fitting rgp lenses

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In the name of God

RGP Lens Design, Fitting and Evaluation

By : hossein alimirzaieoptometrist

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Methods to correct vision:Methods to correct vision:

•Optical:Optical: spectacles

Contact lenses

• Refractive surgery

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Optical methods:Optical methods:

• Spectacles:Spectacles:

High index lenses

Light weight frames

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Optical methods:Optical methods:

• Rigid gas permeableRigid gas permeable

• Soft contact lenses:Soft contact lenses:

conventional Frequent replacement

Contact lenses:

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Disadvantages of Soft Contacts Disadvantages of Soft Contacts vs.. Hardvs.. Hard

• Less durable than hard lensesLess durable than hard lenses

• Unable to correct astigmatism Unable to correct astigmatism completely in some casescompletely in some cases

• Tendency for deposit formationTendency for deposit formation

• Cannot be modified as hard lenses canCannot be modified as hard lenses can

• Requires significant maintenanceRequires significant maintenance

• Lenses can cause decreased vision when Lenses can cause decreased vision when dry or dirtydry or dirty

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BENEFITS OF RGP LENSESBENEFITS OF RGP LENSES

• 1. Good Vision1. Good Vision

•     Automatically corrects most astigmatism Automatically corrects most astigmatism 

• Excellent optical quality Excellent optical quality 

• Good surface wettability Good surface wettability 

• Good bifocal lens designsGood bifocal lens designs

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2. Excellent Eye Health2. Excellent Eye Health•     On average 2-3 times more oxygen provided to On average 2-3 times more oxygen provided to

the cornea as compared to most soft lenses the cornea as compared to most soft lenses 

• Smaller size allows tear exchange and debris Smaller size allows tear exchange and debris removal  removal 

• Fewer deposits due to surface wettabilityFewer deposits due to surface wettability

• 3. Durability3. Durability•  GP lenses last longer and don't need to be replaced as often as soft lenses 

• Lower long-term cost

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4. Ease of Handling4. Ease of Handling

•     Smaller size lenses are easier to insertSmaller size lenses are easier to insert

• 5. 5. Ease of CareEase of Care

•  Simple cleaning and disinfecting systems can be used due to wettable surfaces and deposit-resistant surfaces 

• Lower annual cost

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6. Slow Down Myopia Progression6. Slow Down Myopia Progression

• Studies have shown that GP lenses slow Studies have shown that GP lenses slow down myopia progression in young peopledown myopia progression in young people

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APPLICATIONSAPPLICATIONS

• 1. Critical Vision Needs1. Critical Vision Needs• 2. Astigmatism Correction2. Astigmatism Correction• 3. Children and Teenagers3. Children and Teenagers• 4. Presbyopia - bifocal correction 4. Presbyopia - bifocal correction • 5. Irregular Cornea - keratoconus, trauma, corneal warpage 5. Irregular Cornea - keratoconus, trauma, corneal warpage • 6. Soft Lens Failures - poor vision, eye health compromise6. Soft Lens Failures - poor vision, eye health compromise• 7. Orthokeratology/Corneal Refractive Therapy7. Orthokeratology/Corneal Refractive Therapy

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EMPIRICAL FITTINGEMPIRICAL FITTING

• there are good reasons to consider empirical there are good reasons to consider empirical methods today. Empirical fitting is time-efficient methods today. Empirical fitting is time-efficient and also quite accurate.and also quite accurate.

• corneal topography instrumentation has included corneal topography instrumentation has included sophisticated contact lens design software with the sophisticated contact lens design software with the ability of accurately simulating the fluorescein pattern ability of accurately simulating the fluorescein pattern based upon topography while recommending a lens based upon topography while recommending a lens design which results in a high first-fit success rate.design which results in a high first-fit success rate.

