Soft contact lens Soft contact lens 345345

History of contact lenses (CL)History of contact lenses (CL)

- In 1508, Leonardo da Vinci sketched the first - In 1508, Leonardo da Vinci sketched the first forms of new refracted surface on the cornea.forms of new refracted surface on the cornea.

- He used the example of a very large glass - He used the example of a very large glass bowel filled with water; immersion of the bowel filled with water; immersion of the eyes in water theoretically corrected vision eyes in water theoretically corrected vision (fig.1).(fig.1).

Figure1. A hollow glass semi-spheroid filled with water

H2O

- In 1636, Descartes suggested applying a tube - In 1636, Descartes suggested applying a tube full of water directly to the eye to correct a full of water directly to the eye to correct a refractive error.refractive error.

Figure2. Optical apparatus described by DescartesFigure2. Optical apparatus described by Descartes

H2O

In 1887, Adolf Fick was apparently the first to successfully fit contact lenses, which were made from brown glass

- In 1887, Dr. Fick a physician in Zurich. He - In 1887, Dr. Fick a physician in Zurich. He described the first contact lens with described the first contact lens with refractive power known to have been refractive power known to have been worn.worn.

- The contact lens was made by A.Muller a - The contact lens was made by A.Muller a manufacturer of artificial eyes.manufacturer of artificial eyes.

- Early contact lenses were crude disk of ground - Early contact lenses were crude disk of ground or blown glass and made spherical although or blown glass and made spherical although the cornea is not.the cornea is not.

- - Theses lenses because the developer had Theses lenses because the developer had little knowledge of the metabolic need and little knowledge of the metabolic need and physiology of the cornea.physiology of the cornea.

Did not conform to the shape of the cornea.Did not conform to the shape of the cornea. Causes corneal abrasions.Causes corneal abrasions. Wearing time short and frequently painfulWearing time short and frequently painful..

- This led to the development of a larger lenses - This led to the development of a larger lenses that rested on the sclera (1888 to 1938) and that rested on the sclera (1888 to 1938) and provided a clearance between the lens and the provided a clearance between the lens and the cornea (fig.4). cornea (fig.4).

- However, the edge of the first plastic lenses - However, the edge of the first plastic lenses formed a seal with the sclera, trapping a pool formed a seal with the sclera, trapping a pool of bathing medium under the lens and of bathing medium under the lens and precluding vital metabolic exchange and poor precluding vital metabolic exchange and poor tolerance.tolerance.

The proposed therapeutic uses for contact lenses is to protect and reshape the optical properties of irregular cornea in disease such as keratoconuse

- It become clear that corneal shape is critical importance in CL design, and the cornea obtains the bulk of its oxygen supply from the air and

that the medium of exchange is the tears.

- As result scleral lenses were abandoned and - As result scleral lenses were abandoned and hard contact lenses were redesigned. hard contact lenses were redesigned.

- The newer lenses were smaller, thinner and - The newer lenses were smaller, thinner and more flexible. They ride on the surface of the more flexible. They ride on the surface of the tear film, and each blink of the lids provides a tear film, and each blink of the lids provides a flow of oxygenated tear that supplies the flow of oxygenated tear that supplies the cornea the oxygen. cornea the oxygen.

Tear layer

The cornea

- Even these early suggestions the history of CL - Even these early suggestions the history of CL did not begin until the 19th century.did not begin until the 19th century.

- In 1964 Dr. Wichterle in Czechoslovakia - In 1964 Dr. Wichterle in Czechoslovakia introduced soft hydrophilic plastic contact introduced soft hydrophilic plastic contact lenses. lenses.

- These lenses had the advantage of comfort and - These lenses had the advantage of comfort and permeability by water and oxygen.permeability by water and oxygen.

- The main potential in these lens lay in the - The main potential in these lens lay in the ability of mass production methods that would ability of mass production methods that would bring down the manufacturing costs.bring down the manufacturing costs.

- In 1966 Baush & Lomb introduced the spin-- In 1966 Baush & Lomb introduced the spin-cast soft CL on an experimental basiscast soft CL on an experimental basis

- In 1967, in the USA the first lathe-cut lens was - In 1967, in the USA the first lathe-cut lens was seen.seen.

Contact lens materialsContact lens materials

- Contact lenses can be classified according - Contact lenses can be classified according to their material as to their material as hard or or soft contact contact lenses. Or according to lenses. Or according to hydrophobic (non-loving water) materials or (non-loving water) materials or hydrophilic (loving water) materials.(loving water) materials.

In general these materials should be In general these materials should be Stable.Stable. Clear. Clear. Nontoxic. Nontoxic. Non-allergic. Non-allergic. Optically desirable. Optically desirable.

Hard contact lenses (HCL)Hard contact lenses (HCL)

Polymethylmethacrylate (PMMA)Polymethylmethacrylate (PMMA)

- It is organic plastic compounds stable at room - It is organic plastic compounds stable at room temperature and water content 0.5%.temperature and water content 0.5%.

- It is easily to work with and has excellent - It is easily to work with and has excellent optical clearly, durability, stability, lack of optical clearly, durability, stability, lack of toxicity, resistance to deposit formation. toxicity, resistance to deposit formation.

--The refractive indexThe refractive index 1.48-1.50. 1.48-1.50.

- - DisadvantageDisadvantage lack of oxygen permeability. lack of oxygen permeability.

- It is used to produce hard contact lens.- It is used to produce hard contact lens.

Cellulose acetate butyrate (CAB)Cellulose acetate butyrate (CAB)

- The first rigid gas permeable lenses (RGP) - The first rigid gas permeable lenses (RGP) were made from CAB.were made from CAB.

- CAB has slightly better oxygen permeability - CAB has slightly better oxygen permeability than PMMA.than PMMA.

- It is strong, durable.- It is strong, durable.

- - DisadvantageDisadvantage prone to warpage. prone to warpage.

SiliconeSilicone

- It is a polymer of dimethyl-silicone, permeable - It is a polymer of dimethyl-silicone, permeable to oxygen and glucose. to oxygen and glucose.

- It refractive index 1.43. - It refractive index 1.43.

- There are two types of Silicone contact lenses:-- There are two types of Silicone contact lenses:-

a- a- Silicone rubber lensesSilicone rubber lenses their stiffness and their stiffness and rigidity are intermediate between typical rigidity are intermediate between typical rigidrigid gas permeable RGP and they gas permeable RGP and they hydrogelshydrogels. .

b- b- Silicone resin lensesSilicone resin lenses are in hard state the are in hard state the stiffness to typical rigid gas permeable stiffness to typical rigid gas permeable materials.materials.

Soft contact lenses (SCL)Soft contact lenses (SCL) Hydroxyethlmethacrylate (HEMA)Hydroxyethlmethacrylate (HEMA)

- Soft or hydrophilic contact lenses are - Soft or hydrophilic contact lenses are characterized by the ability to absorb water, characterized by the ability to absorb water, elasticity and flexibility. elasticity and flexibility.

- HEMA is hydrophilic because it contains a - HEMA is hydrophilic because it contains a free Hydroxyl group that bounds with water. free Hydroxyl group that bounds with water.

- The water content range from 38% to 60%.- The water content range from 38% to 60%.

- Its refractive index 1.43. - Its refractive index 1.43.

- Example Bausch&Lomb (Soflens)- Example Bausch&Lomb (Soflens)

Oxygen transmissionOxygen transmission

The passage of oxygen molecules and certain The passage of oxygen molecules and certain other ions and molecules through a contact other ions and molecules through a contact lens is very important in maintaining normal lens is very important in maintaining normal corneal physiology.corneal physiology.

