RGP CONTACT LENS FITTING

Gerald E. Lowther, O.D., Ph.D.

1. no irritation
2. lay flat side of strip gently against conjunctiva (do not rub)

3. no excess solution (shake excess off)

1. use of larger lenses (9.2 to 10.5 mm) to increase comfort by positioning edge under upper lid when upper lid covers part of cornea.

2. use of small diameters (8.0-8.5 mm) with high upper lid-lens center due to surface

tension of tears

3. use of intermediate diameters (8.5-9.0 mm) on small apertures-to prevent lens being

trapped by lower lid.

1. ride high

2. move excessively

3. can cause flare

4. can cause corneal distortion

5. can cause peripheral staining

1. stagnation of tears

2. edema

3. corneal distortion

4. impression ring

5. peripheral staining

1. Spherical base curve and peripheral curves (usually good on corneas of up to

2.50 D. of corneal cylinder)

2. Aspheric base curve-gives slightly better fluorescein pattern

3. Spherical base curve and toric peripheral curves

4. Bitoric (usually best on corneas with over 2.50 D. of cylinder)

1. Fluorescein pattern shows a horizontal band of touch (horizontal bearing) on a

with-the-rule toric cornea (flat meridian in horizontal meridian)

2. Edge stand-off at 12 and 6 o'clock

3. Rocking and excessive movement on blinking

4. Lens tends to ride high or low depending on lid forces

5. Lens easily blinked out of eye

1. Lens rests on cornea in mid-periphery

2. Fluorescein pattern: dumbell or H pattern

3. Best compromise fit

a. lens fitted steeper than flat meridian by about 1/3 of corneal toricity

for a lens with an optical zone approximate 8 mm.

b. example: K-reading 43.00/44.50

Lens BCR: 43.50

4. Can cause cornea distortion

1. may prevent tear exchange

2. may distort cornea

3. lens tends to flex

1. Effect of material

a. PMMA flexes least, silicone-PMMA intermediate, CAB most

2.. Effect of center thickness

a. increasing thickness decreases flexure (must change thickness at least

0.02 to 0.03 mm to have any effect)

3. Fit of lens effects amount of flexure

a. steeper fit flexes more.

b. lenses flatter than K flex least

4. Optical zone effect

a. larger optical zones flex more than smaller

-same effect as BCR due to increase sagittal depth

5. To decrease flexure:

a.. increase center thickness

b. flatter fit

c. smaller optical zone

6. Flexure of a lenses on with-the-rule corneas will correct against-the-rule

residual astigmatism.

a. For example:

K-reading: 43.00 @ 180; 45.00 @ 090

Refraction over non-flexing lens: plano = -0.50 D. x 090

Refraction over lens which flexes 0.50 D.: plano

7. Need for the future: stiffer RGP material that will allow thinner lens designs.

5. Peripheral curve selection

1. Excessive movement

2. Lens rides high if upper lid holds it or rides low

3. Discomfort from excessive edge clearance

4. Surface tension breaks at lens edge causing bubbles (frothing) which can

cause bubbles to be trapped under lens resulting in dimple veiling.

5. Increased peripheral staining

1. impedes tear exchange

2. corneal imprint

3. increased peripheral staining

1. Axial Edge Lift (AEL)-distance from extension of base curve upto the edge of

the lens measured parallel to the lens axis. Most clinically acceptable

values: 0.08 mm to 0.15 mm

(see accompanying chart for values for different lens designs)

2. Radial Edge Lift (REL)-distance from extension of base curve radius up to the

lens edge measured along the radius line.

3. Edge Clearance: Distance from the cornea up to the edge of the lens. This is

not easy to calculate unless one assumes a corneal eccentricity or has a

corneal topographer.

1. Proper clearance gives a definite green band around edge

2. Excessive clearance (peripheral radii are too long):

deep green band at edge

bubbles may form on blinking, especially on up gaze

lens may decenter and move excessively

3. Insufficient clearance (peripheral radii too short):

little to no fluorescein pooling at edge

little lens movement

lens is usually well centered

1. The wider the peripheral curve the steeper (shorter) peripheral radius required.

Example:

Lens #1 BCR 7.50 mm, SCR 9.00, OAD 9.2 mm, OZD 8.0 mm, AEL 0.094.

Lens #2 BCR 7.50 mm, SCR 9.00, OAD 10 mm, OZD 8.0 mm, AEL 0.175.

