leading to oedema or blepharospasm (Van den
Bosch and Lemij 1992). Epstein and Putterman (1981) reported the presence
of a disinsertion of the levator aponeurosis associated with ptosis due to
Chapter 5: Eyelid changes following short-term rigid and soft contact lens wear
129
prolonged rigid lens wear, which they surgically corrected. As our lid aperture
changes occurred after only 8 hours of lens wear, it is unlikely that substantial
trauma or changes to the levator muscle could have occurred in this time. The
changes in palpebral sizes noticed in our subjects could therefore possibly be
due to mechanical irritation leading to lid oedema (Van den Bosch and Lemij
1992) as evidenced by the significant increase in tarsal staining associated with
lens wear, although the change in eyelid position was not significantly
correlated with the tarsal staining. While, we did not follow our subjects to
monitor the recovery in the size of the PA, based on previous studies (Fonn
and Holden 1986; Fonn and Holden 1988; Fonn et al. 1995) we speculate that
the PA size in our group of subjects will increase and normalize after lens
removal.
The PMMA contact lens caused a slightly greater decrease in PA height
compared to the RGP lenses (but not statistically significant, p>0.05) and the
changes with the SiHy contact lenses were smaller again (and not statistically
significant). This differential effect between lens types could arise from one or
more characteristics of the lens. The modulus of elasticity of the PMMA contact
lens is approximately 2000 MPa compared to RGP (1500 MPa) and SiHy (1.1
MPa) contact lenses. The diameters of PMMA/RGP lenses (9.5 and 10.5) were
smaller than the soft (14.0) lens. The differences in lens edge design of contact
lenses is another important factor which determines the interaction between
contact lens and eyelids and thus affects the comfort (La Hood 1988; Picciano
and Andrasko 1989; Andrasko 1991; Caroline et al. 1991). It is conceivable that
one or more of these factors, including the modulus of elasticity, total diameter,
and edge design of the lenses, may influence the changes in eyelid position
after lens wear, presumably due to differences in the mechanical interactions
between the lids and the contact lens.
Changes in the PA with lens wear could also potentially be related to
irritation of the cornea or lids leading to blepharospasm due to the presence of
the contact lenses (Van den Bosch and Lemij 1992) or swelling due to
mechanical trauma. Our group of subjects had no previous experience of
contact lens wear and had no adaptation period to lens wear before
commencement of the study. Therefore, it is likely that our subjects were
sensitive to the initial discomfort caused by the lenses and may have adopted a
narrower PA in order to reduce the mechanical interaction of the lenses with
Chapter 5: Eyelid changes following short-term rigid and soft contact lens wear
130
the eyelids. If the changes are a reflex response (i.e. a blepharospasm), we
would expect the lower eyelid to get higher along with the upper eyelid getting
lower (i.e. since the orbicularis oculi muscle is contracting we would expect
changes in both the upper and lower lid). To investigate this hypothesis, the
mean distance from the limbus to the upper lid and the mean distance from the
limbus to the lower lid was calculated for the right and left eye and compared
for the baseline and the three rigid contact lens wearing days. We found the
distance of both the upper and lower lid from the limbus centre reduced
significantly after lens wear compared to baseline (p<0.05). Thus the origin of
the changes may be partly related to „reflex‟ lid movements associated with
contraction of the orbicularis oculi muscle (changes in the both upper and lower
lid), along with possible inflammation due to mechanical trauma (Van den
Bosch and Lemij 1992) (swelling changes greater in the upper lid compared to
lower lid).
We found a decrease in PA in both right (control eye, no contact lens)
and left eyes (contact lens- wearing eye). This could be attributed to innervation
to the orbicularis oculi which is bilateral (Forester et al. 2008), therefore, the
irritation due to the presence a contact lens in one eye causes blepharospasm
in both eyes (contact lens-wearing as well as control eye).
The tarsal conjunctiva follows the lid-wiper during a blink and rubs
against the ocular/contact lens surface which can result in trauma to this
surface over the course of the day. We investigated the changes in tarsal
conjunctival staining after short-term wear of different contact lenses, increases
in which are likely to indicate micro-trauma to the surface. There was a
significant increase in the amount of tarsal staining after 8 hours of wear of all
three rigid/hard contact lenses (PMMA/9.5, RGP/9.5 and RGP/10.5),
suggesting the relatively stiff lens edge (high modulus of elasticity) and/or lens
edge design of these lenses could be responsible for increased surface trauma
of the palpebral conjunctiva. The soft lens with a lower modulus conforms to
the ocular surface (Holden and Zantos 1981) and has an edge profile that is
thinner and more tapered than standard rigid lens designs and this increase in
tarsal staining over the 8 hours of lens wear only approached significance. The
increase in tarsal staining on the baseline day (without any contact lens) could
be a result of conjunctival surface constantly rubbing against the ocular surface
during blinking and this may be evidence of a diurnal increase in tarsal staining.
Chapter 5: Eyelid changes following short-term rigid and soft contact lens wear
131
Previous research has revealed that the tear film on a contact lens
surface dries more rapidly compared to that on the cornea (Cedarstaff and
Tomlinson 1983; Thai et al. 2004). The increased tarsal staining we found
could also be partly attributed to an increased friction between the lens and
palpebral conjunctiva compared to that between the natural cornea and
palpebral conjunctiva. Studies of longer-term contact lens users have reported
contact lens-related papillary conjunctivitis to be more common and severe in
soft contact lens wearers compared to rigid contact lens wearers (Alemany and
Redal 1991). This difference could be attributed to the use of contact lenses for
a longer duration resulting in increased deposits or protein on lens surface
(Allansmith et al. 1977; Fowler et al. 1985; Dumbleton 2003). However our
short-term data would argue against the likelihood of any increased micro-
trauma of the palpebral conjunctiva as a contributing factor to increased CLPC
in soft lens wearers. Some previous studies using impression cytology have
demonstrated squamous metaplasia and goblet cell loss in the tarsal
conjunctiva which was seen to increase with the duration of rigid contact lens
wear (Saini et al. 1990). In another study by Anshu et al. (2001), it was found
that symptomatic contact lens wearers showed a decrease in goblet cell
density and these changes were more severe in soft contact lens wearers
compared to rigid contact lens wearers. The authors attributed this difference to
larger diameter, larger surface contact area, and increased deposits on soft
lenses compared to rigid lenses.
We found an increase in tarsal conjunctival staining in the morning
measurements with both the RGP contact lenses compared to the morning of
baseline day (no contact lens). This means that the lenses caused some
amount of tarsal staining even in the short period of lens wear of about 40
minutes before the staining measurements were taken. We also found a
significant diurnal increase in tarsal staining even on the baseline day when no
lenses were worn, suggesting that the tarsal conjunctival surface is in constant
friction with the ocular surface (during blinking) resulting in some minor surface
damage over the course of the normal day. A previous cytological study (Hirji et
al. 1984) has found a decrease in the total conjunctival cell count over 9 hours.
This decrease in cell count is possibly related to the increased tarsal staining
we found at the end of the day.
Chapter 5: Eyelid changes following short-term rigid and soft contact lens wear
132
A grading scale for tarsal conjunctival staining has been described and
used for the first time in this study. In the future it would be prudent for this
grading scale to be validated for inter- and intra-observer variability. In this
study, the grading was performed by the one experienced and masked
clinician, so the results are unbiased and should have the ability to discriminate
across a range of levels of tarsal conjunctival staining.
In a contact lens-wearing eye, the lid-wiper region of the upper lid
moves across the edge of the lens with each blink as it spreads the tears
across the lens and ocular surface. This may result in micro-trauma to the
surface epithelial cells in the lid-wiper region that can be identified using
fluorescein and rose bengal staining of the lid margin (Korb et al. 2002; Korb et
al. 2005) or fluorescein and lissamine green staining (Yeniad et al. 2010). Lid-
wiper epitheliopathy has been reported in 13% (Korb et al. 2002) and 32 %
(Yeniad et al. 2010) of asymptomatic long-term soft contact lens wearers. The
prevalence of lid-wiper epitheliopathy is much higher in symptomatic contact
lens wearers (Korb et al. 2002; Yeniad et al. 2010) and dry eye patients (Korb
et al. 2005). In the current study, we found a significant increase in lid-wiper
staining in the afternoon compared to mornings with all of the contact lens
types. There was also an increase in lid-wiper staining on the baseline (no
contact lens) day but this was not statistically significant. On average, there
was also an increase in the magnitude of lid-wiper staining in the afternoons of
the contact lens-wearing days compared to the baseline day afternoons.
However only the changes associated with the rigid lenses were significantly
different to the baseline day afternoon. This is the first study to report lid-wiper
epitheliopathy in as little as 8 hours wear of rigid and soft contact lenses.
The increase in lid-wiper staining during the course of the day is most
likely due to the constant friction between the lid-wiper and the contact lens
surface, edge and/or the ocular surface during each blink. The increase in
staining observed on the contact lens wearing days (particularly with the rigid
lenses) suggests that the contact lens edge and surface cause a greater
amount of friction with the lid-wiper region as compared to the ocular surface
alone (Korb et al. 2005). This highlights the need for smoother contact lens
edges and more lubricious surfaces, to minimise the interactions between the
lid margin and contact lens surface during lens wear. Plasma treatment has
been used for both rigid (Yin et al. 2008; Yin et al. 2009) and silicone hydrogel
Chapter 5: Eyelid changes following short-term rigid and soft contact lens wear
133
(Valint Jr et al. 2001; Valint Jr et al. 2001) contact lenses to increase their
lubricity, thus reducing friction and improving comfort, while the addition of a
wetting agent to soft lenses (for example polyvinyl pyrrolidone) is another
method employed to increase lens lubricity.
