Keratometry: Kerato Cornea Metry Measurement
Keratometry: Kerato Cornea Metry Measurement
Keratometry: Kerato Cornea Metry Measurement
Kerato = cornea
Metry = measurement
Marlya Asri
131810
The ‘keratometry’ or ‘ophthalmometry’ is an
objective method of estimating the corneal
astigmatism by measuring the curvature of central
cornea.
- not of much value in routine refraction for
prescribing glasses;
- of utmost value for prescribing contact lenses and
for calculating the power of intraocular lens to
be implanted.
Principle of keratometer
Keratometer is based on the fact that the anterior surface of the cornea acts as a convex
mirror; so the size of the image produced varies with its curvature. (Inversely proportional)
Therefore, from the size of the image formed by the anterior surface of cornea, the radius
of curvature of cornea can be calculated. The accurate measurement of the image size is
obtained by using the principle of visible doubling
Normal ranges : 43.00 to 44.00 dioptres
Less than 40.00 or more than 47.00 = unsual
Determines :
P = A–2.5L–0.9K
Ask patient to sit down, place chin on chin rest, aligned the
patient’s outer canthus is aligned with the mark on the
instrument. Cover one eye
Instruct patient
• Keep eyes open wide and blink normally
• Try not to move head or speak
• Look at the reflection of own eye in the keratometer barrel
the patient must be fully informed about the procedure. In keratometry, they should be
assured that nothing will touch the eye and they will be in no discomfort. This is particularly
important as any tendency to squeeze the lids may alter corneal contour.
Examiner :
Look into the keratometer and refine the alignment of the image of the mires
(three circles) on the patient cornea
Focus and adjust so that the reticle is centered in the lower rt hand circle
Locate the axis of the cylinder by lining up two cross by totating the barrel with
axis grip.
To locate the two principal meridians of the pt’s cornea,
Focus the horizontal meridian by turning the horizontal measuring drum until the double
crosses are superimposed.
Then we can measure the vertical meridian by rorating the vertical measuring drum until
the double minuses are superimposed
A common practice is to record the horizontal reading first.
However, some sites will record the flattest meridian first followed by the steepest meridian
and its axis
H V flattest steepest
44.00 @160 / 42.75 @ 070 or 42.75 @ 070 / 44.00 @ 160