4 - determining RGP lens power

what is the lacrimal lens?

tear fluid trapped between an RGP and the front surface of the cornea

if the cornea is toric, what kind of power will the lacrimal lens have?

toric power (even if RGP is spherical)

How many contact lenses are patient effectively wearing when wearing GPs?

TWO (GP and lacrimal lens)

Which meridian does the spherical lens align with on a toric cornea?

flat meridian → leaves excessive clearance in steep meridian aka pooling where cornea is steepest

example: WTR cornea → vertical meridian is steeper, horizontal meridian is flatter, → pooling in vertical direction

Which meridian is flatter and steeper in the with-the-rule (WTR) cornea?

steeper → vertical

flatter → horizontal

Which meridian is flatter and steeper in the against-the-rule (ATR) cornea?

steeper → horizontal

flatter → vertical

what is irregular astigmatism?

when the 2 major power meridians are NOT 90° apart

what is regular astigmatism?

when the 2 major power meridians are 90° apart

what does the total ocular (spectacle) astigmatism include?

sum of corneal and lenticular

minor → variations in refractive index of ocular media, oblique astigmatic aberration, irregular shape of the fovea

what are the reasons that cause contact lens induced astigmatism?

1. lens warpage

2. lens flexure

3. lens decentration

4. oblique astigmatic aberration

5. toric CL surfaces

how does oblique astigmatic aberration cause CL induced astigmatism?

CL tilts on the cornea inducing oblique rays

how does toric CL surfaces cause CL induced astigmatism?

1. wrong axis for CL → obliquely crossed cylinders

2. under/over-corrected astigmatic error

what is residual astigmatism?

astigmatism that remains after placing a CL on the eye

what can cause residual astigmatism?

if lacrimal lens doesn't correct some/all → d/t crystalline lens

what is measured residual astigmatism?

astigmatism which is actually measured by over-refraction after fitting a CL

what is predicted residual astigmatism?

astigmatism that is mathematically predicted to remain after fitting a CL

what is the equation for predicted residual astigmatism?

predicted RA = total ocular astigmatism - corneal astigmatism

what does the lacrimal lens neutralize?

corneal astigmatism → NOT lenticular astigmatism

When can't residual astigmatism be predicted?

when spectacle and corneal cylinders are oblique (do not match/wrong axis)

ex: Rx @ 180 and Ks @ 145 → need to use obliquely crossed cylinder techniques to calculate

anything >5-10° off axis of one another

if the patient has oblique spectacle and corneal cylinders, are they a good candidate for GPs?

no → soft toric candidate

what is the diagnostic spherical RGP lens power equation?

= RGP lens power + over-refraction

real world → only use spherical refraction for spherical GPs (skip cyl)

what is the empirical spherical RGP lens power (CLP) equation?

= SpecRxVertexed - LL - RA

what is the lacrimal lens equation?

= base curve radii (BCR) - K readings (Ks)

what is 'need'?

spectacle cylinder measured by refraction (and vertexed)

what is 'have'?

corneal cylinder that creates cyl in the lacimal lens

What if NEED > HAVE

more residual astigmatism → not good candidate for spherical lenses

What is the only way to reduced residual astigmatism?

FIT A TORIC LENS

Can you change the amount of residual astigmatism by changing the BCR?

NO

How does the power of the lens change if you steepen the BCR?

more plus

How do you adjust the lens power if ordering a steeper lens?

add minus (add the amount you are changing the BCR) to the power

SAM FAP

Steeper Add Minus

How does the power of the lens change if you flatten the BCR?

more minus

How do you adjust the lens power if ordering a flatter lens?

add plus (add the amount you are changing the BCR) to the power

SAM FAP

Flatten Add Plus