On further examination of lens edges after the completion of data
collection using a high resolution OCT (Copernicus HR SD OCT, Optopol
Technology SA, Zawiercie, Poland), we found that the lens edges of the PMMA
and RGP lenses were not tapered anteriorly. Figure 5-8 a and b shows the
temporal and nasal lens edge profiles of the RGP/9.5 mm lens. Similar edge
profiles were found with the RGP/10.5 and PMMA contact lens used in the
study. Temporal and nasal lens edge profiles of the soft contact lens are shown
in Figure 5-8 c and d. Lens edge profiles were checked after the lenses were
received from the manufacturer and before the start of the study. However the
OCT technique was not available before the start of the experiment, so we
were unable to make these detailed observations. These lens edge OCT
images suggest that some of the lid changes seen in this experiment could be
attributed to the poor manufacture of lens edges.
Figure 5-8: Lens edge profiles of the RGP/9.5 and soft lenses used in the study. PMMA and RGP/10.5 lenses had similar edge profiles to the RGP/9.5 lens.
Chapter 5: Eyelid changes following short-term rigid and soft contact lens wear
134
The increase in the lid-wiper staining on the baseline day (no contact
lens) suggests that there could be some natural diurnal changes occurring in
the lid-wiper staining due to friction with the ocular surface during each blink
throughout the day. Given the normal blink rate of 12 to 15 blinks per minute
(Carney and Hill 1982; Moses 1987), this will lead to more than 10,000 blinks
per day.
5.6 Conclusion
To conclude, we found small decreases in the vertical PA height in both
the right (control eye) and left (contact lens wearing) eyes after eight hours of
contact lens wear when compared to baseline day measurements, but only the
changes in the hard/rigid contact lens wearing eyes were statistically
significant. There was a significant increase in the amount of tarsal staining
after 8 hours wear of all the three rigid/hard contact lenses (PMMA/9.5,
RGP/9.5 and RGP/10.5), suggesting that the lens edge (due to higher
modulus), poor edge manufacture or surface friction with the rigid lenses
resulted in increased surface trauma of the palpebral conjunctiva. The
comparatively soft lens did not result in a significant increase in tarsal staining.
We also found a significant increase in the magnitude of lid-wiper staining in
the afternoons of the three rigid/hard contact lens-wearing days. These results
highlight the importance of good lens edge manufacture and reducing the
friction between the lens surface/edge and the palpebral conjunctiva/eye-lid
wiper during blinking.
- 135 -
Chapter 6
Tear film surface quality with rigid and soft contact lenses
6.1 Introduction
Contact lens wearers report more dryness symptoms compared to non-wearers
(Millodot 1978; McMonnies and Ho 1987; Brennan and Efron 1989; Orsborn
and Robboy 1989; Doughty et al. 1997; Vajdic et al. 1999), which suggests that
contact lens wear exacerbates marginal tear dysfunction. Up to 50% of all
contact lens wearers have been reported to have some symptoms of dry eye
(Doughty et al. 1997). Additionally, the frequency and severity of dryness
symptoms is higher with contact lenses than without lenses in the same set of
subjects (Cedarstaff and Tomlinson 1983; Lemp 1995; Begley et al. 2000;
Begley et al. 2001).
Contact lens wear alters the structure of the tear film by breaking it into
a pre-lens and a post-lens tear film, where the pre-lens tear film is thought to be
composed of superficial lipid and aqueous layer and the post-lens tear film is
composed of aqueous and mucin layers (Asbell and Uçakhan 2006). Contact
lenses are reported to lead to a variety of changes in the tears including a
reduction in tear break up time (Young and Efron 1991; Guillon and Guillon
1994), increased evaporation of the tear film (Tomlinson and Cedarstaff 1982;
Lemp 1995), increased tear osmolarity (Gilbard et al. 1986; Iskeleli et al. 2002)
and a thinner pre-lens tear film (Wang et al. 2003; Nichols and King-Smith
2004).
The average pre-lens tear film thickness measured using interferometry
has been reported to be 3 µm (King-Smith et al. 2004) with the lipid layer often
being thin or absent (Young and Efron 1991; Craig and Tomlinson 1997;
Nichols and Sinnott 2006). The pre-lens lipid layer thickness is less and
thinning rate is higher in RGP contact lens users with dry eye symptoms than
those without the symptoms (Nichols and Sinnott 2006).
The tear film quality is typically assessed clinically in terms of
fluorescein TBUT, as abnormalities in any of the tear film layers (lipid layer,
aqueous layer or mucin layer) results in an unstable tear film causing reduced
Chapter 6: Tear film surface quality with rigid and soft contact lenses
136
tear break-up time. As the presence of a contact lens divides the tear film into
the pre-lens and post-lens tear film, any break in the pre-lens tear film may not
be readily observed due to the presence of the post-lens tear film in the case of
RGP lenses and absorption of fluorescein by the soft contact lens, resulting in
measurement errors. Therefore, TBUT in contact lens wearers is usually
measured with non-invasive techniques such as the Tearscope (Guillon 1998),
lipid layer interferometry (Nichols et al. 2002; Nichols and King-Smith 2003;
Nichols and Sinnott 2006; Szczesna et al. 2006), wavefront sensing (Thibos
and Hong 1999; Mihashi et al. 2006), meniscometry (Yokoi et al. 2003) and
high-speed videokeratoscopy (Goto et al. 2003). Some of these techniques
have their own limitations such as the Tearscope being a subjective technique,
lipid layer interferometry being sensitive to eye movements, wavefront sensing
depending on pupil size, and high-speed videokeratoscopy often being based
on surface topographic analysis.
Many studies in the past have used videokeratoscopy to estimate the
tear film quality using corneal topography analysis procedures (Nemeth et al.
2002; Goto et al. 2003; Montés-Micó et al. 2004; Iskander and Collins 2005;
Montés-Micó et al. 2005) but these measurements may be inaccurate when the
tear film breaks up (Iskander et al. 2005; Alonso-Caneiro et al. 2008). This is
because videokeratoscopy depends on the specular reflection of the Placido
disc pattern from the surface of tear film and when the tears begin to break up
the topography becomes highly irregular. However, the degree of topographic
irregularity is not closely correlated with the tear stability (Iskander et al. 2005;
Alonso-Caneiro et al. 2008). A newer technique of estimating the TFSQ using
image processing techniques based on the properties of the Placido disk
images has been recently developed and is independent of the surface
topography analysis (Alonso-Caneiro et al. 2008; Alonso-Caneiro et al. 2009).
This method has been used to quantify the TFSQ in eyes with and without soft
contact lenses (Alonso-Caneiro et al. 2009) and has been demonstrated to
exhibit good performance in the detection of patients with dry eye (Szczesna et
al. 2011).
Previous studies have reported no differences in the frequency of
dryness symptoms between RGP and soft contact lens wearers compared to
non-contact lens users (McMonnies 1990; Vajdic et al. 1999). Farris (1986)
suggesting that it is tear disruption which is responsible for dryness symptoms
Chapter 6: Tear film surface quality with rigid and soft contact lenses
137
irrespective of the type of contact lens. However, the thickness of lipid layer is
associated with stability of tear film (Craig et al. 1995), and this layer is very
thin or absent in rigid contact lens wearers (Guillon 1986). Therefore, TFSQ
with rigid lenses is likely be poorer compared to soft contact lenses.
The reports on the effect of rigid contact lenses on TBUT, which is
affected by lipid layer thickness, are inconclusive with one study reporting a
decrease in TBUT with rigid lenses (Hamano 1981) while another found no
difference in the TBUT of rigid lens wearers and non-contact lens wearers
(Bhatia and Singh 1993). However, in these studies TBUT was measured using
fluorescein which can confound the results due to associated reflex tearing
(Mengher et al. 1986). Rigid (RGP and PMMA) lenses have been shown to
cause a significant reduction in non-invasive TBUT at the 3 and 9 o‟ clock
portions of the conjunctiva (Itoh et al. 1999). Therefore, the aim of this study
was to measure the TFSQ with short-term use of PMMA and RGP contact
lenses and to compare with SiHy contact lenses, using the videokeratoscopy
technique described by Alonso-Caneiro et al. (2009).
6.2 Methodology
6.2.1 Subjects
The same group of fourteen subjects as in Chapter 3 (age range: 20 to 33
years, mean 27.8 ± 4.0 years, 5 females, 9 males), who were mainly students
and staff at QUT participated in this study. All subjects had low corneal
astigmatism (≤ 1.50 D corneal cylinder) and no signs of keratoconus or other
ectatic corneal disorders as seen in corneal topography maps acquired using
the Medmont E300 videokeratoscope. A slit-lamp examination was conducted
to ensure that all the subjects had a normal anterior segment and ocular health.
The subjects were screened for any significant dry eye based on the
McMonnies dry eye questionnaire, fluorescein TBUT, Phenol red thread test
and fluorescein and lissamine green staining of the ocular surface with staining
graded using Efron grading scale (Efron 1998). The group mean results from
these screening tests are presented in Table 6-1.
The subjects did not report the use of any ocular or systemic
medications or the presence of any ocular or systemic disease that may affect
the tear film or prevent the wear of contact lenses. Out the fourteen subjects,
two were soft contact lens wearers, but they discontinued the use of their
Chapter 6: Tear film surface quality with rigid and soft contact lenses
138
lenses at least one month prior to the start of the study. None of the subjects
had any history of rigid contact lens wear. A written informed consent was
obtained from all subjects after explanation of the procedures. The study
followed the tenets of the Declaration of Helsinki and was approved by the
Queensland University of Technology (QUT) Human Research Ethics
Committee (Appendix A). Sample size calculations revealed that a sample size
of 14 subjects used in this study would give 80% power to detect 0.02 change
in TFSQ at a 0.05 level of significance.
Table 6-1: Dry eye screening tests, screening criterion and mean scores of the study subjects. Subjects who failed 2 or more dry eye tests were not included in the study.
Test Screening
criterion
Mean score ± SD
McMonnies questionnaire Score ≥ 14 4.29 ± 2.52
Fluorescein TBUT < 10 sec 7.38 ± 4.44
Phenol red thread test < 15 mm 19.36 ± 5.30
Corneal fluorescein/Lissamine staining
staining
> 2 0.75 ± 1.05
6.2.2 Instrument
Dynamic high-speed videokeratoscopy was performed using the Medmont
E300, to derive measurements of non-invasive TFSQ. This technique is based
on specular reflection of a Placido disk pattern that is reflected from the surface
of tear film on the cornea or anterior contact lens. The quality of the reflected
ring pattern depends on the regularity of the surface and gives a measurement
of the TFSQ (Kopf et al. 2008; Alonso-Caneiro et al. 2009; Alonso-Caneiro et
al. 2009).
