Over 120,000 ICL implantation surgeries are performed in over 60 countries annually, which makes vault, an indicator of postoperative safety, particularly important . In the current study, we aimed to assess the early changes in ICL vault in the first month, starting at 2 hours after implantation of the ICL. We analysed preoperative variables, including patient age, ICL size, SE, AL, CCT, flat K, steep K, mean K, ACD, crystalline LT, WTW obtained by three devices, horizontal and vertical STS, BPS and DPS and their difference, to identify factors that influenced or may be used to predict postoperative ICL vault.
Our results demonstrated a significant decrease in vault postoperatively from 2 hours to 1 day after implantation, which then increased from 1 day to 1 week; at 1 month after surgery, vault was still lower than that 2 hours after surgery. Since most surgeons tend to perform ICL implantation at different times, early vault after surgery is the reference for the ICL size selection for the contralateral eye. However, few studies have described changes in vault within 24 hours after surgery. According to the paired sample t test, vault at both 2 hours and 1 day after surgery had a good correlation with vault at 1 month after surgery, but vault at 1 month after surgery was lower than that at 2 hours and higher than that at 1 day after surgery. We speculated that residual viscoelastic agent played a critical role in the relatively high vault value at 2 hours after surgery, as vault then decreased with the removal of the viscoelastic agent by aqueous humour circulation. Garcia-Feijoo et al.  demonstrated that ICL haptics were usually ultimately located in the ciliary sulcus or ciliary body, while Choi et al.  demonstrated that 64.7% of phakic IOL haptics were fixated in the ciliary sulcus. However, by analysing the full-scale UBM of 134 eyes, Zhang et al.  found that the ICL haptics in most cases were not in the ciliary sulcus and that different haptic positions had a significant influence on postoperative vaulting. For example, the eyes with haptics on the top of the ciliary sulcus were likely to have a high vault value, while those with one haptic on the ciliary process and another haptic in the ciliary body tended to have a low vault value. We speculated that the change in the position of haptics may also be a reason for the change in vault after surgery. In addition, previous studies have shown that changes in pupil size were associated with postoperative vaulting [27–30]. Lee et al.  believed that pupil constriction creates anteroposterior vectors through iris constriction, which exerts pressure on the ICL. Because the V4C ICL has a central hole, pressure equilibrium is quickly achieved between the front and rear surfaces of the ICL, facilitating this process (the fountain effect of “aquaport”). In other words, a net effect of placing the ICL closer to the lens was created, followed by a reduction in central vault. Recently, Kato et al.  and Gonzalez-Lopez et al.  demonstrated that ICL vault can be significantly decreased by light-induced pupil constriction. Therefore, we speculated that pupil constriction due to the disappearance of the effect of the mydriatic agent played a very important role in vault reduction within 1 day after surgery. Finally, several studies have proven that the morphology of the crystalline lens also has an influence on vault after ICL implantation [32–34]. ICL vault was affected by changes in crystalline lens rise (CLR) caused by accommodation or light condition changes [20, 35, 36]. We hypothesized that the morphological changes in the crystalline lens caused by accommodation after surgery may also be a reason for decreased vault. Regarding the change from 1 day to 1 month after surgery, our results were highly consistent with those in the study by Chen and colleagues, who believed that changes in the pupil size and the position of haptics were the main reasons for the results .
