week 1 Flashcards
Most people can tolerate up to ___ D uncorrected astigmatism
Most people can tolerate up to 0.75 D uncorrected astigmatism
In Bannon, 1945 study with 2000 patients, how many had astigmatism? % of categories of astig? (WTR ATR oblique)
81% with astigmatism(~80%)
33% WTR
20% ATR
28% oblique (~30%)
In Fledelius, 1986 study, what % had >0.50 D of cyl? >1.50 D of cyl?
- 46% of population w/ >0.50 D (~50%)
* 4.7% of population w/ >1.50 D (~5%)
For Holden 1975 study, what % of pop had >.75 D of astig?
-what % had bw .75-2.50?
> 55%
~40% had bw .75-2.50
GP lenses can correct astigmatism via two
mechanisms:
1) LL of spherical or toric lens corrects all or part of the corneal astigmatism
2) Toric lens power created by cylindrical front and back surfaces that corrects residual
astigmatism
Can use a spherical GP lens to correct corneal astigmatism up to __ D and RA of ___ D?
Can be used for RA up to 0.75 D and
Kcyl up to 3.00 D
what kind of spherical lens should you select to prevent flexure?
Select aspheric back surface, greater
CT and/or material with high modulus
to prevent flexure
what are the 2 options of hybrid lenses Synergeyes offers?
1) Duette
2) Synergeyes A-lots of flexure
* both have GP center and SCL “skirt”
T/F: hybrid lenses offer toric powers and styles
FALSE
*theoretically can correct up to 6D of corneal cyl but in reality pry just 3D
how often do you replace hybrid lenses? what kind of CL soln do you use?
~6 month replacement
-use SCL soln
when is flexure beneficial?
WTR corneal toricity and ATR RA
There must be at least ___ D of corneal
astigmatism for flexure to occur
1.00 D
Thin design lenses can flex ___ of the amount of corneal toricity or more
1/3
List 4 factors you can adjust to increase lens flexure:
1) Decrease lens diameter
2) Decrease thickness profile
(Use lenticular designs if needed)
3) Increase centration
(steepen BC and PCs)
4) Use more flexible (lower modulus) materials
Indications for back surface toric lens:
1) Corneal cyl of >/=2.00 D (<2.00 D will cause rotation of lens (even with back toric)
2) SRcyl roughly 1.5 times Kcyl
3) Refractive and corneal cyl axes are the same
how much more astigmatism is corrected with back surface toric vs spherical GP? what does this depend on?
25% to 60% more
-depends on index of refraction (n)
what is the 1:2:3 rule for back surface toric lenses?
app ratio of ORcyl: BCcyl : CLPcyl
(1:2:3)
*ratio mainly for PMMA which is no longer in GP lenses
also applies to SRcyl = 3 Kcyl = 2 RA (Icyl) = 1
what parameter should you determine first for GP back surface torics?
Base curve
For 9.2 mm diameter, start with what BC?
what if it is a larger or smaller diameter?
start on flat-K or 0.25 D flatter than flat-K
– Flatter for larger diameters
– Steeper for smaller diameters
what is the “Easiest method for Back Surface Toric Base Curve Selection”?
BC toricity selected 0.75 D less than Kcyl
what is the “Back Surface Toric Base Curve Selection for ATR corneas”?
BC toricity equal to full amount of corneal cyl (Decreases lateral decentration)
**Generally fit 0.50 D flatter on both meridians for 9.2 mm lens diameter
when ordering toric GP, order with dot where? how much rotation of dot with blink is acceptable?
order dot on flat meridian
- dot should not rotate more than 10° to 15° on a blink and return rapidly after blink