- The passage of oxygen is one of the most - The passage of oxygen is one of the most important aspects of a contact lens important aspects of a contact lens materialmaterial, , and much attention is directed to this topic by and much attention is directed to this topic by contact lens practitioners and researchers.contact lens practitioners and researchers.

The cornea obtain most of oxygen from the tear The cornea obtain most of oxygen from the tear film. The tear film supply the cornea with film. The tear film supply the cornea with oxygen from the atmosphere when the eyes are oxygen from the atmosphere when the eyes are open.open.

--The cornea has no blood vessels, the oxygen supply necessary for normal metabolism.

O2

O2

During sleep, the eyelids block oxygen from During sleep, the eyelids block oxygen from the atmosphere, and most of the oxygen in the the atmosphere, and most of the oxygen in the tears diffuses from the blood vessels of the tears diffuses from the blood vessels of the limbuslimbus and the and the palpebral conjunctivapalpebral conjunctiva. This. This reduces the amount of oxygen in the tear film reduces the amount of oxygen in the tear film to approximately one third.to approximately one third.

Fig.Fig.

O2

O2

Limbal capillaries

Palpebral conjunctival blood vesselsEyelid

- All contact lenses act as a barrier between the - All contact lenses act as a barrier between the cornea and its oxygen supply. cornea and its oxygen supply.

- The oxygen is able to reach the cornea in two - The oxygen is able to reach the cornea in two different ways:different ways:

In the form of oxygen dissolved in the tears In the form of oxygen dissolved in the tears being being pumpedpumped behind the lens when the lens behind the lens when the lens moves upon blinking, andmoves upon blinking, and

By By diffusingdiffusing directly through the lens directly through the lens material. material.

Fig.Fig. The pumping mechanisum The pumping mechanisum

O2O2

Tear exchange

Contact lens

- Tear exchange not only provides oxygen and - Tear exchange not only provides oxygen and other nutrients to the cornea, but also removes other nutrients to the cornea, but also removes waste products (such as carbon dioxide and waste products (such as carbon dioxide and lactic acid) and dead epithelial cells.lactic acid) and dead epithelial cells.

Fig. 11Fig. 11

Fig.Fig. The diffusion mechanism The diffusion mechanism

O2

The cornea

The contact lens

The pores

- Tear pumping is the major source of corneal - Tear pumping is the major source of corneal oxygenation with PMMA lenses, since these oxygenation with PMMA lenses, since these lenses have almost no oxygen permeability. lenses have almost no oxygen permeability.

- The tear pump alone is insufficient to provide - The tear pump alone is insufficient to provide adequate amounts of oxygen to the cornea.adequate amounts of oxygen to the cornea.

- The PMMA lenses cause unacceptable levels - The PMMA lenses cause unacceptable levels of corneal hypoxia (lack of oxygen; even in of corneal hypoxia (lack of oxygen; even in the presence of an active tear pump.the presence of an active tear pump.

- Diffusion significant amounts of oxygen - Diffusion significant amounts of oxygen directly pass through the lens that is necessary directly pass through the lens that is necessary to provide an adequate oxygen level for to provide an adequate oxygen level for normal cornea metabolism.normal cornea metabolism.

MESUREMENT OF OXYGEN TRANSMISSIONMESUREMENT OF OXYGEN TRANSMISSION

- - Direct diffusionDirect diffusion is the major source of oxygen is the major source of oxygen transmission with soft lenses, transmission with soft lenses,

- it is very important to measure this parameter.- it is very important to measure this parameter.

Oxygen PermeabilityOxygen Permeability

-- PermeabilityPermeability is the is the degreedegree to which a substance to which a substance is able to pass through a membrane other is able to pass through a membrane other maternal. maternal.

- - DiffusionDiffusion is the is the processprocess by which molecules by which molecules pass through a material (such as a contact pass through a material (such as a contact lens); the direction of movement is always lens); the direction of movement is always from the area of higher concentration to the from the area of higher concentration to the area of lower concentration.area of lower concentration.

- Permeability is natural function of the - Permeability is natural function of the molecular composition of the molecular composition of the material. material.

- Permeability is affected by - Permeability is affected by concentration, temperature, pressure, concentration, temperature, pressure, and barrier effects. and barrier effects.

- The permeability of a material is - The permeability of a material is expressed as a expressed as a permeability permeability coefficientcoefficient, denoted Dk. , denoted Dk.

- - The diffusion coefficientThe diffusion coefficient (D) (D) is the speed with is the speed with which gas molecules travel (diffuse) through which gas molecules travel (diffuse) through the material (Figure 15).the material (Figure 15).

- - The solubility coefficient (k)The solubility coefficient (k) defines how defines how much gas can be dissolved in a unit volume of much gas can be dissolved in a unit volume of the material at a specified pressure (Figure the material at a specified pressure (Figure 16).16).

- In order for oxygen to pass through a In order for oxygen to pass through a contact lens material, the molecules contact lens material, the molecules must first dissolve into the material must first dissolve into the material and then travel through it.and then travel through it.

- Permeability is the product of the - Permeability is the product of the diffusion coefficientdiffusion coefficient ( D) and the ( D) and the solubility coefficientsolubility coefficient (k). (k).

- The Dk value is specified in standard - The Dk value is specified in standard units. The actual testing conditions units. The actual testing conditions may vary, but the results must be may vary, but the results must be converted to the standard Dk units.converted to the standard Dk units.

- A typical Dk value, expressed in its - A typical Dk value, expressed in its

standard units: Dk = 8.9 x 10‾¹¹ standard units: Dk = 8.9 x 10‾¹¹ (cm²/sec)(mlO2 / mL x mm Hg) @ (cm²/sec)(mlO2 / mL x mm Hg) @ 25°C 25°C

- The temperature of the testing The temperature of the testing conditions should always be noted conditions should always be noted because Dk increases with increasing because Dk increases with increasing temperature.temperature.

- Because increasing temperature - Because increasing temperature increases the energy of the gas increases the energy of the gas molecules, causing them to travel at a molecules, causing them to travel at a faster rate through the material.faster rate through the material.

- The - The oxygen permeability coefficientoxygen permeability coefficient (the Dk value) of a contact lens (the Dk value) of a contact lens material is an inherent characteristic material is an inherent characteristic of the material, regardless of its of the material, regardless of its thickness. As a rule, Dk is a constant thickness. As a rule, Dk is a constant for a given lens material.for a given lens material.

Oxygen TransmissibilityOxygen Transmissibility

- The Dk value of a material is not how much - The Dk value of a material is not how much oxygen will actually pass through a given oxygen will actually pass through a given contact lens.contact lens.

- The - The actual rateactual rate at which oxygen will pass at which oxygen will pass through a specific contact lens of a given through a specific contact lens of a given thicknessthickness is called its oxygen transmissibility, is called its oxygen transmissibility, denoted Dk/L. denoted Dk/L.

- To calculate the oxygen - To calculate the oxygen transmissibility of a given contact transmissibility of a given contact lens, the Dk value for the material is lens, the Dk value for the material is divided by the lens thickness, denoted divided by the lens thickness, denoted L.L.

- Lens thickness is expressed in - Lens thickness is expressed in centimeterscentimeters, so care must be taken to , so care must be taken to convert lens thickness (which is convert lens thickness (which is typically expressed millimeters) to the typically expressed millimeters) to the proper units.proper units.

