Comparison of corneal epithelial remodeling after small incision lenticule extraction and femtosecond laser-assisted LASIK

Abstract

Objective: To investigate the corneal epithelial thickness changes between small incision lenticule extraction (SMILE) and femtosecond laser-assisted LASIK (FS-LASIK), study the related factors and analyze the relationship between the changes in corneal epithelial thickness and corneal aberration. Methods: This case control study included 59 patients (118 eyes), 27 males and 32 females, aged 25.64±5.57 years, who were scheduled for SMILE or FS-LASIK for treatment of myopia with or without myopic astigmatism at the First Affiliated Hospital of Soochow University between October 2016 and May 2017. All patients were divided into two groups according to the surgery. Epithelial thickness was obtained in nine zones with Fourier-domain optical coherence tomography across a 5-mm diameter centered by the pupil before surgery and at 1 week, 1 month, 3 months and 6 months postoperatively. The observed changes were analyzed by linear-regression analysis with the central corneal thickness decrement, ablation rate and corneal aberration. Comparison between two groups was analyzed by independent sample t-test, analysis of variance and Mann-Whitney U test. Results: There was no significant difference in corneal epithelial thickness of nine zones between SMILE and FS-LASIK for low to moderate myopia (manifest refraction sphere equivalent<-6.00 diopters) and high myopia (manifest refraction sphere equivalent ≥-6.00 to -10.00 diopters), respectively, before surgery (P>0.05). At 1 month, no significant difference existed in corneal epithelial thickness increment in nine zones between FS-LASIK and SMILE for low to moderate myopia (P>0.05), but a larger increase was observed in the superior (t=2.670, P=0.010), superonasal (t=2.506, P=0.015) and nasal (t=2.831, P=0.007) zones following FS-LASIK [(4.2±3.2), (3.4±2.7) and (3.7±2.5) μm] than SMILE [(2.2±2.2), (1.6±2.5) and (1.7±2.6) μm] for high myopia. At 3 months, a larger increase in corneal epithelial thickness was observed in the superior (t=2.703, P=0.009) zone following FS-LASIK than SMILE for low to moderate myopia, and the same increase was observed in the central (t=2.660, P=0.010), superior (t=3.229, P=0.002), nasal (t=2.420, P=0.019), inferonasal (t=2.651, P=0.011), inferotemporal (t=2.153, P=0.036) and superotemporal (t=2.281, P=0.027) zones after FS-LASIK [(6.7±2.6), (5.0±2.1), (3.9±1.9), (4.8±1.8), (7.5±2.5) and (6.4±2.7) μm] than SMILE [(4.7±2.9), (2.9±2.5), (2.4±2.4), (3.3±2.2), (6.0±2.4) and (4.8±2.4) μm] for high myopia. At 6 months, a larger increase in corneal epithelial thickness was observed in the superior (t=3.340, P=0.001) and nasal (t=1.952, P=0.055) zones following FS-LASIK than SMILE for low to moderate myopia, and the same increase was observed in the superior (t=2.332, P=0.024) and inferonasal (t=2.172, P=0.034) zones after FS-LASIK than SMILEfor high myopia. The central corneal thickness decrement after SMILE was much more than FS-LASIK both in the low to moderate myopia and high myopia groups (P<0.05). The average corneal epithelial thickness increment correlated positively with the central corneal thickness decrement and ablation rate at 1 month, 3 months and 6 months postoperatively (P<0.01). For SMILE, the total higher orders aberration (0.81±0.26, 0.79±0.28 and 0.81±0.31) and spherical aberration (0.50±0.21, 0.48±0.20 and 0.52±0.23) were less than FS-LASIK (0.97±0.34, 0.97±0.33 and 0.93±0.32; 0.72±0.25, 0.66±0.30 and 0.71±0.25) at 1 month, 3 months and 6 months postoperatively (P<0.05). Furthermore, the corneal aberration increment correlated positively with the average corneal epithelial thickness increment (P<0.05). Conclusions: The corneal epithelial thickness increment after SMILE was less than FS-LASIK. SMILE had better uniformity of the corneal epithelial thickness increment at the observed zones, which may explain the finding that the postoperative spherical aberration of SMILE was less than FS-LASIK. (Chin J Ophthalmol, 2020, 56:93-102).