Rotational stability of Toric intraocular lenses

Abstract

Background

A prospective study to evaluate the rotational stability of toric intraocular lenses (IOLs).

Methods

A prospective study of 30 eyes in 29 patients. All patients with regular astigmatism of range 0.75–4 D were included in the study. Exclusion criteria included irregular corneal astigmatism, post-refractive surgery and corneal dystrophies. All patients underwent uncomplicated phacoemulsification cataract surgery by the same surgeon. Three eyes with corneal astigmatism of 0.75–1.5 D were implanted with the AcrySof SA60T3 IOL, eight eyes with astigmatism between 1.5 and 2.0 D received the SA60T4 IOL and eight eyes with astigmatism between 2.0 and 2.50 D received the SA60T5 and rest with astigmatism of 2.5 D and higher received SA60T6 and above models. Main outcome measure was the post-operative position of the lens, assessed at day 1, 1 week, 1 month and 6 months, using toric marker and the slit lamp.

Results

There was no significant rotation of IOL observed during a follow-up period of 6 months. Overall, the post-operative rotation was within 5° in 95% of cases. There was no trend for either clockwise or anti-clockwise rotation. However, IOL rotation happens mostly within the first month of surgery, and if it is significant, it requires early repositioning.

Conclusion

Toric IOLs are very effective and consistent in correcting astigmatism during the cataract surgery. Once placed to its position, toric IOLs demonstrate rotational stability in the capsular bag.

Introduction

Cataract surgery, over the last few decades, has evolved from conventional cataract extraction to modern day micro-incision surgery, through incisions as small as 1.4–1.8 mm. A huge advancement in the range of intraocular lenses (IOLs) available has also taken place with the option of correcting astigmatism and also addressing to presbyopia. Modern day cataract surgery thus aims at not only cataract removal but also independence from spectacles. Post-operative emmetropia can be easily achieved for hyperopic and myopic errors by appropriate IOL power calculation; however, astigmatism still needs special attention, which is now being addressed in the form of toric IOLs. A pre-operative corneal astigmatism of 1 D or more may be present in up to one-third of cases undergoing cataract surgery, with 22% having more than 1.5 D of astigmatism and 8% having more than 2.0 D of astigmatism.¹

Currently, the options available for correction of astigmatism include:

• •Spectacle correction
• •Contact lens
• •Arcuate keratotomies
• •Limbal relaxing incisions
• •Opposite clear corneal incisions
• •Toric IOLs
• •Laser in situ keratomileusis

Toric IOLs are specially designed to correct astigmatism and have a toric component at the posterior surface of the IOL optic along with the spherical component. The three dots or a straight line, present on the peripheral part of the optic of the IOL, indicates the cylindrical axis of the IOL and so enables its correct alignment with the axis of astigmatism of the cornea. The AcrySof Toric IOL with open loop and angulated haptics has been one of the most common used toric IOLs.² Toric IOLs have a high quality uncorrected distant visual acuity and greater spectacle independence and lower residual astigmatism.³

Positional instability of toric IOLs had been reported by many surgeons in the past and toric IOLs fell into disrepute. Capsular bag contraction or capsular phimosis due to fibrosis has been most consistent cause of toric IOL rotation after any uncomplicated cataract surgery.⁴

The maximum incidence of IOL rotation occurs in the first hour to 10 days after implantation of IOL.^1,10,14 Even a smaller degree of rotation of toric IOL from its intended axis can result in large reduction of astigmatic correction and patient dissatisfaction.⁵ For example, a deviation of 10° minimizes the potential correction by approximately 35%, in other words every degree of rotation reduces the astigmatic correction by 3.5%.

The aim of this study is to evaluate the post-operative rotational stability of toric IOLs in standard cataract surgery over a 6 months post-operative period.

Materials and method

Sample size estimation was performed by taking into consideration previous studies and proportion of known population (population proportion—sample size formula), where margin of error (ME) and measure of precision (Z) is taken as 1.96 as critical value at 95% confidence interval. However, the past data, which give an idea of variation in the variables, play an important role in calculating the sample size. Since follow-up is included in the study, the sample size was kept at 30 or higher.

All eyes having corneal astigmatism ranging between 0.75 and 4 D undergoing cataract surgery between Jan 2017 and Aug 2017 received a toric IOL (hydrophobic toric acrylic IOL) implant during cataract surgery (Fig. 1).

Fig. 1.

Toric intraocular lens (IOL).

Patients with regular corneal astigmatism with realistic expectations were included in the study.

