One-year evaluation of rotational stability and visual outcomes following horizontal, vertical, and oblique implantation of ICL V4c

Abstract

Background

To evaluate and compare the rotational stability and visual outcomes of Implantable Collamer Lens (ICL) V4c implantation in horizontal, vertical, and oblique orientations and to identify risk factors contributing to postoperative lens rotation.

Methods

In this retrospective study, 502 eyes of 265 patients who underwent ICL V4c implantation for myopia correction were analyzed. Patients were categorized into three groups based on implantation orientation: horizontal, vertical, or oblique. Preoperative parameters included uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), intraocular pressure (IOP), manifest refraction spherical equivalent (MRSE), axial length (AL), anterior chamber depth (ACD), white-to-white (WTW) distance, and sulcus-to-sulcus (STS) measurements. Postoperative evaluations were conducted at 1 year and included UDVA, CDVA, MRSE, IOP, vault measurements, and rotational angles of the ICL. Logistic regression analysis was performed to determine risk factors associated with significant ICL rotation (> 5°).

Results

All three groups achieved excellent visual outcomes, with 100% of cases attaining postoperative UDVA equal to or better than preoperative CDVA. No statistically significant differences were observed among the groups in terms of postoperative MRSE, IOP, or vault measurements. Although the mean rotational angles were comparable across the groups, the oblique implantation group demonstrated a higher incidence of rotations exceeding 5° compared to the horizontal and vertical groups. Furthermore, larger WTW distances were identified as a significant risk factor for increased rotational instability in the oblique group.

Conclusions

ICL V4c implantation is a highly effective procedure for myopia correction, providing excellent visual outcomes regardless of implantation orientation. However, oblique implantation, particularly in eyes with larger WTW distances, is associated with a higher risk of postoperative rotational instability. Careful consideration of WTW measurements may help minimize rotational risks in such cases.

Background

The Implantable Collamer Lens (ICL) has emerged as a well-established and effective option for the correction of myopia, particularly for patients who are not suitable candidates for corneal refractive procedures, such as LASIK or SMILE [1,2,3]. Unlike corneal-based surgeries, ICL implantation preserves corneal tissue and provides a reversible alternative, making it particularly advantageous for individuals with thin corneas, high refractive errors, or other contraindications to laser refractive surgery [4, 5]. Its long-term safety and efficacy have been extensively demonstrated, further establishing its role in modern refractive surgery [6,7,8,9].

The success of ICL implantation largely depends on achieving an appropriate postoperative vault, a key factor in minimizing complications and optimizing visual outcomes. With increasing focus on precision and personalization in surgical results, advancements in imaging technologies, such as ultrasound biomicroscopy (UBM), have significantly improved the understanding of ciliary sulcus anatomy and contributed to refining ICL implantation strategies [10]. Studies have shown that the ciliary sulcus often exhibits a longitudinal ellipse shape, prompting consideration of implanting the ICL in different orientations to better adapt to the varying diameters of the sulcus [11, 12]. However, in addition to achieving optimal vault height, maintaining rotational stability of the ICL within the eye is crucial, particularly for toric ICLs (TICLs). Rotational instability not only increases the risk of complications but also compromises the astigmatic correction efficacy of TICLs. Several factors, including ciliary sulcus morphology, ICL sizing, and implantation techniques, influence rotational stability [13, 14].

While horizontal and vertical orientations have been widely studied [15,16,17,18,19], the impact of oblique TICL implantation remains unclear. Concerns exist that oblique placement may increase the likelihood of rotation, though it remains debated whether implantation orientation is a key determinant of rotational stability [12]. This study seeks to evaluate the rotational stability and visual outcomes of ICL V4c implantation in horizontal, vertical, and oblique orientations. By analyzing a large cohort of patients, this study aims to provide a comprehensive understanding of the factors influencing rotational stability and offer guidance for optimizing outcomes across different implantation angles.

