Currently, femtosecond laser is the mainstream procedure of corneal refractive surgery, and the emergence of femtosecond small-incision lenticule extracion has brought femtosecond laser surgery into the flap-free era, which avoids the possible complications of FS-LASIK ,brings better biomechanics and reduces the occurrence of dry eye[6-7].The safety and effectiveness of SMILE surgery has been recognized by many experts and scholars [1-2]. However, it may have a certain impact on the anterior segment. To date, there have been few related reports, and the observation time was short[8-9]; changes in the anterior chamber structure may have an impact on the occurrence and management of intraocular diseases in patients. Pentacam, one of the most widely used instruments for detecting the corneal morphology, is a new anterior segment analysis system introduced by OCULUS, a German company, which uses a 360° rotating probe to scan the anterior segment of the eye and obtain Scheimpflug images. Rotation allows better access to the data of corneal morphology, especially for the posterior corneal surface . Since the refractive surgery, including SMILE, has been performed in the anterior corneal stromal, it directly changes the curvature and morphology of the anterior corneal surface. However the changes of posterior corneal surface are not directly caused by the surgery but could be caused by intraocular pressure and the thinner cornea . In our study, we found statistically significant changes in the curvature of anterior and posterior corneal surface before and after surgery (F=700.001, P <0.05; F=3.494, P <0.05), indicating that the curvature of posterior surface also changed after SMILE. However the curvature of posterior corneal surface stabilized 1 year after surgery, and the difference was not statistically significant between preoperative and 1 year after operation(preoperative/1 month or 1 year after operation, P=0.117,1.000).That means the curvature of posterior corneal surface became stable in medium and long term after SMILE. The result was in agreement with the study by Manrong Yu et al , who found no statistically significant difference in curvature of posterior corneal surface between preoperative and 3 months after operation.However they also found a trend toward flattening of the posterior surface at 3 years after SMILE (preoperative - 6.22 ± 0.23,3 years after operation - 6.17 ± 0.24), which needs to be further observed.
In our study,we found that there was no statistically significant difference in preoperative and postoperative posterior elevation at the apex corneal surface (F=0.301, P =0.693), but there was statistically significant difference in the posterior elevation at the thinnest corneal point (F=8.374, P =0.001);there was also a statistically significant difference between preoperative and time points after operation（P=0.009，0.008）, and the posterior elevation at the thinnest corneal point decreased (4.5±2.193, 3.47±2.662, 3.270±2.741) gradually over three time points. This was in agreement with the results of Yu Zhao et al. , who found that there was no statistically significant difference in the posterior elevation at the apex corneal surface before and 3 years after SMILE, while there was statistically significant difference in the posterior elevation at the thinnest corneal point, which was considered that the steepening of the peripheral corneal stroma and flattening of the central corneal stroma caused mild hyperopia drift. Changes in posterior elevation at the apex corneal surface and posterior elevation at the thinnest corneal point showed that the corneal posterior surface was relatively stable in the medium and long term after SMILE.
Q value of the corneal surface is a significant parameter to reflect the state of aspherical corneal surface . The normal cornea of human eye becomes flattened from the center to the periphery, which is prolate oval, so the Q value is negative. Amelia Nieto-Bona et al.  measured 118 healthy eyes and found that Q values ranged from -0.83 to -0.01. After different corneal refractive surgery, the central cornea flattens, changing the state of the corneal asphericity, and the Q value of the anterior corneal surface often changes from negative to positive .Zhang Wenwen et al. argued that the change of Q value of the anterior corneal surface after SMILE was smaller than that of SBK, and was negatively correlated with the preoperative spherical equivalent .Our study also found that the negative value of Q value of the anterior corneal surface decreased after SMILE, and the difference was statistically significant compared with the Q value before operation( P < 0.05).Q value of the anterior corneal surface was relatively stable from 1 month to 1 year after SMILE (-0.287 ±0.0916, -0.295±0.082)and there was no significant difference (P =0.805> 0.05).The Q value of the posterior corneal surface also changed (preoperative Q value=-0.339±0.098, Q value 1 month after operation= -0.287±0.0916, Q value1 year after operation=-0.295±0.082 ),and the difference was statistically significant before and after operation (F=28.179, P <0.05)while the difference was relatively stable from 1 month to 1 year after surgery and the difference was not statistically significant (P=0.531>0.05), The data above was in agreement with the observation of Lin Zhang et al. that the negative value of Q value of the posterior corneal surface decreased 6 months after SMILE. It was considered that although the posterior corneal surface was relatively stable after SMILE, there was still a slight backward movement, which was consistent with the changes of posterior elevation at the thinnest corneal point.
Many reports [18-19] found that the ACD became shallower after corneal refractive surgery, and many paper believed that factors affecting ACD included age, corneal diameter, pupil diameter and lens thickness, while there was no significant correlation with intraocular pressure and corneal thickness [20-22]. In our study, we found that both ACD and ACV decreased after SMILE (ACD 2.970±0.231, 2.914±0.250ACV174.267±23.644, 159.867±23.027). Compared with that before operation(ACD3.042±0.237,ACV180.900±23.859), the difference was statistically significant; as time went by, postoperative ACD and ACV were still changing, and the differences were statistically significant (P<0.05). At the same time, we also found that there was no significant difference in ACA between 1 month after operation and preoperative (P =0.297), and there was significant difference in ACA between 1 year after operation and preoperative or 1 month after operation (P=0.001,0.035<0.05). This result ,however ,was slightly different compared to the results of Yang Dandan et al. they found statistically significant difference in ACD, ACV, ACA of high myopia group between 1 month after operation and preoperative , but there was no statistically significant difference in ACA of low and moderate myopia between 1 month after operation and preoperative, which was different from the change of ACA in our study and may be associated with different groups of diopter. The changes of ACD and ACV are believed to be related to the backward movement of the posterior corneal surface;on the other hand, postoperative patients are often in a state of emmetropia or mild hyperopia, which requires accommodation in near vision, resulting in the increasing in lens thickness[24-25], and reduction in ACD and ACV. However, the changes of ACA should be considered if the changes of the posterior corneal surface and the anterior chamber mainly occurred in the central corneal ablation area in the early postoperative period, and had little effect on the peripheral ACA. Some studies found that there was no statistically significant difference in the peripheral anterior chamber angle of patients with low or medium myopia between 3 months after FS-LASIK compared with that before surgery, which was in agreement with what we have found. However, as time went by, the ACD decreased further, which would also have certain influence on the peripheral structure.In addition, whether it was related to the increase of lens thickness and forword movement of lens needs to be further explored.
In addition, we found that although the changes of ACV and ACA were not significantly correlated with the ablation depth, there was a certain correlation between the changes of ACD and the ablation depth one year after surgery . It indicated that the greater the ablation depth is, the greater the changes of ACD will be. It also reminded us to make a reasonable choice when choosing SMILE surgery, and the ablation depth must be strictly controlled.
This study investigated the changes of anterior segment parameters in patients with moderate to high myopia 1 year after SMILE and found that although the curvature of the anterior and posterior corneal surfaces would change in the early postoperative stage after SMILE, it would gradually become stable in the medium and long term. At the same time, the elevation of posterior corneal surface decreased. However, the anterior chamber, especially the ACD, changed significantly, and it was still changing 1 year after the operation. At the same time, there are some limitations in the present study. For example, the sample size was relatively small and this study was not grouped by diopter.We could expand the sample size and prolong the observation time for further observation.