According to our results, vault value at 1 month after surgery was positively correlated with ACD, WTW and ICL size and negatively correlated with crystalline LT. However, we believe that such results are of little clinical significance because ICL size was an important factor affecting postoperative vaulting. In our study, ICL size was not a continuous variable, so it had a significant impact on the results of Spearman’s correlation analysis. We are confident in the results of the multivariate analysis. We found that ICL size, followed by horizontal STS, crystalline LT and vertical STS, significantly influenced 1-month postoperative vaulting. Previous studies have shown that the ciliary sulcus is vertically oval [37–39]. Since the ICL has a flat plate design and a certain width, the supporting points of the lens are located between the horizontal and vertical ciliary sulcus but closer to the horizontal position. Therefore, the influence of horizontal STS distance on postoperative vaulting is greater than that of vertical STS (standardized partial regression coefficient − 0.517 vs. -0.257). As described previously, the morphology of the crystalline lens has a certain influence on vaulting after ICL implantation. Most recent studies have focused on the effect of CLR on vaulting after surgery [31, 34–36, 40]. However, the measurement of CLR is relatively complex. Qi et al.  demonstrated that the crystalline LT had an important influence on postoperative vault, which is highly consistent with our results, and crystalline LT can be easily obtained by an IOLMaster 700. There may be a correlation between CLR and crystalline LT, which needs to be further verified in subsequent studies. Our regression formula used crystalline LT as one of the independent variables, which had a very high degree of fitting (the R, R2 and adjusted R2 of the model were 0.814, 0.660 and 0.643, respectively), indicating that crystalline LT is an excellent predictive variable. Our results also showed that pupil size can influence postoperative vault, but vault cannot be predicted by preoperative pupil size, including BPS, DPS or the difference between the two.
Conventionally, the manufacturer's recommendation for ICL size refers to only two parameters: WTW and ACD (Visian ICL Product Information: Visian ICL For Myopia. Available at http://www.accessdata.fda.gov/cdrh_docs/pdf3/p030016c.pdf)). According to our results, neither WTW measured by any instrument nor ACD was a reliable predictor of postoperative vaulting, which was also the consensus of many similar studies [34, 35, 42]. Lee et al.  obtained the following regression formula after multivariate linear regression analysis of 236 patients with 12.6-mm crystal implantation: central vault (µm) = − 0.784 + (0.171 × preoperative ACD) + (0.038 × preoperative pupil size) + (0.017 × preoperative AL). Unfortunately, the fitting degree of this formula was very low (R2 = 0.144), only one size of ICL was included in this study, and the axial direction of ICL placement was not considered, which would affect the results. Chen et al.  developed the following regression formula in their study: ICL V4 central vault (µm) = (386.51 × ACD) − 718.77, ICL V4c central vault (µm) = (503.43 × ACD) − 1075.64. Similarly, the low fitting degree (adjusted R2 = 0.320 and 0.297) and the small sample size (38 eyes for the V4 group and 39 eyes for the V4c group) make the results unsatisfactory. Recently, Igarashi et al.  developed a relatively good prediction formula on the basis of the angle-to-angle (ATA) measurement: postoperative vault (mm) = 660.9 × (ICL size [mm] – ATA [mm]) + 86.6. However, the fitting degree of the adjusted R2 (0.41) was still not completely satisfactory. The NK formula developed by Nakamura et al. [34, 43] is probably the most accurate prediction formula so far. The formula considers the distance between scleral spurs (ACW) and CLR as independent variables, and the R2 of the multiple regression was 0.68 and the adjusted R2 was 0.666. In subsequent validation, moderate vault was achieved in 92.1% of cases by the application of the formula . The regression formula in this study has some similarities with the NK formula. For example, they have similar fitting degrees (adjusted R2 of 0.666 vs. 0.643), CLR and crystalline LT both describe the morphology of the lens, and the distance between scleral spurs and horizontal and vertical STS describe the anatomical morphology of the posterior chamber. In addition, the sample size of our study was larger (83 eyes vs. 46 eyes) than theirs, and the crystalline LT was easier to measure. To identify the presence of ciliary body cysts, UBM is an important preoperative examination before ICL implantation [18, 44]. Therefore, our formula can be applied conveniently, without an additional anterior segment OCT examination.
There are certain limitations in this study. First, although the sample size was larger than those in some similar studies, it still needs to be supplemented in subsequent studies. Second, we only developed the prediction formula, and verification of the formula still needs to be carried out. We will release our verification work for the first time. Finally, our study was conducted among only Han Chinese, and further study is needed to determine whether ethnic differences will have an impact on the results.