7
The corneal power was calculated in each operated eye using the history-derived
method (Kc.hd) where
Kc.hd = Kpre CRc
It was then compared to the post-Lasik K readings (Kpost) and the difference
between Kc.hd and Kpost per diopter of myopic correction was calculated in each case.
This difference averaged 0.23 ± 0.11 D. The 95% confidence interval around the mean
was (0.22, 0.24). This difference of 0.23 D per diopter of myopic correction at the corneal
level was used to establish the formula used to calculate the keratometric value by the
refraction-derived method where:
Kc.rd = Kpost (0.23 x CRc)
To establish the second formula, the measured values of Kpost and the calculated
ones with Kc.hd were plotted on a scattergram (Fig.1). The best-fit regression equation
was Y = 1.14 x 6.8 and the correlation coefficient was 0.9471. Using this equation and
based only on the post-Lasik keratometric readings (Kpost), the corrected keratometric
value can then be calculated by the clinically-derived method (Kc.cd) where:
Kc.cd = 1.14Kpost 6.8.
The second group of 100 eyes (Group II) was then evaluated to validate the
derived formulas. In each case, we calculated the Kc.rd and Kc.cd values and compared
the results to the Kc.hd values. First, the Kc.rd and Kc.hd values from each eye were
plotted on a scattergram. Then, the same comparison was performed for the Kc.cd and
Kc.hd values. We used the Pearson product-moment correlation coefficient to statistically
evaluate each scattergram correlation. We also calculated the mean keratometric value
(+/- standard deviation) obtained with each formula. Paired t-tests were performed to
