Introduction

To maximize the outcomes of a toric intraocular lenses (IOL), precise axis alignment is of crucial importance. Misalignment of a toric IOL, either due to inaccurate placement or significant rotation after surgery, can jeopardize the merits of astigmatism-reducing technology. Although rotational stability of standard open-loop toric IOLs has been well studied [1,2,3,4,5,6], there is a paucity of data about rotational stability of plate-haptic toric IOLs. Several studies report that plate-haptic toric IOLs showed only small amounts of rotation after surgery [7,8,9,10]; one study reports that, in myopic eyes plate-haptic toric IOLs showed less postoperative rotation than C-loop haptic toric IOLs [11]. On the other hand, extensive misalignment of a plate-haptic toric IOL is reported in the literature [12] as well as in anecdotal reports [13], suggesting that the plate-haptic IOLs tend to become fixed in a meridian of the capsular bag that best fits the diagonal diameter of the IOL [12]. The current multicenter retrospective study was conducted to investigate the occurrence of extensive misalignment of rotationally asymmetric toric IOLs and their association with clinical parameters.

Subjects and methods

Subjects

Data were collected from 332 eyes of 236 patients (72.8 ± 10.0 years) who had undergone phacoemulsification and implantation of a rotationally asymmetric, plate-haptic, refractive segmented multifocal toric IOL with near addition of + 1.5 diopters (D) (Lentis Comfort Toric, Teleon Surgical B.V) from April 2020 to December 2021 at 5 surgical centers in Japan. By reviewing the patients’ charts, data were analysed on age, gender, preoperative keratometry, type of astigmatism, corneal diameter, anterior chamber depth, crystalline lens thickness, axial length, spherical and toric power of an IOL, target toric axis, model of each toric IOL, occurrence of extensive misalignment (≥ 45º from the intended axis), degree of misalignment, and direction of misalignment (clockwise/counter clockwise). Based on the preoperative keratometry, the type of astigmatism was categorized as follows. When the steepest meridian of the anterior cornea was within ± 30º of the vertical axis, the astigmatism was judged to be with-the-rule (WTR). When the steepest meridian was ± 30º of the horizontal axis, the astigmatism was considered to be against-the-rule (ATR). All other cases were regarded as oblique astigmatism.

Surgery

Since this was a retrospective study, examination methods and surgical techniques were not standardized among surgeons. In general, the procedures were as follows. Before cataract surgery, with the patients in an upright seated position to avoid cyclotorsion errors, the corneal limbus was manually marked along the principal meridians at the slit lamp or the reference points on the conjunctiva and limbus were recorded with a digital image-guide system. Standard phacoemulsification and IOL implantation were performed through a 2.2- or 2.4-mm incision. Continuous curvilinear capsulorrhexis (CCC) of approximately 5.0–5.5 mm in diameter was created. The IOL was implanted into the capsular bag with an injector and rotated to the final position by aligning the reference marks on the IOL with the target axis. The ophthalmic viscosurgical devices were completely washed out from the anterior chamber and behind the IOL, and in-the-bag placement of the IOL was confirmed under direct microscopic observation.

Examinations

After surgery, the patients were followed-up for at least a week. On the postoperative visits, the pupil was dilated and IOL orientation was evaluated at a slit-lamp or using the anterior segment optical coherence tomography (CASIA2, Tomey) (Fig. 1) [14]. Misalignment was defined as the difference between the calculated IOL axis before surgery and the real IOL axis postoperatively.

Fig. 1
figure 1

Automatic measurement of toric IOL axis orientation with anterior segment OCT

Corneal topography (left) and color image (right) are presented. A linear marker is automatically aligned on the reference marks of the toric IOL.

IOL intraocular lens. OCT optical coherence tomography

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The study protocol was approved by the Institutional Review Board of Tsukuba University Hospital (R02-184). The committee waived the requirement for informed consent from the patients regarding the use of their medical record data in accordance with the regulations of the Japanese Guidelines for Epidemiologic Study issued by the Japanese Government. Clinical trial registration was not required owing to the observational nature of the study. The study was conducted in accordance with the Declaration of Helsinki, the Ministerial Ordinance Regarding Good Clinical Practice Principles for Medical Devices (2005, Ministry of Health, Labour and Welfare Ordinance No.36).

Statistical analysis

Numerical data were compared between groups using the Mann-Whitney U test. For categorical data, the chi-square test and Fisher’s exact test for trend were used to assess the incidence rates. A p value of less than 0.05 was considered significant and all statistical tests were 2-sided. Statistical analysis was performed using SPSS Statistics for Windows software (version 28, IBM Corp.). Numerical data are presented as the mean ± standard deviation (SD).

Results

Among 332 eyes, extensive misalignment of toric IOLs ≥ 45º occurred in 3.3% (11 eyes of 11 patients). The misalignment was found on the first postoperative visit in all cases, and the IOL orientation did not change thereafter. The amount of misalignment averaged 81.6 ± 22.9º (range 49–118º). Clockwise and counter clockwise rotation was seen in 6 eyes (right/left = 3/3) and 5 eyes (right/left = 2/3), respectively. Repositioning surgery to correct toric IOL misorientation was conducted in 9 eyes between 7 and 28 days after cataract surgery. In 2 eyes, repositioning surgery was carried out twice: 7 and 21 days, and 14 and 21 days following cataract surgery. The amount of axis misalignment at the final visit in these eyes was 7.3 ± 8.2º. Two patients did not consent to the secondary surgery because they found the amount of residual astigmatism caused by IOL misalignment insignificant.

