GP Astig Flashcards
corneal astigmatism is corrected by the ____ surface of a GP lens
back; corrected by the lacrimal lens
residual astig is corrected by the t____ lens power on which side(s) of the GP lens?
toric; BOTH sides of the GP
T/F: LL will still be plano (thus correcting all the corneal astigmatism) with a back surface toric GP
false; toricity will NOT be the same
five GP designs to choose from - IN ORDER you’d choose them: [STBBF]
spherical; thin-flex, back surface toric, bitoric, front surface toric
SPHERICAL GPs:
- T/F: will correct all corneal astig
- can be used for powers up to ___D
true
3D
hybrid CLs provide good vision (GP) with good comfort (SCL) and are available to correct ____ astigmatism up to __D
-ideally you’re looking for what FL pattern under the entire lens?
corneal (can ONLY correct corneal)- up to 6D - BUT: it’ll flex too much after 3D
-i.e. Synergeyes, Duette
-alignment!
Hybrid lenses (spherical GPs):
- remove by pinching where on the lens?
- use what type of care system?
- replacement schedule?
6 o’clock position
- SCL system
- 6 months
THIN FLEX GPs: (flexure)
- helpful for which patients?
- MINIMUM ___D for flexure to occur
pts w/ WTR corneal astig and ATR residual astig (or the opp; but it’s rare)
-1D minimum for flexure - can flex 1/3 of corneal toricity
what should you do to the following to intentionally DECREASE lens flexure?
- LD (increase/decrease)
- thickness
- centration (increase/decrease)
- modulus (high/low)
decrease LD, decrease thickness, increase centration (steepen BC/peripheral curves), low modulus
Brungardt’s thin-flex design has a (small/large) LD with a (small/large) OZD, uses a (bi/tri/tetracurve) design with a (flat/steep) secondary curve, a (small/large) CT, (flatter/steeper) fit, and good centration
small LD; large OZD - makes it a bit thinner
bicurve; STEEP secondary curve (thinner), thin CT, steeper fit
BACK SURFACE TORIC
- indicated when? how much cyl?
- is the LL altered?
- SRcyl is about ____x MORE then Kcyl
- causes a (greater/lesser) refractive effect than a spherical
HIGH corneal toricity; 2.00D or more
- yes; LL is altered (because it’s the back surface)
- 1.5x more - rough. could be less
- greater refractive effect - can correct 25-60% more astig!
what must be accounted for in figuring out how much astigmatism is corrected by a back surface toric CL?
-how do you do the math?
INDEX of the lens!
math: 1) find front surface power (F1=n’-n/r), then back surface power (F2=n-n’/r), and add the two values for the amt of dioptric astigmatism corrected
what “rule” applies to back surface toric lenses?
-how does it work?
1:2:3 rule - applies to ratio of amounts of cyl in the CLP (in air): BC: OR (that can be correction)
CLP:BC:OR
-also applies to SRcyl:Kcyl:RAcyl
What’s the FIRST thing you should figure out in designing a back surface toric lens?
what are the next 3 things (in order)?
BC - select this FIRST! then CLP, PC, CT
(BST design)
-if you’ve chosen the right BC, which type of fit should be mimicked?
sph lens on a low WTR cornea (considered “ideal” for lens fitting)
which meridian is determined first in BST design?
-how is each (flat and steep meridian) calculated?
FLAT, then steep.
flat: dependent on LD. Start on flat-K or 0.25 flatter; go flatter for larger LD, steeper for smaller LD
steep: find the overall BC toricity (based off Kcyl), add it to the BC of the flat
(BST) how is BC selection different for ATR corneas?
fit BOTH meridians 0.50D flatter - what BC toricity to equal the full amt of corneal cyl
(BST)
T/F: BC system is the same for BST lenses as for spherical GPs
true-ish - 1.5mm SCr and 3.0mmPCr, but do it for BOTH meridians
when observing rotational assessment of a BST lens; what’s the MOST the dot should rotate on a blink before rapidly returning to nml position?
10-15 deg
if cyl > 0.75D is found on steep or flat meridian of a BST lens; what lens design might be required as a replacement?
bitoric
bitorics have a ____ and ____ power effect. Which is equal to the toricity of the CLP? Which is used for eyes w/ significant (>0.75D) internal astigmatism?
spherical, cylindrical
sph = toricity of CLP (SPE)
cyl = used for lots of internal astig (CPE)
T/F: A bitoric may affect vision like the other 4 types of astigmatic GPs
FALSE. it’s the only one that WON’T affect vision, because it won’t blur/distort regardless of rotation
SPE bitorics - how do you calculate each surface?
back surface: same way as w/ BST design
front surface: resulting lens has the SAME amt of toricity in the BC and CLP
T/F: SPE bitorics are indicated when the vision is good w/ a spherical GP, but a poor fit is achieved
- corneal astig must be >___D
- refractive astig must be within ___D
true
min 2D of corneal cyl
refractive w/i 0.75D of corneal cyl
do SPE bitorics have an axis?
NO! remember - they’re a type of bitoric - won’t cause changes in VA d/t rotation!…the power rotates w/ the lens
which two toricities must equal w/ SPE bitoric lenses?
BC toricity = power toricity…or else it’s not a bitoric!
CPE (cylindrical) bitorics - used over SPE for lots of internal astig. Which surface corrects corneal astig? How about internal?
corneal - back surface
internal - front surface
T/F: CPE bitorics may affect vision if not rotated properly
TRUE (even though it’s a bitoric - it’s not an SPE)
FRONT SURFACE TORICS
- used for pts w/ significant ____ astig, but only a little ____ toricity
- does this lens require rotational stabilizers?
internal, corneal (b/c BSTs won’t work on eyes w/ <2.00D of corneal cyl)
-yes. big time
FST indications
- less than ___D of corneal astigmatism; requiring a spherical BC
- greater than ___D of RA - so basically, use FST when pt complains VA isn’t perfect w/ only a sph design
2.00; 0.75
Four advantages of sph GP over FST GP:
1) better POSITIONING - FSTs drop inf d/t prism
2) more STABLE VA - no rotation w/ sph
3) better COMFORT - FSTs are thicker and position inferior
4) better O2 PERMEABILITY - FSTs are thicker for prisms
FST design: things to decide (in order)
1) BC (Kave-0.50 or 1.00 for 9.2/6)
2) Power (sphero-cyl form) - put a sph GP and add SCOR
3) PC - determined like sph GPs
4) prism - will orient base DOWN; won’t need as much for ATR corrections (already BD shape)
5) CT - find it the same way you normally would (18+/-) AND THEN increase CT by 0.10mm for each diopter of prism!
in most cases of rotation/mvmt, FSTs tend to rotate so the bottom of the lens rotates _____
nasally - compensate for it using LARS
