A direct method to measure the power of the central cornea after myopic laser in situ keratomileusis

Abstract

Objective: To measure the corneal power after myopic laser in situ keratomileusis (LASIK).

Methods: Six central areas in 6 corneal power maps were studied using the Orbscan II statistical analysis device in 26 eyes that underwent myopic LASIK. Refractive and corneal power changes were compared. Factors related to wrong corneal power measurement were evaluated.

Main outcome measures: Cycloplegic refraction, refractive change at the corneal plane, and Orbscan II corneal power maps.

Results: Preoperatively, only posterior-mean power (P<<.001) and anterior-posterior power ratio (P<<.001) varied according to the size of the analyzed area. Postoperatively, total-optical (P =.03), keratometric-mean (P =.04), total-mean (P<.001), anterior-mean (P =.03), and posterior-mean (P<<.001) powers; and anterior-posterior power ratio (P<<.001) varied according to the area. Postoperatively, the difference between keratometric-mean and total-mean powers became larger (P<.001), and the anterior-posterior power ratio was reduced (P<<.001). A posterior-mean power change occurred (P =.04). Refractive change after myopic LASIK was best estimated by 2-mm total-mean power (mean +/- SD difference, 0.07 +/- 0.62 diopters [D]; P =.55) and 4-mm total-optical power (mean +/- SD difference, -0.08 +/- 0.53 D; P =.37).

Conclusions: Total corneal power is more positive and refractive change is underestimated when deduced from the anterior surface radius and keratometric refractive index. The anterior-posterior power ratio is not a fixed value. The best area to estimate the refractive change depends on the method used to obtain the power in diopters. Refractive change tended to be underestimated in larger areas and higher preoperative myopia. Orbscan II total-mean and total-optical power maps accurately assess the corneal power after myopic LASIK independent of preoperative data or correcting factors, and should improve intraocular lens calculation.