GPs & Astigmatism

corneal

spherical GP lenses correct all _______ astigmatism

1. corneal cyl closely matches SPRx

2. less than 2.5D of corneal astigmatism

what pts do spherical GP lenses work best for?

don’t

the amount a lens flexes is the amount of astigmatism that we _____ correct

1

generally you need at least ___D of corneal astigmatism to get a lens to flex

steep

lenses flex along the _____ meridian

1. steepen BC

2. fit slightly smaller

3. decrease lens thickness

4. use more flexible materials (higher Dk)

what are some ways to alter lens fit to induce flexure?

front surface toric GPs

spherical back surface & toric front surface, designed for pts w/ spherical (or close) corneas, but high levels of internal astigmatism

pts w/ <2.50D of corneal astigmatism & >0.75 of residual astigmatism

what pts are front surface toric GPs best for?

pts who can fit well in a spheric lens but need the toric front surface in order to see well

describe the pt type that front surface toric GPs are best for in terms of fit & vision

T

T/F: stability of front surface toric lenses are very important and LARS should be used when fitting

base down prism ballast

what is the most common front surface toric stabilizer?

larger

larger amounts of astigmatism require ________ amounts of prism in the front surface toric stabilizer

ATR

what type of astigmatism does not require as much prism ballasting as a front surface toric stabilizer?

T

T/F: prism ballasting on front surface toric lenses can induce a bit of cyl

prism ballasting, truncating

what are the options for stabilizing front surface toric lenses?

1. tend to drop with blinks

2. cause blur w/ rotation

3. poorer comfort

4. reduced O2 permeability

5. can promote incomplete blinks

what are the disadvantages of front surface torics?

soft contact lenses

what is a better option for most pts that could be fit in front surface toric lenses?

back surface toric GPs

have a toric back surface & spherical front surface

T

T/F: when back surface toric lenses are fit properly, it will look like a spherical CL fit on a spherical cornea

ATR corneas w/ internal astigmatism

what specific type of pts are back surface toric lenses fit on?

all corneal cyl & 1/3 more of the toricity of the lens

what amount of cyl is corrected by a back surface toric lens?

>2.50D of corneal cyl & SPRx cyl is ~(1/3 x corneal cyl)

what pts are best for back surface toric power lenses?

bitoric GP

have a front surface toric & back surface toric

spherical power effect

• toricity of BC of lens = toricity of power of lens

• used on eyes with >2.50D of corneal cyl & <0.75D residual astigmatism

cylindrical power effect

• toricity of BC of lens does not equal toricity of power of lens

• used on eyes w/ >2.50D of corneal astigmatism & >0.75D residual astigmatism

yes

does rotation matter for SPE lenses?

should look like a spherical lens on a spherical eye

describe the ideal fit of an SPE on eye

T

T/F: SAM FAP still applies if you change the BC of an SPE or CPE lens

each meridian

if a spherical OR is found on an SPE lens, add the power to ______

total power, CPE

if a sphero-cyl OR is found on an SPE lens, add the _______ in each meridian to the lens, you will now have a _____

CPE

• used on pts w/ high levels of corneal astigmatism that does not match the SPRx or the 1.3 x corneal cyl rule

• these pts would not fit well in or see well out of a spherical GP lens

• must be stable on eye

yes

does rotation matter for a CPE?

should look like a spherical lens on a spherical eye (or very slightly WTR cornea)

describe the ideal fit of an CPE on eye

10-15, rapidly

a CPE should not rotate more than _____ degrees w/ a blink & should return ____

each meridian

if a spherical OR is found on a CPE lens, add the power to ______

total power

if a sphero-cyl OR is found on a CPE lens, add the ______ in each meridian to the lens

on flat K or 0.25D flatter than flat K, 0.50-1.00D flatter than steep K

what BC do you generally fit for a bitoric lens?

1. specify powers & BCs in both meridians

2. dot/mark the flat meridian to assess rotation

what do you need to do when ordering bitoric GPs?

higher amounts of back (BC) toricity

what bitoric GPs likely need toric peripheral curves?