Corneal biomechanics will decline after refractive surgeries. In turn, the corneal biomechanical changes result in corneal dilation. Consequently, the changes of corneal biomechanics after refractive surgeries have attracted increasing attention from ophthalmologists. The SMILE procedure does not require flap creation, requires only small incisions, and is free from complications associated with flap creation. Meanwhile, flap creation by FS-LASIK is more precise. Both procedures reduce the corneal thickness, and therefore result in decline of corneal biomechanics. In theory, the SMILE procedure does not generate corneal flaps, does not require cutting of the anterior corneal collagen fibers, uses a femtosecond laser to cut a miniature convex lenticule or convex cylindrical lenticule of a certain size and degree (thickness) at a specific depth in the corneal stroma, and retrieves the lenticule through the small incision around the cornea [11]. Thus, the corneal integrity can be better preserved. The SMILE procedure should provide more stable postoperative corneal biomechanics than FS-LASIK, and is currently regarded as the refractive surgery that brings minimal changes to the postoperative corneal biomechanical properties [12]. However, to date, it remains controversial which of the two procedures enjoy superior corneal biomechanical stability after surgery.
Many researchers have compared the postoperative CH and CRF between the SMILE and FS-LASIK groups. Two reports by Zhang Jun et al. [13, 14] showed no significant postoperative differences in CH and CRF between the SMILE group and the FS-LASIK group. However, Wenjing WU et al. [15] and Elmohamady et al. [16] found that CH and CRF both exhibited postoperative differences between the SMILE group and the FS-LASIK group. Lei Xia et al. [17] and El-Mayah E et al. [18] concluded that while there was no significant difference in the postoperative CH between the SMILE group and the FS-LASIK group, the postoperative CRF was different between the two groups. Therefore, we performed this meta-analysis to clarify whether CH and CRF were different after the surgery in the SMILE and FS-LASIK groups.
This meta-analysis indicated that in both the foreign and the Chinese studies, there was no advantage of CH changes in the SMILE group compared with the FS-LASIK group 3 months after surgery. However, even after the subgroup analysis, the heterogeneity between the two subgroups was still high, possibly due to the small number of articles included. Therefore, a meta-analysis with larger sample size is necessary for further validation. In terms of CRF, the SMILE group showed no advantage over the FS-LASIK group 3 months after surgery in the foreign studies, whereas the SMILE group was superior to the FS-LASIK group in the Chinese studies. The decrease in biomechanics is related to the thickness of the corneal cutting, and greater thickness of the FS-LASIK corneal cut is correlated with more pronounced changes in CH and CRF [19, 20]. In addition, the thickness of the corneal cut is related to the preoperative spherical equivalent. For the SMILE surgery, the one-year postoperative corneal biomechanical changes are correlated with the preoperative spherical power of the patients. The higher the degree of myopia, the greater the biomechanical changes of the cornea. The biomechanical changes are also correlated with the sum of preoperative spherical power and cylindrical power. The higher the degrees of myopia and astigmatism, the greater the biomechanical changes of the patients. Moreover, based on our experience, the biomechanical changes exhibit a positive correlation with the thickness of the lens, and thicker lens exerts greater influence on the biomechanics of the cornea. In the Chinese studies, the postoperative CRF of the SMILE procedure was superior to that of the FS-LASIK procedure. This finding may be attributed to the greater degree of myopia and the consequent greater depth of cutting in the Chinese group. One of the six included studies did not record the preoperative central corneal thickness, and two studies did not document the preoperative spherical equivalent. Accordingly, the current meta-analysis was not grouped according to the preoperative spherical equivalent or corneal thickness, and the results may be influence to a certain degree.
Some researchers [19] believe that the thicker the corneal flap created by the femtosecond laser, the greater the alterations of corneal biomechanics. In the FS-LASIK group, the thickness of the flap was 100 m in four of the included studies, and the thickness in the other two studies was 90-110 m and 100-110 m. A previous report on a pig eye model [21] showed that when the flap thickness was less than 100 m, there was no significant alteration in CH or CRF. Therefore, this study did not use grouping by corneal flap thickness. Four of the studies recorded a light zone of 6 mm, while the other two studies did not record the size of the light zone. Thus, the data were not grouped or analyzed by light zone.
There are certain limitations to this study: (1) only a small number of trials were included in this meta-analysis; (2) the observation time was short; (3) the included articles were all cohort studies; (4) we could not obtain unpublished results, and publication bias could be completely ruled out; (5) the patients were not grouped and analyzed based on the preoperative degree of myopia; (6) since there were few articles on the measurement of corneal biomechanics using Corvis ST, this study included the articles on the measurement of corneal biomechanics with ORA. Few biomechanical indicators were included in this study, and only CH and CRF were measured. This study did not assess corneal deformation parameters, and thus failed to fully reflect the changes in corneal biomechanical properties.
Conclusion
This meta-analysis systematically analyzed the differences in corneal biomechanics between the SMILE surgery group and the FS-LASIK surgery group. Despite the limitations, we believe that the conclusions derived from this meta-analysis are clinically useful for treatment considerations.
Larger sample size is required to determine whether significant differences in corneal biomechanics exist between the SMILE and FS-LASIK procedures. Multi-center trials and studies with longer follow-ups may be necessary to better evaluate the benefits and safety of the SMILE and FS-LASIK procedures. This work provides insights and guidance on the biomechanical considerations for corneal refractive surgery doctors.