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FLUORESCEIN EVALUATIONFLUORESCEIN EVALUATION • The application of fluorescein to the tear film is The application of fluorescein to the tear film is

essential for proper evaluation of the lens-to-essential for proper evaluation of the lens-to-cornea fitting relationship cornea fitting relationship

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Omitting this step can result in any one or more of Omitting this step can result in any one or more of the following problems :the following problems :

a.a. Peripheral sealoffPeripheral sealoff: tear exchange is not possible; therefore, debris removal : tear exchange is not possible; therefore, debris removal and additional oxygen supply is eliminated. and additional oxygen supply is eliminated.

b.b. Areas of bearing or contact of lens against corneaAreas of bearing or contact of lens against cornea: corneal distortion : corneal distortion can result over time. can result over time.

c.c. Regions of excessive edge clearanceRegions of excessive edge clearance: more lid awareness, corneal : more lid awareness, corneal dryness or desiccation and lens decentration can result. dryness or desiccation and lens decentration can result.

d.d. Corneal compromiseCorneal compromise: areas of coalesced cornea staining can be present.: areas of coalesced cornea staining can be present.

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Alignment fit :Alignment fit :• A fluorescein pattern showing the lens evenly A fluorescein pattern showing the lens evenly

contouring the cornea with only light tear pooling contouring the cornea with only light tear pooling between the lens and cornea is termed an between the lens and cornea is termed an alignment fit. alignment fit.

• A steep central fit with excessive fluorescence or tear A steep central fit with excessive fluorescence or tear pooling centrally is termed pooling centrally is termed apical clearanceapical clearance..

• Apical bearingApical bearing represents the opposite form of pattern or represents the opposite form of pattern or direct contact of the lens against the central cornea direct contact of the lens against the central cornea

•

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One should always strive for an One should always strive for an alignment fitting relationship alignment fitting relationship • The only exceptions would pertain to cases in The only exceptions would pertain to cases in

which is an alignment fit can not be achieved which is an alignment fit can not be achieved (i.e., irregular corneas) or is not desired (i.e., (i.e., irregular corneas) or is not desired (i.e., corneal reshaping or orthokeratology). corneal reshaping or orthokeratology).

• Excessive apical bearing should be avoided as Excessive apical bearing should be avoided as corneal molding can occur, possibly resulting in corneal molding can occur, possibly resulting in warping or distortion warping or distortion

1717Figure 1Figure 1: An apical bearing fitting relationship : An apical bearing fitting relationship

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Figure 2Figure 2: An apical clearance, low edge clearance : An apical clearance, low edge clearance fitting relationship fitting relationship

1919Figure 3Figure 3: A high edge clearance fitting relationship : A high edge clearance fitting relationship

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Fluorescein evaluation of the lens-Fluorescein evaluation of the lens-to-cornea fitting relationship should to-cornea fitting relationship should be performed at both the fitting and be performed at both the fitting and all progress evaluation visits. all progress evaluation visits.

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• The ability to accurately assess The ability to accurately assess fluorescein patternsfluorescein patterns comes only after comes only after constant and frequent evaluations. It is constant and frequent evaluations. It is not uncommon to have a lens that not uncommon to have a lens that centers optimally but could induce centers optimally but could induce corneal compromise over time due to corneal compromise over time due to the absence of an alignment fluorescein the absence of an alignment fluorescein pattern pattern

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LENS DESIGN PHILOSOPHYLENS DESIGN PHILOSOPHY• It is important to note that – regardless of the It is important to note that – regardless of the

design philosophy – the most important factor is design philosophy – the most important factor is fluorescein pattern evaluation of the fitting fluorescein pattern evaluation of the fitting relationship. relationship.

• 1. Overall/Optical Zone Diameter.1. Overall/Optical Zone Diameter.

• The optical zone diameter is often approximately The optical zone diameter is often approximately 1.4mm less than the overall diameter. 1.4mm less than the overall diameter.

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The overall diameter and optical The overall diameter and optical zone diameter of a rigid lens are zone diameter of a rigid lens are typically obtained by considering typically obtained by considering the following variables:the following variables:• (a) palpebral aperture size,(a) palpebral aperture size,

• (b) pupil size, (b) pupil size,

• (c) corneal curvature.(c) corneal curvature.

