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1

If given thickness of a contact lens

Calculate back vertex power (same from Geo optics)

BVP = F2 + (F1 ÷ (1-(t/n)F1))

Thickness in METERS

2

Vertex distance/power equation

Same from Geo optics

Fc = Fg ÷ (1-dFg)

3

When we need to worry about vertexing back to corneal plane

Spectacle rx +/- 4.00 D or more

4

GP contact lens equation

Fʟʟ + Fcʟ + For = Rc

Lac lens + RGP power + over-refraction = vertex spec rx (at cornea)

5

RGP lacrimal lens
-back surface is equal to
-front surface is equal to

ROC of the cornea

BC of the contact lens

6

RGP
-keratometer is calibrated to what IR

n = 1.3375 (IR of the cornea)

7

Index of refraction
-tear lens
-cornea

1.336

1.3375

8

RGP
-equation relating radius of curvature and power

F = 337.5 ÷ r

r is in MILIMETERS

For meters use 0.3375/r, which is cornea IR(1.3375) - air IR (1)

9

K readings and lac lens

K readings are equal, but opposite in sign, to the back surface power of the lacrimal lens in air

10

Steps to solve problem:
You decide to flatten the GP lens fit of a pt
Given: corneal curvature/K readings, subjective refraction, new contact lens base curve

1) convert to diopters
2) calculate lacrimal lens
LL = BC(gp) - BC(k)
3) solve GP lens equation

11

Describe SAM FAP

If you STEEPEN the BC of a GP lens, you need to ADD MINUS to compensate for changes in lacrimal lens

If you FLATTEN the BC of a GP lens, you need to ADD PLUS to compensate for changes in lacrimal lens

For every 0.1mm change in BC, total power will change by 0.50D

12

Astigmatism
-pts can often tolerate __ WTR and __ ATR or oblique astig
-if more residual astig is present with a spherical GP, the pt should be fit with __

<1.00 D WTR
= 0.75 D ATR

Toric lens

13

Decribe Javal’s rule

Allows us to empirically predict the TOTAL ASTIGMATISM CORRECTION at the spectacle plane with results from KEATOMETRY

14

Javal’s rule equation

Arx = 1.25*Ac + (-0.50 D x 090)

Total refractive astig = 1.25*corneal astig + (-0.50 D x 090)

Last part is the average amount of internal astigmatism
For WTR patients, use plus cyl: +0.50 x 180

15

Effect of increasing OZD

Incr sag = steepen fit

For every 0.4mm change in OZD, the BC should be adjusted 0.25D

16

OAD
-purpose
-average
-adjusted in what size steps

Selected to minimize flare

9.4-9.6mm

0.4mm steps

17

OZD
-purpose
-average

Usable area of optics

7.6-8.2mm

18

Peripheral curves
-purpose

Prevent bearing
Promote tear exchange
Centration - by supporting tear meniscus

19

Edge lift
-excess

Excessive peripheral fluoro pooling
Decr centration
Incr awareness of lens
Corneal desiccation = 3/9 o’clock staining

20

Edge lift
-inadequate

Minimal pooling of fluoro peripherally
Debris trapped underneath lens
Poor lens movement
Inadequate tear exchange

21

Center thickness
-influences
-significant changes
-thinner
-thicker
-Dk

O2 transmissibility

Significantly changed in 0.03mm steps

Thin: greater O2 transmissibility, better centration, more flexure

Thick: less O2 transmissibility, less flexure, tends to drop inferiorly

In general, higher Dk lenses require thicker CT to minimize flexure

22

Center of gravity

More posterior the center of gravity = better centration

23

Fluoroscein patterns
-alignment/on k fit
-flatter than k
-steeper than k

Evenly distributed, slightly more periphery

Corneal touch/bearing central, broad peripheral ring

Pool in center due to apical clearing, corneal bearing periphery

24

Fluoroscein patterns
-spherical fit over WTR
-spherical fit over ATR

WTR: pooling vertically, touch/bearing horizontally
-horizontal dumbbell-shaped pattern

ATR: pooling horizontally, touch/bearing vertically
-vertical dumbbell-shaped pattern

25

When to use
-bitoric

High corneal astig (>2.50)
High residual astig (≠1.5x corneal cyl)

26

When to use
-back toric

High corneal astig (>2.50)
Spectacle astig MUST = 1.5x corneal astig

27

When to use
-front toric

Low corneal astig
High residual astig
Total: lenticular and corneal < 2.50

Huge problem = rotation
-possible sol’n is prism ballasting

28

When to use
-spherical

No astig
Corneal astig <2.50