6 - Contacts1 Flashcards
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
Vertex distance/power equation
Same from Geo optics
Fc = Fg ÷ (1-dFg)
When we need to worry about vertexing back to corneal plane
Spectacle rx +/- 4.00 D or more
GP contact lens equation
Fʟʟ + Fcʟ + For = Rc
Lac lens + RGP power + over-refraction = vertex spec rx (at cornea)
RGP lacrimal lens
- back surface is equal to
- front surface is equal to
ROC of the cornea
BC of the contact lens
RGP
-keratometer is calibrated to what IR
n = 1.3375 (IR of the cornea)
Index of refraction
- tear lens
- cornea
- 336
1. 3375
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)
K readings and lac lens
K readings are equal, but opposite in sign, to the back surface power of the lacrimal lens in air
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
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
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
Decribe Javal’s rule
Allows us to empirically predict the TOTAL ASTIGMATISM CORRECTION at the spectacle plane with results from KEATOMETRY
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
Effect of increasing OZD
Incr sag = steepen fit
For every 0.4mm change in OZD, the BC should be adjusted 0.25D
OAD
- purpose
- average
- adjusted in what size steps
Selected to minimize flare
- 4-9.6mm
- 4mm steps
OZD
- purpose
- average
Usable area of optics
7.6-8.2mm
Peripheral curves
-purpose
Prevent bearing
Promote tear exchange
Centration - by supporting tear meniscus
Edge lift
-excess
Excessive peripheral fluoro pooling
Decr centration
Incr awareness of lens
Corneal desiccation = 3/9 o’clock staining
Edge lift
-inadequate
Minimal pooling of fluoro peripherally
Debris trapped underneath lens
Poor lens movement
Inadequate tear exchange
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
Center of gravity
More posterior the center of gravity = better centration
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
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
When to use
-bitoric
High corneal astig (>2.50)
High residual astig (≠1.5x corneal cyl)
When to use
-back toric
High corneal astig (>2.50)
Spectacle astig MUST = 1.5x corneal astig
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
When to use
-spherical
No astig
Corneal astig <2.50
