Surgically induced astigmatism after photorefractive keratectomy and laser in situ keratomileusis

Purpose: To compare the axis and magnitude of surgically induced astigmatism in photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK). Setting: Multicenter clinical trial. Methods: In this prospective, randomized trial, 220 eyes of 220 patients entered the study cohort; 105 randomized to PRK and 115 to LASIK. All patients received a single-pass, multizone excimer laser ablation as part of a PRK or LASIK procedure. Attempted corrections ranged from -6.00 to -15.00 diopters (D). The LASIK procedures were performed with nasal hinges. Absolute changes in astigmatism and axis and magnitude of surgically induced astigmatism were analyzed. Patients were followed for up to 6 months. Results: In the PRK group, the mean change in absolute astigmatism was +0.14 +0.16, and +0.32 D at 1, 3, and 6 months, respectively; in the LASIK group, the mean change was -0.15, -0.08, and -0.03 D, respectively At ail time points, a greater proportion of PRK than LASIK eyes had an increase in absolute magnitude of astigmatism. in the PRK group, the axis of vectoral-induced astigmatism ws significantly different from random at 3 and 6 months(P = .01, P < .001, respectively) with a tendency for induced with-the-rule shirts postoperatively. In the LASIK group, the axis of vectoral-induced astigmatism was significantly different from random at only 1 month (P = .04), and the re was no preponderant direction of axis shift. Despite these findings, other analyses showed no statistically significant between-group differences in vectoral axis;or magnitude of surgically induced astigmatism. Conclusions: Induced astigmatism was generally less and more random in axis in LASIK than in PRK; a general trend for induced with-the-rule astigmatism in PRK was not seen in LASIK. Hypothetically, the lamellar corneal flap in LASIK may counteract the tendency toward steepening at 90 degrees-seen in PRK by refracting toward the hinge, by masking underlying induced astigmatism in the ablation zone, or by its mitigating influence on postoperative corneal healing.