Exclusion criteria included irregular corneal astigmatism, axial length less than 22 mm, post-refractive surgery, corneal dystrophy, complicated cataract surgeries and glaucoma or co-existent maculopathy.

Pre-operative counselling of all patients was performed before surgery to address unrealistic patient expectations.

Axial length, K₁ and K₂ as well as IOL power was calculated using ZEISS IOLMaster 500 in all patients. The IOL power calculated was in corroboration with online AcrySof Toric IOL Calculator for steepest axis determination and surgically induced astigmatism (SIA) and calculation of toric IOL placement axis for least astigmatism. The SIA for the surgeon was kept at 0.5 D and incision site was kept constant at 120–130° axis to ensure uniformity in all patients and keeping in view the surgeons preferred choice of incision site.

All patients underwent phacoemulsification cataract surgery by the same surgeon under topical anesthesia using a similar technique. Pre-operative marking of references axis (0, 90 and 180°) was performed using a Nuijts-Solomon pre-operative toric marker (Fig. 2). After cleaning and draping, marking of the axis of incision axis with the help of ASICO toric marker and IOL placement axis with the help of Ron Yeoh toric gauge (Fig. 3) was performed by the surgeon using a sterile marker pen. All surgeries were performed using a 2.8 mm incision, standard size of 5–5.5 mm CCC size was maintained to ensure adequate optic coverage, and toric IOL was injected using the supplied injector and cartridge into a position within 15–20° of the final intended position. Anterior capsular polishing was performed after meticulous I&A to reduce capsular contraction and fibrosis (Fig. 4). Care was taken to remove any ophthalmic viscosurgical devices (OVDs) from pockets behind the IOL by either irrigation & aspiration (IA) below the IOL or by tapping on the center of the IOL with the IA cannula to dislodge any remaining OVD into the anterior chamber (AC). Unless absolutely essential, no air was injected into the AC to ensure stability of the IOL. The final alignment of axis of IOL was performed after complete removal of OVD, preservative-free moxifloxacin 0.5% wash and hydration of wounds and again confirmed at the end of surgery.

Fig. 2.

Toric marking set.

Fig. 3.

(a) Intra-operative marking. (b) Intra-operative axis.

Fig. 4.

Anterior capsular polishing.

Three eyes with corneal astigmatism between 0.75 and 1.5 D were implanted with AcrySof SA60T3 IOL, eight eyes of patients with astigmatism between 1.5 and 2.0 D received the SA60T4 model. Eight eyes with astigmatism ranging from 2.0 to 2.50 D received the SA60T5 and 11 eyes with astigmatism of 2.5 D and higher received the SA60T6 model of IOL (Table 1).

Table 1.

Pre- and post-operative astigmatism.

S. no.	SA60T3	SA60T4	SA60T5	SA60T6 & above
Range of correction	0.75–1.5	1.5–2.0	2.0–2.5	2.5–3 & above
No. of patients	3	8	8	11
Mean pre-operative astigmatism	1.1	1.81	2.35	3.07
Mean post-operative astigmatism at 1 day	0.11	0.19	0.30	0.61
Mean post-operative astigmatism at 28 days	0.1	0.16	0.26	0.52
Mean post-operative astigmatism at 6 months	0.1	0.15	0.24	0.50

In the post-operative follow-up, rotation of the toric IOL was examined by dilatation of pupil and slit-lamp examination, reference marking and axis marking was confirmed with the help of toric marker and the patient in sitting position. The axis of IOL was then compared with the original axis and was recorded for further comparison at 1 week, 4 weeks and 6 months after surgery.

Statistical analysis

The SPSS software II was used for statistical analysis (SPSS Inc., Chicago, IL, USA). The Wilcoxon matched-pairs and Mann–Whitney U tests were used for comparative statistics and independent samples, respectively. For association statistics, the Spearman rank correlation test was applied. P-values less than 0.05 were considered to be statistically significant.

Results

The mean age was 64.16 y ranges between 50 and 76 y (SD = 7.1). After implantation of toric IOL, the change in refractive astigmatism was statistically significant (SA60T3, P = 0.04; SA60T4, P < 0.001; SA60T5, P < 0.001; and SA60T6, P < 0.001).

