Keratometry

Question 1

what does keratometry measure?

Answer 1

central cornea curvature (3mm)

Question 2

the cornea is approximately ___ of the refracting power of the eye

Answer 2

2/3

Question 3

corneal profile is elliptical and central curvature is ___ than periphery

Answer 3

steeper

Question 4

list 4 uses of keratometry

Answer 4

1. to measure corneal curvature
2. to determine corneal toricity
3. to detect presence of corneal irregularity
4. to fit contact lenses

Question 5

what does the corneal act as for the keratometer to work?

Answer 5

acts as a convex (diverging) mirror (requires a good tear film)

Question 6

what type of image and where is the image of the mires in keratometry?

Answer 6

image of mires is erect, virtual, behind corneal surface

Question 7

the smaller the radius of curvature, the surface is more ____ (curved/flat)

Answer 7

curved

Question 8

which radius of curvatures has a more curved surface: 7.50mm or 8.00mm

Answer 8

7.50mm

Question 9

keratometer index of refraction

Answer 9

1.3375

Question 10

why is the keratometer n=1.3375

Answer 10

takes into account negative power of corneal back surface

Question 11

limitations and assumptions of the keratometer

Answer 11

• measures central 3mm of cornea (but cornea 11.5mm wide)

- assumes spherical reflecting surface (but cornea aspheric)

Question 12

pros of a computerized corneal topographer

Answer 12

• larger area of cornea measured

- able to get global view of cornea

Question 13

total astigmatism is =

Answer 13

corneal astigmatism + internal astigmatism (often lenticular)

Question 14

what does corneal toxicity describe?

Answer 14

the difference in curvature of the principle meridians of the anterior corneal surface

Question 15

type of toricity:

cornea is steeper in the vertical meridian (90 +/- 30)

Answer 15

with the rule

Question 16

type of toricity:

cornea is steeper in the horizontal meridian (180 +/- 30)

Answer 16

against the rule

Question 17

where are the major meridians located for oblique toricity?

Answer 17

45 +/- 15 and 135 +/- 15

Question 18

what kind of corneal toricity do infants (3 to 36 months) have?

Answer 18

mostly WTR corneal toricity which decreases over time

Question 19

what kind of corneal toricity do school-aged children (6-14 yo) have?

Answer 19

between 0.25 and 0.75 D of corneal toricity, mostly WTR

Question 20

corneal toricity in adults shifts and at what age

Answer 20

mostly WTR (<20s to 50s) and then shifts to ATR with age (70s and >80s)

Question 21

how to record for a toric cornea

Answer 21

steep x flat curvature (D) @ flat meridian

Question 22

list of clinically significant digits (1/8 diopters)

Answer 22

0.12, 0.25, 0.37, 0.50, 0.62, 0.75, 0.87, 0.00

Question 23

what is the keratometer range in diopters?

Answer 23

36 D - 52 D

Question 24

what is the first step if the cornea is steeper than 52D? (like in keratoconus)

Answer 24

add +1.25 D trial lens, then add 8/9 D to drum reading

Question 25

what is the next step if the +1.25D trial lens isn’t enough for the steep cornea measurement?

Answer 25

add +2.25 D trial lens, add ~16 D to drum reading

Question 26

what do you do if the cornea is too flat, or flatter than 38 D? (like in post-refractive surgery)

Answer 26

extend range with -1.00 D spectacle trial lens and subtract 6D from the drum reading

Question 27

definition of Javal’s rule

Answer 27

predicts the amount of refractive astigmatism based on the corneal toricity

Question 28

equation for Javal’s rule

Answer 28

Total Astigmatism = 1.25 (ΔK) + 0.50D ATR

Question 29

in Javal’s rule, what can the internal astigmatism be represented by?

Answer 29

-0.50 x ATR axis OR +0.50 x WTR axis

Question 30

what is different about Javal’s rule for oblique corneas?

Answer 30

Javal’s rule ignores internal astigmatism, so Total Astigmatism = 1.25 (ΔK)