Results of the AcrySof Toric intraocular lenses implantation

Background

The expectations of patients undergoing cataract removal surgery are large and include complete post-surgical vision acuity. Since more than 15% of people with cataracts also have refractive corneal astigmatism, which uncorrected on the level of 2D or more allows obtaining post-surgical vision acuity of 0.1–0.7, we do not commonly achieve so-called “refractive success” [1]. To gain complete vision acuity among the patients in question, spherical intraocular lens implantation combined with laser astigmatism correction, or the toric intraocular lens implantation, is a necessity.

The first clinical trials of toric lens implantation were undertaken by Shimizu, in 1994 [2]. Toric intraocular lenses were launched in 1990 as an alternative for correction of astigmatism in cataract patients. Initially, the results were poor due to the post-surgical lens rotation. However, at the end of 2005 new and refined designs of toric intraocular lenses (AcrySof Toric) were approved by the US Food and Drug Administration (FDA). The lenses have been recognized as highly stable and bio-compatible, and therefore advisable for astigmatism correction in patients with cataracts [2]. The aim of this study was to estimate the surgical astigmatism correction efficiency after the AcrySof Toric lens implantation.

Material and Methods

STATISTICAL ANALYSIS METHODS:

Statistical estimations were performed accordingly with the T-Student distribution, which is applied in interval estimation, in the parametric tests and especially for the mean values and the variances, as well as for the significance tests of statistical parameters, while dealing with the small test-group (the number N≤30). The T-Student distribution means that continuous probability distribution is frequently applied in the statistical hypothesis testing procedures, as well as for the estimation of measurement errors. Differences were statistically significant when the p value was lower than 0.05.

Discussion

Corneal astigmatism can be corrected through the corneal, relaxation or limbal incision, or by the excimer laser. All these methods have their own limitations depending on the astigmatism degree. Moreover, the post-surgical effects are influenced by patient’s age and the size, amount, depth and length of the incisions. Toric intraocular lens implantation for corneal astigmatism correction is the best surgical option offered for patients with both cataract and corneal astigmatism.

It is important to obtain precise estimation of corneal astigmatism in order to establish proper cylindrical value as well as proper spherical lens’s tensile strength for the toric intraocular lens. In the research of Grabow [5] on the AcrySof Toric lens, manual keratometry was necessary to estimate corneal astigmatism, and 3 different methods were used – manual keratometry, optical cohesion biometry, and corneal topography – with similar results. Additionally, toric intraocular lens’ cylindrical strength and axis correction (calculated by the keratometric cylinder) gave the same value. P values were: 0.375 (manual keratometry and optical biometry), 0.236 (manual keratometry and topography) and 0.485 (optical biometry and topography). The difference between the manual keratometry and the corneal topography was 9.42±11.76 degree. The difference between optical biometry and corneal topography ranged from 10.24±13.33 degree [6]. In our research, the differences between the values estimated by IOL Master and the corneal topography were statistically unimportant. Based on the applied t-Student distribution, in the test group the p value was 0.220, which means it was statistically insignificant. The difference between the IOL Master and the cornea topography was 9.52±11.38 degree.

The main requirement for the toric intraocular lenses is their rotational stability. It is estimated that 1 degree rotation from the axis results in 3.3% cylindrical strength loss, and when lens rotation exceeds 30 degrees, there is complete loss of cylindrical strength [7]. Prior models of the toric intraocular lenses were imperfect due to their high post-surgical rotation, and the necessity of additional procedures aimed at toric intraocular lens axial correction. The procedure’s effectiveness is confirmed not only by the refractive astigmatism correction, but also by the long-lasting toric lens capsular stability [8,9]. Due to the lens instability, many doctors ceased performing toric lens implantation [4,10]. The most frequent reason for lens rotation after complication-free cataract removal surgery is the capsule’s shrinkage caused by its fibrosis [3,11].

The design of the toric intraocular lens turned out to be highly impracticable for the toric lens stability. Weinand [12] proved perfect rotational stability in case of the AcrySof SA60AT toric intraocular lens. The average post-surgical rotation level was 0.7 degree (range from 0.1 to 1.8 degree) in 17 patients. Since that time, only a few studies of the AcrySof Toric lens have been conducted. Zuberbuhler et al. [4] proved that in a study of 44 AcrySof Toric lens implantation, the post-surgical rotation, checked 3 months after the surgical procedure, ranged 5 degrees in 95% of patients, and in 68% of eyes the rotation ranged 2 degrees. Mendicute [13] analyzed 30 eyes and confirmed the toric intraocular lens displacement was 3.63±3.11 degrees, with the rotation less than 10 degrees in 96.7% of eyes with the AcrySof Toric lens. Most of the intraocular lens rotations were observed in the early post-surgical period. Myung Hun Kim [3] found the average rotational axis was 3.40±3.05; however, in our studies, the rotational axis was 3.24±3.41, after 3 months. The possible factors influencing the toric intraocular lens stability might be capsulorhexis level, as well as the toric intraocular lens’ composition. Ruhswurm et al. [14,15] found a relationship between the toric intraocular lens rotation and the lens capsule’s diameter and the eyeball’s length. Myung Hun Kim et al. found that 93.3% of eyes gained 20/40(0.5) or even better UCVA (uncorrected visual acuity), and in 73.3% of eyes 20/25(0.8) or even better UCVA during the final check-up at 13.3 months. As a comparison, early clinical tests by the US FDA found that 66% of patients after 1-sided toric lens implantation achieved UCVA of 20/25 (0.8) or even better, and 41% of subjects in the control group achieved 1-sided UCVA of 20/25 (0.8) at 6 months [10]. In our study, UCVA equal to 0.6 or better was reported in 92.31% of patients, and in 7.69% of cases the best uncorrected visual acuity reached 0.4–0.6 (Figure 3). In another toric intraocular lens study, without a control group, by Mendicute [13] 66.6% of eyes reached UCVA 20/25 or even better, 93.3% of eyes reached 20/40 or better in by 3 months after the surgical procedure. In the research by Myung Hun Kim [3], astigmatism reduction after AcrySof Toric lens implantation was 85.6%, and in our results it was 84.2%. Mendicute [13] demonstrated 70% astigmatism reduction after AcrySof Toric lens implantation. However, research by Sun [15] showed a 54% astigmatism reduction after Staar TF lens implantation. Till [16] proved higher reduction (81%) after Staar AA4203TF or AA4203TL lens implantation. De Silva and others [17] proved the 65% astigmatism reduction after MicroSil 6116TU toric lens application. The differences among the toric intraocular lens values, as well as the pre-surgical astigmatism values, lead to the percentage results’ variability in case of both the astigmatism reduction and the vision acuity. The choice of the proper toric lens is made after calculation based on the pre-surgical corneal astigmatism value, and is used for calculating the lens’ tensile strength and the axial parameters. Keratometry of the cornea’s cylinder, evaluated by the doctor, is necessary for the estimation of the required toric intraocular lenses cylindrical strength. Therefore, in order to obtain the required effect, toric lens implantation should be carried out with small incisions, and based on the keratometry value. In previous research on the toric intraocular lens implantation, the incision’s size ranged from 2.75 to 3.00 mm. Our research concerned cataract surgery with 2.6 mm cornea incision. The results were satisfactory, similar to results of other studies. There were no post-surgical complications caused by micro-incisions in cataract correction. Therefore, combining the cataract surgery using micro-incisions together with the toric intraocular lenses implantation for stigmatism correction should be considered in treatment of patients with cataracts.