Flashcards in 6 - Contacts1 Deck (28)

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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