6.2.3 Contact lenses
Subjects wore 4 different types of contact lenses on 4 different days for 8 hours
on each day in the left eye only. Details of the contact lenses used have been
described in Chapter 3 and are shown in Table 6-2. The PMMA and RGP
lenses were custom ordered from Gelflex Laboratories (Perth, Australia). No
surface treatment was ordered for the PMMA or RGP lenses and the lenses
were stored in Boston conditioning solution (Bausch & Lomb Incorporated, New
York, U.S.A.). The SiHy lens used was the commercially available Bausch and
Lomb PureVision lens, which is manufactured with a standard “Performa”
surface treatment. In order to convert the silicone components on the lens
surface into hydrophilic silicate compounds this lens is surface treated in a gas
Chapter 6: Tear film surface quality with rigid and soft contact lenses
139
plasma reactive chamber (Grobe et al. 1999; Tighe 2004). The exact nature of
this surface treatment is proprietary information. The method of fitting the
lenses is described in Chapter 3.
6.2.4 Protocol
Measurements were performed with the contact lens in eye, in the morning
(between 8 and 11 am) and then repeated in the afternoon (between 4 and 7
pm) just before removal of contact lenses along with the measurements for
Chapter 3 (Figures 3-2 & 3-3). Baseline measurements were also taken both in
the morning and in the afternoon, on a day when no contact lens was worn.
The order of wear of the contact lenses was randomized. Ten to 20 minutes of
lens settling time was allowed before taking the measurements in the morning.
Three sets of 30 - second videokeratoscopy recordings were captured both in
normal (natural blinking) and then suppressed blinking conditions (6 x 30 sec in
total). Each 30 seconds recording consisted of 25 image frames per second.
Thus a total of 750 image frames were captured for each 30 second
measurement.
The subject was positioned in the chin rest of the E300 and was asked
to fixate on the centre of the inner-most ring of the Placido disk. During the
normal blinking condition, the subject was asked to blink normally. During the
suppressed blinking condition, the subject was asked to “take a gentle,
complete blink and then stop blinking for as long as it was comfortable”. The
subject was reminded to take a gentle blink and move back from the headrest
when it became uncomfortable. A gap of 3 - 4 minutes was allowed between
the measurements during which time the subjects could relax and blink
naturally. The measurements were taken in the same room, with approximately
the same humidity (58.0 ± 5.2%) and temperature (24.9 ± 1.0 ºC) and at
approximately the same time of day for all measurement days (both morning 8 -
11 am and afternoon 4 - 7 pm). The main room lights were dimmed during the
measurements.
Chapter 6: Tear film surface quality with rigid and soft contact lenses
140
Table 6-2: Description of the lenses used in the study.
Parameter Lens 1 Lens 2 Lens 3 Lens 4
Lens PMMA/ 9.5 RGP/ 9.5 RGP/10.5 SiHy/ 14.0
Design (centre) Spherical Spherical Spherical Spherical
Design (periphery) Aspheric Aspheric Aspheric B&L PureVision
Material PMMA RGP (Boston XO) RGP (Boston XO) Silicone hydrogel
Power (Dioptre) –0.50 –0.50 –0.50 –0.50
Total diameter (mm) 9.5 9.5 10.5 14.0
BOZD (mm) 8.1 8.1 8.8 8.9
Modulus (MPa) ≈ 2000 1500 1500 1.1
Manufacturing method
Lathe Lathe Lathe Cast moulding
Surface treatment None None None Performa/Plasma
oxidation
PMMA: polymethyl methacrylate, RGP: rigid gas permeable, SiHy: silicone hydrogel, B&L: Bausch and Lomb, BOZR: back optic zone radius, BOZD: back optic zone diameter, Dk: oxygen permeability, MPa: megapascal, unit of modulus of elasticity, mm: millimetres
6.3 Data Analysis
The mean TFSQ was calculated in both the normal and suppressed blinking
conditions. Matlab-based custom written image processing techniques were
used for analysis of the videokeratoscope images from the inter-blink interval.
The details of this method have been described by Alonso-Caneiro et al.
(2009). In order to allow for the tear film to build-up (Nemeth et al. 2002), a
period of one second after the blink was excluded from the analysis. First, the
region of interest (ROI), within the cornea which has the Placido ring pattern, is
estimated. Then the unaltered ring pattern within this area is identified, this
involves differentiating the Placido ring pattern from the interference. The
interference in the ring pattern can be due to shadows from the eyelashes or
poor tear film surface quality. The interference from the eyelashes is removed
to obtain the area of analysis (AOA), which now consists of the area of
unaltered ring pattern and interference due to poor TFSQ. The TFSQ is
estimated in the form of a number (ranging from 0 to 1) in the area of analysis
using image coherence analysis (Alonso-Caneiro et al. 2008; Alonso-Caneiro
et al. 2009). The coherence, which is a measurement of the pattern‟s local
orientation, is estimated as close to 0, when the pattern is poorly oriented (and
the tear film is disrupted and of poor quality) and close to 1, when the pattern is
well oriented (and the tear film is smooth and of high quality). TFSQ for each
image is the average of the coherence measurement in the area of analysis. An
Chapter 6: Tear film surface quality with rigid and soft contact lenses
141
average TFSQ value for all the Placido disc images is then calculated. The
steps involved in the analysis are shown in Figure 6-1. Reflections of the
Placido disc pattern immediately after and a few seconds after blink showing
tear break up on a soft lens is shown in Figure 6-2.
The area of the cornea/tear film that was used for analysis was centred
on the VK rings and had a radius of 2.75 mm (or 5.5 mm diameter). This area
was selected for analysis so that there was no interference from the lens edges
or the front optic zone diameter boundary. For normal blinking conditions the
analysis was carried out on all the data for a duration of 30 seconds (excluding
frames during blinking and 1 sec following the blink) to derive the mean TFSQ
value. For the suppressed blinking condition, the first 6 seconds after the final
blink before start of the recording (excluding the first 1 sec) was selected for
analysis and averaged (i.e. 5 X 25 frames) to derive the mean TFSQ
throughout the 6 seconds. If the subject blinked within 6 seconds, these data
were not used, however at least 2 out of the 3 suppressed blinking trials were
available for the 11 subjects included in the analysis.
A repeated measures analysis of variance ANOVA was used to
investigate the statistical significance of changes in TFSQ, with the lens type (4
different types) and time of day (morning and afternoon) as within-subject
factors in normal and suppressed blinking conditions. Degrees of freedom were
adjusted using the Greenhouse-Geisser correction to prevent any type 1 errors,
where violation of sphericity assumption occurred. Bonferroni adjusted pair-
wise comparisons were carried out for individual comparisons.
Chapter 6: Tear film surface quality with rigid and soft contact lenses
142
Figure 6-1: Steps involved in estimation of TFSQ value on the corneal or contact lens surface. ROI: region of interest. AOA: area of analysis
Figure 6-2: Image frames from high speed videokeratoscopy with an RGP lens on the cornea. Reflections of the Placido disc pattern (a) Immediately after blink (TFSQ value = 0.85) and (b) few seconds after blink showing tear break up (TFSQ value = 0.68). Yellow lines enclose the area of analysis.
Chapter 6: Tear film surface quality with rigid and soft contact lenses
143
The number of blinks per 30 seconds was estimated using the Matlab-
based software to calculate the blink frequency. This was multiplied by 2 to
obtain number of blinks per minute. The subjects are unaware of the blink
frequency being recorded since during the measurements they were informed
that their tear film quality was being measured and were asked to blink
normally.
6.4 Results
6.4.1 TFSQ in natural blinking conditions
The group mean TFSQ value in natural blinking conditions for the baseline day
and with the four different contact lenses are shown in Figure 6-3. The type of
lens (p=0.001, repeated measures ANOVA) had a significant effect on the
mean TFSQ values in the natural blinking conditions. Mean TFSQ values for
the PMMA/9.5, RGP/9.5 and RGP/10.5 lenses were significantly worse than
the baseline day (no lens) in both morning and afternoon (all p<0.05, pairwise
comparison) (Table 6-3). The SiHy lens also showed significant reduction in
TFSQ values in the afternoon after 8 hours of lens wear. Post hoc testing
showed that there was no significant difference between the mean TFSQ
values of the four contact lenses compared to each other (all p>0.05) (Figure 6-
3).
Figure 6-3: Mean TFSQ values in 30 seconds with the four contact lenses and on baseline day (no contact lens), in the morning and afternoon, in natural blinking conditions. The TFSQ is calculated on a scale of 0 to 1 where 0 is very poor and 1 is very good quality. * indicates significant difference compared to baseline. Error bars represent standard error of the mean.
Chapter 6: Tear film surface quality with rigid and soft contact lenses
144
Table 6-3: Mean change in TFSQ values relative to baseline, with the four contact lenses in the morning and afternoon, in natural blinking conditions over a period of 30 seconds. Negative values of TFSQ indicate that TFSQ is worse with contact lenses.
Mean change in TFSQ relative to baseline ± SD (p=value)
Lens Morning (am) Afternoon (pm)
PMMA/9.5 –0.06 ± 0.04 (p=0.002) –0.07 ± 0.04 (p<0.001)
RGP/9.5 –0.06 ± 0.05 (p=0.01) –0.08 ± 0.06 (p=0.002)
RGP/10.5 –0.06 ± 0.06 (p=0.01) –0.05 ± 0.04 (p=0.01)
SiHy/14.0 –0.07 ± 0.09 (p=0.10) –0.04 ± 0.04 (p=0.03)
There was a significant difference in the mean change in TFSQ values
in the afternoon compared to morning for RGP/9.5 lens which showed a
decrease of –0.02 ± 0.03 (p=0.04, pairwise comparison) and RGP/10.5 lens
which showed an increase of 0.02 ± 0.02 (p=0.02, pairwise comparison) (Table
6-4).
