CL - RGP fitting 3 - Week 1 Flashcards
Where is the steep and flat axis for WTR astigmatism?
Steep @ 90deg
Flat @ 180deg
or axis within 30 degrees of those values
Which meridian should the negative cyl be placed in to correct astigmatism? steep or flat? Explain [important]
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.
Define regular astigmatism
2 axes or meridians that are perpendicular
What does delta K represent? How is it calculated?
Keratometric astigmatism
DeltaK = Steep - Flat meridian power
What is the purpose of javal’s rule?
Allows for the inclusion of posterior corneal astigmatism, which isn’t measured by corneal topography/keratometry (topographer/keratometer only measures the anterior cornea)
What is the formula for javal’s modified rule? How does this differ from the original javal’s rule? [important]
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)
What does the -0.50x90 represent in javal’s rule formula? Explain.
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.
How does the -0.50x90 work in javal’s rule formula if DeltaK is at 180 instead?
-0.50x90 is equivalent to +0.50x180 so add +0.50 instead
i.e. DeltaK + (-0.50x90) = DeltaK + (+0.50x180)
Using javal’s rule, what is the Rx astigmatism from keratometry values of:
46.11D@90
41.31D@180
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
What is residual astigmatism and how can we calculate it? [important]
astigmatism not due to the cornea.
= ocular astig - corneal cyl
= spec cyl - deltaK
= astigmatic component to fully correct an eye wearing spherical BOZR CL
What gas permeable lens is indicated if patient has a DeltaK >/= 1 and is an orthoK patient?
OrthoK
Toric periphery
What gas permeable lens is indicated in a patient with DeltaK >/= 2 and regular astigmatism?
Bitoric GP
What gas permeable lens is indicated in a patient with DeltaK >/= 5 and has PMD?
Bitoric GP
What gas permeable lens is indicated in a patient with Rx >/= 0.75DC residual astigmatism
Front surface toric
Define Spherical Power Equivalent (SPE)
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.
How will lens rotation affect astigmatism in a spherical power equivalent (SPE) bitoric?
it won’t
Where are the markings on a compensated (SPE) bitoric?
On the flat axis
How do compensated (SPE) bitorics correct for refractive cylinder? What about induced astigmatism?
Correct refractive cylinder via corneal toricity.
Correct induced astigmatism via front CL surface toricity
How do compensated (SPE) bitorics fit on the eye? (in terms of alignment) How is this achieved?
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)
Do SPE bitorics require a prism ballast? Explain
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
Define induced astigmatism [important]
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)
What is the formula for estimating induced astigmatism from a toric lens?
~0.4x CL toricity
(CL toricity = deltaK, in diopters obv.)
Estimate the induced astigmatism for the following lens:
BOZR 8.50 (flat) and 7.90 (steep)
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
What is the formula for exactly calculating induced astigmatism from a toric lens?
[1000(1.336-ncl)/BOZRflat] - [1000(1.336-ncl)BOZRsteep]
