Comparison of Changes in Manifest Refraction and Orbscan Power Maps after Photorefractive Keratectomy and Laser in Situ Keratomileusis

Kim, Su Young; Lew, Ho Min; Chang, Yoon Hee; Ahn, Jae Hong

Abstract

PURPOSE
To determine whether the refractive change obtained using the Orbscan-derived power maps is in concordance with the manifest refractive change produced after photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK). METHODS: Forty eyes of 21 PRK patients and 40 of 21 LASIK patients were included in the study. Orbscan topographical analysis and manifest refraction were performed preoperatively and 1 month postoperatively. This study evaluated the concordance between the change in manifest refraction (corrected to the corneal plane) and the change measured by Orbscan power maps before and after PRK and LASIK. We also compared the concordance of the change in manifest refraction (corrected to the corneal plane) with the change calculated using the Gaussian optics formula, radius of anterior and posterior corneal surface , and corneal thickness measured by ultrasound pachymetry. RESULTS: The 3.0-mm zone axial power anterior map gave the best correlation between manifest refractive change, and Orbscan-measured corneal power change in PRK (r =0.733, P<0.001). The 4.0-mm zone of calculated refractive change using Gaussian optic formula map gave the best correlation with manifest refractive change in LASIK (r =0.531 , P<0.001). The change of anterior corneal surface maps in PRK and the change of total power maps in LASIK represented the best correlation with the change in manifest refraction, but they did not reliably represent the changes in manifest refraction due to high standard deviation value. CONCLUSIONS: The 3.0-mm zone axial power anterior map gave the best correlation between manifest refractive change, and Orbscan-measured corneal power change in PRK. The 4.0-mm zone of calculated refractive change using Gaussian optic formula map gave the best correlation with manifest refractive change in LASIK. Nevertheless, these maps did not reliably represent the changes in manifest refraction because of the high standard deviation value.