- The lens thickness chosen to - The lens thickness chosen to calculate Dk/L is usually the center calculate Dk/L is usually the center thickness of a -3.00 D lens, as this is thickness of a -3.00 D lens, as this is typically the midrange power of the typically the midrange power of the minus lens range for many minus lens range for many manufacturers; +3.00 lenses are manufacturers; +3.00 lenses are typically used as the midrange of plus typically used as the midrange of plus lenses.lenses.

- The lens thickness chosen to calculate - The lens thickness chosen to calculate Dk/L is usually the center thickness of a Dk/L is usually the center thickness of a -3.00 D lens, as this is typically the -3.00 D lens, as this is typically the midrange power of the minus lens range midrange power of the minus lens range for many manufacturers; +3.00 lenses are for many manufacturers; +3.00 lenses are typically used as the midrange of plus typically used as the midrange of plus lenses.lenses.

- - It is important to remember that most It is important to remember that most published Dk/L values represent only published Dk/L values represent only -3.00 D lenses. -3.00 D lenses.

- It is significant that as lens thickness - It is significant that as lens thickness increases, the oxygen transmissibility increases, the oxygen transmissibility decreases. This means that plus lenses decreases. This means that plus lenses (which are thickest at the center of the (which are thickest at the center of the lens) will have lower calculated oxygen lens) will have lower calculated oxygen transmissibilities than minus lenses transmissibilities than minus lenses (which are thinnest at the center of the (which are thinnest at the center of the lens) of the same material.lens) of the same material.

- Dk is a function of - Dk is a function of water content in hydrogelwater content in hydrogel lenses. As a general rule, this is a linear lenses. As a general rule, this is a linear function with Dk increasing at the same rate as function with Dk increasing at the same rate as water content.water content.

Figure 19. Figure 19.

- Although lenses with higher water content - Although lenses with higher water content typically have higher Dk values, they often typically have higher Dk values, they often must be made thicker than lower water must be made thicker than lower water content lenses for several reasons:content lenses for several reasons:

They dry out, or dehydrate, more rapidly in They dry out, or dehydrate, more rapidly in

thin designs, leading to corneal drying which thin designs, leading to corneal drying which is observed as corneal desiccation staining. is observed as corneal desiccation staining.

High water lenses are generally more fragile High water lenses are generally more fragile m thin designsm thin designs..

- - The thicker designs of high water lenses often The thicker designs of high water lenses often result in Dk/L values that are similar to thinner result in Dk/L values that are similar to thinner

lenses with lower water content. Table 6 lenses with lower water content. Table 6

There are 2 main types of water content There are 2 main types of water content materials in soft CLsmaterials in soft CLs

- Low water content materials (Bausch & Lomb, - Low water content materials (Bausch & Lomb, water content 38.6%, trade-name lens Optima water content 38.6%, trade-name lens Optima 38)38)

- High water content materials (Bausch & Lomb, - High water content materials (Bausch & Lomb, water content 70%, trade-name lens B&L 70 water content 70%, trade-name lens B&L 70 Minus)Minus)

Contact Lens OpticsContact Lens Optics

- The general principle of correction of - The general principle of correction of refractive errors with contact lenses is refractive errors with contact lenses is substituting a new refractive surface (contact substituting a new refractive surface (contact lens) for the old surface (cornea). lens) for the old surface (cornea).

- The new surface is uniform, with a different - The new surface is uniform, with a different index of refraction and anterior radius index of refraction and anterior radius curvature. This substitutes for the cornea curvature. This substitutes for the cornea which may be irregular.which may be irregular.

RadiusRadiusThe power of the eye is dependent upon The power of the eye is dependent upon - The radius (r) of curvature of the cornea and The radius (r) of curvature of the cornea and

lenslens- The index of refraction (n)- The index of refraction (n)- The length of the eye.- The length of the eye.

The dioptric value of each surface can be The dioptric value of each surface can be calculated with the formulacalculated with the formulaD = n2 – n / rD = n2 – n / rD = dioptric powerD = dioptric power;;n = index of refraction of first mediumn = index of refraction of first medium ; ;

n2= index of refraction of second mediumn2= index of refraction of second medium..

Air =n1

The cornea

The lens

Light

The power of the typical anterior corneal surfaceThe power of the typical anterior corneal surface D = 1.376 – 1.000 / 0.007D = 1.376 – 1.000 / 0.007D = 0.376 / 0.007D = 0.376 / 0.007D = + 48.83 DD = + 48.83 D

The power of the typical posterior corneal surfaceThe power of the typical posterior corneal surface D = 1.336 – 1.376 / 0.0068D = 1.336 – 1.376 / 0.0068D = 0.040 / 0.0068D = 0.040 / 0.0068

DD= =

The total corneal power in round numbers isThe total corneal power in round numbers is + +48.8348.83 - - 5.885.88 + = + = 42.9042.90 DD

- -The typical anterior human lens surface powerThe typical anterior human lens surface power D = 1.41– 1.336 / 0.010D = 1.41– 1.336 / 0.010D = 0.074 / 0.0010D = 0.074 / 0.0010D = + 7.40 DD = + 7.40 D

- -The typical posterior human lens surface powerThe typical posterior human lens surface power D = 1.376 – 1.000 / 0.0006D = 1.376 – 1.000 / 0.0006D = - 0.0074 / 0.0006D = - 0.0074 / 0.0006D = + 12.33 DD = + 12.33 D

- -The total corneal power in round numbers isThe total corneal power in round numbers is + +7.407.40 + + 12.3312.33 + = + = 19.7019.70 DD

The total power of the eye is arrived at by use of The total power of the eye is arrived at by use of the formula for combination of lensesthe formula for combination of lenses..

If the total power of the eyes s 58.00 diopters, If the total power of the eyes s 58.00 diopters, then the total length isthen the total length is

F = 1/DF = 1/D

f = 1/58.00f = 1/58.00

f = 0,017 = 17 mmf = 0,017 = 17 mm

Where f = focal length; D = dioptric powerWhere f = focal length; D = dioptric power..

Index of RefractionIndex of RefractionIf the radius (r ) of the refractive surface and length If the radius (r ) of the refractive surface and length of the eye are constant, then the variations of the of the eye are constant, then the variations of the index of refraction (n) (Table 2.4) will change the index of refraction (n) (Table 2.4) will change the power of the refracting surface. Using the power of the refracting surface. Using the formulaformulaD = nD = n22 – n / r – n / rassigning a value to r of r = 7.50 mm, then if n is assigning a value to r of r = 7.50 mm, then if n is given, the amount of change dioptric power can given, the amount of change dioptric power can be calculatedbe calculated..

For example, if nFor example, if n2=2= 1.33, then 1.33, thenD = 1.33 – 1.00/0.0075D = 1.33 – 1.00/0.0075D = 0.33/0.0075D = 0.33/0.0075D = 44.0D = 44.0

- AS the index of refraction (n) increases, the - AS the index of refraction (n) increases, the refractive power increases (Table 2.5).refractive power increases (Table 2.5).

Radius of CurvatureRadius of Curvature

The power of the refractive surface is dependent The power of the refractive surface is dependent upon the radius of curvature ( r) and the index of upon the radius of curvature ( r) and the index of refraction (n). refraction (n).