There was no difference observed in the rotational stability of the four types of IOLs. It was observed that the maximum amount of rotation happens between the first 4 weeks post-operative ranging from 2.5 ± 2° (Chart 1). However, it has been observed in past that IOL rotation can occur as early as 1 h post-operative to initial 10 days.^1,10,14 Between 4 week and 6 months, the overall mean absolute rotation of all 30 implanted IOLs was insignificant. Ten IOLs (33%) rotated anti-clockwise with a maximum rotation of less than 5°, five IOLs (17%) rotated clockwise with rotation of less than 5° and the rest 15 IOLs (50%) did not rotate. It is imperative to note that the mean rotation of IOL was more in eyes with high axial length >25 mm (Table 2).⁶ No patient required a redialing procedure to correct the axis of the IOL. The residual astigmatism ranged from 0 to 0.75 D at 1 week post-operative to 0 to 0.5 D at 6 months post-operative. The association between the rotation of the IOL and the change in the refractive astigmatism was insignificant (P > 0.1).

Chart 1.

Pre- and post-operative pattern of astigmatism.

Table 2.

Relation of axial length with mean rotation.

Axial length (mm)	Mean rotation at 4 weeks	Mean rotation at 6 months
<23	1.80	1.85
23–23.5	2.00	2.10
23.5–24	2.28	2.40
>24	3.35	3.39

Discussion

In our prospective study, the AcrySof toric IOLs provided successful visual restoration and lower amount of residual astigmatism. At the end of 6 months follow-up, IOL rotation of all 30 implanted IOLs was insignificant. Maximum rotation measured up to 4 weeks post-operative was only 2.5 ± 2°. Of these 30 implanted IOLs, only 15 implanted IOLs showed statistically insignificant rotation and 15 implanted IOLs showed no rotation. Inspite of this, there was no requirement for redialing as no patient had significant residual astigmatism (0.5 D at 6 months). In previous studies, the mean residual astigmatism was at 6 months to 1 year after surgery ranged from 0.59 to 0.73 D, similar to our results.^{11, 12, 13, 14}

The degree of residual astigmatism increases with increasing toric IOL rotation. Therefore, it is important to identify the factors associated with rotational instability to reduce the rotation and improve efficacy of toric IOL.

It is imperative to note that the mean rotation of IOL was more in eyes with high axial length >25 mm.^1,10,14 Similarly, in our study, we found a positive correlation between IOL rotation and axial length at 6 months post-operative period.

Some earlier studies have also assumed that the capsular fibrosis and anterior capsular opacification also contribute to rotation of toric IOL in post-operative period. The post-operative IOL rotation or misalignment has been identified to be caused by factors such as the size of the capsulorhexis, asymmetrical contraction of the capsular bag, design of IOL, inadequate OVD removal, axis of alignment and Nd-YAG capsulotomy by various authors.^{3,5,7, 8, 9, 10}

Conclusion

Over the years, cataract surgery no longer only addresses a diseased lens but also the problems of astigmatism and presbyopia and is now a form of refractive surgery. Correction of astigmatism using toric IOLs at the time of cataract surgery helps achieve better refractive outcomes and reduces the need to address the issue of astigmatism later by surgical and non-surgical means. Toric IOLs have thus become an essential tool required in the armamentarium of all ophthalmologists. Post-operative rotational instability of a toric IOL, has in the past, been recognized as a significant problem, which often leads to less desirable results. Improved surgical techniques, better IOL materials and manufacturing have all led to a reduction of such surprises and improvement in refractive outcomes. Capsular phimosis because of a small CCC or inadequate cortical cleanup is an important cause of decentration of IOL and potential cause of rotation of the IOL. In our study, we found out that the surgeons need to keep in mind a number of minor but important factors or surgical steps to ensure good rotational stability in the immediate 4–6 weeks after surgery, which is in corroboration with the studies performed in past:

• •Correct reference marking.
• •Accurate incision and IOL axis marking.
• •Good cortical cleanup to reduce Yag rates. Anterior capsular polishing of at least 2 mm of the CCC margins to reduce capsular contraction and phimosis.
• •Meticulous OVD removal.
• •Avoid over-inflation of the AC using air because it tends to enhance rotation as the AC volume normalizes during the first 48–72 h as the air gets absorbed.
• •Do a slow controlled stromal hydration to prevent inadvertent rotation.
• •And always confirm the IOL axis at the end of surgery.

In the right hands and with meticulous surgery and precautions, toric IOLs are an effective tool to manage pre-existing astigmatism and improve the visual outcome in these patients. Pre-operative patient counselling is also essential to explain the facts and also to highlight the possibility of a minor degree of residual astigmatism in such patients. Keeping in view the above data, we can also consider customizing toric IOLs with different overall diameters, that is, 11.5, 12.0 and 13.0 mm based on axial length or white-to-white diameter to ensure better stability.

Disclosure of competing interest

The authors have none to declare.