Surgically induced astigmatism after hyperopic and myopic photorefractive keratectomy

Abstract

Purpose To compare the axis and magnitude of surgically induced refractive astigmatism (SIA) after hyperopic and myopic photorefractive keratectomy (PRK). Setting Department of Ophthalmology, University of Texas Southwestern Medical Center, Dallas, Texas, USA. Methods In this single-center retrospective study, the VISX Star S2 excimer laser was used to create a peripheral annular ablation profile to correct spherical hyperopia in 23 eyes of 16 consecutive patients. Attempted corrections ranged from +0.50 diopter (D) to +4.25 D with 0 to 1.00 D of astigmatism. The same laser was used to create a central ablation profile to correct spherical myopia in 25 eyes of 17 consecutive patients. Attempted corrections ranged from –2.25 to –6.50 D with 0 to 1.00 D of astigmatism. The absolute change in refractive astigmatism was calculated by taking the difference in magnitudes of astigmatism before and after laser treatment without regard to axis. Axis and magnitude of SIA were analyzed by vector differences. Magnitudes were compared using the Student t test, and axial shifts were compared using the chi-square test. All patients were followed for a minimum of 6 months. Results The mean changes in absolute astigmatism were 0.29 ± 0.28 D at 3 months and 0.34 ± 0.29 D at 6 months after hyperopic PRK and 0.40 ± 0.35 D at 3 months and 0.39 ± 0.36 D at 6 months after myopic PRK. The mean vectoral magnitudes were 0.49 ± 0.29 at 3 months and 0.52 ± 0.25 at 6 months after hyperopic PRK and 0.48 ± 0.39 at 3 months and 0.44 ± 0.38 at 6 months after myopic PRK. The mean values for SIA (the centroid) were 0.10 ± 0.57 D × 113 degrees at 3 months and 0.15 ± 0.57 D × 131 degrees at 6 months after hyperopic PRK and 0.04 ± 0.63 D × 160 degrees at 3 months and 0.08 ± 0.58 D × 171 degrees at 6 months after myopic PRK. There was no statistically significant difference between the 2 groups in vectoral axis or magnitude of SIA. Conclusion Surgically induced astigmatism after hyperopic PRK was comparable to astigmatism induced by myopic PRK. A peripheral annular ablation for hyperopic correction, similar to a central ablation in myopic PRK, did not appear to result in uneven corneal healing causing astigmatism.