If n is kept constant and r is changed, the power If n is kept constant and r is changed, the power will also changewill also change

If D = nIf D = n22 – n / r, – n / r,

n = air n = air nn22 = PMMA contact lens (n = 1.49) = PMMA contact lens (n = 1.49) then the effect of changes in r can be then the effect of changes in r can be

calculated. calculated. If r = 7.50If r = 7.50 D = 1.49 – l-00/0-0075 D = 1.49 – l-00/0-0075 D = 0.49/0.0075D = 0.49/0.0075 D = 65.333 dioptersD = 65.333 diopters The smaller the radius or the steeper the The smaller the radius or the steeper the

refractive surface, the greater the refractive refractive surface, the greater the refractive power of the lens (Table 2.b).power of the lens (Table 2.b).

Vertex DistanceVertex Distance The power of a lens is the reciprocal of the focal The power of a lens is the reciprocal of the focal

length, the relative or effective power of a length, the relative or effective power of a corrective lens changes with the placement of the corrective lens changes with the placement of the lens or the distance between the lens and the eye.lens or the distance between the lens and the eye.

This relationship is expressed by the formulaThis relationship is expressed by the formulaD= 1 /fD= 1 /f

Where D = power in diopters, f = focal length in Where D = power in diopters, f = focal length in meters.meters.

Example. In a +10.00 diopter lens, the focal Example. In a +10.00 diopter lens, the focal length is 10 cm (0.1 m):length is 10 cm (0.1 m):

D = 1/0.1 D= 10D = 1/0.1 D= 10

- The closer a lens comes to the corneal surface - The closer a lens comes to the corneal surface and nodal point of the eye, the greater the plus and nodal point of the eye, the greater the plus power required and the less the minus power power required and the less the minus power needed to correct the refractive error. needed to correct the refractive error.

- These vertex power changes can be arrived at - These vertex power changes can be arrived at in four ways: The formula for the change in in four ways: The formula for the change in vertex power of the lens isvertex power of the lens is

Δ= D²dΔ= D²d

Where Δ = change in power due to vertex Where Δ = change in power due to vertex distance; D = lens power; d = distance lens in distance; D = lens power; d = distance lens in meters.meters.

Example 1. An aphakic spectacle correction of Example 1. An aphakic spectacle correction of +13.00 diopters at 13 mm in from the eye. +13.00 diopters at 13 mm in from the eye. What power should the contact lens beWhat power should the contact lens be??

Δ = 13² x 0.013Δ = 13² x 0.013

Δ = 169 X 0.013Δ = 169 X 0.013

Δ =2.197 dioptersΔ =2.197 diopters

The power of the required contact lens corrected The power of the required contact lens corrected for vertex distance isfor vertex distance is

+13.00 +2.20= +15.20 D+13.00 +2.20= +15.20 D

Example 2. If a -10.00 lens is at 15 lens the power Example 2. If a -10.00 lens is at 15 lens the power of the required contact is calculated as follows:of the required contact is calculated as follows:

Δ =D²dΔ =D²d

Δ = -10² X 0.015Δ = -10² X 0.015

Δ = 1.5 DΔ = 1.5 D

The total required power isThe total required power is

-10.00 + 1.50= -8.50 diopters-10.00 + 1.50= -8.50 diopters

- The power and position of the correcting lens must - The power and position of the correcting lens must be such that the focal point of the lens is conjugate be such that the focal point of the lens is conjugate to the focal point of the eye.to the focal point of the eye.

General examinationGeneral examination

1- External examination.1- External examination.

- The external examination can be done with a - The external examination can be done with a penlightpenlight combined with combined with hand magnifierhand magnifier or a or a slit slit lamplamp which is better choice. which is better choice.

- CL is contraindicated if there is any active - CL is contraindicated if there is any active pathology of the eye, e.g. inflammation, injury of pathology of the eye, e.g. inflammation, injury of the cornea, conjunctiva or lid.the cornea, conjunctiva or lid.

1- 1- Procedure for penlight:Procedure for penlight:

- Seat the patient comfortably in room with good - Seat the patient comfortably in room with good light condition.light condition.

- Direct the penlight illumination at the area to be - Direct the penlight illumination at the area to be examined while you look through the magnifier.examined while you look through the magnifier.

- Examine the eyelid skin, lid margin (blepharitis - Examine the eyelid skin, lid margin (blepharitis marginalis), conjunctiva, cornea (scars), sclera, marginalis), conjunctiva, cornea (scars), sclera, anterior chamber and iris. anterior chamber and iris.

- You may need to avert the lids to examine the - You may need to avert the lids to examine the conjunctiva properly for follicles or papillae. conjunctiva properly for follicles or papillae.

- Note any inflammation or injury of the area - Note any inflammation or injury of the area examined.examined.

22 - -Procedure for slit lampProcedure for slit lamp::

- -Seat the patient comfortably at the slit lamp by Seat the patient comfortably at the slit lamp by adjusting the patient seat, the slit lamp height adjusting the patient seat, the slit lamp height

and chine rest or bothand chine rest or both . .

- -Examine all the tissue mentioned above. Examine all the tissue mentioned above. Particularly those directly related to contact lens Particularly those directly related to contact lens

fittingfitting . .

- -Diffuse illumination used to examine the Diffuse illumination used to examine the conjunctiva and the lidsconjunctiva and the lids . .

Direct illumination used to examine the cornea Direct illumination used to examine the cornea and limbusand limbus . .

- - The Patient tear quality and quantity should be The Patient tear quality and quantity should be tested, because wearing CL on dry cornea can tested, because wearing CL on dry cornea can cause poor tear circulation, corneal edema, cause poor tear circulation, corneal edema, blurry vision and burning sensation. blurry vision and burning sensation. Therefore, there are two tests commonly used Therefore, there are two tests commonly used analysis tear.analysis tear.

- Tear quality- Tear quality→→ Tear breakup time (TBUT) Tear breakup time (TBUT) - Tear quantity- Tear quantity→→ Schirmer test Schirmer test

◊ ◊ Tear breakup time (TBUT)Tear breakup time (TBUT)

- Blinking helps in distributing tear over the - Blinking helps in distributing tear over the cornea, immediately after a blink, evaporation cornea, immediately after a blink, evaporation begins and tear film begin to thin. Therefore, begins and tear film begin to thin. Therefore, the tear breakup time is often used as an index the tear breakup time is often used as an index for an abnormal tear formation. for an abnormal tear formation.

- Tear breakup time is the interval time between - Tear breakup time is the interval time between a complete blink and the first randomly a complete blink and the first randomly distributed dry spot. distributed dry spot.

- - If an eye is kept open without blinking for If an eye is kept open without blinking for 15-34 seconds the tear will show dry spot 15-34 seconds the tear will show dry spot areas.areas.

- When fluoresecin applied these dry areas - When fluoresecin applied these dry areas appear black when examined with ultraviolet appear black when examined with ultraviolet light. light.

- Any dry areas occur in less than 10 seconds is - Any dry areas occur in less than 10 seconds is considered a negative factor in patient considered a negative factor in patient selection for CL fitting.selection for CL fitting.

◊ ◊ Schirmer testSchirmer test

- It used to evaluate the rate of tear flow. It - It used to evaluate the rate of tear flow. It provides information on hypo and hyper provides information on hypo and hyper secretion of tears.secretion of tears.

- A special filter paper (5x35mm) is used; this - A special filter paper (5x35mm) is used; this paper has an indentation at the uppers 5mm of it paper has an indentation at the uppers 5mm of it length.length.

- After 5 minutes the paper is removed and the - After 5 minutes the paper is removed and the length moistened by tear is measured with a length moistened by tear is measured with a ruler. ruler.

