In the present study, we confirmed the effect of SMILE and FS-LASIK procedures on subbasal nerve cutting in the cornea. Nerve parameters decreased significantly in both groups one month after surgery and increased slowly during that period but still did not return to preoperative levels until 6 months after surgery. In the study by Patel et al., it was demonstrated that 12 months after FS-LASIK, postoperative subbasal nerve density remained significantly lower than preoperative density. There are relatively few long-term studies on SMILE. In a cross-sectional study of the post-SMILE eye, it was demonstrated that even 5.5 years after surgery, the neurological status had not returned to the normal range.3
In a previous six-month study, it was observed that nerve density was significantly greater in SMILE eyes than in FS-LASIK eyes at one week and three months postoperatively, but no difference was observed at six months., However, our study showed that the nerve density in SMILE eyes was still significantly greater than that in FS-LASIK eyes at six months postoperatively. The previous study may have amplified the individual effect by including both eyes of the same patient, resulting in a difference in findings. Vestergaard AH et al. reached the same results as we did. In our SMILE group, eyes three months postoperatively presented greater CNFD, CNBD, CNFL, CTBD, CNFA, CNFW, and CFracDim. We further analyzed the correlation between the cap (flap) diameter and its correlation, and we found that all parameters at three months decreased with increasing eyelid (flap) diameter, except CNFW. The cap diameter of the SMILE group was significantly smaller than the flap diameter of the FS-LAISK group. This confirmed that the difference in nerve parameters between the two groups was caused by the size of the cross-section of the nerve cutoff diameter. Approximately 44 thick nerve bundles enter the human corneal limbus in a relatively equal distribution and move randomly toward the central cornea. Nerves from all directions converge centrally toward the inner cornea, forming a typical clockwise rotation pattern. It also further illustrates that the larger the diameter of the cut in the anterior matrix, the greater the damage to the nerve. There are many factors affecting the corneal nerve that may not only be influenced by differences in surgical approach, but in our study, we also found a positive correlation between the patient's preoperative SE and the CNFD, CNBD, CNFL, CTBD, and CNFA three months postoperatively; that is, they decreased as the patient's myopia degree increased. the absolute values of preoperative SE in SMILE eyes were significantly smaller than those in the FS-LASIK group. This may also be the reason for the difference in corneal nerves between the two groups. This phenomenon was also found in the study by Yu-Chi Liu et al.3 There was no significant difference in the thickness of the corneas cut (ablated) between the two groups in the study. In a large population study, SE was found to be negatively correlated with mean keratometry and corneal thickness. That is, the higher the absolute value of SE, the greater the mean keratometry and the thinner the corneal thickness, and the ratio of the volume of cut (ablation) to corneal thickness increased with the absolute value of SE, even at the same cut (ablation) thickness. We speculate that this may contribute to the association of SE with neurological status. Further studies are needed to confirm this.
Anterior IF keratocyte density and posterior IF keratocyte density in both groups decreased immediately after surgery and showed no significant signs of recovery, which is consistent with the decrease in corneal density in the ablated area after LASIK reported by Erie et al., , Lee et al. found a strong correlation between subbasal nerve density and keratinocyte density in their prospective LASIK and PRK series. However, we did not observe this phenomenon. The cause of this discrepancy may be related to differences in the way corneal cells are counted. Meiyan Li et al. obtained the same results as ours, and we used the same method in calculations for corneal cells.15
Finally, we also observed changes in DC density, which decreased significantly in both groups at one month postoperatively. However, patients in the SMILE group still showed a decreasing trend in DC density between 1 and 6 months, but it was not statistically significant, while the opposite was true for the FS-LASIK group. In an animal experiment, DCs were found to be sufficiently activated to stimulate T cells involved in DED pathogenesis and progression. We hypothesized that the decrease in postoperative DC density was associated with intensive postoperative eye drops in patients. There was no significant difference in DC density between the two groups at one month, three months and six months postoperatively, and and it can be inferred that there is no significant difference between FS-LASIK and SMILE procedures for DC activation. The mechanism of dry eye production is not only the activation of DC but also the reduction of basal nerve fiber density,therefore many studies have shown that FS-LASIK surgery leads to more severe dry eye compared to SMILE surgery.,,
Currently, a better understanding of corneal changes after refractive surgery is critically needed for two main reasons. First, it allows a better understanding of the complications associated with refractive surgery, which further helps in the development and evaluation of refractive surgery. Second, it is important to determine the cause of the decrease in the subbasal nerve of the cornea due to surgery so that nerve damage can be minimized during the surgical design. In this study, FS-LASIK was shown to cause more significant damage to the subbasal nerve in the early postoperative period than SMILE. However, due to the different diameter sizes of the fabrication caps (flaps) in the two procedures, the cause of nerve reduction may not only be due to the different surgical approaches. Further studies will expand the sample size to determine the effect of the diameter size of the fabrication cap (flap) on the subbasal corneal nerve during the procedure.