The clinical data of patients are compared between two groups (Table 1). The eyes with extensive misalignment showed significantly greater axial length (p < 0.001), larger corneal diameter (p = 0.034), and smaller corneal power (p = 0.044) than those without extensive misalignment. Other parameters were not significantly different between the two groups. The stepwise logistic regression analysis revealed that axial length was significantly associated with the occurrence of extensive misalignment of a toric IOL (p < 0.001). The amount of misalignment in eyes without extensive misalignment was 3.0 ± 2.7º.

Table 1 Comparison of patients’ data between groups
Full size table

Discussion

Extensive misalignment of a standard open-loop toric IOL is rarely seen. In the current study, extensive misalignment ≥ 45° was found in 3.3% (11 eyes) of cases that were treated with a plate-haptic toric IOL. Amigó et al. [12] report 5 eyes of 3 patients in which plate-haptic toric IOLs (Lentis Mplus Toric) rotated by 15–45° away from the intended orientation, but further rotation of IOLs did not occur after 24 h postoperatively. These phenomena may represent a sign that the perimeter of the capsular bag is not round but elliptical, and the IOL takes a position which finally prevents further rotation, fitting into the capsular bag diameter that best suits the diagonal diameter of the IOL [12]. It is known that the anterior segment of the human eye is not circular with the horizontal corneal diameter being significantly greater than its vertical diameter [15]. The perimeter of the sulcus is also significantly ellipsoidal with its vertical diameter larger than the horizontal one [16]. There is also evidence indicating that the ciliary ring is ellipsoid [17].

Except for the 11 eyes that showed extensive misalignment, the average amount of toric IOL misalignment was 3.0 ± 2.7° in 321 eyes, comparable with the results of previous studies on the standard open-loop toric IOLs [1,2,3,4,5,6]. In general, plate-haptic toric IOLs are reported to have satisfactory rotational stability [7,8,9,10,11]. It seems that despite good outcomes in most cases with a plate-haptic toric IOL, extensive misalignment occurs occasionally depending on anatomical, clinical, and other conditions. Amigó et al. [12] reveal that extensive rotation of a plate-haptic toric IOL was seen in eyes with axial length greater than 24 mm. We also found that the risk factors of extensive misalignment included greater axial length, flatter corneas, and larger corneas. These are among the known risk factors for postoperative rotational instability of open-loop toric IOLs [13, 18,19,20].

In order to prevent extensive misalignment of toric IOLs, use of a capsular tension ring (CTR) at the time of cataract surgery may be considered. Rastogi et al. [21] report that co-implantation of a CTR with an open-loop toric IOL significantly reduced the amount of IOL rotation, but visual outcomes did not improve. In patients with axial myopic astigmatism, Zhao et al. [22] indicate that the use of a CTR effectively enhanced rotational stability of an open-loop toric IOL, achieving improvement in astigmatism and visual acuity. On the other hand, one randomized clinical trial denied the effects of a CTR to limit postoperative rotation or avoid outliers in cohorts with an open-loop toric IOL [23]. For a plate-haptic toric IOL, Ma et al. [24] demonstrate that the use of a CTR significantly improved rotational stability of a toric IOL by reducing the impact of crystalline lens thickness, and CTR co-implantation was recommended in patients with lens thickness ≥ 4.5 mm, white-to-white ≥ 11.6 mm, or high preexisting astigmatism. Since the use of a CTR in eyes without the risk of zonular instability is off-label, we did not think of the use of a CTR in our study. Asymmetric or oval shape of the capsular bag can cause decentration of an IOL, especially a plate-haptic IOL which is less likely to absorb the centripetal force of the capsular bag. Asymmetry and ovality of the capsular bag could be counteracted by a CTR, which appears more beneficial with a plate-haptic than an open-haptic toric IOL for the prevention of unexpected displacement.

In this study, we analyzed the incidence of eyes with extensive misalignment ≥ 45°. While axis misalignment of 30° can occasionally be found with other toric IOLs, extensive misalignment of 45° or more is rarely seen with the conventional open-loop type toric IOLs. Thus, we used this cut line. When we had analyzed the data using the cut line of 30°, the incidence rose to 4.8% (16/332), but the results were similar.

The current study has several limitations. First, because this was not a prospective study, some important parameters were not pre-determined or recorded, such as the size and coverage status of CCC over the IOL optic, which are known to play an important role in preventing postoperative rotation of toric IOLs [25]. In general, surgeons tend to create a larger CCC for safer and smoother implantation of plate-haptic IOLs. Such propensity might contribute to the occurrence of major rotation of this type of toric IOLs. Second, in the current study, extensive misalignment was seen mainly in eyes with ATR astigmatism, but the type of astigmatism (WTR, ATR, and oblique) was not a significant risk factor for rotation (Table 1). This may be because most of the eyes included in the study had ATR astigmatism in the first place, and that the number of eyes with extensive misalignment was limited. This can be an important point because a plate-haptic IOL is placed horizontally in eyes with ATR, vertically in eyes with WTR, and obliquely in eyes with oblique astigmatism. With the open-loop toric IOLs, it is reported that more postoperative IOL rotation occurred when vertically placed at the time of cataract surgery [26, 27], whereas greater rotation occurred with oblique placement [28]. In another study, no correlation existed between initial IOL placement and its postsurgical rotation [29]. The association between intraoperative placement orientation and postoperative misalignment of a plate-haptic toric IOL is an interesting theme, which will be the subject of future studies.