Corneal Topography

Question 1

what is the quantitative reading of a keratometer?

Answer 1

K readings, curvature values

Question 2

what is the qualitative reading of a keratometer?

Answer 2

mire image quality, 1+ to 4+ mire distortion

Question 3

the klein keratoscope that is hand held and qualitative only?

Answer 3

Placido’s disk

Question 4

“polaroid” where you measure distances on pictures and figure out curvature from it is called

Answer 4

Gene Reynolds-corneascope

Question 5

video to capture image and computer to interpret image is called a

Answer 5

videokeratoscope

Question 6

the videokeratoscope projects rings or grid pattern and uses what type of detections?

Answer 6

either edge detection (bowl, width of black or white rings) or image separation (cone, ring to ring)

Question 7

which one has the shorter capture distance: cone design or bowl design?

Answer 7

cone design has shorter capture distance

Question 8

when is alignment and focus more critical: with shorter or longer capture distance?

Answer 8

more critical with shorter capture distance

Question 9

with videokeratoscopy, a topographic contour map is produced where rings closer together=

Answer 9

they are steeper

Question 10

list of 5 indices a videokeratoscoper can give

Answer 10

1. simulated keratometry values (Sim K)
2. surface asymmetry index (SAI)
3. surface regularity index (SRI)
4. potential visual acuity (PVA)
5. corneal eccentricity index (CEI)

Question 11

positive (+) value for CEI means

Answer 11

prolate

Question 12

0 value for CEI means

Answer 12

sphere

Question 13

negative (-) value for CEI means

Answer 13

oblate

Question 14

which colors have flatter curvature areas

Answer 14

blue (cool colors)

Question 15

hot colors (red, orange) mean:

Answer 15

steeper curvature areas

Question 16

what is the absolute color scale

Answer 16

35-52 D range, same curvature always same color

Question 17

describe features of the absolute color scale

Answer 17

• fixed scale
• can compare universally
• same curvature always the same color
• usually lower resolution due to larger increments

Question 18

describe the features of the normalized scale

Answer 18

• individual scale

- specific to patient and patient visit

Question 19

in data analysis, axial=

Answer 19

• sagittal

- values relative to the visual axis

Question 20

pros/cons of axial data analysis

Answer 20

• extreme values are averaged
• less noise
• less detail (more lipstick)

Question 21

in data analysis, tangential=

Answer 21

• instantaneous rate of curvature

- value obtained at the foci

Question 22

pros/cons of tangential data analysis

Answer 22

• more accurate for distorted corneas and in periphery

- more local detail

Question 23

how is elevation used in data analysis ?

Answer 23

elevation= height data (not curvature) and is relative to computer generated best fit reference sphere 
(red= higher elevation, blue= lower elevation)

Question 24

what are difference maps useful for

Answer 24

they map progression/change- useful for orthoK or post surgery corneal thinning

Question 25

what is an example of a screening indices

Answer 25

Rabinwiitz I-S for keratoconus- 1.5D difference b/w above center and below center good indication of keratoconus

Question 26

3 examples of other display possibilities

Answer 26

1. difference maps
2. screening indices
3. contact lens design, including fluorescein patterns

Question 27

what is the orbscan (Scheimpflug slit)

Answer 27

• uses Placid disk plus scanning slit

- performs posterior K topography (posterior float) and K thickness measurements