As the most vital postoperative parameter after EVO-ICL surgery, vault has drawn wide concern of researchers. Prior studies developed a variety of formulas and algorithms to explore the relationship between preoperative parameters and vault, but the conclusions and factors that significantly affect the vault were still not clear. For instance, NK and KS formula12–14 have been commercially available for predicting the ICL vault, but the factors they used to build the formulas are completely different: according to NK formula, the predicted ICL vault = 0.5 + 1.1 × (implanted ICL size − optimal ICL size using the NK formula)12, while in KS formula, predicted ICL vault (µm) = 660.9 × (ICL size (mm) − ATA (mm)) + 86.613. However, the traditional tool to recommend the optimal ICL size, the manufacturer’s nomogram was based on WTW and ACD. A study aiming to compare the achieved vault using formulas above showed that the achieved vault was significantly smaller than the predicted vaults using the NK and KS formulas respectively, and the agreement rate of the recommended ICL size using the manufacturer’s nomogram, the NK formula, and the KS formula was 50.0%.18 Many researches have been carried out to predict vault more precisely: Trancon et al built a formula that explained 34% of the vault variance15; 36% found by Zheng et al 19; and Lee et al reported a formula that explained37% of the vault variance20, but there is still no general agreement about the parameters that should be identified as risk factors of abnormal vault.
The reason previous studies fail to build a wide-accepted model to predict vault effectively is complicated. In theory, the parameters that measure the size of anterior chamber may show a liner correlation of the vault in the patients implanted with particular size of ICL. While the accuracy of parameters could be a challenge when trying to explore the factor that influence vault since it has been reported that biometric measurements such as WTW with different technologies showed large discrepancies with low correlation levels21,and interexaminer variance in STS measurement should be considered22. In addition, researchers have found that the vault could be unpredictable due to immeasurable posterior chamber anatomic factors such as ciliary body23. Chen et al reported eyes with an anteriorly positioned ciliary body were associated with a higher rate of excessive vault24. Other researchers categorized the iris shape into three groups, and they found concave shape iris had a higher risk of low vault and convex shape iris were more likely to demonstrate high vault in eyes with thick lens25,26.Therefore, a new tool that based on different perspective need to be developed.
In this retrospective study, we innovatively divided the vaults into two categories: optimal vault (250 to 750 µm) and abnormal vault (< 250 or > 750 µm). Further, we identified independent risk factors of abnormal vault, including WTW, ACA, pupil size, and ICL-width. On the basis of these preoperative parameters, we established a nomogram to estimate risk of abnormal vault. The discrimination and calibration of the nomogram were proved, and this nomogram shows a good predictive effect. These results therefore need to be interpreted with caution since that the risk factors we found couldn’t be simply explained by traditional opinion such as there are linear interrelationship between preoperative parameters and precise value of vault. This new prediction model could contribute to our understanding of the reason why abnormal vault still exist after detailed calculation.
WTW is generally acknowledged as an indirect parameter to measure the size of posterior chamber. According to the nomogram we built, WTW is positively correlated with the risk of abnormal vault, but its points are relatively low, which indicates that after selecting optimal ICL size based on WTW, the influence of WTW has been partly diluted. Given the ICL haptic footplates are located on the ciliary sulcus, some researches took anatomic factors such as horizontal compression by the iris into consideration20. Therefore, the optimal ICL size should be adapted to the structure of posterior chamber, our study revealed that on condition that the optimal ICL has been selected, patients who have lager WTW along with lager width of ICL are more likely to have abnormal vaults after surgery.
In practice, there are only four types of ICL-width: 12.1mm, 12.6 mm, 13.2 mm, and 13.7 mm, which may explained why the ICL-width was found to be a risk factor of abnormal vault. The bigger ICL we choose, it is more likely to achieve abnormal vault, which may support the theory that other confounding factors such as the friction and softness of the ciliary sulcus structure would make the vault unpredictable20, and with larger size of ICL implanted, the influence of confounding factors is amplified. In the idealized model, horizontal compression generated by iris is the main factor that forms the vault. However, the iris also has vertical compression on the ICL, pushing the ICL close to the crystalline lens and inserting the ICL footplates into the ciliary sulcus, especially when the bigger size of ICL was chosen 20,27, leading to the deviation of the predicted and actual vault and higher risk of abnormal vault.
In addition, the angle between peripheral cornea and the root of iris, ACA, was a preventive factor, bigger ACA-T suggested lower risk of abnormal vault according to the nomogram. Eyes with large ACA have flat iris. As is elaborated above, when the ICL lens was implanted behind the iris, more flat iris would generate greater vertical compression and enlarge the friction of haptic footplates against the iris, which causing more risk of unpredictable achieving vault.
Moreover, our predictive model included pupil size as a risk factor. The relationship between pupil and vault changes has been widely investigated, several studies have revealed that pupil constriction leads to decrease of vault under photopic conditions28,29. It could be explained that as the pupil construct, the iris tension increase, pushing the ICL towards the crystalline, which leads to lower vault. Such theory are unsatisfactory because the changes of vault caused by pupil and iris movement are also influenced by other complicated factors. An example of these is the study carried out by Xiong et al. which suggested that an extremely low vault has a big rate of vault change, while an extremely high vault would constrict the posterior movement of pupil on mesopic condition.30 On the other hand, the ciliary muscle contraction along with pupil construction makes ICL bend and adapt to the posterior surface of the iris, which could also cause unpredictable outcomes and eventually leads to abnormal vault. In our study, the pupil size were measured on mesopic condition, which partly represents the general tension of iris. A possible theory is that the bigger pupil is related to inclination of ciliary and iris transformation, which were also supported by another research that showed increasing pupil size is associated with vault change postoperatively from 1 day to 1 week.31
In conclusion, our prediction model could be useful when selecting the size of ICL before surgery. If a patient have high points according to the nomogram, additional consideration is required since the risk of abnormal vault is relatively high. On the other hand, surgeons could select the size of ICL on the basis of nomogram to reduce the risk of abnormal vault and improve and decrease the rates of ICL re-change after EVO-ICL surgery. Furthermore, the parameters we identified to build prediction model may shed a light on future researches that aim to explore the method to predict the vault more precisely.
There are some limitations in our research. First of all, for a multivariable prediction model, our research was on the basis of a relatively small number of cases, so we plan to conduct further study and enlarge the study population. Secondly, our follow-up time is relatively short, the vault we analyzed was measured in one week after surgery. Although it is reported that the vault was meanly change in one week after surgery 32,33, it would be more convincible if we increase follow-up time and get more comprehensive data., which means the predictability may be imprecise with time goes by.