SMILE has been widely accepted for the correction of myopia less than − 12.0 D; LASEK has also been shown to have good visual outcomes in the correction of low myopia.[2, 9, 11] In the present study, we compared for the first time the visual quality and corneal aberrations obtained by the two procedures applied for low myopia correction.
Both SMILE and LASEK showed good safety, efficacy, and predictability in the correction of low myopia. The efficacy indexes of the SMILE group and LASEK group were 1.19 ± 0.17 and 1.23 ± 0.20, respectively; the safety indexes were 1.24 ± 0.17 and 1.28 ± 0.18, correspondingly, which was consistent with the results of previous studies.[9, 10] Reinstein et al reported that after the application of SMILE (mean SE: -2.61 ± − 0.54 D) for low-myopia with 1-year follow-up, 96% of the patients had UDVA of 20/20 or better and UDVA of 20/25 or better with mean residual SE of -0.05 ± 0.36D. In terms of safety, no patient lost two or more lines. The authors concluded that SMILE had safety and efficacy similar to those of LASIK used for low-myopia. In another study, Autrata et al performed LASEK for low to moderate myopia with 20% alcohol inside the alcohol solution cone for 25-30s. LASEK had faster vision recovery, milder pain, and lower incidence of haze than PRK. At 2-year follow-up visit, the safety and efficacy indexes were 1.04 and 0.98, respectively; 62% of the patients had a postoperative SE within ± 0.5D and 92% within ± 1.0D. Spadea et al used flap-preserved LASEK without alcohol to correct low to moderate myopia and obtained good outcomes with an efficacy index of 0.87 and a safety index of 1.25 after a follow-up of 60 months.
In addition to safety and efficacy, other considerations also should be considered when selecting surgical methods for patients. SMILE and LASEK have their own advantages and disadvantages. For example, SMILE maintains the integrity of the corneal epithelium and Bowman's layer and therefore mild postoperative ocular discomfort, faster recovery and free of flap-related complications. However, SMILE required 10–30 µm of additional base thickness in corneal stroma, and thus more corneal tissue was removed using this technique than the application of excimer laser surgery. Furthermore, the cooperation of patients was critical during the femtosecond laser scanning. In addition, the design of the parameters in the SMILE set for low myopia treatment should be more carefully determined since the outcomes were affected by the adjustment of the nomograms and the applied laser energy and femtosecond laser scanning quality. LASEK was also a flapless procedure, which prevented flap-related complications. Compared with SMILE, less corneal tissue was removed in LASEK, and therefore it was more suitable for patients with relatively thin cornea. Additionally, the excimer laser machine had an eye-tracking system that was valuable in the process of excimer laser scanning, and was hence more appropriate for patients with poor cooperation or large angle kappa. Previous studies had shown that postoperative pain of LASEK might be related to the time and concentration of alcohol used in the procedure.[1, 12, 15] In this study, 20% alcohol infiltration was applied for 12s, after which the patients reported mild postoperative pain or discomfort. In addition, the medication time after LASEK is longer, and more frequent follow-up is needed.
We found that the increment of total corneal spherical aberrations (SA) and SA of the anterior cornea surface after SMILE were less than those after LASEK. These results were in accord with previous studies indicating that the SA induced by SMILE was less while the coma error was higher than that of FS-LASIK.[16, 17] The increase of spherical aberration would augment the post-operative occurrence of halo in the dark. Zhu et al and Yu et al also reported significantly lower HOAs and SA after SMILE than after LASEK, with no significant differences in the coma and trefoil aberrations between groups. In the current study, the vertical coma was significantly increased after SMILE, which was in line with the results of previous studies. Lack of an eye-tracking system as well as involuntary Bell phenomenon in SMILE procedure may contribute to the increase of vertical coma.
In clinic, many patients who complain of visual disturbances after refractive surgeries have visual acuities of 1.0 or better and refractions close to plano, therefore, it is inadequate to assess their symptoms with conventional measures of acuity or refraction. Subjective visual quality after laser surgery should be given full attention. In this study, the main visual disturbances were starburst, halo, and vision fluctuation two years after the surgery. These symptoms were not reported to disturb patients' daily life except for inconvenience when driving at night postoperatively, which is consistent with the findings of Wei et al.
Our study has some limitations. First, we observed only the changes in the corneal aberrations, whereas the effect of total ocular aberrations on visual quality was not analyzed. Second, we included only patients with spherical equal to -3.00D or less and astigmatism equal to 1.50D or less, and thus the results could not be extrapolated to patients with spherical − 3.00D and astigmatism − 1.50D or more. Third, the sample size was small, and thus larger samples of observation with a longer follow-up period should be implemented in the future.