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ABSTRACT
PURPOSE: To describe and evaluate a refraction-derived method and a clinically-
derived method to calculate the correct corneal power for IOL power calculations after
Lasik, and to compare the results to the commonly used history-derived method.
DESIGN: Interventional case series.
METHODS: Retrospective analysis of consecutive cases.
Setting: clinical practice
Study population: 200 randomly selected eyes from 200 patients were evaluated before
and after Lasik surgery. For each case, we established the pre- and post-Lasik spectacle
refraction, the pre-Lasik (Kpre) and post-Lasik K readings (Kpost). We then calculated
for each case the pre- and post-Lasik refraction at the corneal plane and the amount of
correction obtained by the refractive surgery (CRc). The cases were divided into 2
groups. Group I was used to derive the two formulas. The K values were calculated using
the history-derived method (Kc.hd) where Kc.hd = Kpre CRc. Kc.hd was compared to
Kpost. The average difference was 0.23 D for every diopter of myopia corrected at the
corneal plane. This value was used to calculate the corneal power using the refraction-
derived method (Kc.rd) where Kc.rd = Kpost 0.23CRc. A regression equation was also
established by comparing on a scattergram the values of Kc.hd and Kpost in each case.
This equation was used to develop a clinically-derived method (Kc.cd) where Kc.cd =
1.14Kpost 6.8. The values obtained with the two methods were compared to the Kc.hd
values in Group II to validate the results.
