10
In our study, the K readings need to be decreased by a factor of 0.23 D for each
diopter of myopic correction obtained by the refractive surgery. Our results confirm the
findings of Hamed et al (12) that showed a total error of 24%; half of this error (12%) is
due to changes in the net index of refraction after Lasik and the additional 11 to 12 %
error is due to sampling in the periphery of an aspherical corneal surface. The variation in
the measurements is due to the fact that the corneal location of keratometry readings
depends on the steepness of the cornea and is not fixed for all eyes, and that the readings
obtained by corneal topography depend on the number of rings evaluated.
Lasik surgery decreases the corneal thickness. Theoretically, such a decrease has
a neglectable effect on the corneal power (12). Also, the posterior corneal surface is
presumed unaltered with Lasik surgery and its refractive power does not affect the
changes noted after refractive surgery. In all calculations, the posterior corneal surface is
considered to have a fixed power of 5.90 D. In a study enrolling 263 normal
participants, Seitz and Langenbucher (7) found a wide inter-individual variability in
posterior surface keratometric diopters ranging from 2.10 to 8.50 D. Therefore adding
only the mean value of the posterior surface keratometric diopters to the anterior surface
keratometric diopters may cause a considerable error in a given patient. Further studies
and newer technology are needed to accurately measure not only the anterior corneal
radius but also the posterior one to obtain an exact measurement of the total corneal
power (12).
The contact lens over-refraction method (17) re-measures the corneal
power instead of re-calculating its value. The patient's refraction is first determined. A
Plano hard contact lens of a known base curve is then placed on the eye and an over-
