6 - Contacts1

Question 1

If given thickness of a contact lens

Answer 1

Calculate back vertex power (same from Geo optics)

BVP = F2 + (F1 ÷ (1-(t/n)F1))

Thickness in METERS

Question 2

Vertex distance/power equation

Answer 2

Same from Geo optics

Fc = Fg ÷ (1-dFg)

Question 3

When we need to worry about vertexing back to corneal plane

Answer 3

Spectacle rx +/- 4.00 D or more

Question 4

GP contact lens equation

Answer 4

Fʟʟ + Fcʟ + For = Rc

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

Question 5

RGP lacrimal lens

• back surface is equal to
• front surface is equal to

Answer 5

ROC of the cornea

BC of the contact lens

Question 6

RGP

-keratometer is calibrated to what IR

Answer 6

n = 1.3375 (IR of the cornea)

Question 7

Index of refraction

• tear lens
• cornea

Answer 7

1. 336

1. 3375

Question 8

RGP

-equation relating radius of curvature and power

Answer 8

F = 337.5 ÷ r

r is in MILIMETERS

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

Question 9

K readings and lac lens

Answer 9

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

Question 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

Answer 10

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

Question 11

Describe SAM FAP

Answer 11

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

Question 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 __

Answer 12

<1.00 D WTR
= 0.75 D ATR

Toric lens

Question 13

Decribe Javal’s rule

Answer 13

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

Question 14

Javal’s rule equation

Answer 14

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

Question 15

Effect of increasing OZD

Answer 15

Incr sag = steepen fit

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

Question 16

OAD

• purpose
• average
• adjusted in what size steps

Answer 16

Selected to minimize flare

9. 4-9.6mm
10. 4mm steps

Question 17

OZD

• purpose
• average

Answer 17

Usable area of optics

7.6-8.2mm

Question 18

Peripheral curves

-purpose

Answer 18

Prevent bearing
Promote tear exchange
Centration - by supporting tear meniscus

Question 19

Edge lift

-excess

Answer 19

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

Question 20

Edge lift

-inadequate

Answer 20

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

Question 21

Center thickness

• influences
• significant changes
• thinner
• thicker
• Dk

Answer 21

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

Question 22

Center of gravity

Answer 22

More posterior the center of gravity = better centration

Question 23

Fluoroscein patterns

• alignment/on k fit
• flatter than k
• steeper than k

Answer 23

Evenly distributed, slightly more periphery

Corneal touch/bearing central, broad peripheral ring

Pool in center due to apical clearing, corneal bearing periphery

Question 24

Fluoroscein patterns

• spherical fit over WTR
• spherical fit over ATR

Answer 24

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

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

Question 25

When to use

-bitoric

Answer 25

High corneal astig (>2.50)

High residual astig (≠1.5x corneal cyl)

Question 26

When to use

-back toric

Answer 26

High corneal astig (>2.50)

Spectacle astig MUST = 1.5x corneal astig

Question 27

When to use

-front toric

Answer 27

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

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

Question 28

When to use

-spherical

Answer 28

No astig

Corneal astig <2.50