CL - RGP fitting 4 - Week 1 Flashcards
List 5 indications for toric gp’s
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
How should you select the BOZRs for the principle meridians for a toric gp?
Flat meridian: on-K for WTR
Steep meridian: slightly flatter than K
When choosing BOZRs for a toric gp, what should CL toricity be?
> /=1.50D
How can you calculate the ocular rx from BVPs?
Oc Rx = BVPtears + BVPcl
(likewise, BVPcl = Oc Rx - BVPtears)
How can you calculate tear lens power?
336/BOZR - 336/K
What is the approximation for tear lens power? When does this approximation break down?
0.25D for every 0.05mm difference
Approximation breaks down for differences >/= 0.2mm
What is the main difference between a spherical power equivalent bitoric and a cylindrical power equivalent bitoric? (in terms of what they correct)
cylindrical power equivalent (CPE) bitoric corrects for residual (non-corneal) astigmatism whereas SPE does not
Define residual astigmatism (in the context of rgps)
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)
Define induced astigmatism
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)
What makes a spherical power equivalent bitoric lens a bitoric?
they are bitoric because the front surface contains a cylinder solely for the correction of induced astigmatism
Explain the purpose behind an SPE (compensated) bitoric
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.
Explain the purpose behind an CPE bitoric lens
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
Give an example of residual astigmatism
Astigmatism from the lens
Is vision affected by lens rotation in CPE wearers?
Yes. Because the lens contains residual cylinder (like a soft loric)
How do you convert mm to diopters (or vice-versa)?
337.5/x, where x is the number (mm or diopters)