Table 6-4: Mean change in TFSQ values in the afternoon relative to morning, with the four contact lenses in natural blinking conditions. Negative values of TFSQ indicate that TFSQ is worse in the afternoon.
Mean change in TFSQ values (pm - am) ± SD
Baseline –0.0001 ± 0.03 (p=0.99)
PMMA/9.5 –0.01 ± 0.03 (p=0.41)
RGP/9.5 –0.02 ± 0.03 (p=0.04)
RGP/10.5 0.02 ± 0.02 (p=0.02)
SiHy/14.0 0.03 ± 0.07 (p=0.15)
6.4.2 Blink frequency in natural blinking conditions
The time of day had a significant effect (p=0.01) on the number of blinks per
minute in natural blinking conditions. The group mean number of blinks per
minute (or blink frequency) in natural blinking conditions for the baseline day
and with the four different contact lenses is shown in Figure 6-4. On the
baseline day, the blink frequency was significantly faster in the afternoon
compared to morning, with a mean increase of 4.24 ± 5.39 blinks/min (p=0.01,
repeated measures ANOVA). The blink frequency did not change significantly
during the day with any of the contact lenses. There was also no statistically
significant difference (all p>0.05) in the blink frequency with the different types
of contact lenses compared to baseline, although all lenses caused some
increase in mean blink frequency compared to baseline morning
measurements.
Chapter 6: Tear film surface quality with rigid and soft contact lenses
145
Figure 6-4: Mean blink frequencies (number of blinks per minute) in natural blinking conditions with and without contact lenses. Error bars represent standard error of the mean.
6.4.3 TFSQ in suppressed blinking conditions
The TFSQ in suppressed blinking conditions was studied for 11 subjects, since
data with at least 2 measurements up to 6 seconds was available only for these
subjects. The type of lens (p=0.01) also had a significant effect on the mean
TFSQ values under suppressed blinking conditions. The group mean TFSQ
values in suppressed blinking conditions for the baseline day and with the four
different contact lenses in the morning and afternoon are shown in Figure 6-5.
The figure illustrates the mean TFSQ values for each second immediately after
blinking for up to 6 seconds. The mean TFSQ values show significant
increases (improvement), in the first second after the blink (build-up time) with
all lenses and during baseline measurement, both in the morning (Figure 6-5 a)
as well as the afternoon (Figure 6-5 b). Mean TFSQ values for the three rigid
lenses (PMMA/9.5, RGP/9.5 and RGP/10.5) as well as SiHy/Soft lens was
significantly worse than the baseline day (no lens wear) both in the morning
and afternoon (all p<0.05, pairwise comparison) (Figure 6-5 and Table 6-5).
Post hoc testing showed that there was no significant difference between the
mean TFSQ values of the four contact lenses compared to each other (all
p>0.05). There were no significant differences in mean TFSQ values in the
afternoon compared to morning on the contact lens-wearing or the baseline
days.
Chapter 6: Tear film surface quality with rigid and soft contact lenses
146
Figure 6-5: The group mean TFSQ values in suppressed blinking conditions with time for the 6 seconds after a blink, for the baseline day and with the four different contact lenses in the morning (a) and afternoon (b).
Table 6-5: Mean changes in TFSQ values in suppressed blinking conditions relative to baseline.
Mean change in TFSQ ± SD in suppressed blinking conditions, relative to baseline
Lens Morning (am) Afternoon (pm)
PMMA/9.5 –0.04 ± 0.03 (p=0.01) –0.05 ± 0.02 (p<0.001)
RGP/9.5 –0.05 ± 0.04 (p=0.02) –0.06 ± 0.02 (p<0.001)
RGP/10.5 –0.04 ± 0.03 (p=0.04) –0.04 ± 0.02 (p=0.003)
SiHy/14.0 –0.05 ± 0.05 (p=0.04) –0.05 ± 0.04 (p=0.01)
Chapter 6: Tear film surface quality with rigid and soft contact lenses
147
There was a clear trend towards reduction in mean change in TFSQ
with time over the 5 second period of recording (Figure 6-5) both in the morning
and afternoon.
6.4.4 Trend of TFSQ with time in suppressed blinking conditions
TFSQ generally decreased with time. Subjects showed four different types of
trends in the change in TFSQ over a period of 30 seconds. The different
patterns that were most commonly observed are shown in Figure 6-6 for four
representative subjects. Type 1 pattern was a steady decline in TFSQ with time
till the end of recording time [Figure 6-6 (Type 1) & Figure 6-7]. The second
type of trend showed a slower decrease with time with a small improvement
towards the end of recording time (Figure 6-6, Type 2). The third trend showed
a steep decline in TFSQ following a stabilization of tear film (after the blink) in
the first few seconds forcing the subject to blink well before the end of
recording time (time of last frame from Figure is at 9.84 seconds, Figure 6-6,
Type 3). The fourth and the most unexpected trend was a decline in TFSQ with
time for first 7 to 9 seconds after the blink, but then TFSQ begins to improve
and gets even better than the baseline values by 12 to 13 seconds before
stabilising [Figure 6-6 (Type 4) & Figure 6-8].
Chapter 6: Tear film surface quality with rigid and soft contact lenses
148
Figure 6-6: The four different types of representative patterns of TFSQ with time over 30 seconds.
Figures 6-7 and 6-8 show the first and the fourth types of TFSQ
patterns respectively over time and the corresponding Placido disk images at 3
different times. The trend shown in Figure 6-8 was seen most commonly with
rigid contact lenses compared to the soft lens and never noticed during
baseline measurements (no contact lens) (Table 6-6).
Table 6-6: Analysis of recordings showing an increase in TFSQ value with time.
Lens Total number of
recordings Number of recordings
with an increase in TFSQ Percentage (%)
Baseline 84 0 0
PMMA/9.5 84 57 68
RGP/9.5 84 55 65
RGP/10.5 84 56 67
SiHy/14.0
84 16 19
A total of 14 subjects, with 3 recording each performed in both morning and afternoon. (Total recording is 84). Pattern of increase in TFSQ is shown in Figure 6-8.
Chapter 6: Tear film surface quality with rigid and soft contact lenses
149
Figure 6-7: TFSQ over time for a representative subject, showing an increase during the first second post-blink (build-up time) and then a constant reduction in TFSQ over time till the end of the measurement. Corresponding Placido disc maps can be seen at the beginning (clear rings), middle (breaks in the ring pattern) and end (severe distortion of the ring pattern) of the measurement. Yellow lines enclose the area of analysis.
Figure 6-8: TFSQ over time for a representative subject, showing an increase during first second post-blink, then a reduction is seen with time till a certain point after which it shows an improvement and reaches a value more than the baseline. Corresponding Placido disc maps can be seen at the beginning (clear rings), middle (few breaks in the ring pattern) and end (very clear and regular ring pattern) of the measurement. This later period seems to correspond to complete drying of the lens surface which now acts like a mirror to produce a high TFSQ value. Yellow lines enclose the area of analysis.
Chapter 6: Tear film surface quality with rigid and soft contact lenses
150
6.4.5 Association between TFSQ value and blink rate
To study the association between mean TFSQ values and blink frequency
(number of blinks per minute) in normal blinking conditions, a Pearson‟s
correlation was calculated between these variables. A very weak negative
correlation (R2 = 0.07, p=0.33) between these parameters was found (i.e. there
was a weak tendency for an increase in blink frequency with decrease in
TFSQ) but this was not significant (Figure 6-9).
Figure 6-9: Correlation between mean TFSQ values and blink rates (number of blinks per minute) in the morning and afternoon, for all the lenses combined. (Afternoon measurements only)
6.4.6 Association between TFSQ value and tarsal conjunctival and lid-wiper staining
To study the association between the TFSQ value and tarsal conjunctival
staining Spearman‟s correlation was calculated between changes in these
variables compared to baseline. There was no correlation (R2 = 0.01, p=0.43)
between the mean TFSQ values and tarsal staining for all lenses combined
(Figure 6-10 a). Similarly, there was no correlation between mean TFSQ values
and lid-wiper staining all lenses combined (R2 = 0.03, p=0.51) (Figure 6-10 b).
y = -0.0007x + 0.8472 R² = 0.0789
0.78
0.80
0.82
0.84
0.86
0.88
0 20 40 60
Me
an
TF
SQ
Blink Rate
Mean TFSQ vs blink rate (pm)
Chapter 6: Tear film surface quality with rigid and soft contact lenses
151
Figure 6-10: Correlation between mean TFSQ values and (a) Tarsal staining (b) Lid-wiper staining for all the lenses combined for morning and afternoon.
6.5 Discussion
This study shows that all types of contact lenses adversely affect the TFSQ.
The mean TFSQ value was worse with all the lenses both in morning and
afternoon compared to measurements on the baseline day and the changes
were statistically significant with all the rigid lenses (PMMA/9.5, RGP/9.5 and
RGP/10.5) during normal blinking conditions. The SiHy lens also showed a
significant reduction in TFSQ value in the afternoon after 8 hours of lens wear.
This is in agreement with previous studies (Kopf et al. 2008; Alonso-Caneiro et
al. 2009) that found significant differences in TFSQ values with both hydrogel
and SiHy contact lenses compared to baseline after one day of lens wear. The
mean TFSQ value with the SiHy lens in this study was 0.83 ± 0.04 in
suppressed blinking conditions and 0.85 ± 0.08 in normal blinking conditions
(after 8 hours of lens wear) compared to 0.84 ± 0.02 (after one day of lens
wear) in suppressed blinking, in the study by Alonso-Caneiro et al. (2009) This
is the first study to use dynamic videokeratoscopy to measure TFSQ with rigid
lenses and demonstrates that similar magnitude reductions in TFSQ occur with
rigid lenses of different materials and designs.