Lens #3 BCR 7.50 mm, SCR 8.20, OAD 10 mm, OZD 8.0 mm, AEL 0.095.

1. If BCR of lens is correct: add spherical equivalent over-refraction (O.R.)

power to diagnostic lens power

-example: lens BCR 7.50 mm, power -3.00 D. O.R. -1.00 = -0.50 x 090 Order: BCR 7.50 mm, power -4.25 D.

2. If BCR to be ordered is different than BCR of diagnostic lens then in addition

to adding the power of the over-refraction one must compensate for the

BCR change.

-example: diagnostic lens BCR 7.50 mm, power -3.00 D.

O.R. -1.00 = -0.50 x 090

lens to order: BCR 7.42 mm

7.50 mm =45.00D.; 7.42 mm=45.50 D.

Lacrimal lens power change +0.50 D.

Power to order: -4.75 D.

(-3.00 in diag.lens, -1.25 from O.R., -0.50 to

compensate for the BCR change)

3. Note: with refraction over diagnostic lenses you do not need to know the

K-reading. This may be the required way to determine lens power when there is corneal distortion.

1. Convert lens BCR to be ordered to dioptric power (337.5/BCR in mm) and

take difference between this and flat K-reading. If lens is steeper than flat K, add minus power equal to this difference to spectacle lens sphere power (minus cylinder form)

2. Example: K-reading 44.50 @180; 44.50 @ 090

Spectacle Rx: -2.00

BCR 7.5 mm (45.00 D.)

Power to order: -2.50 D

(-2.00 from spectacle Rx plus -0.50 D. to compensate for

the lens being steeper than the cornea giving a +0.50 D.

tear lens)

3. Example: K-reading 44.50 @180; 44.50 @ 090

Spectacle Rx: -2.00

BCR 7.67 mm (44.00 D.)

Power to order: -1.50 D

(-2.00 from spectacle Rx plus +0.50 D. to compensate for

the lens being steeper than the cornea giving a -0.50 D.

tear lens)

4. Example (case with corneal toricity and spectacle cylinder):

K-reading 44.50 @180; 45.50 @ 090

Spectacle Rx: -2.00 = -1.00 x 180

BCR 7.5 mm (45.00 D.)

Power to order: -2.50 D

(-2.00 from spectacle Rx plus -0.50 D. to compensate for

the lens being steeper than the cornea giving a +0.50 D.

tear lens)

This lens would correct the spectacle cylinder since the tear layer

compensates for the corneal toricity. Over-refraction would be

plano.

5. Example (case with corneal toricity and spectacle cylinder of different

amounts):

K-reading 44.50 @180; 45.50 @ 090

Spectacle Rx: -2.00 = -1.50 x 180

BCR 7.5 mm (45.00 D.)

Power to order: -2.50 D

This lens would correct 1.00 D. of the spectacle cylinder since the

tear layer compensates for the corneal toricity of 1.00 D.. Over-refraction would be plano = -0.50 x 180. The 0.50 D. of cylinder is called residual astigmatism.

2. Center Thickness

1. 0.08 mm to 0.12 mm prior to shaping ideal.

1. minus lenses (over about -3.00 D.) will have thicker edges

2. effect of peripheral curve radius

a. the longer the radius the thinner the edge

3. effect of optical zone diameter

a. keeping all other parameters constant: the smaller the optical zone

the thinner the edge.

1. silicone

a. increase oxygen diffusion

b. hydrophobic-decreases wettability

c. increase lipid deposits

2. PMMA

a. increase stability and machine ability

b. no oxygen permeability

3. Methylacrylic acid, NVP or HEMA

a. increase wettability

b. increase protein adhesion

c. decrease stability

4. Fluorine

a. decrease deposits

b. increase oxygen diffusion

1. Most important since it gives the amount of oxygen going through lens (Dk divided by

lens thickness).

2. Effect of thickness:

if Dk=36 and ct=0.15 then Dk/L=24

ct=0.20 Dk/L=18

if Dk=100 and ct=0.15 then Dk/L=66.6

ct=0.20 Dk/L=50

3. Dk/L required to prevent overnight edema: 75 to 100

Dk/L required to prevent open eye edema: 22

1. BUT of the tear film on lens surface varies with lens material

a. trend is decrease BUT with increased Dk

2. Surface debris collection also varies with material

Copyright 1997, The Trustees of Indiana University
Web Administrator: Jennifer Steele
Last updated: 2/11/97