We investigated the ACD and vault for horizontal and vertical fixations of the ICL. There was no change in the ACD from before to 3 m after surgery in both groups. However, the vault in the horizontal fixation group was greater than that in the vertical group throughout the course, and the difference between the two groups was stable from the next day onward and was approximately 110 µm at the final measurement. Comparing the preoperative prediction using the KS formula and actual measurement after surgery, the horizontal fixation almost matched the preoperative prediction in the final measurement, whereas vertical fixation value was lower than the predicted one. The vault was low immediately after surgery, increased once on the next day, and then gradually decreased over the next month in both fixation methods.
Normally, temporal incision and horizontal fixation are standard surgical procedures used for ICL implantation.
However, because most young patients undergoing ICL surgery have with the rule astigmatism, some surgeons prefer superior incision2, and there are also cases where the non-toric model is fixed in the vertical orientation. Additionally, this report demonstrated safety at 1 year postoperatively2.
Furthermore, to take the advantage of the vertical ciliary sulcus, which is longer than the horizontal ciliary sulcus, to prevent the rotation of the toric ICL, some surgeons prepare the ICL for the reverse toric axis and fix it vertically. Kamiya et al.2 reported safety, efficacy, predictability, and stability of the vertical fixation of ICL in terms of refractive outcome and uncorrected visual acuity. However, in 95% of Caucasian patients, the ciliary sulcus is larger in the vertical direction than in the horizontal one1. Furthermore, Kojima et al. reported that postoperative vault changes following ICL surgery may also be associated with postoperative rotation7. This difference may affect the determination of the lens size in vertical fixation because low vault can be an incidence from anterior capsular opacification and cataract formation and excessively high vault is a risk factor of inflammation, high intraocular pressure, angle-closure glaucoma and pigment dispersion syndrome. Thus, it is important to consider the postoperative vault in the case of vertical fixation and its relationship with currently frequently used prediction formula for ICL size based on the horizontal WTW and angle-to-angle (ATA) distances. However, the abovementioned reports on vertical fixation of ICLs discuss postoperative refraction and rotational stability2,3 but do not mention postoperative vault.
The ICL can be manufactured in four sizes, ranging between 12.1 and 13.7 mm, and must be selected from preoperative biometric measurements. Haptics is fixed at four points on the elliptical ciliary sulcus, although it is difficult to measure the STS distance preoperatively without specialized inspection equipment, such as an ultrasound biomicroscope4. Moreover, the STAAR surgical calculator, which supplies ICLs, also determines the ICL size with reference to the WTW distance only. On the other hand, in cases of postoperative high vault, the technique of vertical rotation of the implanted ICL,8 and the option of vertical fixation of the same size lens in the second eye surgery,9 have also been reported. These concepts have received more attention in recent years because the central port has eliminated the need for laser iridotomies and surgical iridectomy.
However, the STS and WTW distances are correlated in emmetropic eyes, and the correlation is low in myopic eyes1. In such situation, CASIA2 is equipped with two types of ICL size determination software. Because the STS distance cannot be measured using this machine, the expected vault and expected ACD (corneal endothelium–ICL anterior distance) are calculated from other measurement data to determine the ICL size. The present study found that vertical fixation of the ICL reduced the vault by > 100 µm on average compared with the horizontal fixation. This corresponds to approximately 20% of the CT; thus, in cases where vertical fixation is planned from the beginning, it would be advisable to consider subtracting this value from that of the predicted postoperative vault from the current formula when determining the size. According to the manufacturer’s instructions the recommended vault after ICL insertion is 250–750 μm, which is set with a considerable range. There are not many cases where the size is reduced by planning to fix it vertically, although this value is useful as a reference. However, while maintaining the aforementioned differences, changes in the vault over time for both vertical and horizontal fixations were similar to those shown in previous reports10, where the vault was increased from immediately after surgery to the next day and then decreased again. Therefore, there is no need to pay special attention to changes in the vault over time, depending on the direction of fixation. Factors that affect the postoperative vault include variations in the shape of the ciliary sulcus11, anteroposterior movement of the lens due to accommodation12,13, and miosis, which causes the ICL to be pushed backward by the iris, resulting in a lowered vault14. Regarding changes in the vault over time, Chen et al. reported that the vault was slightly high 2 h after ICL surgery, decreased significantly over the next day, and then approached its original height again over the next week and month15. In our study, the vault was lowered immediately after operation when the pupil was still in the state of mydriasis. Although the results were different from those reported by Kato et al.14 and Zhu et al.15, in the early postoperative period, the influence of residual viscoelastic substances appears to have been greater than the interference caused by the movement of the iris.
One of the limitations of this study is that there were unavoidable differences between patients and surgeons, especially in the aspiration and removal of viscoelastic substances in the data immediately after surgery. However, in this study, clinically useful information was obtained regarding the vault at the final observation period. Although the vault does not change significantly from 1 week to 1 month after surgery and is already stable at this time, it is also desirable to confirm vault changes throughout the postoperative period. Another fundamental problem is that the vault undergoes dynamic changes due to intraocular dynamics, including the pupillary light reflex16,17,18, so it would be ideal to measure the illuminance at the time of measurement and keep the conditions constant, but in this study, no illuminance measurement was performed. Also it should be added that this study is based entirely on Asian data.
In conclusion, when the ICL is vertically fixed, the postoperative change over time shows a similar trend to that when ICL is fixed horizontally; however, it is desirable to select the ICL size considering the vault would be lowered by > 100 µm immediately afterward and throughout the course of treatment. Ultimately, a calculation formula should be constructed for vertical fixation.