CL - RGP fitting 4 - Week 1 Flashcards

1
Q

List 5 indications for toric gp’s

A

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

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2
Q

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

A

Flat meridian: on-K for WTR
Steep meridian: slightly flatter than K

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3
Q

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

A

> /=1.50D

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4
Q

How can you calculate the ocular rx from BVPs?

A

Oc Rx = BVPtears + BVPcl

(likewise, BVPcl = Oc Rx - BVPtears)

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5
Q

How can you calculate tear lens power?

A

336/BOZR - 336/K

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6
Q

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

A

0.25D for every 0.05mm difference

Approximation breaks down for differences >/= 0.2mm

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7
Q

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

A

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

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8
Q

Define residual astigmatism (in the context of rgps)

A

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)

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9
Q

Define induced astigmatism

A

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)

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10
Q

What makes a spherical power equivalent bitoric lens a bitoric?

A

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

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11
Q

Explain the purpose behind an SPE (compensated) bitoric

A

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.

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12
Q

Explain the purpose behind an CPE bitoric lens

A

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

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13
Q

Give an example of residual astigmatism

A

Astigmatism from the lens

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14
Q

Is vision affected by lens rotation in CPE wearers?

A

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

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15
Q

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

A

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

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16
Q

What types of CPE torics are there? (4)

A

Alignment bitoric
Back surface bitoric
Oblique bitoric
Front surface toric

17
Q

Describe an alignment bitoric

A

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)

18
Q

Explain why an alignment bitoric is called an alignment bitoric

A

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

19
Q

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

A

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

20
Q

Describe a back surface toric

A

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

21
Q

When can you use back surface torics? Explain

A

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

22
Q

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

A

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

23
Q

Describe an oblique bitoric

A

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.

24
Q

How do you prescribe an oblique bitoric?

A

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

25
Q

Describe front surface torics

A

Toroidal front surface, spherical back surface.

26
Q

When are front surface torics used?

A

if significant residual astigmatism but minimal corneal astigmatism.

27
Q

What do front surface torics require?

A

lens stabilisation mechanisms such as prism ballast or truncation

28
Q

Are front surface torics prescribed frequently?

A

Rarely prescribed due to sucess of soft torics

29
Q

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

A

Can do.