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Flashcards in Lecture 3 - corneal topography Deck (19)
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1

what is the e-value represent?

the descriptor for the amount of peripheral flattening - no unit

2

what is the typical e value for a human cornea?

0.4 - 0.6

3

what does it mean if the e value is higher?

very flat periphery (low e value = closer to spheres)

4

what is the e-value for keratoconus patients?

0.9 - 1.5 (steep center and very flat periphery)

5

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

6

what units are the axial and tangential maps in?

mm or diopters

7

what unit is the elevation map in?

microns - used for simulating fluorescein images (wavefront aberrations is also in microns)

8

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)

9

what is the average central K-reading of a cornea?

7.84mm (43.00D)

10

what is an aspheric CL?

back surface radii progressively flatten from center to periphery - follows aspheric corneal shape

11

what is a multi-curve CL?

back surface has spherical radii in distinct zones - zone radii are larger in periphery (approximates elliptical corneal shape)

12

which is steeper in a multi-surface CL, the BCR or the SCR?

the SCR is steeper and then flattens SCR

13

what is the shape factor?

the e-value squared (linear change with linear C-T change)

14

how much of the cornea is measured with a keratometer?

the central 3mm surface area

15

what are the components for a keratoconus diagnosis?

K > 47.20D, I-S difference > 1.40D and KISA index

16

when is the axial map used?

when fitting CL - identifying type and extension of astigmatism

17

what is the tangential map used for?

to detect subtle changes and monitoring corneal changes

18

how do you know if a patient has unstable tearfilm in topography measurements?

when the patient blinks it makes the irregular mires regular again

19

what should the topographer be aligned to when taking a measurement?

the patients line of sight