Corneal ray tracing versus simulated keratometry for estimating corneal power changes after excimer laser surgery

Abstract

Purpose: To evaluate whether the refractive changes induced by excimer laser surgery can be accurately measured by corneal ray tracing performed by a combined rotating Scheimpflug camera-Placido-disk corneal topographer (Sirius).

Setting: Private practices.

Design: Evaluation of diagnostic test.

Methods: This multicenter retrospective study comprised patients who had myopic or hyperopic excimer laser refractive surgery. Preoperatively and postoperatively, 2 corneal power measurements--simulated keratometry (K) and mean pupil power--were obtained. The mean pupil power was the corneal power calculated over the entrance pupil by ray tracing through the anterior and posterior corneal surfaces using Snell's law. Agreement between the refractive and corneal power change was analyzed according to Bland and Altman. Regression analysis and Bland-Altman plots were used to evaluate agreement between measurements.

Results: The study evaluated 72 eyes (54 patients). The difference between the postoperative and preoperative simulated K values underestimated the refractive change after myopic correction and overestimated it after hyperopic correction. Agreement between simulated K changes and refractive changes was poor, especially for higher amounts of correction. A proportional bias was detected (r = -0.77; P<.0001), and the 95% limits of agreement (LoA) were -0.15 -0.14 × ±0.62 diopters (D). The difference between the postoperative and preoperative mean pupil power showed an excellent correlation with the refractive change (r(2) = 0.98). The mean pupil power did not overestimate or underestimate the refractive change. The 95% LoA ranged between -0.97 D and +0.56 D.

Conclusion: Corneal ray tracing accurately measured corneal power changes after excimer laser refractive surgery.

Financial disclosures: Dr. Calossi is consultant to Costruzione Strumenti Oftalmici. Dr. Carones is consultant to Wavelight Laser Technologie AG. No other author has a financial or proprietary interest in any material or method mentioned.