SMILE surgery has been widely implemented for myopia correction. It has been shown that SMILE exhibits better predictability and stability for its smaller changes than femtosecond laser–assisted laser in situ keratomileusis (LASIK)15–17, including smaller aberrations and better biomechanics. However, there are still complaints of problems with night vision-like ghosting and glare. Previous studies suggested that the appearance of these symptoms is directly related to the reductions in the EOZ.2,18,19. At present, EOZ has been evaluated by multiple methods, including region-growing algorithms20, ray-tracing analysis,21 corneal power distribution analysis22 and Modulation Transfer Function Method2. These methods have different measurement principles. However, the method performed by the corneal topography appears to be the most practical2,23. In this study, we adopted the Scheimpflug corneal topography system to measure and compare the EOZ and POZ before and after SMILE surgery by different measurement methods.
EOZ was defined as the diameter of the largest ring that the corneal refractive power difference between the ring and the pupil center was smaller than 1.50 D22. In our study, four measurement methods showed that EOZs were significantly reduced in eyes with high myopia as compared to the POZ at 3 months after SMILE surgery (Table 1), which is consistent with the findings in other reported studies2,3,11,19,24. This reduction could be attributed to the postoperative corneal oblate changes15 and the corneal healing responses, including the increased epithelial thickness25, the loss of laser energy around the cutting zone and the changes in biomechanical properties after surgery8,26. We observed that EOZV was the closest to the POZ, followed by EOZD, EOZP and EOZ4 (Figure 1). There was no significant difference between EOZV and EOZP. Since EOZV, EOZP and EOZ4 are easy to be determined, they could be measured in patients even with preoperative data loss. On the other hand, since both preoperative and postoperative data are clearly visible, EOZD is suitable to measure the eccentricity. In our study, it was noted that the difference between EOZP and EOZV was 0.02 mm with no statistical difference (Figure 2), which was in the range of clinically acceptable difference. This is likely due to the small differences between the position of corneal apex and pupil center.
In the current study, eccentricity was positively correlated with corneal aberrations, except Trefoil (Table 2). The changes in eccentricity and aberration were independent of EOZ, which was similar to the results from an earlier study22. In the eyes undergoing SMILE surgery, Qian et al. found that there was no correlation between the size of EOZ between high and low myopia, but the EOZ in myopic eyes with less than -7.50 D was significantly smaller as compared to those between -6.00 D and -7.50 D. However, no correlation was noted between the change in EOZ and dioptric power. Hou et al.15 found that there was no significant correlation relationship between the attempted refractive correction and the reduction of EOZ. For the LASIK patients with myopic spherical equivalent less than -10.00 D, Nepomuceno et al.21 showed that increase in the attempted refractive correction leads to further reduction of EOZ. Moreover, we also found that there was no significant difference between EOZ reduction and aberrations. Likewise, similar to the previous results, Dan et al. found that postoperative corneal aberrations changes shows no differences among the 3 EOZ measurement groups24.
There were few limitations in this study. This study focused on the patients with high myopia. Larger sample size with wide diopter range and higher myopia (> -7.50 D) are warranted in future studies. Many studies showed that the reduction of EOZ was related to the changes of Q values. On the other hand, the EOZ would not necessarily to be round, but oval in most cases. Our study did not analyze the transverse and vertical diameters of the EOZ, which could make further improvements in the measurement.