Contact Lenses Flashcards
How is the equation for Shape factor different from the Equation for Back Vertex Power
Both use F1 in the denominator but the Shape Factor Equation has a numerator of 1 and the Fv equation has a numerator of F1
Contact Lenses are _____ lenses so to find their power use the equation for _______
Thick; Back Vertex Power
When using the Effective Vergence equation going from CL to Specs or vise versa do you have to adjust the vertex distance?
NO!!
You only have to adjust the vertex distance when calculating Power Factor of SM because it is measured to the entrance pupil
How does power in the LL change per every 0.1mm change in BC?
0.50D
Residual Astigmatism Equation
Spec Cyl - K Cyl
-remember to vergence SRx if >4.00
Javals Rule
Estimate of refractve cyl based on K readings
Refractive cyl = 1.25 (K cyl) + (-0.50 x 090)
Parameter: Optical Zone Diameter (OZD)
Usable area of optics
Adjust in steps of 0.4mm (adjust BC by 0.25D)
-Increase OZD -> increase Sag -> steeper
Parameter: Overall Diameter (OAD)
Adjusted in steps of 0.4mm (Adjust BC by 0.25D)
Chosen to minimize flare, lid attach and maximize comfort (bigger)
Parameter: Peripheral Curves
Contribute to alignment b/t edge and peripheral cornea
- prevents bearing on K, promotes tear exchange, and centration
Parameter: Edge Thickness
Ideal is similar to a -3.00 lens
Parameter: Edge Lift
Change in steps of 1.00mm
Too little edge lift: Traps debris, no movement
Too much edge lift: decreased centration, increased 3 & 9 desication
Parameter: Center Thickness
Change in steps of 0.03mm
Thinner = better oxygen transmission
Higher Dk materials need a thicker CL to minimize flexure
Parameter: Center of Gravity
More posterior = Better centration
Describe fluorescein pooling in WTR vs ATR
Dumbell pattern corresponds to touch
WTR: horizontal
ATR: vertical
When to fit Bitoric
When K cyl is > 2.50D AND spec cyl =/= 1.5Kcyl
When to fit front surface toric
-How to adjust for rotation
K cyl is < 2.50D but Rx cyl is high
Prism Ballasting with BD prism helps prevent rotation
When to fit Back Surface Toric
Corneal cyl is > 2.50D and spec cyl is = 1.5Kcyl
-usually used in ATR corneas
Bitoric Fitting philosophies
Saddle Fit: both meridians fit equally flat (usually 0.25)
-used for ATR corneas
Low Toris Simulation: fit one meridian 0.25 flat and the other 0.75 to 1.00 flat to give a WTR tear film
-used for WTR corneas, most used
Advantages and Disadvantages of Aspheric GP CLs
Advantages: better centration and comfort, decreased spherical aberration
Disadvantage: decreased tear exchange
Types of Aspheric CLs
Back Surface: Typically Distance center, GPs, and more positive aberrations
-High, ATR, or irregular K astigmatism
Front Surface: Typically Near center, Soft, and more negative aberrations
-Excessive residual astigmatism (most fits are these)
Lens Designs of MF CLs
Simultaneous: increases aberration but also increases depth of focus (Aspheric lenses give functional fuzzy vision at all distances)
Translating: move up as pt looks down, prism ballasted for rotation
(usually IP fit and spheric lenses -> good optics)
Lens flexure is a GP lens _____ the eye and contributes to ______
ON; Residual astigmatism (provides less cyl correction via LL)
Warpage is ______ of a GP lens ________ the eye that occurs due to _______
Permanent toricity; ON and OFF; excessive digital cleaning
Equation for Oxygen Transmission through a CL
Dk/t
t in cm