Methods

This retrospective cohort study included 502 eyes from 265 patients (82 males, 183 females; mean age: 28.22 ± 6.23 years) who underwent ICL V4c implantation for myopia correction at Shenzhen Shendong Aier Eye Hospital between August 2020 and December 2023. Inclusion criteria were: (1) age between 18 and 45 years; (2) stable refractive error (defined as a spherical equivalent change of ≤ 0.50 diopters annually over the past two years); (3) endothelial cell count of ≥ 2,000 cells/mm²; (4) anterior chamber depth (ACD) of ≥ 2.6 mm; and (5) a minimum follow-up of 12 months. Exclusion criteria included: (1) previous ocular trauma or surgery; (2) active ocular diseases, such as glaucoma, cataracts, or uveitis; (3) systemic diseases that might affect ocular health; and (4) non-adherence to follow-up protocols. The study adhered to the ethical principles outlined in the Declaration of Helsinki and received approval from the Ethics Committee of Shenzhen Shendong Aier Eye Hospital (approval number: SDLL-SCPJ-2024-03). Informed consent was obtained from all participants prior to data collection and surgical procedures.

Examinations and measurements

Preoperative evaluations included measurements of uncorrected and corrected distance visual acuity (UDVA and CDVA), manifest refraction, and intraocular pressure (IOP). The axial length was measured using the IOLMaster 700 (Carl Zeiss Meditec), and slit-lamp examination was conducted to assess the anterior segment. Corneal endothelial cell density was measured using non-contact specular microscopy (NSPC; KONAN MEDICAL), and a fundus examination was performed to ensure retinal health. Parameters such as anterior chamber depth (ACD), anterior chamber volume (ACV), white-to-white (WTW) distance, flat and steep keratometry (K-flat and K-steep), were obtained using Scheimpflug tomography (Pentacam HR; Oculus Optikgeräte GmbH). Sulcus-to-sulcus (STS) measurements in both horizontal and vertical directions were assessed using ultrasound biomicroscopy (SW-3200 L; Suowei Electronic Technology).

Postoperative evaluations were conducted at 6 h, 6 months, and 1 year post-surgery. At the postoperative visit and 1-year follow-up, patients were positioned at a slit-lamp microscope, with the chin and forehead rests adjusted to maintain a standardized horizontal meridian reference line, based on corneal reflection and slit beam alignment across both eyes. Anterior segment images of the operated eye were captured using a digital imaging module (S390L; Mediworks) mounted on a slit-lamp microscope. The ICL implantation angle (θ) was measured using ImageJ software (National Institutes of Health). It was defined as the angle between the line connecting the circular holes at each end of the ICL and the horizontal meridian reference line (Fig. 1). The rotation angle was defined as the absolute difference between the postoperative and 1-year implantation angles (θ), quantifying rotational changes over time. Vault measurements were acquired using the Pentacam HR.

Fig. 1

Measurement of ICL implantation angle and Angle-STS. A, Slit-lamp image illustrating the ICL implantation angle (θ), defined as the angle between the line connecting the circular holes at each end of the ICL (yellow line) and the horizontal meridian (red line). B, Schematic illustration of Angle-STS calculation. Angle-STS was derived from preoperative H-STS, V-STS, and the implantation angle (θ) relative to the horizontal meridian and calculated using the ellipse formula (lower equation). The horizontal meridian (red line) and implantation angle (θ, -90° to 90°; clockwise as +, counterclockwise as -) are indicated

Surgical procedures

ICL power was calculated using the manufacturer’s program (STAAR Surgical AG, Nidau, Switzerland), selecting the spherical power closest to plano according to the program’s recommendations. Angle-STS was derived from preoperative H-STS, V-STS, and the implantation angle relative to the horizontal meridian (θ). The ciliary sulcus was assumed to follow a longitudinal ellipse shape, and the length of Angle-STS was calculated mathematically using the ellipse formula (Fig. 1B). ICL size and implantation orientation were determined based on Angle-STS, anterior chamber width (ACW), crystalline lens rise (CLR), STSL (the distance between the STS plane and the anterior crystalline lens surface), anterior chamber depth (ACD), and white-to-white (WTW) distance. Horizontal alignment was defined as 0° ± 22.5°, vertical alignment as 90° ± 22.5°, and oblique positioning as 22.6° to 67.4° from the horizontal meridian.

Preoperative corneal markings were placed at 0° (horizontal meridian) and the designated implantation orientation. After achieving full pupil dilation and administering topical anesthesia, a 2.8 mm clear corneal incision was created along the steepest meridian of astigmatism. A viscoelastic agent (Qi Sheng Biologics) was injected into the anterior chamber to protect intraocular structures, followed by ICL insertion using a calibrated push injector. The ICL was rotated to align the circular marker holes with the predetermined axis, and the four haptics were positioned in the ciliary sulcus using an ICL positioning instrument. A handheld slit-lamp was used to evaluate vault height, and minor angle adjustments were performed as needed. Following irrigation of the viscoelastic agent, the incision was sealed, and standard postoperative care was initiated, including corticosteroid, antibiotic, and non-steroidal anti-inflammatory eye drops.

