Introduction
Currently, the toric implantable collamer lens (TICL) is widely considered a safe and effective procedure for correcting myopia and astigmatism [
1‐
5]. However, its correction efficacy for low astigmatism within 1.0 D is affected by several factors. The TICL power calculation software provided by the manufacturer does not consider surgically induced astigmatism (SIA) when predicting postoperative refractive outcomes [
2], and rotation can significantly reduce the correction efficacy of TICL in astigmatism [
5]. The influence of these factors on astigmatism correction may reduce patient visual quality and satisfaction, particularly for low astigmatism. Therefore, it is worth exploring a simpler, safer, and more economical method to correct pre-existing low astigmatism during ICL implantation.
The steep-meridian corneal relaxing incision (SM–CRI) is made at the steep meridian of the cornea [
6]. The basic principle is that the relaxing incision of the cornea flattens the steep meridian and steepens the flat meridian to reduce corneal astigmatism. This simple and economical surgical approach has been widely used in cataract surgery to correct low-to-moderate corneal astigmatism [
7]. In a study by He et al. concerning the SM–CRI, they found that postoperative astigmatism showed good correction efficacy in patients with preoperative corneal astigmatism between 0.37 D and 1.0 D [
8]. He et al. also found that compared with patients with non-steep-meridian corneal relaxing incision (NSM–CRI), patients with SM-CRI had significantly lower postoperative corneal astigmatism and incidence of postoperative irregular astigmatism, and better postoperative visual quality. They suggested that SM–CRI might be an ideal option for toric IOL implantation [
8].
To date, there is no consensus on the type of incision that should be used for ICL implantation. Therefore, this prospective study sought to evaluate the efficacy of SM–CRI and NSM–CRI in ICL surgery for low-to-moderate astigmatism correction.
Methods
Patients
In this prospective cohort study, we recruited individuals with myopia or myopic with astigmatism who underwent ICL V4c implantation at the Eye and ENT Hospital of Fudan University (Shanghai, China) between May 2021 and October 2021. The inclusion criteria were as follows: (1) age ≥ 20 years, (2) stable refractive error, (3) preoperative cylinder between −1.50 and −0.50 diopters, and (4) the minimum anterior chamber depth (ACD) was 2.8 mm and the minimum endothelial cell density was 2500 cells/mm2. The exclusion criteria were as follows: (1) concurrent infections of the cornea, (2) concomitant autoimmune diseases, (3) severe dry eye disease, (4) history of other pre-existing ocular diseases, or (5) severe mental disorders, including anxiety and depression.
Seventy eyes from 70 participants were included in the current study. The participants were classified into two groups: SM–CRI and NSM–CRI. Each patient provided written informed consent prior to surgery, after a detailed explanation of the benefits and risks of the study was provided. All subjects were treated in accordance with the tenets of the Declaration of Helsinki. The Ethical Committee of the Eye and ENT Hospital of Fudan University Review Board approved the study protocol.
Surgical Techniques
All surgeries were performed by an experienced surgeon (X.Z.) using the same technique as used in this study. The size of the implanted ICL, V4c, was determined according to the ACD and horizontal white-to-white corneal diameter. Dilating and topical anesthetic agents were administered on the day of surgery. For patients in the SM–CRI group, a clear corneal incision of 3.0 mm was made in the subjective steep meridian of refraction, while for patients in the NSM–CRI group, an incision of the same size was made deviating 15–75° from the steep meridian that would cause oblique torque effects, mostly on the temporal part of the cornea. An ICL V4c was implanted after injection of a viscosurgical substance into the anterior chamber. The ICL was then placed into the posterior chamber. Finally, the viscosurgical substance was replaced with Ringer’s lactate solution. Postoperatively, antibiotics (0.5% levofloxacin; Santen Pharmaceutical, Osaka, Japan) and 1.0% prednisolone acetate (Pred Forte; Allergan, Irvine, CA, USA) were topically administered four times daily for 7 days and pranoprofen (Senju, Osaka, Japan) four times daily for 14 days.
Postoperative Assessment
All patients underwent postoperative ocular examinations at 1 and 6 months after surgery. The following parameters were evaluated: uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), manifest refraction, slit-lamp evaluation, intraocular pressure (IOP), corneal topography, and endothelial cell density. Vector analysis was performed for eyes with astigmatic correction using the Alpins method [
9,
10]. As suggested by Alpins, the target-induced astigmatism (TIA), surgically induced astigmatism (SIA), difference vector (DV), magnitude of error (ME), angle of error (AE), correction index (CI), and index of success (IOS) were analyzed. Keratometric surgically induced astigmatism (SIA
K) was calculated online using the assort vector calculator for ophthalmic refractive surgery techniques (
http://www.assort.com/assort-vector-calculator-0).
Statistical Analyses
All statistical analyses were performed using IBM SPSS Statistics (version 24.0; SPSS Inc., Armonk, NY, USA). The mean ± standard deviation was used for quantitative variables. Differences were considered statistically significant when P < 0.05. The Kolmogorov–Smirnov test for normality was used to confirm data normality. Independent sample t-tests were used to compare clinical variables and astigmatic vector components between the two groups.
