CL - RGP fitting 4 - Week 1

Question 1

List 5 indications for toric gp’s

Answer 1

Regular corneal astig deltaK >/= 2.00D
Residual astigmatism
Semi-regular high astig (deltaK >/= 5.0D) (PMD)
Poor fit with spherical gp
soft toric refit for better vision or GPC

Question 2

How should you select the BOZRs for the principle meridians for a toric gp?

Answer 2

Flat meridian: on-K for WTR

Steep meridian: slightly flatter than K

Question 3

When choosing BOZRs for a toric gp, what should CL toricity be?

Answer 3

> /=1.50D

Question 4

How can you calculate the ocular rx from BVPs?

Answer 4

Oc Rx = BVPtears + BVPcl

likewise, BVPcl = Oc Rx - BVPtears

Question 5

How can you calculate tear lens power?

Answer 5

336/BOZR - 336/K

Question 6

What is the approximation for tear lens power? When does this approximation break down?

Answer 6

0.25D for every 0.05mm difference

Approximation breaks down for differences >/= 0.2mm

Question 7

What is the main difference between a spherical power equivalent bitoric and a cylindrical power equivalent bitoric? (in terms of what they correct)

Answer 7

cylindrical power equivalent (CPE) bitoric corrects for residual (non-corneal) astigmatism whereas SPE does not

Question 8

Define residual astigmatism (in the context of rgps)

Answer 8

the astigmatic component of a lens required to fully correct an eye wearing a spherical powered rgp with a spherical BOZR

(in simple terms it’s basically non-corneal astig)

Question 9

Define induced astigmatism

Answer 9

the astigmatic effect created in the contact lens/tear lens system by the toroidal back optic zone bounding two different surfaces of different refractive indices (namely, the lens and the tears)

Question 10

What makes a spherical power equivalent bitoric lens a bitoric?

Answer 10

they are bitoric because the front surface contains a cylinder solely for the correction of induced astigmatism

Question 11

Explain the purpose behind an SPE (compensated) bitoric

Answer 11

Designed to correct all of the refractive cylinder created from corneal toricity. If the corneal toricity is = to spectacle astigmatism (I.e deltaK = CL cyl power) –> a compensated bitoric will fully correct the patient.

Question 12

Explain the purpose behind an CPE bitoric lens

Answer 12

All non-SPE bitorics are classified as CPE. Their unifying feature is they incorporate a correction for residual astigmatism (in addition to induced and refractive)

If the deltaK =/= spec astig (i.e. BVP of the meridian): then using a CPE instead of an SPE will fully correct the patient

Question 13

Give an example of residual astigmatism

Answer 13

Astigmatism from the lens

Question 14

Is vision affected by lens rotation in CPE wearers?

Answer 14

Yes. Because the lens contains residual cylinder (like a soft loric)

Question 15

How do you convert mm to diopters (or vice-versa)?

Answer 15

337.5/x, where x is the number (mm or diopters)

Question 16

What types of CPE torics are there? (4)

Answer 16

Alignment bitoric
Back surface bitoric
Oblique bitoric
Front surface toric

Question 17

Describe an alignment bitoric

Answer 17

Toroidal front and back surface, where the front surface incorporates corrections for both residual and induced astigmatism.

Correction for resdiual astigmatism is along one of the principle meridians of the lens (hence why it’s called alignment)

Question 18

Explain why an alignment bitoric is called an alignment bitoric

Answer 18

The axes of the spectacle refraction over the lens correspond with the principle meridians of corneal curvature, so the correction for the residual astigmatism will be along one of the principle meridians of the lens

Question 19

What is the tolerance on the principle axis in over-refraction to assume it corresponds with the principle meridians of the cornea?

Answer 19

Within about 25 degrees. So it should be within 25 degrees for an alignment bitoric fit.

Question 20

Describe a back surface toric

Answer 20

Spheroidal front, toroidal back surface. Front surface corrects for both residual and induced, and the corrections cancel out, leaving a spherical front surface.

Question 21

When can you use back surface torics? Explain

Answer 21

Only when the residual astigmatism is equal to the induced astigmatism. Because that way when the toroidal back surface corrects the two, they cancel each other out (equal and opposite)

NB: this is very rare

Question 22

Mathematically, explain what the residual astigmatism must be like for a back surface toric lens to be indicated?

Answer 22

Residual astigmatism must (in negative cyl form) have the same axis as that of the corneal cylinder

and be = to around 0.4 x CL toricity

Question 23

Describe an oblique bitoric

Answer 23

Toroidal front and back surface. Correction for oblique residual astig is NOT along a principle meridian of the lens. And the principle meridians of the toroidal front and back surfaces are NOT parallel.

Question 24

How do you prescribe an oblique bitoric?

Answer 24

prescribe by getting oblique over-rx ground onto the front surface of a back-surface toric

Question 25

Describe front surface torics

Answer 25

Toroidal front surface, spherical back surface.

Question 26

When are front surface torics used?

Answer 26

if significant residual astigmatism but minimal corneal astigmatism.

Question 27

What do front surface torics require?

Answer 27

lens stabilisation mechanisms such as prism ballast or truncation

Question 28

Are front surface torics prescribed frequently?

Answer 28

Rarely prescribed due to sucess of soft torics

Question 29

NB: there are some case examples that I can’t really put into cards. Make sure to read these examples etc.

Answer 29

Can do.