Statistical analysis

Data were analyzed using GraphPad Prism 9 (GraphPad Software, San Diego, CA). The distribution of continuous variables was assessed using the Kolmogorov-Smirnov test to evaluate normality. Parametric data were analyzed with one-way ANOVA, and post-hoc pairwise comparisons were made using Tukey’s HSD test. Non-parametric data were analyzed with the Kruskal-Wallis test, and pairwise comparisons were made using Dunn’s test. Categorical variables were compared using the chi-square test. The distribution of ICL rotation angles across groups was analyzed with the Kruskal-Wallis test. Logistic regression analysis was performed to identify risk factors associated with significant rotation (> 5°), with a significance threshold of P < 0.05.

Results

Baseline characteristics

A total of 502 eyes from 265 patients were included, distributed across three groups: 177 eyes in the horizontal group, 160 in the vertical group, and 165 in the oblique group. There were no significant differences in demographic and baseline ocular characteristics, including manifest refraction spherical equivalent (MRSE), axial length, endothelial cell count, anterior chamber depth (ACD), WTW distance, anterior chamber volume, intraocular pressure, and keratometry values across the groups (P > 0.05). However, there was a significant difference in ICL size among the groups (P < 0.001), with mean values of 12.68 ± 0.37 mm in the horizontal group, 13.00 ± 0.39 mm in the vertical group, and 12.82 ± 0.35 mm in the oblique group (Table 1).

Table 1 Baseline characteristics of patients in horizontal, vertical, and oblique ICL groups^{a, b}

Visual and safety outcomes

At the 1-year follow-up, 99% of eyes in the horizontal group and 100% of eyes in both the vertical and oblique groups achieved UDVA of 20/20 or better (Fig. 2A). No group experienced a loss of two or more lines of CDVA, and 100% of eyes demonstrated UDVA equal to or better than preoperative CDVA (Fig. 2B/D). Efficacy indices and safety indices did not significantly differ between the groups (P > 0.05), with efficacy indices of 1.18 ± 0.13, 1.16 ± 0.13, and 1.19 ± 0.16, and safety indices of 1.15 ± 0.15, 1.15 ± 0.13, and 1.14 ± 0.13 for the horizontal, vertical, and oblique groups, respectively (Table 2).

Fig. 2

Visual and refractive outcomes at 1-year follow-up across horizontal, vertical, and oblique implantation groups. A, Preoperative CDVA and 1-year postoperative UDVA. B, Change in Snellen lines of UDVA at 1-year follow-up compared to preoperative CDVA. C, Postoperative MRSE, with the percentage of eyes achieving MRSE within ± 0.50 D and ± 1.00 D. D, Change in Snellen lines of CDVA at 1-year follow-up compared to preoperative CDVA. E, Postoperative refractive astigmatism, with the percentage of eyes achieving astigmatism ≤ 0.50 D and ≤ 1.00 D. F, Scatter plot of attempted versus achieved SEQ with linear regression analysis for each orientation. D = diopters; CDVA = corrected distance visual acuity; UDVA = uncorrected distance visual acuity; MRSE = manifest refraction spherical equivalent; SEQ = spherical equivalent

Table 2 Postoperative outcomes, vault, and rotation angle in vertical, horizontal, and oblique ICL groups^{a, b}

No statistically significant differences were found in postoperative MRSE, with means of -0.16 ± 0.42 D, -0.06 ± 0.36 D, and − 0.15 ± 0.37 D in the horizontal, vertical, and oblique groups, respectively (P > 0.05, Table 2). Postoperative MRSE within ± 0.50 D was achieved in 75.71% of eyes in the horizontal group, 75.76% in the oblique group, and 83.12% in the vertical group, while accuracy within ± 1.00 D was reached in 96.61%, 99.39%, and 99.38% of eyes, respectively (Fig. 2C). Additionally, postoperative refractive astigmatism of ≤ 0.50 D was observed in 74.58% of eyes in the horizontal group, 86.67% in the oblique group, and 89.38% in the vertical group, with all groups achieving ≤ 1.00 D in over 98% of cases (Fig. 2E). Regression analysis of achieved versus attempted SEQ indicated high predictability across all groups, with strong linear correlations (Y = 0.9769X − 0.0059, R² = 0.9708 for horizontal; Y = 0.9962X + 0.1192, R² = 0.9732 for oblique; and Y = 1.0163X + 0.1792, R² = 0.9635 for vertical) (Fig. 2F).