Discussion
SM–CRI, as an economical and simple surgical method, has been widely used to correct mild and moderate astigmatism in cataract surgery [
11,
12]. In the ICL study, we made an incision according to the steep meridian of the manifest refraction to correct the preoperative astigmatism. SM–CRI in ICL surgery can avoid misalignment of the TICL, thus reducing the risk and cost of complications of ICL exchange. The results of our prospective randomized study indicate that both the SM–CRI and NSM–CRI groups achieved good refractive outcomes in terms of predictability, efficacy, and safety. All surgeries were uneventful and no obvious intraoperative or postoperative complications were observed during the follow-up period. Moreover, UDVA and CDVA were significantly increased after surgery. However, the postoperative refractive astigmatism in the SM–CRI group was significantly less than that in the NSM–CRI group (DV: 0.25 versus 0.39 D).
Six months after surgery, we found that the CI of both groups was less than 1, suggesting undercorrection of astigmatism. However, since most young patients have with-the-rule (WTR) astigmatism, slight undercorrection is acceptable. The CI of the SM–CRI group was 0.84, while that of the NSM–CRI group was only 0.67, suggesting that the SM–CRI has significant advantages in astigmatism correction. In addition, we also found that SM–CRI had a smaller effect on the axis of astigmatism (AE: 23.88° versus 10.13°) than NSM–CRI, which is also an advantage of SM–CRI over other incision sites. Therefore, the SM–CRI can reduce the cylindrical diopter of the ICL. As for the stability of astigmatism correction, no statistically significant changes were found between the two groups during the follow-up period of 1 and 6 months after surgery. Therefore, we can conclude that SM–CRI has better benefits for ICL with low astigmatism.
In our study, SIA was 0.53 D in the SM–CRI group and 0.38 D in the NSM–CRI group. In a comparative study of incision sites for cataracts, He et al. reported that SIA of the steep-meridian incision and non-steep-meridian incision were 0.50 and 0.54 D, respectively, with no significant difference [
8]. Borasio observed a significant reduction in SIA through a temporal incision (0.34 D) compared with a steep meridian incision (0.63 D) in cases of mild-to-moderate astigmatism [
13,
14]. In addition, since young patients with WTR were the main subjects of this study, the superior incision was mainly used for SM–CRI, while the temporal incision was mainly used for NSM–CRI. The study by Kamiya on low-to-moderate astigmatism showed that SIA of superior and temporal incisions were 0.57 and 0.48 D, respectively, which was consistent with our study findings [
2]. Previous studies have assumed that making an incision at the steep meridian can flatten it, while making an incision at the flat meridian can steepen it, thus theoretically it may correct some astigmatism without changing the axis of the astigmatism [
13]. In our study, corneal astigmatism in the SM–CRI group was significantly alleviated 6 months after surgery, while that in the NSM–CRI group was slightly increased, which was related to the corneal relaxing effect of the incision site. The vector analysis results of SIA also indicated that there was a tendency for corneal flattening towards the incision site, with the introduction of against-the-rule (ATR) (0.38 at 8°) and WTR(0.20 at 96°) for the superior and temporal incisions, respectively, which is consistent with the results of Tejedor [
15]. To some extent, this conclusion explains why the SM–CRI group had better astigmatism correction efficacy as a non-toric ICL was included in this study.
Notably, although corneal astigmatism in the NSM–CRI group increased slightly, the manifest astigmatism decreased significantly, which is consistent with the conclusion of Kamiya’s previous study [
16]. In their study, the horizontal incision caused corneal astigmatism to increase from 1.16 D at 90.5° to 1.45 D at 90.2°, while the manifest astigmatism decreased from 0.93 D at 93.2° to 0.72 D at 87.4°. They believed that the role of lens accommodation in lens astigmatism may lead to a difference between the cornea and manifest astigmatism after ICL implantation. In addition, ICLs, such as corneal contact lenses, appear to compensate for small amounts of lenticular astigmatism, thus contributing to the reduction in manifest astigmatism.
Limbal relaxing incision (LRI) is also used for astigmatism correction. Riaz et al. demonstrated that LRI during cataract surgery achieved an effective and sustained reduction of both refractive and keratometric astigmatism, regardless of the meridian or magnitude of astigmatism; this correction persisted for at least 1 year postoperatively [
17]. Compared with on-axis incision, the level of astigmatism reduction achieved at the intended meridian was significantly more favorable with the LRI technique [
18]. Li et al. also reported that LRI effectively reduces corneal astigmatism during ICL surgery [
19]. Although recent research has demonstrated the usefulness of LRI in reducing astigmatism, this technique is an additional method that is not a part of the ICL surgery procedure.
There are still some limitations to this study, including the small sample size and the lack of randomization. A previous study on cataract surgery indicated that the flattening effect of temporal incisions, but not superior incisions, performed on the preoperative corneal steep meridian differed between the right and left eyes [
20]. Additionally, we did not observe the visual quality of the entire eye. Future studies should include a larger number of cases in randomized control trials and assess the ipsilateral eye.
Acknowledgements
We thank all study participants for their involvement in the study.