We found no significant differences between the mean TFSQ values
with the different lenses (i.e. all the lens materials caused reductions in TFSQ
of similar magnitude compared to the baseline) and this correlates with
previous studies of ocular symptoms that have reported no differences in the
frequency of dryness symptoms between RGP and soft contact lens wearers
(McMonnies 1990; Vajdic et al. 1999). Less dryness is reported by subjects
Chapter 6: Tear film surface quality with rigid and soft contact lenses
152
using SiHy contact lenses compared to conventional hydrogel lenses (Fonn et
al. 2000) and this is attributed to less lens dehydration seen with the SiHy
lenses than with conventional hydrogel materials. Whereas in another study, no
differences were found between SiHy and hydrogel lenses, in terms of pre-lens
tear film thinning times (Thai et al. 2004). The method of measuring TFSQ
described in this study may not be sensitive enough to distinguish differences
between lenses but has shown significant differences between lens wear and
baseline (bare eye) conditions.
We also found a reduction in mean TFSQ values in the afternoon
compared to the morning on the baseline day, and with PMMA/9.5 and
RGP/9.5 lenses. On the other hand, RGP/10.5 and SiHy/14.0 lenses showed a
slight improvement in mean TFSQ value in the afternoon compared to morning.
This is in contrast to studies that have shown an increase in severity of
symptoms at the end of contact lens wearing time in contact lens wearers
(Begley et al. 2000; Begley et al. 2001).
The mean change in TFSQ value in the afternoon compared to morning
showed an opposite effect with the RGP/9.5 lens (a decrease of –0.02 ± 0.03)
and RGP/10.5 lens (an increase of 0.02 ± 0.02). This small difference in the
TFSQ values with the two lenses, though not statistically significant could be
partly attributed to the total diameter of these lenses, since it has been reported
that larger diameter lenses are more comfortable than smaller diameter lenses
(Williams-Lyn et al. 1993). The larger diameter contact lenses, being slight
more comfortable could lead to less blinking and reflex tearing and thus could
result in a more stable tear film. An increase in TFSQ value was also seen with
the larger diameter SiHy lens (SiHy/14.0) but this increase was not statistically
significant.
We noticed four different patterns of deterioration in TFSQ values over
the period of 30 seconds in suppressed blinking conditions (Figure 6-6). First a
gradual linear decrease in TFSQ, second a slower rate of decrease with a little
improvement in TFSQ towards the end of recording time, third a rapid decrease
in TFSQ to reach minimum in about 11 seconds and the fourth showing a
decrease till 9 seconds followed by an improvement at about 13 seconds to
result in TFSQ even better than the baseline. This improvement is unlikely to
be a real improvement in TFSQ and appears to be due to the lens surface
evenly drying and acting as a mirror. This difference in the pattern of
Chapter 6: Tear film surface quality with rigid and soft contact lenses
153
deterioration in TFSQ has not been previously reported since the measurement
time used by (Alonso-Caneiro et al. 2009) was not long enough (8 seconds),
compared to the present study (30 seconds) to reveal this pattern. This pattern
was most frequently noticed with rigid/hard lenses and a few silicone hydrogel
lenses and is something that would be expected to occur in a clinical setting in
RGP contact lens wearers showing a full break-up of the tear film. The results
are related to the differences in thickness of the pre-lens tear film (Nichols and
King-Smith 2003; Wang et al. 2003; Nichols et al. 2005) and pre-lens lipid layer
(Guillon 1986) of contact lenses. Based upon previous results of a mean pre-
lens thinning rate of 6.97 µm/min, for a mean PLTF (pre-lens tear film)
thickness of 2.54 µm of a soft contact lens (Nichols et al. 2005), it will take
about 22 seconds to dry up completely. It is possible that a thinner or absent
lipid layer on the rigid lens will result in the rate of evaporation with these
lenses being much greater and causes the lens surface to dry out completely
and act as a mirror. Since 22 seconds is slightly longer than the 13 seconds
after which we observed complete drying, it is likely that some factors other
than evaporation may also be involved in this.
6.6 Conclusion
In summary, a significant decrease in TFSQ values is shown with all
types of contact lenses compared to baseline (bare eye) in normal blinking
conditions, though no differences were noticed between the lens types. This
indicates the need for better contact lens materials and surfaces and highlights
the importance of plasma coating on the lenses to improve their wettability and
hydrophilicity. An interesting pattern of change in TFSQ over time was found in
suppressed blinking conditions, in which TFSQ was reduced until a certain time
after which it improved to become even better than the baseline. This
phenomenon (seen more frequently with rigid lenses) has never been reported
earlier and could be attributed to tear film drying completely over the surface of
contact lenses. Our technique presented in this study does not measure tear
film thickness, so in order to test the hypothesis that the tears are drying
(thinning) completely further studies are needed to measure tear film thickness
using interferometry techniques in suppressed blinking conditions.
Chapter 6: Tear film surface quality with rigid and soft contact lenses
154
- 155 -
Chapter 7
Conclusions We have studied the effects of the short-term wear of soft and rigid contact
lenses on various structures of the anterior eye in front of and behind the
contact lenses. This included the tear film, anterior corneal topography,
posterior corneal topography, regional corneal thickness, palpebral conjunctiva,
eyelid margin and eyelid position. These studies have shown that as little as 8
hours of contact lens wear can lead to measurable changes in all of these
structures of the anterior eye. The location and magnitude of corneal changes
were largely determined by the regional oxygen transmissibility of the lens for
both soft and rigid lenses, while mechanical changes involving the eyelids were
generally more common with the higher modulus rigid contact lenses. Figure 7-
1 summarises the structures and parameters of the anterior eye that were
investigated in this thesis.
Figure 7-1: Schematic representation of ocular structures and parameters affected by short-term use of contact lenses presented in this thesis.
Chapter 7: Conclusions
156
7.1 Changes in ocular structures posterior to the contact lens
Important changes were found in ocular structures posterior to the contact lens
including regional corneal thickness, anterior corneal topography, posterior
corneal topography and wavefront error changes. These changes are
described below and summarised in Figure 7.2.
Figure 7-2: Changes in ocular structures and parameters (posterior to contact lenses) affected by short-term use of contact lenses, in comparison to baseline day changes.
7.1.1 Corneal thickness changes and contact lenses
We systematically studied the effect of soft contact lens power, design and
material on regional corneal thickness after short-term lens wear (Chapter 2).
Soft toric lenses have their thickest areas in the periphery and might therefore
be expected to have greatest influence on the underlying regional corneal
thickness. This is the first study to systematically investigate the effect of short-
term use of soft toric contact lenses on corneal thickness. Using a rotating
Chapter 7: Conclusions
157
Scheimpflug technique (Pentacam), we were able to study regional changes in
corneal thickness with these lenses and to compare the changes with those
found with spherical soft lenses. We observed greater swelling towards the
periphery after wear, for all soft lenses tested. However, the swelling was
greatest in the regions corresponding to the thick stabilization zones of the
hydrogel toric lens (47 µm or 8.2% directly under the stabilization zone
compared to 19 µm or 2.9% in the centre). These findings indicate that these
corneal regions are suffering the greatest hypoxia and in the long term are
likely to be susceptible to various secondary complications of compromised
corneal metabolism (e.g. neovascularization at the limbus).
The SiHy spherical and toric lenses resulted in small overall changes in
corneal thickness and led to slight central corneal thinning. Clearly the
increased oxygen permeability of SiHy lenses is providing sufficient oxygen to
the underlying cornea and this leads to little change in corneal thickness profile.
The corneal swelling seen in this study after 8 hours of lens wear was similar in
magnitude to that seen after eyelid closure associated with overnight sleep,
which is not likely to have an adverse effect on the cornea. However, the
corneal changes associated with longer term hydrogel toric lens wear are
expected to be larger (Hagan et al. 1998; Schornack 2003). Future research
involving controlled clinical studies of longer term soft toric lens wear of
different designs is required to improve our understanding of the nature and
magnitude of the longer term corneal effects.
In this study we also found significant diurnal changes in corneal
thickness from mid-morning to afternoon, which were usually larger than those
caused by the SiHy lenses. These natural diurnal variations should be
considered while investigating contact lens induced corneal changes (for both
daily and extended wear conditions) since these can have potentially
confounding effects on any analysis of the effects of the contact lenses.
We also investigated the effect of the short-term wear of PMMA and
RGP lenses on corneal thickness (Chapter 3). Previous studies have shown
significant corneal swelling with the wear of PMMA contact lenses (Carney
1974; Fonn et al. 1984; Wang et al. 2003; Moezzi et al. 2004), and our results
are consistent with these findings. The mean central corneal swelling with the
PMMA contact lenses was 27.9 ± 15.49 µm (4.77%) and was much greater in
the centre than in the peripheral cornea (17.78 ± 12.11 µm, 2.71% swelling).
Chapter 7: Conclusions
158
The corneal swelling was much less pronounced following 8 hours of RGP lens
wear and did not reach statistical significance, however on average greater
swelling was observed in peripheral corneal regions compared to central
regions with both of the RGP lenses (varied in diameter but the same Boston
XO material). Similar results were seen with the spherical and back surface
toric RGP lenses (both Boston XO material) that were studied in a group of
subjects with toric corneas after short-term wear (Chapter 4). For rigid contact
lenses as with the soft lenses, the degree of corneal swelling appeared to be
primarily driven by the regional oxygen transmissibility characteristics of the
lenses.
7.1.2 Anterior corneal curvature changes and contact lenses
The changes in anterior corneal curvature that we observed with the various
types of contact lenses seem to have been mostly the result of mechanical
pressure from the lens, with little association with the underlying corneal
swelling (with the possible exception of PMMA lens wear). SiHy lenses
(Chapter 2) resulted in slight but statistically significant flattening of the
peripheral anterior curvature which caused a decrease in anterior corneal
refractive power of about 0.15 D. The hydrogel lens caused little change in
anterior curvature, even though this lens caused greater overall corneal
swelling. These results are in accordance with previous studies suggesting that
there can be substantial changes in corneal thickness without any significant
change in anterior corneal topography with the use of contact lenses (Bailey
and Carney 1972; Carney 1972; Carney 1975).
