Principles and practice of corneal astigmatic surgery
-
Upload
laurence-sullivan -
Category
Health & Medicine
-
view
291 -
download
1
description
Transcript of Principles and practice of corneal astigmatic surgery
Dr Laurie Sullivan FRANZCOMelbourne, AustraliaCorneal Clinic, RVEEH, East MelbourneBayside Eye SpecialistsLasersight
To you, for your interest To Gerard and the University of Sydney for
asking me to contribute
There is a commentary for this presentation so I hope you have your sound working
Dr Laurie Sullivan 2009 [email protected] 2
No financial interest, apart from the odd free travel to the B&L Technolas user group meeting
I use the B&L Technolas Z 100 excimer laser, and the Amadeus mechanical microkeratome, and Intralase femtosecond laser keratome
Dr Laurie Sullivan 2009 [email protected] 3
I understand that by now, you have already covered:
Overview of Refractive Surgery: History, classification, terminology
Anatomy, Physiology, Pharmacology, Pathology ,Corneal Wound Healing
Corneal Imaging: Topography, Orbscan, Pentacam, Aberrometry Ocular and Systemic Disease relevant to Refractive Surgery Patient Assessment and Evaluation Principles of Laser Technology including Excimer, Femtosecond,
Thermal and Conductive Keratoplasty Principles and Practice of LASIK Principles and Practice of PRK/LASEK
So I am going to assume a fair bit or prior knowledge on your part.
Dr Laurie Sullivan 2009 [email protected] 4
= Differing refractive powers at differing axes (meridia)
Due to non-spherical (often “toric”) refractive surface(s)
Synonyms: “cylinder”, “toric”
Dr Laurie Sullivan 2009 [email protected] 5
Corneal astigmatism◦ Regular or Irregular. Only regular astigmatism is remediable with
refractive surgery. Irregular corneal astigmatism requires rigid contact lens or surgery to regularise the surface.
◦ Symmetrical or non-symmetrical Intraocular (“lenticular”) astigmatism
◦ Refractive astigmatism without corresponding corneal curvatures
Dr Laurie Sullivan 2009 [email protected] 6
Dr Laurie Sullivan 2009 [email protected] 7
Cornea: Manual or automated keratometry Corneal topography (videokeratography)Refractive: Subjective and objective refraction Wavefront analysis
Dr Laurie Sullivan 2009 [email protected] 8
Computerised video-keratography A means of describing, depicting
(“mapping”), and quantifying corneal shape and/or power
Placido disc imaging the basis of many topographers.
Also slit scanning and Scheimpflug imaging in more recent machines.
These later technologies include posterior corneal shape and thickness maps
Dr Laurie Sullivan 2009 [email protected] 9
Warm (redred) colours indicate a relatively steepersteeper curvature or more anterioranterior elevation
Cool (blueblue) colours indicate a relatively flatterflatter curvature or more posteriorposterior elevation
Corneal curvatures are numerically described in dioptres (which are derived mathematically from the radii of curvature which are what is actually measured)
Dr Laurie Sullivan 2009 [email protected] 10
Keratoconus (KCN) and pellucid marginal corneal degeneration (PMCD) will often evidence astigmatism
Ectasia is a risk if these conditions are operated upon
Beware asymmetric corneas, drooping Beware asymmetric corneas, drooping against-the-rule astigmatism (“C sign” or against-the-rule astigmatism (“C sign” or “pinch sign”), and exaggerated posterior “pinch sign”), and exaggerated posterior corneal surface elevationcorneal surface elevation
Dr Laurie Sullivan 2009 [email protected] 11
Dr Laurie Sullivan 2009 [email protected] 12
A means of obtaining a detailed refractive map of the entire optical system through the pupil
Can quantify higher order optical aberrations (especially coma and spherical) as well as sphere and cylinder
Most excimer lasers may use this information to drive the refractive correction, but it should be used only if it corresponds to the patient’s subjective refraction.
Iris registration (imaging) improves the accuracy of the alignment of the laser’s astigmatic correction
Dr Laurie Sullivan 2009 [email protected] 13
Toric (“cylindrical”) spectacle lens Contact lenses
◦ Rigid: imposes a spherical surface over the cornea +/- toric surface of the contact lens itself
◦ Soft: toric contact lens surface(s) Refractive surgery: mainly excimer laser,
intraocular lenses, or corneal incisions.
Dr Laurie Sullivan 2009 [email protected] 14
Dr Laurie Sullivan 2009 [email protected] 15
Corneal options◦ Steepen the flat axis (“remove tissue”; excimer
laser, corneal shrinkage techniques E.g. thermal keratoplasty)
◦ Flatten the steep axis (“add tissue”; astigmatic keratotomy incisions, intracorneal ring segment insertion
Dr Laurie Sullivan 2009 [email protected] 16
Prior to the advent of the excimer laser AK was the main method of correcting astigmatism, often in association with Radial Keratotomy (RK) for correction of myopia
Early excimer lasers were only able to treat spherical error so AK was often used in conjunction
Dr Laurie Sullivan 2009 [email protected] 17
AK nomograms were developed to improve predictability.
