In this retrospective study of 218 eyes with myopia that underwent FS-LASIK or SMILE, we reported changes in the anterior segment using the DRS analyzer. Both the FS-LASIK and SMILE procedures treat myopia by removing the anterior corneal stroma; however, SMILE preserves the anterior segment of the cornea and is performed using only the FS laser, whereas FS-LASIK requires flap creation and ablation using an excimer laser; thus differences in the degree of change in the cornea after each procedure would be expected.6,7
We compared eyes that underwent FS-LASIK with those that underwent SMILE. Notably, there were significant preoperative differences in the ablation depth (AD) or lenticule thickness (LT) and estimated residual bed thickness (RBT) between the two groups, although there was no significant difference in the spherical equivalent refraction or CCT. The estimated LT in the SMILE group was significantly greater than the AD in the FS-LASIK group, although we targeted refraction emmetropia, and all eyes had a spherical equivalent refraction within ± 0.75 D at postoperative 6 months. There has been no comprehensive report describing the AD or LT difference between LASIK and SMILE, but some differences have been reported. Lazaridis A et al.15 compared the corneal clarity and visual outcomes between FS-LASIK and SMILE, reporting that the preoperative mean spherical equivalent refraction did not show a significant difference (–4.80 ± 2.4 D and –5.51 ± 1.86 D, respectively; p = 0.136), but the mean estimated LT was significantly thicker in the SMILE group (99 ± 38 µm and 116 ± 28 µm, respectively; p = 0.017). Wang et al.16 reported postoperative differences in the degree of corneal biomechanical change between SMILE and LASIK, where the estimated LT was significantly thicker in the SMILE group when the spherical equivalent refraction was less than –6.0 D even if there was no significant difference in preoperative spherical equivalent refraction and postoperative biomechanical strength of the cornea between the two groups. However, in our study, the difference in the CCT at 6 months after the procedure showed no statistical significance between the two groups (p = 0.063; Table 4). The correlation between the preoperative estimated AD or LT and change in CCT after the procedure in the SMILE group was strong, but slightly weaker compared to that in the FS-LASIK group (Figure 1). Luft N et al.17 reported discrepancy between preoperative estimation of LT and the decrease in CCT after SMILE using spectral domain-optical coherence tomography. In their study, mean surgical correction of spherical equivalent refraction was –4.94 ± 1.75 D, and the decrease in CCT was 18.7 ± 5.7 µm smaller than the preoperative estimated LT. This was consistent with our results. When we considered changes in the CCT after a procedure as actual AD or LT, we concluded that the reason for such changes was overestimation of the LT by the Visumax® program; therefore, the method used for LT estimation should be revised after further investigation.
Because the FS-LASIK and SMILE procedures are designed to flatten and weaken the anterior surface of the cornea, a change in the posterior cornea should be expected.18 Posterior corneal changes are less pronounced than anterior corneal changes after the procedure, because the anterior corneal surface is reshaped directly during the procedure. Meanwhile, the posterior corneal surface may change, but not directly in response to the procedure; rather, the change arises from intraocular pressure and post-operative corneal thinning.19 Forward displacement of the posterior surface shows similar characteristics to corneal ectasia, and is thought to represent subclinical ectasia.20 In our study, the maximal posterior elevation (MPE) was significantly increased after the procedure in both groups (Table 3), and the change in the MPE after the procedure was greater in the FS-LASIK group than in the SMILE group even though it was not significant (Table 4). This was consistent with the report by Wang et al.21, whose study compared postoperative changes in posterior corneal elevation between FS-LASIK and SMILE. In their study, FS-LASIK was associated with a greater increase in posterior corneal elevation than SMILE, as well as a greater reduction in the corneal resistance factor, showing that SMILE caused less weakening of the cornea than did FS-LASIK. This was because SMILE leaves the anterior-most stromal lamellae intact except for the region with the small incision, which acts to stabilize the cornea after the procedure.7
Because changes in the posterior corneal surface after keratorefractive procedures usually occur, posterior corneal elevation, posterior corneal curvature, posterior asphericity, and posterior K can change.21,22 However, because posterior corneal surface changes occur indirectly due to remodeling of the cornea after anterior corneal flattening, changes in the posterior cornea have shown different results.20–24 Khairat et al.23 did not observe significant changes in the keratometric power of the posterior cornea after LASIK, but Seitz et al.21 reported a significant increase in the negative corneal power of the posterior cornea after the LASIK procedure. We observed a statistically significant increase in the negative power of the posterior cornea after FS-LASIK compared to the preoperative results, with no significant change seen after SMILE. This difference may be explained by preservation of a strong Bowman’s layer and the compact anterior corneal stroma in SMILE,16 resulting in more subtle changes in the posterior corneal surface.
Several studies have evaluated changes in the ACD after LASIK and photorefractive keratectomy (PRK).25–28 Most studies found that the ACD was significantly decreased after myopic corneal ablation. However, LASIK and PRK do not affect ocular structures other than the cornea, and a decrease in the ACD is inconsistent with forward shifting of the posterior corneal surface after LASIK or PRK. Nawa29 suggested that the ACD decrease was the result of a decrease in the magnification effect of the cornea after myopic LASIK, and some authors have reported that an ACD decrease was associated with age and accommodation. Nishimura et al.26 reported that the ACD significantly decreased after myopic LASIK in patients younger than 40 years, and Wang et al.27 reported that the crystallin lens thickness increased significantly after LASIK, thus leading to a decrease in the ACD. In our study, the ACD was significantly decreased after both FS-LASIK and SMILE, and the decrease in the ACD after SMILE was significantly greater than that after FS-LASIK. Considering that there was no significant difference in age, preoperative spherical equivalent refraction, or the extent of the postoperative decrease in CCT between the groups, two mechanisms that could explain these findings include a difference in the amount of forward shifting of the cornea, and anterior segment remodeling. Zhang and Wang30 reported that displacement in the central and peripheral parts of the cornea showed a slight backward and forward shifting tendency, respectively, after PRK. Because we did not investigate regional posterior corneal shifting, we could not conclude that ACD changes were the result of postoperative changes in the posterior cornea. However, because we found that the MPE was more protruded after FS-LASIK compared to SMILE, there may be other differences in the posterior corneal surface specific to the procedure. Rosa et al.28 suggested that a decrease in the ACD after PRK was related not only to corneal change, but also to anterior segment remodeling. In their study, there was no significant change until 3 months after PRK, but a significant change occurred at between 3–6 months postoperatively. The exact mechanism of anterior segment remodeling was not clear; however, this phenomenon could be explained by a continuous change in the anterior segment over time caused by a different postoperative response to intraocular pressure of the newly structured cornea. Because we used the DRS analyzer 6 months after the procedure, we determined that the ACD change could be caused by anterior segment remodeling after FS-LASIK or SMILE, and there could have been a difference in the newly structured cornea according to the procedure that was applied.