This was a retrospective observational paired-eye case series conducted at B&VIIT Eye Center, Seoul, Republic of Korea. The study protocol followed the tenets of the Declaration of Helsinki and good clinical practice. Yonsei University College of Medicine institutional review board approved the study, and the same institutional review board waived the requirement for informed consent as this was a retrospective study. All patients underwent SMILE between January 2017 and March 2019. All surgeries were performed by two experienced surgeons (IHR and ISL). Patients who underwent uneventful SMILE in one eye (the Uneventful group) and the immediate reapplication of SMILE without adjustment of the surgical parameters following suction loss in the contralateral eye (Suction loss group) were enrolled.
The inclusion criteria included aged 20–45 years, stable myopia for ≥ 1 year, a corrected distance visual acuity (CDVA) of 20/25 or better, a spherical equivalent (SEQ) refraction of − 2.00 to − 8.00 diopters (D), and refractive astigmatism of < 3.00 D. Patients with any ocular surface diseases; a history of corneal or intraocular surgery; ocular trauma, keratoconus; cataract; or collagen, vascular, or autoimmune diseases were excluded.
All examinations were performed before and 6 months after surgery. Patient evaluations included measurement of the logarithm of the minimum angle of resolution (logMAR), uncorrected distance visual acuity (UDVA), CDVA, manifest refraction, slit-lamp examination (Haag-Streit, Köniz, Switzerland), central corneal thickness (CCT), and Scheimpflug-based corneal tomography (Pentacam HR; Oculus Optikgeräte GmbH, Wetzlar, Germany). Corneal wavefront aberrations were assessed using a Pentacam HR in a 6 mm zone.
Uneventful SMILE (uneventful group)
SMILE was performed using a VisuMax 500-kHz system (software version 2.4.0; Carl Zeiss Meditec AG, Jena, Germany) with standardized techniques17. Laser spot spacing was set at 4.5 μm. The upper and lower edges of the refractive lenticule were delineated and a 2 mm incision was made at the 145° meridian. Once the upper and lower interfaces were separated, the lenticule was extracted using microforceps. The diameter of the cap was between 6.7 and 7.8 mm. The optical zone diameter was between 5.7 and 6.8 mm. The intended cap thickness was 90–135 μm. The default cap thickness was set to be 120 μm. The cap thickness was modified according to the residual stromal thickness, the amount of refractive correction, and the preoperative pachymetry. Postoperative medications included 0.5% moxifloxacin, and 1% prednisolone four times a day for 1 months.
Immediate reapplication of SMILE after suction loss (suction loss group)
When suction loss occurred during the posterior lenticule surface cut, the SMILE procedure was repeated from the beginning. The surgical parameters were re-entered without modification. When suction loss occurred during or after the cap was cut, the SMILE procedure was repeated from the point at which it was stopped. In these cases, the surgical parameters did not need to be re-entered into the laser control platform.
Statistical analyses were performed using IBM SPSS Statistics for Windows, version 25.0 (IBM Corp., Armonk, NY, USA). The Kolmogorov–Smirnov test was used to confirm data normality. Two measurements were compared using the paired t-test for normally distributed data or the Wilcoxon signed-rank test for non-normally distributed data. Categorical variables were compared using the chi-square test, and linear regression analysis was used to investigate refractive predictability. All data are presented as the mean ± standard deviation. Statistical significance was set at P < 0.05.