Increased patient age, incision length, depth and number are important determinants of increased effect
Incisions closer to the corneal apex have more effect Incisions near the limbus are called Limbal Relaxing
Incisions (LRIs) and are used in cataract surgery “Coupling” describes the observation that an incision
will cause flattening in the axis of the incision and also steepening in the axis at 90° to the incision. This occurs because the cornea is a closed physical system, limited by the limbus
Lindstrom RL, Lindquist TD. Surgical correction of postoperative astigmatism. Cornea 1988; 7:138–148
Dr Laurie Sullivan 2009 [email protected] 19
Dr Laurie Sullivan 2009 [email protected] 20
Dr Laurie Sullivan 2009 [email protected] 21
A guarded micrometer diamond blade is generally set at 95-100% of thinnest local pachymetry in virgin corneas – always cuts less deep than intended
AK in corneal transplants is unpredictable – one should always aim for significant undercorrection
Lindstrom RL, Lindquist TD. Surgical correction of postoperative astigmatism. Cornea 1988; 7:138–148
Price FW, Grene RB, Marks RG, Gonzales JS. Astigmatism reduction clinical trial: a multicenter prospective evaluation of the predictability of arcuate keratotomy; evaluation of surgical nomogram predictability. Arch Ophthalmol 1995; 113:277–282; correction, 577
Dr Laurie Sullivan 2009 [email protected] 22
Astigmatism (D) Incision Type Length (mm) Optical Zone
1.00 One LRI 6.0 At limbus1.00 to 2.00 Two LRIs 6.0 At limbus2.00 to 3.00 Two LRIs 8.0 At limbus>3.00 Two LRIs 8.0 and CRIs as
indicated at 3 months postop
LRI = limbal relaxing incision; CRIs = corneal relaxing incisions
Dr Laurie Sullivan 2009 [email protected] 23
“Always cut on the red” (steep axis) Always plan your surgery before you get to
the operating room, and draw a diagram on the patient’s topographic map for intraoperative reference
Mark the patient’s eye preop at the slit lamp with the eye in the primary position
Dr Laurie Sullivan 2009 [email protected] 24
Dr Laurie Sullivan 2009 [email protected] 25
102° = steep axis
Paired AK incisions planned @ 7mm optical zone. Blade set at 640 microns
Dr Laurie Sullivan 2009 [email protected] 26
Dr Laurie Sullivan 2009 [email protected] 27
Excimer laser (PRK or LASIK) is effective, particularly if there is a spherical refractive error as well
Most lasers can sculpt either plus or minus cylinder, or a mixed correction, depending on the starting refraction
This flexibility allows planning for minimum tissue removal with adequate refractive effect
Dr Laurie Sullivan 2009 [email protected] 28
Most surgeons are happy treating 4 or 5 dioptres of regular cylinder, more in corneal transplants
Regression of effect may occur with astigmatic corrections, just as with spherical corrections
Epithelial hyperplasia and subepithelial haze are the main causes of regression
Many surgeons use Mitomycin C in PRK to minimise regression
Dr Laurie Sullivan 2009 [email protected] 29
This is absolutely critical for optimum results Cornea or limbal conjunctiva can be marked
at the slit lamp with 25g needle and gentian violet ink
Iris registration images (in some laser platforms) enable the most accurate alignment
Dr Laurie Sullivan 2009 [email protected] 30
Higher astigmatic corrections may be required
Retreatments are more common Astigmatic keratotomy is unpredictable PRK seems to get better results than LASIK
(Lawless M, unpublished data 2008)
Dr Laurie Sullivan 2009 [email protected] 32
Cytotoxic, crosslinks DNA Kills keratocytes, amongst other cells
(which later repopulate the stroma over 6-12 months)
Prevents haze formation after PRK Common dose is 0.02% soaked on a sponge
and applied to the stroma for 10-20 seconds
Dr Laurie Sullivan 2009 [email protected] 33
Intraocular option◦ Insertion of toric intraocular lens, either phakic or
pseudophakic Use nomograms provided by lens
manufacturers
Dr Laurie Sullivan 2009 [email protected] 34
Toric Visian phakic IOL for myopic astigmatism
The Visian TICL* is available for patients with myopia between -4.0 and -20.0 and astigmatism of 1D to 4D.
Posterior chamber insertion, anterior to crystalline lens
3.0 mm ACD required (crystalline lens growth throughout life)
1-2% rate of anterior cortical cataract formation
Eyes treated with LASIK on average have three times more spherical aberration and two times more coma than the Visian ICL eyes
*Sarver EJ, Sanders DR, Vukich, JA. Image quality in myopic eyes corrected with laser in situ keratomileusis and phakic intraocular lens. J Refract Surg. 2003;19(4):397-404.
Dr Laurie Sullivan 2009 [email protected] 35
Relevant because of the increasing use of clear lens extraction (“refractive lens exchange”) for treating high hyperopic refractive error
Dr Laurie Sullivan 2009 [email protected] 36
Dr Laurie Sullivan 2009 [email protected] 37
Steep axis of postoperative corneal astigmatism
Alcon Toric IOL calculation Sheet (online)
Recommends power & axis of placement Considers surgically-induced astigmatism Again, place the IOL marks on the steep (red) axis
Dr Laurie Sullivan 2009 [email protected] 38
Ocular astigmatism of 0.75 D or more will decrease VA.
Most surgeons will treat the refractive astigmatism rather than the corneal astigmatism (unless lens extraction is part of the surgery).
Patients often dislike having their astigmatism over-corrected (axis reversal), or having a significant axis change.
Small amounts of astigmatism (<= 0.5D) may improve depth of focus (and reading ability) in presbyopes and pseudophakes.
Dr Laurie Sullivan 2009 [email protected] 39
Any further questions can be directed to the email address below
Dr Laurie Sullivan 2009 [email protected] 40