Lecture 3 - corneal topography Flashcards
what is the e-value represent?
the descriptor for the amount of peripheral flattening - no unit
what is the typical e value for a human cornea?
0.4 - 0.6
what does it mean if the e value is higher?
very flat periphery (low e value = closer to spheres)
what is the e-value for keratoconus patients?
0.9 - 1.5 (steep center and very flat periphery)
what does a negative e value mean?
indicative of Lasik, PRK, ortho-K when myopia was treated (0 to -0.9) = cornea is oblate or oblong
what units are the axial and tangential maps in?
mm or diopters
what unit is the elevation map in?
microns - used for simulating fluorescein images (wavefront aberrations is also in microns)
what unit is the refractive power map in?
diopters - used for prediction of optical power changes in front of pupil (keratoconus, ortho-K and LASIK)
what is the average central K-reading of a cornea?
7.84mm (43.00D)
what is an aspheric CL?
back surface radii progressively flatten from center to periphery - follows aspheric corneal shape
what is a multi-curve CL?
back surface has spherical radii in distinct zones - zone radii are larger in periphery (approximates elliptical corneal shape)
which is steeper in a multi-surface CL, the BCR or the SCR?
the SCR is steeper and then flattens SCR
what is the shape factor?
the e-value squared (linear change with linear C-T change)
how much of the cornea is measured with a keratometer?
the central 3mm surface area
what are the components for a keratoconus diagnosis?
K > 47.20D, I-S difference > 1.40D and KISA index