The relationship between anterior axial curvature change and corneal
swelling is not straight forward. If the cornea swells predominantly in a
backward direction (i.e. the anterior chamber becomes narrower) as suggested
by many authors (Read and Collins 2009), then we would expect no
association between corneal swelling and anterior corneal curvature change.
Even if the corneal swelling was entirely in the anterior direction (i.e. anterior
chamber depth remains unchanged) then a substantial uniform change in
corneal swelling of say 3.5% (20.3 microns) will only lead to a small flattening
of corneal curvature (7.70 mm axial radius becomes 7.72 mm) (see Figure 7-3).
The standard deviation of typical anterior axial curvature measurements is ±
Chapter 7: Conclusions
159
0.02 mm, so small changes in anterior curvature related to corneal swelling
would be barely detectable with modern videokeratoscopes.
On comparing the RGP and PMMA lenses in Chapter 3, we found that
these lenses have opposite effects on anterior corneal curvature. RGP lenses
resulted in anterior corneal flattening, which caused a decrease in corneal
refractive power ranging from 0.998 to 0.01 D. These changes in anterior
curvature appear to be related to mechanical pressure from the lens, since
there was little sign of corneal swelling with the RGP lenses. Further research
examining long-term changes in corneal refractive power using RGP lenses are
essential to understand how these changes progress with time.
PMMA contact lenses, on the other hand, showed corneal steepening
which was greater in the centre than the periphery of the cornea. Since the
mechanical pressure exerted by these PMMA lenses is expected to have been
identical to the RGP lens we used (identical fitting), the most obvious cause of
the steepened central corneal curvature is the associated central corneal
swelling seen with the PMMA lenses. This would suggest that at least in the
case of the PMMA lenses, the corneal swelling was predominantly in the
anterior direction (i.e. anterior chamber depth remained unchanged) and
because it occurred to a greater extent in the central cornea, it created a
steeper central curvature (see Figure 7-3).
While rigid contact lenses altered the shape of the cornea to produce
substantial changes in corneal refractive power, SiHy contact lens (used for
comparison in Chapter 3) resulted in only small changes in corneal refractive
power, which were clinically insignificant. These results support the view that
soft contact lenses predominantly conform to the shape of the cornea and
therefore produce little refractive power change due to lens pressure.
We were able to demonstrate a moderate correlation between the
location of regions of corneal flattening on the anterior corneal surface and the
regions of minimum fluorescein clearance behind the rigid lens. The
mechanical forces due to a contact lens are potentially distributed across the
cornea based on the clearance between the lens and corneal surface. The type
of fluorescein pattern indicated the expected changes in corneal curvature due
to mechanical forces and this principle is routinely used in orthokeratology lens
fitting. No previous report has quantitatively analysed the fluorescein pattern of
Chapter 7: Conclusions
160
standard rigid contact lens fittings and correlated this to the short-term corneal
topographic changes. Information provided by this analysis may be useful in
predicting the changes in corneal curvature that a rigid contact lens is likely to
produce in clinical practice after longer term wear.
Figure 7-3: Schematic diagram showing difference between central and uniform anterior swelling.
We further explored corneal curvature and refractive changes after
short-term use of back surface toric compared to spherical RGP lenses on
subjects with toric corneas for the first time (Chapter 4). We found corneal
flattening was slightly greater with the spherical lens compared to the back toric
lens, which resulted in a decrease in refractive power (about 0.25 D with the
spherical lens and 0.15 D with the back toric lens). The spherical lens also
caused a significant decrease in WTR astigmatism (0.13 D). This confirmed our
hypothesis that the mechanical effects due to a spherical lens would differ from
those of a back toric lens which more closely aligned to the corneal curvature,
however it should be noted that the magnitude of difference in corneal changes
between the two lens designs was small.
Chapter 7: Conclusions
161
7.1.3 Posterior corneal curvature changes and contact lenses
The magnitude and nature of posterior corneal change associated with daily
wear of various contact lenses has not been reported earlier. Previous studies
have reported posterior corneal changes with contact lenses only after lens
wear in closed eye conditions (Moezzi et al. 2004; Martin et al. 2009). In
Chapters 2 and 3 we have attempted to gain a better understanding of the
effects of soft and rigid contact lens wear on posterior corneal shape. All of the
soft lenses used (Chapter 2) resulted in a steepening of posterior curvature
which is likely to be due to greater peripheral swelling seen with these lenses
compared to the centre of the cornea. The largest magnitude of change was
observed with the HEMA/Toric/–3 lens, with a mean steepening of –0.07 mm of
the central posterior cornea. The steepening of the central posterior curvature
in this study correlated with the corneal swelling. This is in agreement with
previous reports that posterior corneal curvature changes are associated with
corneal edema (Kikkawa and Hirayama 1970; Lee and Wilson 1981; Erickson
et al. 1999; Read and Collins 2009).
PMMA and RGP lenses were observed to have opposite effects on
corneal curvature (Chapter 3), with PMMA lenses showing a posterior flattening
and the RGP lenses showing steepening. These changes correlated well with
the changes in central and peripheral corneal thickness (i.e. greater central
corneal swelling with PMMA and greater peripheral swelling seen with RGP
lenses) (Chapter 3, Figure 3-9). The spherical and back surface toric RGP
lenses on toric corneas (Chapter 4) also caused steepening similar to changes
seen with the spherical RGP lenses in Chapter 3.
7.1.4 Wavefront aberrations and rigid contact lenses
In the past, a number of authors have described the effects of contact lenses
(on-eye) on the ocular wavefront aberrations (Hong and Himebaugh 2001;
Dorronsoro et al. 2003; Lu et al. 2003). The effect of short-term contact lens
wear on ocular aberrations measured after removal of the lens has been
reported for the first time in Chapter 3. PMMA lenses produced a significant
increase in HO RMS, 2nd, 3rd and 4th order RMS wavefront error for a 4 mm
pupil diameter and in 3rd, 4th and HO RMS for a 5.5 mm pupil. These changes
may cause a reduction in ocular image quality after lens removal and may be
partly responsible for “spectacle blur” reported with PMMA lenses (Levenson
Chapter 7: Conclusions
162
1983; Wilson et al. 1990). RGP and SiHy lenses resulted in small and
insignificant changes in higher order aberrations after lens removal.
7.2 Changes in ocular structures anterior to the contact lens
A variety of clinically and statistically significant changes were also found in
ocular structures anterior to the contact lens including changes in the position
of the eyelids, lid-wiper epitheliopathy, tarsal staining and changes in the tear
film surface quality. The changes are described below and summarised in
Figure 7.4.
Figure 7-4: Changes in ocular structures and parameters (anterior to contact lenses) affected by short-term use of contact lenses, in comparison to baseline day changes. PA: Palpebral aperture.
7.2.1 Lid related changes and contact lenses
Previous studies have shown a reduction in PA height with long-term (2 weeks
to 10 years) rigid contact lens wear (Fonn and Holden 1988; Van den Bosch
and Lemij 1992; Fonn et al. 1996). The small but significant decrease in the PA
height with PMMA/9.5 and RGP/10.5 lenses in Chapter 5 shows that a subtle
Chapter 7: Conclusions
163
blepharoptosis can occur even after short periods of lens wear. This is the first
study to report the effect of short-term (few hours) contact lens wear on PA
height. The difference in the blepharoptosis seen with different lenses is most
likely due to one or more of the factors such as the modulus of elasticity, total
diameter and edge manufacture of the lenses.
The changes in lid position could potentially be related to irritation of the
cornea or lids leading to blepharospasm due to the presence of the contact
lenses (Van den Bosch and Lemij 1992) or eyelid swelling due to mechanical
trauma. On further investigation, we found that changes in the position of both
upper and lower lids contributed to the reduction in PA but the upper lid
changes were larger. This implies that the origin of the PA height changes may
be partly related to „reflex‟ lid movements associated with contraction of the
orbicularis oculi muscle (changes in the both upper and lower lids), along with
possible inflammation and swelling of the lids due to mechanical micro-trauma.
While we did not follow our subjects to record a recovery in size of PA , based
on previous studies (Fonn and Holden 1986; Fonn and Holden 1988; Fonn et
al. 1995), we speculate that PA size in our group of subjects will increase and
normalize after lens removal.
The lid-wiper region of the upper lid moves across the edge and surface
of the contact lens during each blink as it spreads the tears across the lens and
ocular surface. This may result in micro-trauma to the surface epithelial cells in
this region of the marginal conjunctiva. Lid-wiper epitheliopathy has been
reported in long-term soft contact lens wearers and dry eye patients (Korb et al.
2002; Korb et al. 2005; Yeniad et al. 2010) but this is the first study to report
increases in lid-wiper staining in as little as 8 hours wear of rigid and soft
contact lenses. A significant increase in the magnitude of lid-wiper staining was
associated with the short-term wear of all the rigid lenses. The magnitude of lid-
wiper staining increased during the course of the day with all types of contact
lenses (soft and rigid).
The increase in lid-wiper and tarsal staining during the course of the day
is most likely due to the constant friction between the lid-wiper/tarsal surfaces
and the contact lens surface, edge and/or the ocular surface during each blink.
The increase in staining observed on the contact lens-wearing days
(particularly with the rigid lenses) suggests that the contact lens edge and
Chapter 7: Conclusions
164
surface cause a greater amount of friction in the lid-wiper region as compared
to the ocular surface alone (Korb et al. 2005).
We also examined the tarsal conjunctiva for signs of micro-trauma since
it follows the lid-wiper during a blink and rubs against the ocular/contact lens
surface. We developed a grading scale (Figure 5.4) to investigate the changes
in tarsal conjunctival staining after short-term wear of different contact lenses,
increases in which are likely to indicate greater micro-trauma to the surface. An
increase in the amount of tarsal staining (Chapter 5) with all three rigid/hard
contact lenses (PMMA/9.5, RGP/9.5 and RGP/10.5) was found. However the
SiHy lens did not cause any significant increase in tarsal staining over the 8
hours of lens wear. Previous research has revealed that the tear film on a
contact lens surface dries more rapidly compared to that on the cornea
(Cedarstaff and Tomlinson 1983; Thai et al. 2004), thereby increasing the
friction between the surfaces. The increased tarsal staining we found could
therefore be partly attributed to an increased friction between the lens and
tarsal conjunctiva compared to that between the natural cornea and tarsal
conjunctiva. However later OCT imaging of the PMMA and RGP lens edges
suggest that poor lens edge manufacture had contributed to the increased
tarsal staining with these lenses.
