Thirty-five patients (70 eyes) who underwent SMILE between July 2015 and January 2017 at the Department of Ophthalmology, Peking Union Medical College Hospital, Beijing, China were included in this retrospective study. Among them, 18 patients (36 eyes) and 17 patients (34 eyes) underwent bilateral SMILE with small incision (4 mm) and microincision (2 mm or 2.5 mm) hinge width, respectively. Based on the definition of myopia, two eyes in the small incision group and one eye in the microincision group had low myopia (-3 to 0 D), 15 eyes in the small incision group and 17 eyes in the microincision group had moderate myopia (-6 to -3 D), and 19 eyes in the small incision group and 16 eyes in the microincision group had high myopia (more than − 6 D). The study protocol followed the guidelines of the Declaration of Helsinki, which was approved by the Institutional Review Board for Human Studies of the Peking Union Medical College Hospital Institutional Ethics Committee. Written informed consent was obtained from all the patients before the commencement of the study.
Patients enrolled in the study underwent SMILE surgery for the correction of myopia and myopic compound astigmatism. All patients demonstrated at least 1 year of stable refraction before undergoing refractive surgery, with a follow-up period of at least 6 months postoperatively. The exclusion criteria included amblyopia, ocular pathology, retinal disorders, previous ocular surgery, or insufficient follow-up. Patients were required to discontinue the use of soft contact lenses for at least 2 weeks and rigid gas-permeable lenses for at least 4 weeks prior to surgery.
Preoperative examination
All patients underwent a routine standard ophthalmologic examination preoperatively. The investigation included manifest refraction, cycloplegic refraction, slit-lamp examination, ultrasound pachymetry, dilated funduscopy (Optos California, Marlborough, MA, USA), and intraocular pressure (IOP) measurement using a Goldmann applanation tonometer. Uncorrected distance visual acuity (UDVA) and corrected distance visual acuity (CDVA) were assessed using Snellen charts. The CDVA was always assessed using trial frames and not contact lenses. Central corneal thickness (CCT) was measured by ultrasonic pachymetry (TOMEY Ltd, Aichi, Japan), in which each measurement was the average of five consecutive measurements. Corneal topography was measured by TMS-4N (TOMEY, Erlangen, Germany). For high myopia, the macular and optic thickness was measured by Topcon DRI OCT Triton (Topcon, Corp., Tokyo, Japan), and visual field assessments were performed using Octopus 900 perimeter (Haag-Streit Inc, Koenic, Switzerland).
Surgical procedure
All surgical procedures targeted individuals with emmetropia and were performed by the same experienced surgeon (Y.L.). Topical anaesthesia was administered with 0.5% proparacaine (Alcaine, Alcon-Couvreur, Puur-Sint-Amands, Belgium, USP). SMILE surgery was performed by VisuMax Femtosecond Laser Platform (Carl Zeiss, Meditec AG, Germany, 500-kHz repetition rate). A small curved interface cone was used during each surgical procedure. The anterior and posterior surfaces of lenticules (spiral-out and spiral-in patterns, respectively) were followed by a side cut of the cap. The options value of power and spot distances for lenticule creation were 140 nJ and 4.30 μm, respectively. VisuMax Femtosecond Laser ablation options for SMILE surgery were as follows: 6.0–6.5-mm lenticule diameter; 110–120-μm cap thickness; 4-mm (small incision) or 2–2.5-mm (microincision) hinge width at 120 degrees for lenticule extraction; and 7.5–7.6-mm cap diameter with 90-degree side cut angle. A spatula was inserted through the side cut over the roof of the refractive lenticule to dissect this plane, followed by the bottom of the lenticule. The lenticule was subsequently grasped with modified McPherson forceps (Geuder, GmbH, Heidelberg, Germany) and removed.
After surgery, topical tobramycin-dexamethasone (Tobradex, Alcon, Fort Worth, Texas, USA) was administered for 4 times/day for one week. Flumetholon (0.1% fluoromeholone, Santen Pharmaceutical, Osaka, Japan) was used for 4 times/day during the second week, after which the frequency was decreased to once per day each week for 1 month. Finally, topical eye drops of 0.5% antibiotic (Ofloxacin, Santen Pharmaceutical, Osaka, Japan) were administered 4 times/day for 2 weeks.
Postoperative evaluation
Patients were followed up at 1 week and 1, 3, and 6 months, postoperatively. All postoperative follow-up visits included the assessment of UDVA, IOP and corneal topography (TMS-4N, TOMEY, Erlangen, Germany). Polar SIA was analysed by the polar value analysis method [12].
Statistical analysis
Data were recorded in an Excel spreadsheet (Microsoft, Redmond, Washington, USA), and statistical analyses were performed using SPSS for MAC version 25.0 (IBM SPSS, Somers, New York, USA). The normality of the data was tested with the Shapiro–Wilk test. If the data were not normally distributed, the Wilcoxon rank–sum test and Mann–Whitney test were performed. Friedman’s Analysis of variance (ANOVA) with the Bonferroni correction was applied when data did not show normal distribution. Fisher exact or chi-square test (asymptotic, Yates corrected, or exact permutation as appropriate) for categorical variables. P-values <0.05 were considered significant.