Keratometry Flashcards

1
Q

what does keratometry measure?

A

central cornea curvature (3mm)

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2
Q

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

A

2/3

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3
Q

corneal profile is elliptical and central curvature is ___ than periphery

A

steeper

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4
Q

list 4 uses of keratometry

A
  1. to measure corneal curvature
  2. to determine corneal toricity
  3. to detect presence of corneal irregularity
  4. to fit contact lenses
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5
Q

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

A

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

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6
Q

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

A

image of mires is erect, virtual, behind corneal surface

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7
Q

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

A

curved

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8
Q

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

A

7.50mm

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9
Q

keratometer index of refraction

A

1.3375

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10
Q

why is the keratometer n=1.3375

A

takes into account negative power of corneal back surface

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11
Q

limitations and assumptions of the keratometer

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

- assumes spherical reflecting surface (but cornea aspheric)

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12
Q

pros of a computerized corneal topographer

A
  • larger area of cornea measured

- able to get global view of cornea

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13
Q

total astigmatism is =

A

corneal astigmatism + internal astigmatism (often lenticular)

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14
Q

what does corneal toxicity describe?

A

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

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15
Q

type of toricity:

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

A

with the rule

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16
Q

type of toricity:

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

A

against the rule

17
Q

where are the major meridians located for oblique toricity?

A

45 +/- 15 and 135 +/- 15

18
Q

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

A

mostly WTR corneal toricity which decreases over time

19
Q

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

A

between 0.25 and 0.75 D of corneal toricity, mostly WTR

20
Q

corneal toricity in adults shifts and at what age

A

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

21
Q

how to record for a toric cornea

A

steep x flat curvature (D) @ flat meridian

22
Q

list of clinically significant digits (1/8 diopters)

A

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

23
Q

what is the keratometer range in diopters?

A

36 D - 52 D

24
Q

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

A

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

25
what is the next step if the +1.25D trial lens isn't enough for the steep cornea measurement?
add +2.25 D trial lens, add ~16 D to drum reading
26
what do you do if the cornea is too flat, or flatter than 38 D? (like in post-refractive surgery)
extend range with -1.00 D spectacle trial lens and subtract 6D from the drum reading
27
definition of Javal's rule
predicts the amount of refractive astigmatism based on the corneal toricity
28
equation for Javal's rule
Total Astigmatism = 1.25 (ΔK) + 0.50D ATR
29
in Javal's rule, what can the internal astigmatism be represented by?
-0.50 x ATR axis OR +0.50 x WTR axis
30
what is different about Javal's rule for oblique corneas?
Javal's rule ignores internal astigmatism, so Total Astigmatism = 1.25 (ΔK)