FormalPara Key Summary Points

Why carry out this study?

The steep-meridian corneal relaxing incision (SM–CRI) has been widely used in cataract surgery to correct low-to-moderate corneal astigmatism. However, there is no consensus on the type of incision that should be used for implantable collamer lens (ICL) implantation to correct the myopia with low-to-moderate astigmatism.

This study aimed to evaluate the efficacy of SM–CRI and non-steep-meridian corneal relaxing incision (NSM–CRI) in ICL surgery for low-to-moderate astigmatism correction.

What was learned from this study?

SM–CRI can alleviate corneal astigmatism and decrease the cylindrical diopter of the ICL, thus improving postoperative visual quality compared with NSM–CRI.

SM–CRI might be ideal for ICL surgery for low-to-moderate astigmatism correction.

Introduction

Currently, the toric implantable collamer lens (TICL) is widely considered a safe and effective procedure for correcting myopia and astigmatism [1,2,3,4,5]. 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 (SIAK) 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.

Results

All surgeries were uneventful and did not involve any intraoperative complications. The vaults at 6 months postoperatively were 366.71 ± 132.44 µm and 353.97 ± 176.10 µm in the SM–CRI and NSM–CRI groups, respectively. The intraocular pressure in all patients was normal. No complications, such as secondary glaucoma or cataract, occurred in either group throughout the 6 month follow-up period. The preoperative clinical characteristics are presented in Table 1, and no significant difference in preoperative baseline characteristics was observed between the two groups.

Table 1 Patients’ demographic data
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Refractive Outcomes

At the postoperative 6 month visit, all participants in both the SM–CRI and NSM–CRI groups achieved an UDVA of 20/20 or better (Figs. 1A and 2A). Compared with the preoperative CDVA, 71% (25/35) and 66% (23/35) of treated eyes in the SM–CRI and NSM–CRI groups, respectively, exhibited a gain of one or more lines in the postoperative UDVA (Figs. 1B and 2B). Similarly, 97.0% (34/35) of treated eyes in the SM–CRI group and 100.0% (35/35) of treated eyes in the NSM–CRI group exhibited unchanged or improved CDVA (Figs. 1C and 2C). Scatter plots of the attempted versus achieved spherical equivalent correction are shown in Figs. 1D and 2D. After surgery, the SE in 86.0% (30/35) of treated eyes in the SM–CRI group and 91% (32/35) of treated eyes in the NSM–CRI group were within ± 0.50 D (Figs. 1E and 2E). The change in manifest SE is shown in Figs. 1F and 2F. As for astigmatism correction, 89.0% (31/35) of the treated eyes in the SM–CRI group and 80.0% (28/35) of the treated eyes in the NSM–CRI group demonstrated postoperative astigmatism within 0.50 D (Figs. 1G and 2G). Scatter plots of the TIA versus SIA vectors and the distribution of AE are shown in Figs. 1H, 2H, 1I, and 2I, respectively.

Fig. 1
figure 1

Visual outcomes of the SM–CRI group at 6 months postoperative. A uncorrected distance visual acuity (UDVA) outcomes. B postoperative UDVA and preoperative corrected distance visual acuity (CDVA). C change in CDVA. D distribution of achieved spherical equivalent outcomes. E spherical equivalent refractive accuracy. F stability of spherical equivalent refraction. G refractive astigmatism. H target induced versus surgically induced astigmatism vectors, and I refractive astigmatism angle of error distribution at 6 months postoperatively. D = diopters

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Fig. 2
figure 2

Visual outcomes of the NSM–CRI group at 6 months postoperative. A uncorrected distance visual acuity (UDVA) outcomes. B postoperative UDVA and preoperative corrected distance visual acuity (CDVA). C change in CDVA. D distribution of achieved spherical equivalent outcomes. E spherical equivalent refractive accuracy. F stability of spherical equivalent refraction. G refractive astigmatism. H target induced versus surgically induced astigmatism vectors, and I refractive astigmatism angle of error distribution at 6 months postoperatively. D diopters

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Vector Analysis

The TIA of the SM–CRI and NSM–CRI groups were 0.67 ± 0.16 and 0.61 ± 0.21 D, respectively (P = 0.157). At 6 months postoperatively, the mean magnitude of the SIA was 0.56 ± 0.24 and 0.39 ± 0.22 D (P = 0.002), respectively. The DV showed that residual astigmatism in the SM–CRI group was much smaller than that in the NSM–CRI group (P = 0.021). Figure 3 shows single-angle polar plots of the SM–CRI and NSM–CRI groups to display the vector parameter distributions at the 6 month follow-up.

Fig. 3
figure 3

Single-angle polar plots at 6 months postoperative. A surgically induced astigmatism vector of SM–CRI group. B target induced astigmatism vector of SM–CRI group. C difference vector of the SM–CRI group. D correction index of SM-CRI group. E surgically induced astigmatism vector of the NSM–CRI group. F target induced astigmatism vector of the NSM–CRI group. G difference vector of the NSM–CRI group. H correction index of the NSM–CRI group

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At 6 months after surgery, the CI was 0.84 ± 0.30 and 0.67 ± 0.35 for the SM–CRI and NSM–CRI groups, respectively, which reached a statistically significant difference (P = 0.013). From the distributions in Figs. 1 and 2, approximately 71% of eyes in the SM–CRI group had an AE within ± 15°, whereas 55% of eyes in the NSM–CRI group were within that range. The absolute mean AE was 10.13 ± 14.57° in the SM–CRI group, compared with 23.88 ± 28.22° in the NSM–CRI group (P = 0.038) (Table 2).

Table 2 Vector analysis results after SM–CRI and NSM–CRI at 1 and 6 months
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Surgically Induced Astigmatism

At 6 months after surgery, the corneal astigmatism (Astig) showed a significant reduction (−0.33 ± 0.27 D) in the SM–CRI group and a slight increase (0.17 ± 0.30 D) in the NSM–CRI group. The mean magnitude of the SIAK was 0.53 ± 0.33 D, and the flattening effect was 0.41 ± 0.27 D in the SM–CRI group. In the NSM–CRI group, the mean magnitude of the SIAK was 0.38 ± 0.19 D, and the flattening effect was 0.30 ± 0.21 D. The mean SIAK values were 0.38 at 8° in the SM–CRI group and 0.20 at 96° in the NSM–CRI group (Table 3).

Table 3 SIA analysis results after SM–CRI and NSM–CRI at 1 and 6 months
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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.

Conclusions

SM–CRI can alleviate corneal astigmatism and decrease the cylindrical diopter in the ICL, thus improving postoperative visual quality compared with that achieved by NSM–CRI. Therefore, SM–CRI might be better to address low-to-moderate astigmatism during ICL implantation.