- Normal tear secretion moistens 10-15mm of the - Normal tear secretion moistens 10-15mm of the strip, yet older patient have less reading. strip, yet older patient have less reading.

- - The The corneal diametercorneal diameter has effect on the has effect on the specification of the CL prescribed especially specification of the CL prescribed especially lens diameterlens diameter. Because the. Because the corneal diameter corneal diameter is is assumed to be equal the diameter of the iris.assumed to be equal the diameter of the iris.

- The actual measurement is made with a P.D. - The actual measurement is made with a P.D. ruler. The pupil diameter can be approximated ruler. The pupil diameter can be approximated by using the iris as reference scale.by using the iris as reference scale.

- For - For older childrenolder children and and adultadult the iris is usually the iris is usually about 12mm in diameter.about 12mm in diameter.

- - The The palpebral Aperturepalpebral Aperture height height is important is important factor in determining corneal contact lens factor in determining corneal contact lens dimension.dimension.

- the - the palpebral Aperture heightpalpebral Aperture height is measured by is measured by instructing the patient to relax and fixating instructing the patient to relax and fixating straight ahead, and measurement should be straight ahead, and measurement should be made of the maximum vertical distance when made of the maximum vertical distance when the lids are separated. the lids are separated.

- This measurement is difficult to obtain since - This measurement is difficult to obtain since the lid aperture is under voluntary control, so the lid aperture is under voluntary control, so patient tend to squint when ruler is placed near patient tend to squint when ruler is placed near their eyes.their eyes.

- - The refractive error of the patient must be The refractive error of the patient must be measured and final prescription is written in measured and final prescription is written in minus cylinder formminus cylinder form for ordering the contact for ordering the contact lens.lens.

- - ThreeThree reading of reading of keratometer keratometer measurement for measurement for the patient is obtained for maximum accuracy, the patient is obtained for maximum accuracy, and then the median value of the three is and then the median value of the three is recorded.recorded.

Contact fitting procedure lensContact fitting procedure lens

1- 1- Selection of lens diameter (Dia)Selection of lens diameter (Dia)- Obtain patient's horizontal visible iris diameter - Obtain patient's horizontal visible iris diameter

(HVID) measurement (HVID) measurement - CL diameter = HVID + (1 to 3mm, average = - CL diameter = HVID + (1 to 3mm, average =

2mm) 2mm) - Increase or decrease lens diameter in - Increase or decrease lens diameter in 0.50mm0.50mm step step

if necessary during evaluation process. if necessary during evaluation process. - Lens must completely cover cornea. - Lens must completely cover cornea. - Most soft CLs are available from diameter of - Most soft CLs are available from diameter of

13.50mm to 15mm13.50mm to 15mm. However, large diameter e.g. . However, large diameter e.g. 15mm tends to tighten on the cornea and may 15mm tends to tighten on the cornea and may result in complication e.g. ulcers or result in complication e.g. ulcers or neoneo-vasculariztion-vasculariztion..

2- 2- Selection of Base curve (BC)Selection of Base curve (BC)

- SCL are usually fitted flatter than the flattest K- SCL are usually fitted flatter than the flattest K

- The flattest K minus 3.00 diopter- The flattest K minus 3.00 diopter

BC = flattest K - 3.00DBC = flattest K - 3.00D

- Convert the- Convert the diameter value to millimeters diameter value to millimeters using a converting table.using a converting table.

- Increase or decrease BC in - Increase or decrease BC in 0.30mm0.30mm steps if steps if necessary. necessary.

- Clinical experience shows that majority of - Clinical experience shows that majority of patient can be fitted with an average or median patient can be fitted with an average or median BC. This is usually is the BC. This is usually is the 8.50-9.00mm8.50-9.00mm range. range.

As a guide and an alternative method of BC As a guide and an alternative method of BC selection. The following table can be usedselection. The following table can be used::

K-reading Soft CL Base Curve

Less < 41.00D

Between 41.00– 45.00 D

Larger > 45.00D

Flat ( >9.00mm)

Medium (8.00-.00mm)

Steep (< 8.00mm)

33 - -Selection of lens powerSelection of lens power - Refraction prescription must be converted to - Refraction prescription must be converted to

minus cylinderminus cylinder from: from:

1- If cylinder in refraction is (less than or equal to) 1- If cylinder in refraction is (less than or equal to) less or equal less or equal ≤≤ 0.50D, power = spherical 0.50D, power = spherical componentcomponent

2 - If cylinder in refraction is 0.750D to 1.00D, the 2 - If cylinder in refraction is 0.750D to 1.00D, the contact lens power = spherical equivalent contact lens power = spherical equivalent ((spherical component + 1/2 Cylspherical component + 1/2 Cyl))

- If overall spherical component in 1 and 2 is - If overall spherical component in 1 and 2 is greater than ± 4.00D, compensate for vertex greater than ± 4.00D, compensate for vertex distance using either method 1 or 2.distance using either method 1 or 2.

Method 1Method 1Fc = Fs / 1- d FsFc = Fs / 1- d FsWhere Fc = power of CL, Fs = power of spectacle Where Fc = power of CL, Fs = power of spectacle

lens (D)lens (D)d = distance between spectacle lens and CL in d = distance between spectacle lens and CL in

meter meter Ref; -5.00 +1.00 x 90 (plus cyl form)Ref; -5.00 +1.00 x 90 (plus cyl form)-4.00 -1.00 x 180 (minus cyl form) -4.00 -1.00 x 180 (minus cyl form) vertex distance = 13mm » 13 / 1000 = 0.013mvertex distance = 13mm » 13 / 1000 = 0.013mFc = 4.5 / 1- (0.013 x 4.5)Fc = 4.5 / 1- (0.013 x 4.5)Fc = 4.5 / 1- 0.0585 Fc = 4.5 / 1- 0.0585 Fc= 4.5 / 0.9415 = 4.249 Fc= 4.5 / 0.9415 = 4.249 ≈≈ 4.25D 4.25D

Method 2Method 2

Add 1/2 of cyl to sphereAdd 1/2 of cyl to sphere , ,

e.g.2e.g.2

Ref; -5.00 + 1.00 x 90 (plus cyl form)Ref; -5.00 + 1.00 x 90 (plus cyl form)

- -4.004.00 - - 1.001.00 x 180 (minus cyl form)x 180 (minus cyl form)

contact lens power = - 4 + - 0.50 = - 4.50Dcontact lens power = - 4 + - 0.50 = - 4.50D

The contact lens power from the table = - 4.25DThe contact lens power from the table = - 4.25D

This is greater than ± 4.00D, so compensate for This is greater than ± 4.00D, so compensate for vertex distance e.g. 13mm (by calculation or vertex distance e.g. 13mm (by calculation or using table)using table)

Types of Soft contact lensesTypes of Soft contact lenses

Soft contact lenses divided into four categories Soft contact lenses divided into four categories

1- Daily ware 1- Daily ware

- These lenses are worn on daily basis for 12 to - These lenses are worn on daily basis for 12 to 14 hours and removed before bedtime for 14 hours and removed before bedtime for cleaning and disinfection. cleaning and disinfection.

- Methods of disinfection are heating, chemical - Methods of disinfection are heating, chemical and oxidation (hydrogen peroxide).and oxidation (hydrogen peroxide).

2- Flexible/Extended ware2- Flexible/Extended ware

- Usually corresponds to patients who wear - Usually corresponds to patients who wear lenses overnight only on an occasional basis lenses overnight only on an occasional basis such as weekends, and should be cleaned and such as weekends, and should be cleaned and disinfection upon removal. disinfection upon removal.