Intraocular pressure (IOP) showed no statistically significant differences among the groups, with mean values of 13.14 ± 2.55 mmHg, 13.64 ± 2.16 mmHg, and 13.30 ± 2.12 mmHg in the horizontal, vertical, and oblique groups, respectively (P > 0.05, Table 2). Vault measurements were also similar across groups, with mean values of 392.60 ± 109.37 μm in the horizontal group, 383.31 ± 128.72 μm in the vertical group, and 379.27 ± 136.08 μm in the oblique group (P > 0.05, Table 2).

Rotational stability

At the 1-year postoperative follow-up, the mean rotation angle of the ICL was 2.29 ± 2.27˚ in the horizontal group, 2.17 ± 2.11˚ in the vertical group, and 2.92 ± 3.09˚ in the oblique group, with no statistically significant differences observed among the groups (P > 0.05, Table 2)​. To further assess rotational stability, the distribution of eyes across different rotation thresholds was analyzed, with statistically significant differences observed for rotations exceeding 5˚ (Table 3). Specifically, the oblique group had a higher incidence of rotation > 5˚ compared to the horizontal and vertical groups (P = 0.004 and P = 0.032, respectively, Table 3). These results indicate that, although the mean rotation angles were comparable across groups, the horizontal and vertical groups had higher proportions of eyes with no rotation and rotations confined to ≤ 5˚, suggesting greater rotational stability compared to the oblique group.

Table 3 Comparison of postoperative rotations in vertical, horizontal, and oblique ICL groups^a

Risk factors for rotation of ICL

At the 1-year follow-up, the relative risk (RR) of postoperative rotation > 5˚ was assessed across the horizontal, vertical, and oblique groups. Eyes in the oblique group showed a significantly higher risk of rotation > 5˚ compared to both the horizontal and vertical groups, with RRs of 2.789 (95% CI: 1.300, 5.983) when compared to the horizontal group and 2.101 (95% CI: 1.020, 4.326) relative to the vertical group (Table 4)​. This indicates that eyes in the oblique group are more than twice as likely to experience rotation > 5˚ compared to the other groups.

Table 4 Relative risk of postoperative rotation following ICL surgery

To further evaluate factors associated with postoperative rotation > 5˚, logistic regression analysis was conducted, incorporating parameters such as anterior chamber volume and depth, axial length, white-to-white (WTW) distance, endothelial cell count, preoperative and postoperative IOP, keratometry (K-flat and K-steep), ICL size, Angle-STS, and vault. Among these factors, a significant association was observed only with WTW distance in the oblique group (coefficient: 2.707; P = 0.008), suggesting that a larger WTW distance may contribute to an increased likelihood of postoperative rotation > 5˚ in this group (Table 5)​.

Table 5 Logistic regression analysis of factors for rotation > 5° in ICL groups^a

Discussion

In our consecutive retrospective analysis, we examined 502 eyes from 265 patients who underwent ICL V4c implantation for myopia correction. Our results demonstrated favorable visual outcomes across all implantation orientations at the 1-year follow-up. Postoperative uncorrected distance visual acuity (UDVA) was equal to or better than preoperative corrected distance visual acuity (CDVA) in all cases, with no significant differences in efficacy, safety indices, postoperative manifest refractive spherical equivalent (MRSE), intraocular pressure (IOP), or vault measurements among the groups. This finding indicates that visual outcomes were consistent and stable, irrespective of the implantation orientation. Regarding rotational stability, mean rotation angles did not differ significantly across the horizontal, vertical, and oblique implantation groups. Notably, rotations exceeding 5° were more frequent in the oblique group, with statistical significance observed when compared to the horizontal (P = 0.004) and vertical groups (P = 0.032). These results suggest that horizontal and vertical placements are associated with greater rotational stability compared to oblique placements.

