The number of the patients who received SMILE has been increasing enormously, which makes it crucial for the ophthalmologists to comprehend its impact on the eye, not just on the cornea. There have been many studies in the literature evaluating the outcomes of corneal refractive surgery regarding corneal thickness and anterior chamber depth; however, most of them addressed LASIK patients and only a few focused on the anterior chamber angle structures. Moreover, the data in the majority of those studies were obtained from other tomography systems rather than Sirius Scheimpflug-Placido disc tomographer [6–8, 12, 13]. This study shows the change in the anterior segment dimensions including the angle-related measurements, and the possible factors affecting the outcome after SMILE.
In this study, CCT, and ACD significantly decreased at postoperative 6-month, similar to previous studies with other tomography systems (p < 0.05 for both) [6, 9, 13]. In addition, we observed a significant decrease at HACD (p < 0.05), but no change at ICA (p = 0.54). Yassa et al. suggested that this discrepancy might be due to the accommodation of the patients while fixating to the tomography device for imaging, which causes the central lens shift more anteriorly whereas the periphery remains flatter . However, although not statistically significant, in current study both temporal and nasal ACA increased postoperatively (p = 0.118, and p = 0.255, respectively).
Similar to our results, Yassa et al. and Zhou et al. reported that after LASIK, anterior chamber angle showed no difference postoperatively by using the data obtained from Sirius and Pentacam (Oculus, Wetzlar, Germany) tomography systems, respectively [7, 8]. In their study comparing different techniques to assess the status of ICA, Källmark et al. concluded that despite the lack of good agreement with gonioscopy, Sirius still might be a good option, since it is more comfortable for the patient, and it requires less skills to perform in an objective manner . In terms of refractive surgery, it might also provide an essential insight for the glaucoma evaluation of the patient preoperatively.
Our analysis confirmed that age was positively, and Δ-tACA was negatively correlated with the Δ-HACD (Fig. 1, 2). HACD shows the distance between the two angles, which might be modified by both the angle structures and the anterior chamber dimension. This is the first time HACD was evaluated after refractive surgery; however a few studies evaluated the angle to angle diameter following cataract surgery, and they concluded that horizontal angle to angle diameter increased significantly more than the vertical diameter . Prior to phakic intraocular lens (IOL) implantation, anterior segment dimensions, especially the horizontal corneal dimension (white-to-white or horizontal visible iris diameter) is checked regularly . We believe HACD might also provide a critical contribution in the decision-making process of the appropriate size of the IOL for the eye. Due to new technological developments in the design and implantation techniques of intraocular lenses (IOL), together with the advancements of corneal refractive surgery, there will probably be more patients requiring subsequent and more sophisticated IOL implantation procedures in near future.
There was a significant negative correlation between Δ-ICA and Δ-ACD (r=-0.282, p = 0.016) (Fig. 3). In their study, Nishimura et al. evaluated the effect of age on change in ACD after LASIK, and reported that there was a significant negative correlation, which was interpreted as ACD decreased in the younger patients (less than 40 years of age), but not in older patients . In our study, the population consisted almost exclusively by young patients, which might explain the difference. Chen et al. found that despite the lack of correlation after 1 day, Δ-LT and Δ-ACD had a negative correlation following SMILE after 1 week . However, this follow-up time is also shorter than ours, and this might have been a temporary outcome until the accommodative process adopts to the changes due to procedure. Our results suggest that even though ICA does not change after SMILE, its alteration has a significance on the change of ACD.
There are some limitations to this study. First, Sirius images were acquired by single measurement. However, the intrasession repeatability of Sirius has been already reported as successful by other studies , and the technician (HK) was very experienced, and we included the images only with good acquisition quality. Second, since the Sirius system uses a fixation target and light for imaging, this might have affected the accommodation and caused pupil constriction, and thusly the measurement of anterior chamber depth and the iridocorneal angle of the eyes. However, the room itself was dark during acquisition, and the conditions were similar at both preoperative and postoperative imaging. The retrospective nature of our study is the other limitation.
SMILE is now a frequently performed procedure, hence ophthalmologists require more and better insight about the possible impact of it on the eye. The current study supports the findings that CCT, ACD, and HACD significantly decreases after SMILE. Age and Δ-tACA are correlated with Δ-HACD, and there is a negative correlation between Δ-ICA and Δ-ACD. Therefore, it is important to evaluate the anterior segment structures of the eyes including the anterior chamber angle, before and after SMILE. This may especially be helpful for patients regarding glaucoma assessment and a possible subsequent IOL implantation.