CL - RGP fitting 3 - Week 1

Question 1

Where is the steep and flat axis for WTR astigmatism?

Answer 1

Steep @ 90deg
Flat @ 180deg

or axis within 30 degrees of those values

Question 2

Which meridian should the negative cyl be placed in to correct astigmatism? steep or flat? Explain [important]

Answer 2

The flat meridian is the axis of the negative cyl in Rx. This is known as the reference axis, and is the cyl axis we’d get during subjective refraction.

Question 3

Define regular astigmatism

Answer 3

2 axes or meridians that are perpendicular

Question 4

What does delta K represent? How is it calculated?

Answer 4

Keratometric astigmatism

DeltaK = Steep - Flat meridian power

Question 5

What is the purpose of javal’s rule?

Answer 5

Allows for the inclusion of posterior corneal astigmatism, which isn’t measured by corneal topography/keratometry (topographer/keratometer only measures the anterior cornea)

Question 6

What is the formula for javal’s modified rule? How does this differ from the original javal’s rule? [important]

Answer 6

Rx astigmatism = 1.00xDeltaK + (-0.50x90)

(More simply Rx astig = Deltak + (-0.50x90), or
Rx astig = keratometry astig + (-0.50x90)
(original rule was 1.25 rather than 1.00)

Question 7

What does the -0.50x90 represent in javal’s rule formula? Explain.

Answer 7

This is an allowance of 0.50D ATR astigmatism. The typical eye has about 0.50D ATR astigmatism at the posterior cornea. So we subtract for this when using javal’s rule.

Question 8

How does the -0.50x90 work in javal’s rule formula if DeltaK is at 180 instead?

Answer 8

-0.50x90 is equivalent to +0.50x180 so add +0.50 instead

i.e. DeltaK + (-0.50x90) = DeltaK + (+0.50x180)

Question 9

Using javal’s rule, what is the Rx astigmatism from keratometry values of:
46.11D@90
41.31D@180

Answer 9

Flatter axis @180 (WTR). Flatter axis is reference axis for negative cyl. DeltaK = 46.11-41.31 = 4.8 diopters of astigmatism.

Rx astig = DeltaK + (+0.50x180) = 4.8 + 0.50
= -5.30x180

Question 10

What is residual astigmatism and how can we calculate it? [important]

Answer 10

astigmatism not due to the cornea.

= ocular astig - corneal cyl
= spec cyl - deltaK
= astigmatic component to fully correct an eye wearing spherical BOZR CL

Question 11

What gas permeable lens is indicated if patient has a DeltaK >/= 1 and is an orthoK patient?

Answer 11

OrthoK
Toric periphery

Question 12

What gas permeable lens is indicated in a patient with DeltaK >/= 2 and regular astigmatism?

Answer 12

Bitoric GP

Question 13

What gas permeable lens is indicated in a patient with DeltaK >/= 5 and has PMD?

Answer 13

Bitoric GP

Question 14

What gas permeable lens is indicated in a patient with Rx >/= 0.75DC residual astigmatism

Answer 14

Front surface toric

Question 15

Define Spherical Power Equivalent (SPE)

Answer 15

SPE is when the dioptric difference in a toric BOZR is = to the dioptric difference in BVP

i.e. CL BOZR toricity is similar to CL cylindrical power

i.e. if the difference between the 2 BOZRs is equal to the astigmatism (difference in BVP) then it’s an SPE.

Question 16

How will lens rotation affect astigmatism in a spherical power equivalent (SPE) bitoric?

Answer 16

it won’t

Question 17

Where are the markings on a compensated (SPE) bitoric?

Answer 17

On the flat axis

Question 18

How do compensated (SPE) bitorics correct for refractive cylinder? What about induced astigmatism?

Answer 18

Correct refractive cylinder via corneal toricity.
Correct induced astigmatism via front CL surface toricity

Question 19

How do compensated (SPE) bitorics fit on the eye? (in terms of alignment) How is this achieved?

Answer 19

Toric back surface of the lens allows the “saddle principle”, where you have a full alignment of both principle corneal meridians (i.e. fit on K for each meridian)

Question 20

Do SPE bitorics require a prism ballast? Explain

Answer 20

No. Because the back surface of the lens is toric/curved, it will latch onto the eye anyway and doesn’t need additional stability from a prism ballast

Question 21

Define induced astigmatism [important]

Answer 21

the astigmatic effect introduced into the system every time a toric BOZ surface is used due to tear/lens refractive index difference

i.e. the astigmatic effect created by a toric BOZR bounding two surfaces of different refractive index (lens, tears)

Question 22

What is the formula for estimating induced astigmatism from a toric lens?

Answer 22

~0.4x CL toricity

(CL toricity = deltaK, in diopters obv.)

Question 23

Estimate the induced astigmatism for the following lens:
BOZR 8.50 (flat) and 7.90 (steep)

Answer 23

7.90mm = 42.72D; 8.50mm = 39.71D
CL toricity = deltaK = 42.7 - 39.7 = 3.00

Induced astig (estimate) = 0.4 x 3.00 = 1.2D

Question 24

What is the formula for exactly calculating induced astigmatism from a toric lens?

Answer 24

[1000(1.336-ncl)/BOZRflat] - [1000(1.336-ncl)BOZRsteep]

Question 25

List 3 general indications for GP torics [important]

Answer 25

Significant cyl & corresponding regular corneal toricity (>/= 2.00D)
Residual (non-corneal) astigmatism
PMD and (rarely for) KC

Question 26

List 4 refractive indications for GP torics

Answer 26

High Rx (>5.00D &/or 3.00DC)
Dual correction of astig AND presbyopia (via translating bifocals)
High semi-regular astigmatism (e.g. PMD)
Upper limit: cornea =15.0DC & CL = 10.0DC

Question 27

List 5 indications for GP torics in spherical GP wearers

Answer 27

Poor lens centration
Harsh bearing along flatter meridian
3&9 staining
Significant residual cylinder
Corneal distortion/spectacle blur

Question 28

List 5 indications for GP torics in toric SCL wearers

Answer 28

Unstable SCL fitting/rotation (visual)
Large ocular cyl out of disposable SiH toric range
Hypoxic signs
GPC
New wearer

Question 29

What should we keep in mind for CL toricity when choosing a BOZR for a toric GP?

Answer 29

Choose BOZR such that CL toricity >/= 1.50D

Question 30

How much should we correct CL toricity in a toric GP patient with moderate toricity? Why?

Answer 30

Under correct by about 0.50D to enhance tear exchange

Question 31

When should we select a Lid attachment fitting?

Answer 31

if upper eyelid covers superior limbus, then larger TD so do lid attachment fitting

Question 32

When should we select an intrapalpebral fitting?

Answer 32

if upper eyelid does not cover the limbus (then smaller TD)

Question 33

How do we calculate BVP of contact lens empirically?

Answer 33

BVPcl = OcRx - BVPtears

Question 34

How do you calculate tear lens power?

Answer 34

336/BOZR - 336/K

Question 35

How do you estimate tear lens power changes?

Answer 35

0.25D for every 0.05mm difference

Approximation breaks down for differences greater than or equal to 0.2mm

Question 36

What is SAM and FAP?

Answer 36

SAM = Steepen add minus
FAP = Flatten add plus

This will be my last card in this deck, but later I should explain this better. It relates to tear lens power or something. I dunno. Am very tired. Want to sleep. Let’s do that.