Comparison of Keratometric Changes After Myopic Ablation: Ray Tracing Versus Simulated Keratometry

PURPOSE: To compare the changes in corneal power (Delta K) induced by myopic abiations estimated by Placido-disk and total corneal power (TCP) ray tracing methods to the refractive change (Delta SE). METHODS: Manifest refraction, simulated keratometry from a Placido-disk based system, and TCP from a dual-Scheimpflug analyzer were obtained from 35 patients (58 eyes) before and 3 months after a myopic ablation. The change in the manifest refraction that occurred after surgery was then vertex distance corrected and compared to the changes observed in corneal power with the various systems analyzed. RESULTS: The Placido-based simulated keratometry overestimated the corneal power after the procedure by 0.50 +/- 0.53 diopters (D) when compared to refractive change in the corneal plane induced by the laser surgery (Delta SE-Delta K). The ray tracing method showed the opposite trend, with the TCP simulated keratometry showing an underestimation of the corneal power of -0.25 +/- 0.48 D. The Placido system showed a direct correlation between the overestimation of the corneal power and the level of myopia, whereas the ray tracing method showed an underestimation, which was more pronounced in higher levels of myopia. CONCLUSIONS: Ray tracing methods underestimate corneal power as opposed to the overestimation in corneal power after refractive surgery, directly related to the level of myopia, observed in Placido-based systems. They have the potential to overcome the errors in calculations based on anterior curvature alone. However, ray tracing methods need to be validated and optimized before it can be used routinely in IOL calculation.