We also noted a significant diurnal increase in tarsal staining even on
the baseline day when no lenses were worn. This suggests that the tarsal
conjunctival surface is in constant friction with the ocular surface during each
blink resulting in some minor surface damage over the course of the normal
day. A larger controlled study in a group of dry eye and non-dry eye subjects is
required to investigate diurnal changes in these parameters to confirm these
effects.
This is the first study to report blepharoptosis, tarsal conjunctival
staining and lid-wiper staining in as little as 8 hours after use of a variety of
contact lenses. The overall findings related to eyelid changes associated with a
short period of contact lens wear highlight that there is still a need for better
and smoother contact lens edges to minimize the interactions between the lid
margin and contact lens edge during lens wear. There is a need to measure
and better understand the lubricity of a range of available contact lenses and
the effect of different types of plasma treatment on the lubricity of contact lens
materials. The lens edges should be inspected by the practitioner if any signs
Chapter 7: Conclusions
165
of lid trauma are observed. There is also scope for the development of more
lubricious contact lens surfaces to reduce lid surface trauma due to friction
especially for patients with dry eye problems.
7.2.2 Tear film surface quality (TFSQ) and contact lenses
The tear film, in addition to providing the first optical surface of the eye is also
responsible for lubrication between the lids and the ocular surface. The
increased evaporation of the pre-contact lens tear film during lens wear
(Cedarstaff and Tomlinson 1983; Thai et al. 2004) potentially increases the
amount of friction thereby, causing increased lid-wiper and tarsal conjunctival
staining. Therefore, a poor TFSQ may increase damage to the ocular surface
because of the contact lens and reduce the optical quality of the eye. Non-
invasive assessment of TFSQ with RGP contact lenses using dynamic
videokeratoscopy has not been reported earlier. We studied the effect of short-
term lens wear on TFSQ (Chapter 6) using a high speed videokeratoscopy
technique and demonstrated that all types of contact lenses adversely affect
TFSQ in both natural as well as suppressed blinking conditions. The mean
TFSQ value was worse with all the lenses (soft and RGP) in the afternoon in
both normal and suppressed blinking conditions The SiHy lens also showed a
significant reduction in TFSQ in the afternoon after 8 hours of lens wear which
is in agreement with previous studies (Kopf et al. 2008; Alonso-Caneiro et al.
2009) that showed significant differences in TFSQ values with both hydrogel
and SiHy contact lenses compared to baseline after one day of lens wear. The
mean TFSQ values with the SiHy lens in our study (0.83 ± 0.04 in suppressed
and 0.85 ± 0.08 in normal blinking conditions) are comparable to that reported
by Alonso-Caneiro et al. (2009) (0.84 ± 0.02 in suppressed blinking conditions).
This is the first study to use dynamic videokeratoscopy to measure TFSQ with
rigid lenses and demonstrates that a similar magnitude of reduction in TFSQ
occurs with rigid contact lenses irrespective of their material (PMMA versus
Boston XO). This agrees with previous studies of ocular symptoms that have
reported no differences in the frequency of dryness symptoms between RGP
and soft contact lens wearers (McMonnies 1990; Vajdic et al. 1999). The
method of measuring TFSQ described in this study has shown significant
differences between the bare cornea and during contact lens wear, but was not
sensitive enough to distinguish any potential differences between lens
types/materials, if differences do exist.
Chapter 7: Conclusions
166
We found an interesting and unexpected pattern of change in TFSQ in
suppressed blinking conditions. TFSQ value was found to gradually reduce and
then improve to a value even better than the baseline bare eye measurements.
This is the first study to report this phenomenon which was seen more
frequently with rigid lenses and is likely to be due to the tear film drying
completely over the surface of the contact lenses to create a perfect “mirror-
like” reflection directly from the lens surface. In order to test the hypothesis that
the tears are drying (thinning) completely, further studies measuring tear film
thickness using interferometry techniques in suppressed blinking conditions
using various contact lenses are required. The overall findings of this study of
tear film quality over the surface of contact lenses in the eye show that there is
a need for better contact lens materials and surfaces with improved wettability
and hydrophilicity in order to improve the pre-lens tear film surface quality.
7.3 Conclusion and clinical implications
Contact lenses are in close contact with the ocular surface and lead to complex
mechanical and physiological effects on the tissue of the anterior eye. In this
series of studies we have examined the short-term effects of various types
(materials and designs) of contact lenses on anterior and posterior corneal
curvatures, thickness, lid-wiper, tarsal conjunctiva and TFSQ. The experimental
paradigm adopted in this study aimed to utilize lenses of identical design or
material and then systematically vary one or more parameters to understand
the influence of lens characteristics on the ocular surface. The lenses that we
designed and used in the various studies reported in this thesis showed that
measurable changes of each of the anterior eye parameters in as little as 8
hours of lens wear.
In general, the soft lenses made with modern SiHy material caused
minimal changes in the anterior eye after short-term wear compared with the
older HEMA lens material. This was particularly evident with the significant
corneal swelling seen beneath the thicker stabilizing zones of the HEMA toric
soft lens, whereas the identical lens design in SiHy material caused very little
corneal change. In the case of the rigid contact lenses, the differences between
the effects of modern lens materials (Boston XO) and the older lens material
(PMMA) was most clearly illustrated in the case of corneal swelling after short-
term lens wear. The PMMA lens led to significant central corneal swelling and
anterior corneal steepening, whereas an identical design in Boston XO material
Chapter 7: Conclusions
167
caused minimal corneal swelling or curvature changes. All the lenses caused
signs of micro-trauma to the eyelid wiper and tarsal conjunctiva, although rigid
lenses appeared to cause more significant changes than the SiHy lens. Tear
film surface quality was also significantly reduced with all types of contact
lenses.
These results show that even following the advances that have been
made in the last few decades, contact lenses still behave as a foreign object in
the eye and affect ocular health. These short-term changes in the anterior eye
are potential markers for further long-term changes. While modern contact lens
materials have clearly improved the impact of the lenses on the ocular surface,
aspects of lens wear such as tear film surface quality and micro-trauma to the
eyelids still show an obvious opportunity for improvements in lens designs and
materials in the future. Improvements in lens materials and designs will help to
reduce the physiological impact of contact lenses on the anterior eye.
Chapter 7: Conclusions
168
- 169 -
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Appendices
Appendix A: Ethics and Consent form
Appendix B: Conference abstracts arising from this thesis
Tyagi, G., Collins, M., Read, S., Davis, B. “Soft contact lens wear and regional changes in corneal
thickness and topography”. American Academy of Optometry, San Francisco, 19th November,
2010.
Tyagi, G., Collins, M., Read, S., Davis, B. “Corneal topography after short term use of RGP
contact lenses”, American Academy of Optometry, San Francisco, 19th November, 2010.
Tyagi, G., Collins, M., Read, S., Davis, B. “Corneal optical changes with short term contact lens
wear”, IHBI Inspires student conference, Gold Coast, 25th November, 2010.
Appendix C: Publications arising from this thesis
Tyagi G, Collins M, Read S and Davis B (2010). "Regional Changes in Corneal Thickness and
Shape with Soft Contact Lenses." Optometry & Vision Science 87(8): 567-575.
Tyagi G, Collins M, Read S and Davis B (2011). “Corneal changes following short-term rigid
contact lens wear.” Contact Lens and Anterior Eye. Under revision.
Tyagi G, Alonso-Caneiro D, Collins M and Read S (2011). ―Tear film surface quality with rigid and
soft contact lenses” Eye and Contact Lens. Under revision.
Appendices
200
l0!ti PARTICIPANT INFORMATION for QUT RESEARCH PROJECT ~---------------1
"Corneal Topography and Contact Lenses"
Research Team Contacts
Description
Garima Tyagi (PhD Candidate) Phone: 07 31385716
Email: g.tyagi@qut.edu.au
Or Scott Read (Associate supervisor) Phone: 07 31385714
Email: sa .read@qut.edu.au
Prof. Michael Collins (Supervisor) Phone: 07 3138 5702
Email: m.collins@qut.ed u.au
Brett Davis (Associate supervisor) Phone: 07 3138 5721
Email: b.davis@qut.edu.au
This project is being undertaken as part of PhD research project by Garima Tyagi.
The purpose of this project is to investigate the changes in corneal topography (shape) and thickness with the use of different types of contact lenses. The relative influence of different contact lens materials and designs on the shape of the front and back surface of the eye (cornea), the thickness of the cornea and the total optics of the eye will be investigated.
Participation Your participation in this project is voluntary. If you do agree to participate, you can withdraw from participation at any time during the project without comment or penalty. Your decision to participate will in no way impact upon your current or future relationship with QUT (for example your grades, employment or ongoing clinical care).
In this study, you will be asked to wear contact lenses of different materials and designs. Your participation will involve a series of measurements to determine the shape and optical characteristics of your eyes before and after the wear of contact lenses for up to 8 hours. A drop of local anaesthetic (0.4% Benoxinate) may be used to decrease reflex tearing. Benoxinate is very safe; however, it is possible to scratch the eye without feeling sore because of the anaesthetic effect. You are advised not to rub your eyes for at least 45 minutes after drug instillation. The shape of the front and back surface of your eye (cornea) will be measured using the Medmont and Pentacam instruments, a wavefront sensor will be used to measure the total optics of your eye, and the IOL master instrument will be used to measure t he length of your eye. You will be asked to look into each of the instruments as they take their measurements. These measurements will be carried out a number of times over the course of the study. The Medmont, Pentacam, wavefront sensor and IOL master are all standard clinical instruments that do not touch your eyes and pose no risk to the health of your eyes. Digital images of your eye (cornea) may be taken for our records.
