In accordance with the tenets of the Declaration of Helsinki, the Ethics Committee of Fudan University Eye and ENT Hospital Review Board (Shanghai, China) approved the study protocol. Written informed consent was provided by each subject before entering the study.
In this prospective controlled study, 80 eyes of 47 patients undergoing SMILE at the Department of Ophthalmology, Eye and ENT Hospital of Fudan University (Shanghai, People’s Republic of China) were recruited. The patients had no ocular disease other than a refractive error and met the inclusion criteria. All patients underwent a comprehensive preoperative ophthalmologic examination, including slit-lamp examination, measurement of uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), intraocular pressure, and Pentacam HR imaging.
Eyes were categorized into the following groups based on the preoperative manifest spherical equivalent: high myopia group (40 eyes; range, -6.25D to -9.00D; mean, -7.49±0.70D) and moderate myopia group (40 eyes; range, -3.00D to -6.00D; mean, -4.43±0.87D).Detailed information is shown in Table 1.
2.2 Surgical Procedure
The VisuMax femtosecond laser system (Carl Zeiss Meditec AG, Germany) was used to perform all the surgeries. After topical anesthesia, the patient was positioned under the curved contact glass and instructed to focus on the internal target light. The surgeon then achieved correct corneal centration and initiated suction, followed by femtosecond laser scanning. Once the laser scanning was completed, the surgeon inserted a spatula into the cornea, dissected the lenticule interface, and manually extracted the lenticule. The femtosecond laser settings were as follows: repetition rate 500 kHz, 100 µm intended cap thickness, 5.8 to 6.5 mm optical zone (lenticule diameter), 7.3 to 7.5 mm cap diameter, and a 2-mm side cut at the 12 o’clock position. The same experienced surgeon performed all the procedures (XZ).
2.3 Pentacam Scheimpflug Imaging
All eyes were examined using the Pentacam imaging system. The patient was instructed to position their head on the headrest and fixate on the target light. After attaining alignment, the device captured 25 images and automatically recorded 12,500 elevation points within 2 seconds. To avoid miscalculations due to poor imaging, the quality of each measurement was shown in the specification window, and only results with “OK” statements were accepted. The examination was repeated if the statement did not meet the requirement (marked yellow or red). Only maps with at least 10 mm of corneal coverage and no deduced data in the central 9-mm zone were accepted.
2.4 Postoperative Examination
Follow-up appointments were scheduled at 6 months and 5 years after surgery. Postoperative examinations included Pentacam imaging examinations, slit-lamp examination, measurements of UDVA, CDVA, spherical equivalent refraction and intraocular pressure.
2.5 Data Collection
Elevation data of the posterior corneal surfaces were acquired from the Pentacam images. The reference best-fit sphere (BFS) was defined in the center 8.0-mm region of the preoperative data, to ensure it was the same across all images. For points above the reference, values were positive; for points below the reference, values were negative. Calculated values of single points were obtained for the posterior central elevation (PCE), posterior maximum elevation (PME), and posterior elevation at the preoperative thinnest point (PTE) in the central 4-mm area above the BFS. The other 26 determined points in the central 6-mm zone were obtained as follows: 4 points at a 1-mm distance from the center along the 45°, 135°, 225° and 315° meridians (0° was defined as the horizontal semi-meridian on the right, and rotating counterclockwise in both eyes), 8 points at a 2-mm distance from the center at 0°, 45°, 90°, 135°, 180°, 225°, 270°, and 315°, and the other 14 points at a 3-mm distance from the center at 15°, 45°, 75°, 90°, 105°, 135°, 165°, 195°, 225°, 255°, 270°, 285°, 315° and 345°. Posterior corneal elevation in the central 4-mm area and in various optical zones (2-mm diameter, MPE-2mm; 4-mm diameter, MPE-4mm; 6-mm diameter, MPE-6mm) was calculated as the mean value from the points in the corresponding area. Changes in the posterior elevation were found by subtracting the preoperative data from the postoperative data (difference elevation map). The change in elevation was the shift of the posterior corneal surface. Elevation data were recorded in an Excel Spreadsheet (Microsoft Corp, Redmond, WA, USA) for further analysis.
2.6 Statistical Analysis
The descriptive results were reported as the mean and the standard deviation. The Kolmogorov-Smirnov normality test and test for homogeneity of variances were performed for all data. The analysis of variance (ANOVA) for repeated measures with the Bonferroni correction was employed to compare pre- and post-operative values. If the data were not normally distributed, the Friedman’s rank test for k correlated samples was used instead of the ANOVA. Bivariate normal analysis was performed before analysis of correlations. Analysis of variance (ANOVA) or the Mann-Whiney U test was applied to compare the differences between groups. The Pearson or Spearman correlation was applied subsequently to determine the association between the changes in posterior corneal elevation and the residual bed thickness (RBT). Statistical analyses were performed using SPSS ver.20.0 (SPSS Inc, Chicago, IL, USA). A P value < 0.05 was considered to be a statistically significant difference.