- Most practitioners now recommend lens wear - Most practitioners now recommend lens wear without overnight removal 3 to 7days. without overnight removal 3 to 7days.

- Methods of disinfection involve chemical and - Methods of disinfection involve chemical and oxidative. oxidative.

- These lenses also available in planned - These lenses also available in planned replacement, tinted and toric forms. replacement, tinted and toric forms.

3- Planned replacement3- Planned replacement

- These lenses allow the patient to replace lenses - These lenses allow the patient to replace lenses weekly or biweekly depend on their weekly or biweekly depend on their preferences. preferences.

- For example, disposable lenses are worn for a - For example, disposable lenses are worn for a one- week extended wear period and one- week extended wear period and discarded, therefore, a care regimen is not discarded, therefore, a care regimen is not required. required.

4- 4- TintsTints

These are usually cosmetic Soft contact lensThese are usually cosmetic Soft contact lens

Disinfection Disinfection

- The tear film is vital. Not only does it provide - The tear film is vital. Not only does it provide oxygen exchange as the lens is moved, but it oxygen exchange as the lens is moved, but it also passes also passes lysozymelysozyme, an antibacterial enzyme , an antibacterial enzyme that inhibits bacterial proliferation.that inhibits bacterial proliferation.

- Patients with a tear deficiency are more prone - Patients with a tear deficiency are more prone to infections and often cannot be fit to infections and often cannot be fit comfortably with lenses.comfortably with lenses.

- - Many complications with soft contact lens wear Many complications with soft contact lens wear occur after lenses are successfully fit, occur after lenses are successfully fit, when when patients care and handle their lensespatients care and handle their lenses. .

- Problems arise due to nature of hydrogel lens - Problems arise due to nature of hydrogel lens materials which materials which vulnerable to contamination by vulnerable to contamination by bacteria and fungi. bacteria and fungi.

- Routine soft contact lens care including - Routine soft contact lens care including disinfectiondisinfection and and cleaningcleaning. .

- There are three methods of disinfection used with - There are three methods of disinfection used with SCL: SCL: thermalthermal, , chemicalchemical, and , and oxidativeoxidative, each of , each of these methods has advantages and disadvantages these methods has advantages and disadvantages which will aid the practitioner in selecting the which will aid the practitioner in selecting the care regimen best suited for each patient and care regimen best suited for each patient and lens. lens.

Thermal disinfection Thermal disinfection

- This technique is - This technique is not expensivenot expensive and most and most effectiveeffective system in the system in the short termshort term. .

- The thermal disinfection technique is - The thermal disinfection technique is contraindicated with lenses containing greater contraindicated with lenses containing greater than 55% water. than 55% water.

- Thermal care regimen consists of - Thermal care regimen consists of salinesaline, , surfactant cleanersurfactant cleaner, , enzymatic cleanerenzymatic cleaner and and rewetting rewetting oror lubricating drops lubricating drops. .

After soft contact lens removalAfter soft contact lens removal:- :- 1- The CL should be cleaned with 1- The CL should be cleaned with surfactant surfactant

cleanercleaner. . 2- Stored in a case filled with saline. 2- Stored in a case filled with saline. 3- Enzymatic cleaner should be used weekly.3- Enzymatic cleaner should be used weekly.

The advantages of thermal disinfectionThe advantages of thermal disinfection- Q- Quick (20 min) require very few stepsuick (20 min) require very few steps- Preservative-free solution - Preservative-free solution for patients sensitive to for patients sensitive to

preserved solutions preserved solutions - Effective against all form of bacteria - Effective against all form of bacteria such as such as

pseudomonas and AIDS virus.pseudomonas and AIDS virus.

The disadvantage of thermal disinfectionThe disadvantage of thermal disinfection - The - The heat bakes on the deposits so lens not cleanedheat bakes on the deposits so lens not cleaned- The - The lens life shortenedlens life shortened- Not interchangeable with other care systems- Not interchangeable with other care systems-Complications such as giant papillary conjunctivitis -Complications such as giant papillary conjunctivitis

(GPC), or red eye occurs due to deposited lens(GPC), or red eye occurs due to deposited lens. .

Chemical disinfection Chemical disinfection

- This method consist of a - This method consist of a disinfecting solutiondisinfecting solution that contain that contain preservativespreservatives, , surfactant cleanersurfactant cleaner, , enzymatic cleanerenzymatic cleaner and and rewettingrewetting or or lubricating lubricating dropsdrops, and many of these solutions may be used , and many of these solutions may be used for for rinsingrinsing and and to storto store of the lenses. e of the lenses.

- For example, - For example, ReNu Multi-purposeReNu Multi-purpose solutionsolution can be can be used as a used as a cleanercleaner, , salinesaline and with the and with the enzymatic enzymatic tabletstablets; however disinfection must still be ; however disinfection must still be performed following enzymatic cleaning.performed following enzymatic cleaning.

The advantages of chemical systemsThe advantages of chemical systems- It can be used for all type of SCL.- It can be used for all type of SCL.- Little effect on lens life.- Little effect on lens life.- It remove 90% of a measured amount of - It remove 90% of a measured amount of

bacteria.bacteria.- The solutions number and steps are less make it - The solutions number and steps are less make it

simple and convenient for patient. simple and convenient for patient. The disadvantage of chemical systemsThe disadvantage of chemical systems- The use of - The use of preservativespreservatives such as such as thimerosalthimerosal and and

chlorhexidinechlorhexidine that are toxic to some patient so it that are toxic to some patient so it is more likely that the lens will have to be is more likely that the lens will have to be replaced.replaced.

33 - -Oxidative disinfectionOxidative disinfection

- This technique consists of a 3% hydrogen - This technique consists of a 3% hydrogen peroxide solution, neutralizing (solution, tablet, peroxide solution, neutralizing (solution, tablet, and disc), saline, surfactant cleaner, enzymatic and disc), saline, surfactant cleaner, enzymatic cleaner and rewetting or lubricating drops.cleaner and rewetting or lubricating drops.

- - Hydrogen peroxideHydrogen peroxide - Effective against bacteria - Effective against bacteria - It can be used in a disinfection cycle of 10min - It can be used in a disinfection cycle of 10min

with 10min of neutralization.with 10min of neutralization.- Yet longer exposure time is recommended to be - Yet longer exposure time is recommended to be

effective against fungi.effective against fungi.

Disadvantages this systemDisadvantages this system

- - Large number of solutions and stepsLarge number of solutions and steps

- Storing the lens in hydrogen peroxide for - Storing the lens in hydrogen peroxide for lengthily periods may affect the base curve lengthily periods may affect the base curve radius of the lens, especially with high-water radius of the lens, especially with high-water content lens materials.content lens materials.

- The - The acidity of hydrogen peroxideacidity of hydrogen peroxide could cause could cause mild to moderate punctuate keratitis. To prevent mild to moderate punctuate keratitis. To prevent this there are many methods to neutralizing it.this there are many methods to neutralizing it.

Advantages of this systemAdvantages of this system

- Safe, effective, and preservatives-free.- Safe, effective, and preservatives-free.

Examples of Oxidative disinfectionExamples of Oxidative disinfection

1- 1- Allergan OpticalAllergan Optical has two systems has two systems OxyseptOxysept and and UltraCareUltraCare both are preservatives-free. both are preservatives-free.

