In this study, we compared the accuracy of post-operation vault which predicted by NK formula and KS formula in the AS-OCT respectively. The prediction deviation of the KS formula was smaller than that of the NK formula, and both NK formula and KS formula showed poor predictability in larger ICL size group. The predicted ICL vault tended to be overestimated by using AS-OCT, especially when selecting a larger ICL size. Our finding also demonstrated that the achieved vault was influenced by ACD and STS. And ACD and ICL size were the predict error factors in vault prediction when using the KS formula. But for the NK formula, ICL size was the only predict error factor in vault prediction. It should be noted that the absolute prediction error showed no significant difference between the two formulas. It would be noticed that the prediction formulas need to be modified in different ICL size, especially when using the larger one.
Generally, the ICL size is recommend base on WTW and ACD by the online calculator system support by the manufacturer[6]. However, STAAR's recommended approach only achieved an ideal vault in 69% of patients. The measurement of white-to-white (WTW) can be conducted using various devices such as Orbscan, IOL Master, Pentacam and Lenstar. Factors like limbal pigmentation, pannus, or wide greyish limbal edges can impact the positioning of the corneal limbus and affect ICL sizing calculation[9]. While the haptics of the ICL are typically placed in the ciliary sulcus, determining the appropriate ICL size is ideally based on STS measurements. However, studies have shown that there is limited correlation between STS diameter and WTW measurement[10–13]. UBM-based measurement of STS may vary depending on examiner experience and skills. Incorrect STS measurement could lead to selecting an inappropriate ICL size or extreme variations in ICL vault height that may necessitate an exchange procedure. AS-OCT has demonstrated excellent repeatability and accuracy when measuring ocular biological parameters[14]. The NK formula and the KS formula are two predictive formulas for vault which preloaded on the AS-OCT (CASIA2, Tomey, Nagoya, Japan). AS-OCT shows great repeatability and accuracy in measurements of ocular biological parameters[9]. The KS formula installed in the AS-OCT provides the predicted vault of different ICL size using ATA [4]. Although a meta-analysis revealed no statistically significant differences in vault size between WTW-based and STS distance-based sizing methods[11], this finding may have been influenced by potential inaccuracies in measuring the STS distance and by heterogeneity among the studies included. In the current study, our results showed that the achieved vault was significantly different from the predicted vault using the NK formula and the KS formula when selecting a 13.2 mm ICL size. In addition, the achieved vault was smaller than the predicted vault using these two formulas when selecting a 13.2 mm ICL size. However, the achieved vault was not different from the predicted vault using these two formulas when selecting a 12.6 mm ICL size. It suggested that the larger ICL size might show more errors in predicting vault when comparing with smaller ICL size. These results were similar with previous studies[15, 16]. Santiago et al[17]. proposed that eyes implanted with 13.7 mm ICL were seven times more likely to present high vaults compared to eyes implanted with 13.2 mm ICL. However, when an ICL size of 12.1 mm was selected, the postoperative vault tended to be smaller than expected. When ICL sizes of 13.2 mm or more were selected, the postoperative vault tended to be larger than expected. Our results are consistent with Akihito's study[16]. The differences in the compression among the different ICL size may be a major cause for related higher vault effect[17]. The compression may cause by iris, dampening effect of ciliary sulus structure and innate ICL lens vault[12]. Therefore, ICL prediction formulas of different sizes should be considered to be corrected for different sizes. The adjusted KS formula matched by ICL sizes presented better consistency to achieved vault than that before[16]. In our study, the KS formula showed higher prediction consistency in different conditions among different ICL sizes, CLR and STS when comparing with the NK formula.
The distance separating the STS line from the anterior apex of the crystalline lens represents CLR, an amplitude relevant to the vault[18]. The larger the CLR was, the more it would occupy the postoperative vault, which might lead to a small postoperative vault. Therefore, the prediction formula should be adjusted for CLR in different ranges. Studies have shown that CLR increases with age, and vault decreases by 5µm each year with age[19, 20]. However, age was not a risk factor for vault in our study. Our results showed that CLR, STS and ACD were factors influencing the achieved vault, which were similar with previous studies[17, 20, 21]. The greater the CLR, STS and ACD were, the greater the deviation between the predicted value and the achieved vault would be. Therefore, the vault prediction adjusted by the personalized parameters would make the prediction formula more accurate.
The present study has two notable limitations. Firstly, the sample size was relatively small. Secondly, we included both eyes of the same patient in this study. Moving forward, we aim to enhance the regression equation by accumulating data from a larger cohort and refining individual biological parameters to augment accuracy. We obtained similar findings even when selecting only one eye randomly from each patient, indicating that the bilateral issue may not be significantly severe (as including both eyes mainly amplifies type 1 errors) within this study population. Therefore, we included both eyes of the same patient in this case series, aligning with numerous published studies on refractive surgery.