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a. Palpebral Aperture Size(PAL)a. Palpebral Aperture Size(PAL).. Normal Normal PAL values range between 9mm-10.5mm. In PAL values range between 9mm-10.5mm. In most lid-to-cornea relationships there is, at most lid-to-cornea relationships there is, at minimum, slight overlap of the lids over the minimum, slight overlap of the lids over the limbus. limbus.

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Figure 4: A lid attachment fitting relationship

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c. Corneal Curvaturec. Corneal Curvature. . • Steeper-than-average corneal curvatures Steeper-than-average corneal curvatures

(i.e., >45D) often necessitate a smaller-than-(i.e., >45D) often necessitate a smaller-than-average overall diameter to maintain average overall diameter to maintain centration on a cornea that typically flattens centration on a cornea that typically flattens at a higher rate than normal. at a higher rate than normal.

• An average overall diameter/optical zone An average overall diameter/optical zone diameter of 9.4/8.0mm should be successful diameter of 9.4/8.0mm should be successful in most cases, particularly when a lid in most cases, particularly when a lid attachment fitting relationship is desired. 0attachment fitting relationship is desired. 0

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2. Base Curve Radius2. Base Curve Radius. .

• The base curve radius (BCR) to be selected for a The base curve radius (BCR) to be selected for a given patient depends upon several factors, given patient depends upon several factors, including corneal curvature, lid-to-cornea including corneal curvature, lid-to-cornea relationship and fluorescein pattern evaluation. relationship and fluorescein pattern evaluation.

• . As the cornea is aspheric and tends to flatten . As the cornea is aspheric and tends to flatten at a greater rate at an increasing distance from at a greater rate at an increasing distance from the center, the selection of a base curve radius the center, the selection of a base curve radius that is slightly flatter than “K” should result in that is slightly flatter than “K” should result in an alignment fitting relationship. an alignment fitting relationship.

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• Gas permeable lenses will always move Gas permeable lenses will always move along the steeper meridian of the cornea along the steeper meridian of the cornea as well as to the steepest region of the as well as to the steepest region of the cornea (i.e., apex). cornea (i.e., apex).

• A simple rule of thumb is to flatten the BCR by A simple rule of thumb is to flatten the BCR by 0.25D for every increase in overall diameter of 0.25D for every increase in overall diameter of 0.5mm and to steepen the BCR by 0.25D for 0.5mm and to steepen the BCR by 0.25D for every decrease in overall diameter of 0.5mm. every decrease in overall diameter of 0.5mm.

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3. Peripheral Curve Radii and Width3. Peripheral Curve Radii and Width. .

• The purpose of the peripheral curve radii is to The purpose of the peripheral curve radii is to allow good tear flow while compensating for an allow good tear flow while compensating for an aspheric corneal shape. aspheric corneal shape.

• The distance from the edge of the lens and the The distance from the edge of the lens and the cornea is termed the edge clearance. cornea is termed the edge clearance.

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4. Center Thickness4. Center Thickness. .

• Overall lens thickness is dependent on a Overall lens thickness is dependent on a number of variables, but primarily lens power number of variables, but primarily lens power and overall diameter and overall diameter

• The center thickness is greater and the center of The center thickness is greater and the center of gravity more anterior for plus lenses, while the gravity more anterior for plus lenses, while the edge thickness is greater and the center of edge thickness is greater and the center of gravity more posterior for minus lenses. gravity more posterior for minus lenses.

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When making lens design changes, it is When making lens design changes, it is important that this change is significant important that this change is significant enough to result in an improvement in enough to result in an improvement in the lens-to-cornea fitting relationship the lens-to-cornea fitting relationship

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PowerPower

• . After obtaining an optimum lens-to-. After obtaining an optimum lens-to-cornea fitting relationship, accurate cornea fitting relationship, accurate determination of the final lens power is determination of the final lens power is important. important.

• Vertex DistanceVertex Distance..

• Another important consideration in predicting the final lens Another important consideration in predicting the final lens power is the effective power at the corneal plane for power is the effective power at the corneal plane for patients exhibiting a high amount of myopia or hyperopiapatients exhibiting a high amount of myopia or hyperopia –

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