Effective Optical Zone Following Small Incision Lenticule Extraction for Myopia Calculated With Two Novel Methods

Abstract

PURPOSE: To evaluate the effective optical zone (EOZ) following small incision lenticule extraction in myopic eyes using two novel methods and investigate factors influencing postoperative EOZ. METHODS: In this prospective study, 45 patients (45 eyes) with a mean spherical equivalent of −5.82 ± 1.58 diopters underwent SMILE and were observed during a 6-month follow-up. Postoperative EOZ was calculated using custom software that automatically distinguishes EOZ on the tangential curvature difference map (EOZc) and total corneal refractive power map (EOZp) of the Pentacam HR (Oculus Optikgeräte GmbH). The agreement between the two methods, the difference between postoperative EOZ and programmed optical zone (POZ), and its relationship with parameters including corrected spheres, cylinders, ablation ratio (ablation depth/central cornea thickness), and Q-value change were investigated. RESULTS: The EOZc area was 20.76 ± 2.43 mm 2 (diameter: 5.04 ± 0.60 mm) and the EOZp area was 20.22 ± 4.70 mm 2 (diameter: 5.13 ± 0.30 mm). Both were significantly smaller than POZ ( P < .001). Bland-Altman plots showed 4.44% (2/45) points located outside the 95% limits of agreement. EOZc and EOZp reductions were negatively related to corrected cylinders ( r = −0.631, P < .001 and r = −0.594, P < .001, respectively). EOZp reduction was positively correlated with corrected spheres ( r = 0.336, P = .024). Subgroup analysis revealed significant differences in EOZc and EOZp reduction between low and high astigmatism groups despite myopia degree. CONCLUSIONS: EOZ after incision lenticule extraction, measured using two novel methods, was smaller than POZ. The reduction of EOZ was negatively correlated with the corrected cylinders. [ J Refract Surg. 2022;38(7):414–421.]