Impact of ICL Fixation Direction on Postoperative Vault and Prediction Accuracy Using the KS Formula

Purpose To compare postoperative vault measurements between horizontal and vertical fixation of the Implantable Collamer Lens (ICL) (KS-AquaPORT; STAAR Surgical) when its size is determined using the KS formula. Methods This retrospective study analyzed 2,343 eyes from 1,275 patients who underwent myopic ICL implantation. Pre-operative anterior segment optical coherence tomography (AS-OCT) (CASIA 2; Tomey Corporation) was performed in both horizontal and vertical orientations. ICL size was determined using the KS formula integrated into the imaging device, and postoperative vault measurements were compared between horizontal and vertical fixation groups. Results The mean postoperative vault was significantly larger in the horizontal fixation group (0.50 ± 0.20 mm) compared to the vertical fixation group (0.43 ± 0.17 mm; P < .001). The proportion of eyes with an extraordinarily high vault (⩾ 1 mm) was significantly higher in the horizontal fixation group (7 of 827 eyes; 0.85%) than in the vertical fixation group (2 of 1,516 eyes; 0.13%; P = .012). Prediction error was also significantly greater in the horizontal group (0.08 ± 0.16 vs 0.06 ± 0.14 mm; P < .001). Similarly, the median absolute error was significantly larger in the horizontal group (0.108 vs 0.085 mm; P < .001). Conclusions When ICL sizing is determined using the KS formula, horizontal fixation results in a larger postoperative vault and lower prediction accuracy compared to vertical fixation. These findings support vertical fixation as a potentially safer and more predictable approach for ICL implantation.