Patients
We conducted a retrospective analysis of 218 eyes (109 patients) that underwent FS-LASIK or SMILE at the Onnuri Smile Eye Clinic, Seoul, Republic of Korea between July 2015 and April 2017. Of these eyes, 98 (49 patients) underwent FS-LASIK and 120 (60 patients) underwent SMILE. Inclusion criteria for the study were as follows: myopia up to -6.0 diopters (D), astigmatism up to -3.0 D, minimum age of 18 years, corrected distance visual acuity (CDVA) of 20/40 or better, and a minimum calculated postoperative residual bed thickness (RBT) of 300 µm. Exclusion criteria were as follows: active or residual ocular disease, a prior history of ocular surgery, immunocompromised, or status of pregnant or nursing. The study was approved by the Public Internal Regulatory Board of the Ministry of Health and Welfare, Korea (P01-201802-21-002). All procedures adhered to the tenets of the Declaration of Helsinki, and written informed consent for study participation was obtained from all participants.
Patients underwent a preoperative examination that included measurement of their CDVA, manifest and cycloplegic refraction, and intraocular pressure via tonometry (CT-80®; Topcon, Tokyo, Japan). Patients also underwent slit-lamp microscopy examination, fundus examination, autokeratometry (KR-8900®; Topcon), specular microscopy (noncom Robo-ca®; Konan Medical, Hyogo, Japan), and DRS analyses. Central corneal thickness (CCT) was defined as the thinnest corneal thickness using the DRS analyzer.12 The ablation depth (AD) value was exported from the Mel 90® program in FS-LASIK, and the lenticule thickness (LT) was exported from the Visumax® program in SMILE. The estimated RBT was obtained by subtracting the cap or flap thickness and AD or LT from the preoperative CCT.
Galilei Dual Rotating Scheimpflug Analyzer
All eyes underwent corneal topographic analyses using a Galilei® G4 DRS analyzer (software version 6.1.4), preoperatively and at 6 months after SMILE. Measurements with the DRS analyzer were performed according to the manufacturer’s guidelines. All measurements were performed between 10 AM and 3 PM with non-dilated pupils under identical lighting conditions. The device was brought into focus and the patient’s eye was aligned along the visual axis by a central fixation light. The patients were then asked to blink completely just before the measurement. Measured variables included the CCT, anterior chamber depth (ACD), anterior and posterior keratometry (K), anterior and posterior best-fit sphere (BFS) radius, and maximum posterior elevation (MPE). The BFS was determined using the central 8.0-mm zone of the preoperative cornea, which was identical for both the preoperative and postoperative maps. The change in MPE was defined by the maximum forward protrusion of the posterior cornea above the BFS in the 4.0-mm zone. The MPE was calculated by subtracting the preoperative elevation data from the postoperative elevation data based on the maximum difference in the central 4.0-mm zone. Forward protrusion of the posterior cornea was recorded as a negative number.
Surgical Procedures
All surgeries were performed by two experienced ophthalmologists (CYT and KBK), and the target refraction was emmetropia in all eyes.
FS-LASIK was performed using a Visumax® FS laser for flap creation, followed by a Mel 90® excimer laser for stromal ablation. FS-LASIK was performed with topical anesthesia using 0.5 % proparacaine hydrochloride (Alcaine®; Alcon, Fort Worth, TX, USA) applied 2–3 minutes before the procedure. The flap had a 9.0-mm diameter, 110-µm thickness, and a hinge position at the superior position. The optic zone diameter was 6.0~6.5 mm. The target refractive error treatment was applied using an excimer laser after manual lifting of the flap. The stromal bed was washed using a balanced salt solution and the flap was gently repositioned over the stromal bed. A sponge was used to sweep gently radially along the flap edge.
SMILE was performed using a Visumax® FS laser with a repetition rate of 500 kHz, pulse energy of 140 nJ, and spot spacing of 3.5 µm. SMILE was performed with topical anesthesia using 0.5% proparacaine hydrochloride applied 2–3 minutes before the procedure. The lenticule diameter was 6.0~6.8 mm and the cap diameter was 0.8 mm larger than the lenticule diameter. The intended thickness of the cap was 120 µm, and the incision was 2.0 mm long at the 11 o’clock position. The lenticule was separated with a blunt spatula using Chung’s swing technique, as described previously.13
After the procedure, 0.5% moxifloxacin (Vigamox®; Alcon) was applied four times daily for 5 days, 0.1 % fluorometholone (Opti-V®; Reyon Pharmaceutical, Seoul, Republic of Korea) was used six times daily in the first postoperative 5 days and was gradually tapered for 4 weeks, and preservative-free hyaluronic acid lubricating drops (Tearinfree®; DHP Korea, Seoul, Republic of Korea) were applied for at least 4 weeks.
Statistical Analysis
Statistical analyses were performed using SPSS for Windows statistical software (ver. 15.0; SPSS Inc., Chicago, IL, USA) and the graphs were drawn using Microsoft Excel 2016 software (Microsoft Corp., Redmond, WA, USA). Paired t-tests were used to identify differences between pre- and postoperative values. The independent t-test was used to compare differences between the two groups. Pearson’s correlation analysis was used to evaluate the relationships between variables. A value of p < 0.05 was considered to indicate a statistically significant difference, and the Bonferroni correction was used to account for multiple comparisons,14 for a value of p < 0.01 (= 0.05/5).