A- A- The Oxysept is two step processesThe Oxysept is two step processes

1- The CL is placed in the case containing 1- The CL is placed in the case containing hydrogen peroxide after proper time interval of hydrogen peroxide after proper time interval of disinfection (10min-12hours).disinfection (10min-12hours).

2- Place neutralizing tablet in the case to 2- Place neutralizing tablet in the case to neutralize hydrogen peroxide acidity. neutralize hydrogen peroxide acidity.

B- B- The UltraCare is one step processThe UltraCare is one step process

- The CL is placed in the case containing - The CL is placed in the case containing hydrogen peroxide and place UltraCare hydrogen peroxide and place UltraCare neutralizing tablet at the same time. neutralizing tablet at the same time.

- The UltraCare neutralizing tablet is coated with - The UltraCare neutralizing tablet is coated with a viscosity agent that prevents activation of a viscosity agent that prevents activation of tablet for 20-30mins this allow disinfection with tablet for 20-30mins this allow disinfection with hydrogen peroxide to occur prior to hydrogen peroxide to occur prior to

neutralization.neutralization.

3. 3. Ciba VisionCiba Vision has one system called has one system called AODisc.AODisc.

- The CL is placed in the case containing - The CL is placed in the case containing hydrogen peroxide after proper time interval of hydrogen peroxide after proper time interval of disinfection. disinfection.

- The platinum disc attached to the lens cage - The platinum disc attached to the lens cage begins neutralizing hydrogen peroxide begins neutralizing hydrogen peroxide immediately upon contact when the lens cage is immediately upon contact when the lens cage is placed in the case. placed in the case.

- The disc should be replaced after 3 months of - The disc should be replaced after 3 months of daily use. daily use.

1- 1- Saline solutionSaline solution - It is necessary part of hydrogel CL care, - It is necessary part of hydrogel CL care,

because the hydrogel CL must stay hydrated.because the hydrogel CL must stay hydrated. - Saline is non toxic to eye and sterile used to - Saline is non toxic to eye and sterile used to

rinse the lens from foreign body as well as to rinse the lens from foreign body as well as to dissolve enzyme tablets.dissolve enzyme tablets.

- Distilled water not suitable since it not sterile - Distilled water not suitable since it not sterile and easily contaminated. and easily contaminated.

- Saline solution is not capable of disinfecting the - Saline solution is not capable of disinfecting the lens when used a lonelens when used a lone

- It available in preserved (with thimersol or - It available in preserved (with thimersol or sorbic acid) and unpreserved (e.g. aerosol sorbic acid) and unpreserved (e.g. aerosol saline) solutions. saline) solutions.

2- 2- Surfactant cleanersSurfactant cleaners - It prevents buildup of lens deposition thus it - It prevents buildup of lens deposition thus it

should be used after every lens removal.should be used after every lens removal.- It acts as a soap to remove debris, unbound - It acts as a soap to remove debris, unbound

proteins, lipid deposits and some microbial proteins, lipid deposits and some microbial contamination.contamination.

- The lens placed in the palm of the hand with - The lens placed in the palm of the hand with few drops of the cleaner, the lens rubbed gently few drops of the cleaner, the lens rubbed gently back and forth for 20 to 30 seconds, and then back and forth for 20 to 30 seconds, and then the lens rinsed and soaked in disinfection the lens rinsed and soaked in disinfection solution.solution.

AlconAlcon introduced three generations of cleaners introduced three generations of cleaners for hydrogel CL:-for hydrogel CL:-

- Opticlean (preservative was Thimerosal )- Opticlean (preservative was Thimerosal )

- Opticlean II (preservative was Polyquad )- Opticlean II (preservative was Polyquad )

- Opti-Free Dialy Cleaner (preservative was - Opti-Free Dialy Cleaner (preservative was Polyquad)Polyquad)

Ciba VisionCiba Vision

- Introduce - Introduce Mira FlowMira Flow contain among other contain among other cleaning ingredients, cleaning ingredients, isopropyl alcoholisopropyl alcohol. .

- - Isopropyl alcoholIsopropyl alcohol eliminates the need for a eliminates the need for a preservative because of its broad-spectrum preservative because of its broad-spectrum antimicrobial effects. antimicrobial effects.

- It excellent cleaner especially for patients with - It excellent cleaner especially for patients with the tendency toward lipid deposits, but the lens the tendency toward lipid deposits, but the lens should be rinsed to avoid the risk of parameter should be rinsed to avoid the risk of parameter changes. changes.

3- 3- Enzymatic cleanerEnzymatic cleaner

- It is used once a week to break down peptide - It is used once a week to break down peptide bonds, allowing bonds, allowing proteinprotein to be rubbed off to be rubbed off mechanically. mechanically.

- The proper care sequence when enzyming - The proper care sequence when enzyming hydrogel lenses are cleaning, rinsing, enzymatic hydrogel lenses are cleaning, rinsing, enzymatic cleaning, rinsing, and disinfecting. cleaning, rinsing, and disinfecting.

4- 4- Lens lubricants/ rewettingLens lubricants/ rewetting

- It is optional, but may be beneficial in cases of - It is optional, but may be beneficial in cases of dry eyes, foreign body sensation, irritations and dry eyes, foreign body sensation, irritations and for morning and evening use in extending wear.for morning and evening use in extending wear.

- Lens lubricants used directly in the eye with and - Lens lubricants used directly in the eye with and without the lenses. without the lenses.

- It is not suitable to use ophthalmic medication - It is not suitable to use ophthalmic medication as lubricants because this could cause as lubricants because this could cause discoloration and cause toxic reaction. discoloration and cause toxic reaction.

Fitting evaluation Fitting evaluation Normal fit Normal fit

- Soft lens should be fitted with what is known as - Soft lens should be fitted with what is known as three-point touch.three-point touch.

1- The lens should 1- The lens should parallelparallel the the superiorsuperior and and inferiorinferior sclera as well is the sclera as well is the corneal apexcorneal apex. .

2- When the lens 2- When the lens restsrests only on the only on the superior and superior and inferior sclera inferior sclera and and jumpjump the the corneal apexcorneal apex, the , the lens is lens is too steeptoo steep (Fig. 61). (Fig. 61).

- - If the lens If the lens restsrests on the on the corneal apexcorneal apex and the and the edges stand off from the scleraedges stand off from the sclera, the lens is , the lens is too too flatflat (Fig. 6-3). (Fig. 6-3).

- - All soft lenses, regardless of power, size, or All soft lenses, regardless of power, size, or manufacturer, should be fitted to obtain this manufacturer, should be fitted to obtain this three-point touch (Fig. 6-4). three-point touch (Fig. 6-4).

- - A well-fitted lens will show five basic qualities; A well-fitted lens will show five basic qualities; good centrationgood centration, , adequate movementadequate movement, , stable stable visionvision, , crisp retinoscopic reflexcrisp retinoscopic reflex, , clear clear undistorted keratometry miresundistorted keratometry mires, and , and clear clear endpoint over-refractionendpoint over-refraction..

1- 1- Good centration.Good centration.

- The lens will center itself well easily after - The lens will center itself well easily after insertion in the eye. After the patient blinks, it insertion in the eye. After the patient blinks, it will not show more rim of lens on one side of will not show more rim of lens on one side of the cornea than on the other side. the cornea than on the other side.

- Lens decentration requires refitting with either a - Lens decentration requires refitting with either a steeper base curvesteeper base curve or a or a larger diameterlarger diameter (Fig. 6- (Fig. 6-5). 5).

