Clinical Outcomes of Small Incision Lenticule Extraction in Myopia: Study of Vector Parameters and Corneal Aberrations

Abstract

Purpose: To investigate clinical outcomes of small incision lenticule extraction (SMILE) including vector parameters and corneal aberrations in myopic patients.

Methods: This retrospective, observational case series included 57 eyes (29 patients) that received treatment for myopia using SMILE. Visual acuity measurement, manifest refraction, slit-lamp examination, autokeratometry, corneal topography, and evaluation of corneal wavefront aberration were performed preoperatively and at 1 and 3 months after surgery. We analyzed the safety, efficacy, vector parameters, and corneal aberrations at 3 months after surgery.

Results: Preoperatively, mean manifest refraction spherical equivalent refraction was -4.94 ± 1.94 D (range, -8.25 to 0 diopters [D]), and the cylinder was -1.14 ± 0.82 D (range, -3 to 0 D). Mean manifest refraction spherical equivalent improved to -0.10 ± 0.23 D at 3 months postoperatively, when uncorrected distance visual acuity was 20 / 20 or better in 55 (96%) eyes. The linear regression model of target induced astigmatism vector versus surgically induced astigmatism vector exhibited slopes and coefficients (R²) of 0.9618 and 0.9748, respectively (y = 0.9618x + 0.0006, R² = 0.9748). While total corneal root mean square higher order aberrations, coma and trefoil showed statistically significant increase, spherical aberration did not show statistically significant change after SMILE.

Conclusions: SMILE has proven to be effective and safe for correcting myopia and astigmatism. We showed that SMILE did not induce spherical aberrations. A small increase in postoperative corneal higher order aberration may be associated with increase in coma and trefoil.

Keywords: Astigmatism; Corneal wavefront aberration; Refractive surgical procedures; Small incision lenticule extraction; Vector analysis.

Fig. 1. Visual outcomes after small incision lenticule extraction. (A) Cumulative 3-month postoperative uncorrected distance visual acuity (UDVA) and preoperative corrected distance visual acuity (CDVA). Changes in Snellen lines of (B) postoperative UDVA and (C) CDVA relative to preoperative CDVA. (D) Attempted versus achieved changes in spherical equivalent refraction (SEQ) at 3 months after surgery. (E) The accuracy of SEQ to the intended target. (F) Comparative distribution of preoperative and 3-month postoperative cylinder and (G) target induced astigmatism (TIA) versus surgically induced astigmatism (SIA) vectors at 3 months. (H) Refractive astigmatism angle of error distribution at 3 months after surgery. D = diopters.

Fig. 2. Single-angle polar plots of (A) target induced astigmatism vector, (B) surgically induced astigmatism vector, (C) difference vector, and (D) correction index at 3 months after small incision lenticule extraction (SMILE). D = diopters.

Fig. 3. Changes in higher order aberrations (HOA) at 3 months after small incision lenticule extraction. Data are presented as mean values ± standard error of the mean. RMS = root mean square; SphAb = spherical aberration; preop = preoperative; postop = postoperative. ^*Significant difference.