Prior to the experiments, we will conduct a screening examination to determine your suitability for the study and ensure that your eyes are healthy. This will involve routine clinical t ests such as the measurement of your visual acuity (with letter chart) and the examination of the front of the eye with a biomicroscope (slit lamp).
In this study, measurements will be taken a number of times over the 8 hour period. Contact lenses will be removed for the measurements. Each measurement session will take up to 30 minutes. All measurements will be conducted at the School of Optometry at QUT. You will be reimbursed for out of pocket expenses following the study.
Expected benefits lt is expected that this project will not benefit you directly. However, we are interest ed in the changes which occur in the shape and optics of the human eye after the wear of contact lenses. Data collected from this study are expect ed to improve understanding of this area and aid in the better design of contact lenses.
Risks There are no greater risks in this study than those associated with routine eye examinations or the wear of contact lenses. There are minor risks associated with wearing contact lenses at any time. Contact lenses often cause some mild discomfort when they are first inserted into the eyes. Occasionally the lens can irritate your eyes if it is not inserted correctly or if the solutions used to clean the lenses have not been properly rinsed from the lens surface. If the contact lenses cause you undue discomfort we will immed iately remove them from your eyes. An optometrist will then give you advice about the health of your eyes and offer any ongoing eyecare that you may need (at no cost).
The instruments used to measure the shape and optical characteristics of your eye are standard clinical instruments.
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201
Confidentiality The research data we gather from the experiments will not personally identify you by name, or in any way that allows you to be identified. We will use a code, known only to the investigators listed above, to identify your data. Any publication of data arising from
this research will use a code system which does not identify you personally. The data will be stored securely in the School of Optometry.
Consent to Participate We would like to ask you to sign a written consent form (enclosed) to confirm your agreement to participate.
Questions I further information about the project Please contact the researcher team members named above to have any questions answered or if you require further information about the project.
Concerns I complaints regarding the conduct of the project QUT is committed to researcher integrity and the ethical conduct of research projects. However, if you do have any concerns or complaints about the ethical conduct of the project you may contact the QUT Research Ethics Officer on 3138 5123 or email ethicscontact@qut.edu.au. The Research Ethics Officer is not connected with the research project and can facilitate a resolution to your concern in an impartial manner.
Thank you for helping with this research project. Please keep this sheet for your information.
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202
~~--------C_O_N_S_E_N_T_F_O_R_M_f_o_rQ __ UT __ R_ES_E_A_R_C_H_P_R_O_JE_CT ________ ~
"Corneal Topography and Contact Lenses"
Statement of consent
By signing below, you are indicating that you:
• have read and understood the information document regarding this project
• have had any questions answered to your satisfaction
• understand that if you have any additional questions you ea n contact the research team
• understand that you a re free to withdraw at any time, without comment or penalty
• understand that you can contact the Research Ethics Officer on 3138 51230 or email ethicscontact@qut.ed u.au if you have concerns about the ethical conduct of the project
• agree to participate in the project
• understand that the project may include digital video photography of the eye (cornea)
Name
Signature
Date I I ...................................................................
Please return this sheet to the investigator.
Appendices
203
Event Code 105762
Title SOFT CONTACT LENS WEAR AND REGIONAL CHANGES IN CORNEAL THICKNESS AND TOFOGRAFHY
Author Tyagi, Garima B.S. (Optometry) (Queensland University of Technology)
Coauthor( s) Michael J. Collins (Queensland University of Technology), Scott Read (Queensland University of Technology), Brett Davis (Queensland University of Technology)
Topic Cornea and Contact Lens
Day Fri, Nov 19, 2010
Time 9:00AM-5:00PM
Room Third Floor Foyer
Details Purpose: To examine the effect of short term soft contact lens wear on regional corneal thickness and shape while taking into consideration natural diurnal variations. Method: Four dif ferent types of soft contact lenses were worn by 12 young unadapted subjects, on 4 different days. The lenses were of two different materials (silicone hydrogel or hydrogel), designs (spherical or toric) and powers (-3.00 or -7.00 D). The Pentacam HR system was used to measure corneal thickness and topography before and after 8 hours of lens wear. Additionally, measurements were also carried out on two days without lens wear. Results: A significant diurnal corneal thinning was observed on days when contact lenses were not worn, and this was accounted for when calculating the contact lens induced corneal changes. Significant changes in corneal thickness and curvature were observed following contact lens wear. The greatest magnitude of corneal swelling was seen with the hydrogel toric contact lens in the central (20.3 ± 10.0 microns) and peripheral cornea (24.1 ± 9.1 microns) (p < 0.001) with an obvious regional swelling of the cornea beneath the stabilizing zones. The anterior corneal surface generally showed a slight flattening with lens wear. All contact lenses resulted in central posterior corneal steepening and this was weakly correlated with central (R2 = 0.17, p = 0.03) and peripheral corneal swelling (R2 = 0.27, p = 0.01). Conclusions: The hydrogel soft toric lenses caused an obvious regional corneal swelling under the location of the stabilization zones, the thickest regions of the lenses . However, the magnitude of corneal swelling induced by the silicone hydrogel contact lenses over the 8 hours of wear was typically less than the natural diurnal thinning of the cornea over this same period. These natural diurnal variations in corneal thickness observed from mid-morning to afternoon should therefore be considered when studying contact lens induced corneal swelling.
Attachments Rlename Size Attach Date
Key Words Corneal topography, Corneal anatomy/physiology
Disclosures No Conflicts Exists
Appendices
204
Event Code 105032
Hie CORNEAL TORJGRAPHY AFTffi SHORTTffiM USE OF RGPCONTACT LENSES
Author Tyagi, Garirrra B.S. (Optorretry) (School of Optorretry, Queens~nd Unr-tersity of Technology)
Michael Collins PhD, FAAO (School of Optometry, Queensland Unwersity of Technology), Scott Read PhD Coauthor(s) (School of Optometry, Queens~nd Unr-t erstty of Technology), Brett Davis BAppSc (School of Optorretry ,
Queensland Universtty of Technology)
Topic Cornea and Contact Lens
Day Fri, Nov 19, 2010
Time 9:00AM-5:00PM
Room Third Floor Foyer
Details Purpose: The purpose of this study was to investigate the changes in anterior corneal topography after short term use of rigid gas perrreable (RGP) contact lenses and to ex a nine the correlation between these changes with the fluorescein pattern under the lens. Method: Anterior corneal topography was measured ( 4 maps) before and after 8 hours of RGP lens wear using the Medmont E300 video keratoscope system on a group of 12 young heatthy unadapted subjects. Custom software was used to calculate the statistical significance of the changes in corneal topography and corneal w avefront aberrations. The fluorescein pattern observed under the lens was captured with digital sltt-~mp photography through a Wratten 128 fitter. The klcation of the contact lens w tth respect to the limbus was derwed from these images, while the location of the corneal topography map w tth respect to the limbus was also calculated. Results: The topography tangential curvature maps following lens wear demonstrated a ring of mid peripheral f~ttening which was statistically significant for all subjects. A highly significant posttive correlation (R2 = 0.8, p = 0.003) was observed between the location of minimal fluorescein pooling beneath the RGP lens ( i.e. midperipheral bearing) and the location of corneal flattening after lens wear. Some higher order corneal aberrations (Zernike terms up to 4th order) showed significant changes (p s 0.05) after RGP contact lens wear. Conclusions: A region of slight but significant corneal f~ttening was noticed in the mid peripheral region of the cornea after 8 hrs use of RGPcontact lenses. These corneal topographical changes correlated w ith the fluorescein pattern seen with the contact lens on eye. Small but signif ic ant changes also occurred in some of the higher order corneal aberrations after the use of RGP lenses.
Attachments Rlename Size Attach Date
Key Words Corneal topography
Disclosures No Conf licts Exists
Appendices
205
Abstract #7
Corneal optical changes with short term contact lens wear
Tyagi G.', Collins M1, Read S.1 , Oavis 8 1
1Vision Domain, Institute of Health and Biomedical Innovation, School of Optometry, Queensland University of Technology, Brisbane, OLD
Introduction: Anecdotal reports suggest that corneal optics can be influenced by contact lens wear, however the relative influence of different contact lens parameters on these corneal changes is unclear. This study aimed to examine the effect of different types of contact lens materials, designs and powers on corneal optics after a short period of wear, using the Medmont videokeratoscope.
Methods: Four different types of soft contact lenses and one spherical RGP lens were worn by 12 young unadapted subjects, on 5 different days. The soft lenses were of two different materials (silicone-hydrogel or hydrogel), designs (spherical or toric) and powers (-3.00 or - 7.00 D). The Medmont E300 videokeratoscope was used to measure corneal optics before and after 8 hours of lens wear. The change in best fit sphere-cylinder from the corneal refractive power and higher-order corneal wavefront aberrations were calculated for 4 mm (photopic) and 6 mm (mesopic) corneal diameters.
Results: Significant changes in corneal optics were observed following eight hours of spherical silicone-hydrogel (-7) lens wear. A significant hyperopic shift in anterior corneal best fit sphere (M) forthe4 mm (-0.20 ±0.14 D) and 6 mm (-0.16 ± 0.12 D) analyses were found
(both, p < 0.008). Small but significant changes in corneal vertical coma ( c;') were also
found with this lens (p < 0.05). The changes in corneal best fit sphere with the toric siliconehydrogel lens also approached significance for the 6 mm corneal diameter. The other lenses caused no significant changes in corneal sphere-cylinder or higher-<>rder aberrations.
Conclusions: This study compared the changes in corneal optics caused by a variety of contact lenses and found that the high power silicone-hydrogel lens caused the largest changes.
'Real World Implications': When contact lenses are worn they can cause subtle, temporary changes in the shape and thickness of the underlying cornea that result in changes in corneal optical power. For one of the lens types we examined in this study, these changes were statistically and clinically significant after only 8 hours wear. An optometrist can use this information to modify the lens power to provide optimal vision.
~ ~n~u~fi Healtl1 and Biomedical Innovation