2- Adequate movement

- The slit lamp is very useful for evaluation of proper movement.

- Fitting should be evaluated while the patient looks straight ahead, upward, and laterally. The patient should be asked to blink under slit lamp observation.

- Evaluation should then be made clinically as to

whether the movement is excessive, negligible, or adequate.

- A standard-thickness lens may show movement of 0.5 to 1 mm on upward gaze after a blink, and it should show no greater movement on lateral gaze.

- If the lens is even with tears and does not move, the person should be switched to a lens with a flatter base curve (Fig. 6-6, A).

-If the lens moves excessively, a lens with a steeper base curve (Fig. 6-7) series or one with a larger diameter should be substituted (Fig. 6-6, C and D)

33 - -Stable visionStable vision . .- When he patient blinks, the vision should remain - When he patient blinks, the vision should remain

equally clear before and during the blink and equally clear before and during the blink and visual acuity should be as sharp as possible (Fig. visual acuity should be as sharp as possible (Fig. 6-8). If trial-set lenses re used for fit evaluation, 6-8). If trial-set lenses re used for fit evaluation, an over-refraction should be performed.an over-refraction should be performed.

- If visual acuity is not adequately sharp after - If visual acuity is not adequately sharp after changing the lenses or holding over low-plus or changing the lenses or holding over low-plus or low minus lenses. It is useful to have the patient low minus lenses. It is useful to have the patient view an astigmatic clock. If some of the clock view an astigmatic clock. If some of the clock lines are significantly blurred, residual lines are significantly blurred, residual astigmatism is present and vision cannot be astigmatism is present and vision cannot be improved soft lenses. improved soft lenses.

- - Variable vision initially may be caused by a Variable vision initially may be caused by a lens that is either lens that is either too loosetoo loose or or too tighttoo tight. If . If the fit is found to be adequate and the the fit is found to be adequate and the patient still complains of fluctuating vision, patient still complains of fluctuating vision, such factors as such factors as drynessdryness of the eye or from of the eye or from the environment, lack of blinking, or excess the environment, lack of blinking, or excess mucus secretions must be considered as mucus secretions must be considered as causative factors.causative factors.

- Normally, blinking may be reduced with - Normally, blinking may be reduced with driving and reading. The patient should be driving and reading. The patient should be warned of soft lens variable vision. It is warned of soft lens variable vision. It is easily reduced by a series of blinks or easily reduced by a series of blinks or artificial tears.artificial tears.

4- 4- Crisp retinoscopic reflex.Crisp retinoscopic reflex.

- As confirmatory evidence of a good fit, the - As confirmatory evidence of a good fit, the retinoscope, streak is flashed in all meridians retinoscope, streak is flashed in all meridians while the patient blinks. When the patient is while the patient blinks. When the patient is adequately fitted, the retinoscopic reflex will be adequately fitted, the retinoscopic reflex will be sharp and crisp as if no lens were in place, both sharp and crisp as if no lens were in place, both before and after blinking (Fig. 6-9, A). before and after blinking (Fig. 6-9, A).

- - If the lens is steep, there will be a spreading of If the lens is steep, there will be a spreading of the streak centrally in the rest position, which the streak centrally in the rest position, which will clear after a blink because of ironing out of will clear after a blink because of ironing out of the apical jump (Fig. 6-9, C).the apical jump (Fig. 6-9, C).

- - If the lens is flat, it may ride low; a position that If the lens is flat, it may ride low; a position that can be detected by the retinoscopic shadow may can be detected by the retinoscopic shadow may be blurry immediately after a blink (Fig. 6-9, be blurry immediately after a blink (Fig. 6-9, B). B).

5- 5- Clear, undistorted keratometry mires.Clear, undistorted keratometry mires.

The mires that are reflected from the keratometer The mires that are reflected from the keratometer while the person is wearing the soft lens will while the person is wearing the soft lens will often indicate if the fit is adequate. With the often indicate if the fit is adequate. With the correct fit, the mires of the keratometer should correct fit, the mires of the keratometer should not be distorted either before or after a blink not be distorted either before or after a blink (Fig. 6-8, A). (Fig. 6-8, A).

- - If the mires are blurred, the patient should blink If the mires are blurred, the patient should blink several times; if the mires are still distorted, the several times; if the mires are still distorted, the lens should be changed (Fig. 6-10, B and C). lens should be changed (Fig. 6-10, B and C).

Characteristic of steep fitting Characteristic of steep fitting

There are characteristics of steep fitting for soft There are characteristics of steep fitting for soft contact lens:contact lens:

- Little or no movement either on blinking or as the - Little or no movement either on blinking or as the eye change fixation.eye change fixation.

- Tight fit is quit comfortable, sometimes more so - Tight fit is quit comfortable, sometimes more so than a correct fit, because a complete immobile than a correct fit, because a complete immobile lens produces the minimum of lid sensation.lens produces the minimum of lid sensation.

- Usually good centration - Usually good centration

- - The slit lamp may show irritation of the The slit lamp may show irritation of the conjunctival or limbal vessels and, with very conjunctival or limbal vessels and, with very light lenses an annular ring of conjunctival light lenses an annular ring of conjunctival compression may be seen this often visible when compression may be seen this often visible when lens removed.lens removed.

- Vision unstable and poor because momentary - Vision unstable and poor because momentary pressure on the eye during blinking occurred with pressure on the eye during blinking occurred with steep fitting jump the corneal apex.steep fitting jump the corneal apex.

- Subjective refraction is difficult with no clearly - Subjective refraction is difficult with no clearly defined end point, and more negative power than defined end point, and more negative power than predicated may be required because of a positive predicated may be required because of a positive liquid lens.liquid lens.

- Retinoscopy and keratometer mires both show - Retinoscopy and keratometer mires both show irregular distortions these mires improve with irregular distortions these mires improve with blinking.blinking.

Characteristic of loss fittingCharacteristic of loss fitting

There are characteristics of loss fitting for soft There are characteristics of loss fitting for soft contact lens: contact lens:

- Easily to diagnose because of poor centration, - Easily to diagnose because of poor centration, greater lens mobility on blinking and excessive greater lens mobility on blinking and excessive lag on lateral eye movements.lag on lateral eye movements.

- Very uncomfortable especially on looking - Very uncomfortable especially on looking upwards, lower lid sensation experienced if the upwards, lower lid sensation experienced if the lens drops lens drops

- Vision and over refraction are variable, but - Vision and over refraction are variable, but nevertheless may still give satisfactory results.nevertheless may still give satisfactory results.

- - The retinoscopy reflex may be clear centrally but The retinoscopy reflex may be clear centrally but with peripheral distorted.with peripheral distorted.

- The keratometry mires change according to lens - The keratometry mires change according to lens movement.movement.

CorrectionCorrection The following steps should be taken to correct a The following steps should be taken to correct a

loose lens:loose lens:- Either changing the base curve by decreasing it by - Either changing the base curve by decreasing it by

0.2 to 0.3 mm 0.2 to 0.3 mm OROR- Increasing the diameter of the lens by 0.5mm up to - Increasing the diameter of the lens by 0.5mm up to

15mm.15mm.The following steps should be taken to correct a light The following steps should be taken to correct a light

lens:lens:- Either changing the base curve by increasing it by - Either changing the base curve by increasing it by

0.2 to 0.3 mm 0.2 to 0.3 mm OROR - Decreasing the diameter of the lens by 0.5mm.- Decreasing the diameter of the lens by 0.5mm.