Implantable Collamer Lens (ICL) implantation is widely regarded as an effective, safe, and stable procedure for correcting refractive errors, particularly myopia [20,21,22]. As its clinical application expands, various factors that contribute to optimal surgical outcomes have garnered increasing attention. One such factor is the accurate selection of ICL size, which is crucial for maintaining the safety and stability of the lens within the eye [23]. An insufficient vault may result in anterior subcapsular cataract formation due to lens-crystalline lens contact, while an excessively high vault can lead to angle closure and elevated intraocular pressure [24]. By leveraging the differences in horizontal and vertical sulcus-to-sulcus diameters, ICLs can be oriented to optimize vault height, thereby ensuring optimal lens positioning. This raises the important question of whether the alignment of the ICL along various sulcus axes—horizontal, vertical, or oblique—affects its rotational stability and overall surgical effectiveness [25, 26]. Even minor rotational misalignments in toric ICLs can lead to residual astigmatism and a marked reduction in visual quality [27]. Previous research indicates that each 3° of rotation in toric ICLs results in approximately a 10% reduction in astigmatism correction effectiveness, with rotations exceeding 10° causing significant degradation of visual outcomes [17]. Thus, achieving and maintaining optimal rotational stability is critical.

Although the mean rotation angle differences between the vertical and horizontal groups were not statistically significant in our study, the vertical group exhibited a smaller mean rotation angle, which suggests superior rotational stability in this orientation. This finding is consistent with previous studies, which have attributed the stability of vertical implantation to better anatomical alignment with the ciliary sulcus [15, 18]. Additionally, factors such as anterior chamber volume (ACV) and vault height also play a role in maintaining rotational stability. Larger ACVs have been associated with increased rotation, likely due to insufficient fixation, whereas a higher vault height helps stabilize the lens and reduce rotation [16]. It is also well-documented that most rotational adjustments occur within the first three months postoperatively, after which stability tends to plateau [28]. One study incorporated oblique placements (22.5° to 90°) along with horizontal orientation, suggesting that TICL stability is not determined by implantation angle but is instead influenced by supporting forces from the ciliary body, iris, and zonules [12]. However, this study did not differentiate the vertical group separately. Our study also included an oblique implantation group, allowing for a more comprehensive comparison of rotational stability across these three orientations. The oblique group had a significantly higher incidence of rotation > 5°, with RR analysis showing over twice the likelihood of significant rotation compared to the horizontal and vertical groups. This finding suggests that the rotational instability in oblique implantation may result from unequal ciliary sulcus distances where the diagonal ICL haptics are positioned, leading to uneven force distribution and an increased likelihood of rotation. Notably, TICL rotation has been linked to footplate malposition, underscoring the importance of precise positioning to enhance long-term stability [19].

Furthermore, logistic regression analysis in our study revealed that white-to-white (WTW) distance was a significant factor influencing postoperative rotations exceeding 5° in the oblique implantation group. The WTW distance, an important parameter in preoperative assessments, is typically considered alongside anterior chamber depth (ACD) and sulcus-to-sulcus (STS) measurements when selecting the appropriate lens size [29]. Discrepancies between the selected lens size and the actual sulcus dimensions, especially in patients with larger WTW distances, may lead to suboptimal fixation within the ciliary sulcus, thereby increasing the risk of postoperative lens rotation [19, 30]. These findings are consistent with the work of Mori et al., who documented a mean postoperative rotation of 4.82 ± 6.98° in toric ICL implantations, highlighting a significant correlation between intraoperative fixation angle and postoperative rotation [31]. Incorrect preoperative measurements or insufficient vault height can exacerbate the risk of lens misalignment, which compromises postoperative stability [27].

This study provides valuable insights into the rotational challenges associated with oblique implantation, particularly in patients with larger WTW distances and high astigmatism. Our findings suggest that oblique implantation should be approached with caution in such cases, as horizontal or vertical orientations may provide more stable outcomes. However, research on oblique implantation is still limited, and further studies are needed to evaluate its feasibility and long-term stability in larger cohorts.

Several limitations of this study must be acknowledged. The relatively small sample size and one-year follow-up period may not fully capture the long-term effects of rotational instability on visual outcomes. Additionally, this study did not account for the potential influence of ciliary body morphology on rotational stability, which may play a significant role. Future studies should include a more comprehensive range of factors, such as ciliary body morphology, and involve larger sample sizes, extended follow-up periods, and multicenter clinical trials to refine our understanding and establish evidence-based guidelines for optimizing ICL implantation.

Conclusions

ICL V4c implantation provides excellent visual outcomes across horizontal, vertical, and oblique orientations. However, oblique implantation, especially in eyes with larger white-to-white (WTW) distances, is associated with an increased risk of postoperative rotational instability. Careful preoperative assessment of WTW measurements may help minimize rotational risks in such cases.