Keratometry Flashcards
what does keratometry measure?
central cornea curvature (3mm)
the cornea is approximately ___ of the refracting power of the eye
2/3
corneal profile is elliptical and central curvature is ___ than periphery
steeper
list 4 uses of keratometry
- to measure corneal curvature
- to determine corneal toricity
- to detect presence of corneal irregularity
- to fit contact lenses
what does the corneal act as for the keratometer to work?
acts as a convex (diverging) mirror (requires a good tear film)
what type of image and where is the image of the mires in keratometry?
image of mires is erect, virtual, behind corneal surface
the smaller the radius of curvature, the surface is more ____ (curved/flat)
curved
which radius of curvatures has a more curved surface: 7.50mm or 8.00mm
7.50mm
keratometer index of refraction
1.3375
why is the keratometer n=1.3375
takes into account negative power of corneal back surface
limitations and assumptions of the keratometer
- measures central 3mm of cornea (but cornea 11.5mm wide)
- assumes spherical reflecting surface (but cornea aspheric)
pros of a computerized corneal topographer
- larger area of cornea measured
- able to get global view of cornea
total astigmatism is =
corneal astigmatism + internal astigmatism (often lenticular)
what does corneal toxicity describe?
the difference in curvature of the principle meridians of the anterior corneal surface
type of toricity:
cornea is steeper in the vertical meridian (90 +/- 30)
with the rule
type of toricity:
cornea is steeper in the horizontal meridian (180 +/- 30)
against the rule
where are the major meridians located for oblique toricity?
45 +/- 15 and 135 +/- 15
what kind of corneal toricity do infants (3 to 36 months) have?
mostly WTR corneal toricity which decreases over time
what kind of corneal toricity do school-aged children (6-14 yo) have?
between 0.25 and 0.75 D of corneal toricity, mostly WTR
corneal toricity in adults shifts and at what age
mostly WTR (<20s to 50s) and then shifts to ATR with age (70s and >80s)
how to record for a toric cornea
steep x flat curvature (D) @ flat meridian
list of clinically significant digits (1/8 diopters)
0.12, 0.25, 0.37, 0.50, 0.62, 0.75, 0.87, 0.00
what is the keratometer range in diopters?
36 D - 52 D
what is the first step if the cornea is steeper than 52D? (like in keratoconus)
add +1.25 D trial lens, then add 8/9 D to drum reading
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
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
definition of Javal’s rule
predicts the amount of refractive astigmatism based on the corneal toricity
equation for Javal’s rule
Total Astigmatism = 1.25 (ΔK) + 0.50D ATR
in Javal’s rule, what can the internal astigmatism be represented by?
-0.50 x ATR axis OR +0.50 x WTR axis
what is different about Javal’s rule for oblique corneas?
Javal’s rule ignores internal astigmatism, so Total Astigmatism = 1.25 (ΔK)
