The goal in modern cataract surgery is emmetropia, and this can be achieved for patients with myopic or hyperopic refractive errors by selecting the appropriate spherical lens power. Toric IOLs, a recent innovation, now provide the opportunity to reduce or eliminate corneal astigmatism as well, offering patients with preexisting astigmatism the potential for optimal distance vision without the use of spectacles or contact lenses with a cylindrical correction. Approximately 22% of patients undergoing cataract surgery have substantial corneal astigmatism of more than 1.25 D and would benefit from toric IOL implantation.1
PATIENT SELECTION CRUCIAL
Achieving success with toric IOLs depends on selecting suitable patients and performing accurate preoperative corneal astigmatism measurements. Taking into account the minimal available toric IOL power of 1.00 D at the corneal plane and the amount of astigmatism induced by surgery, patients must have a corneal astigmatism of at least 1.25 D to be candidates for a toric IOL.
Patients with regular bow-tie astigmatism are the most suitable for toric IOL implantation. Corneal topography is important for ruling out irregular astigmatism and keratoconus. Patients with Fuchs' endothelial dystrophy who might need keratoplasty in the future are poor candidates for toric IOL implantation.
Accurate keratometry (K) values and astigmatism axis measurements must be obtained to ensure success with toric IOLs. Our preferred method to measure K values is with the IOLMaster (Carl Zeiss Meditec, Jena, Germany). Its automated keratometry feature minimizes problems related to human error, and it has been shown to be a reliable biometric tool for spherical power calculations.
To determine the astigmatic axis, we compare the values obtained with the IOLMaster and corneal topography. If these values are within 5° of each other, we use the axis measurements obtained by the IOLMaster. If the discrepancy is more than 5°, we use the Javal-Schiotz keratometer (Rodenstock, Dusseldorf, Germany), which we still regard as the gold standard for axis determination.
Implantation of toric IOLs requires only slight modifications from the standard cataract surgical procedure. For the AcrySof Toric IOL (Alcon Laboratories, Inc., Fort Worth, Texas), the toric IOL with which our center has the most experience, cylinder power and alignment axis can be calculated using a Web-based toric IOL calculator program (available at http://www.acrysoftoriccalculator.com). This program takes into account the patient's K values and axis of astigmatism, and the surgeon's expected surgically induced astigmatism, in calculating the optimal IOL power and axis for implantation. It also predicts the postoperative residual astigmatism.
Preoperatively, corneal reference marks are placed at the 0°, 90°, and 180° positions, with the patient sitting upright to correct for recumbent cyclotorsion. This can be done using a slit lamp or a Nuijts/Lane Toric Reference Marker (Figure 1A; Model No. AE-2791TBL, ASICO, Westmont, Illinois). The advantage of this marker is that the extensions of the marker leave slight impressions on the cornea, even if the ink marks may have faded away after the surgical preparation.
Intraoperatively (Figure 2), the implantation axis is determined using the corneal reference marks and the alignment axis obtained from the toric calculator program. This can be done using a Mendez ring and the Nuijts Toric Axis Marker (Figure 1B; Model No. AE-2740, ASICO).
The surgeon's standard phacoemulsification technique can be performed, after which the foldable toric IOL is inserted through a 2.2- to 2.8-mm limbal incision. After the ophthalmic viscosurgical device is removed, the IOL is rotated to its final position by exact alignment of the reference marks on the toric IOL with the limbal axis marks.
In the event of a complication during surgery that might compromise the stability of the toric IOL (eg, zonular damage, vitreous loss, capsulorrhexis tear, or capsular rupture), conversion to a standard, nontoric IOL may be required. Postoperatively, the orientation axis of the toric IOL must be verified to confirm optimal alignment and ensure no postoperative IOL rotation.
TORIC IOL PERFORMANCE
Crucial to the safety and efficacy of toric IOLs are accurate surgical placement and the subsequent rotational stability of the IOL. As little as 10° of axis misalignment reduces the efficacy of astigmatic correction by 33%. Misalignment by more than 30° actually induces astigmatism. Table 1, which is available exclusively in the online article at www.crstodayeurope.com, shows the refractive stability of toric IOL models as recorded in the literature.
The refractive stability of older toric IOL models such as the STAAR toric IOL (STAAR Surgical, Monrovia, California) and the MicroSil toric IOL (HumanOptics AG, Erlangen, Germany), made with silicone materials, can be seen in Table 1.2-8
The AcrySof toric IOL, which became available in 2006, is made of foldable hydrophobic acrylic material instead of silicone. An advantage of this toric IOL is its rotational stability in the bag due to adhesions between the acrylic material of the IOL and the posterior capsule.9
Other toric IOLs currently available in Europe include the T-flex and the multifocal M-flexT IOLs (Rayner Intraocular Lenses Ltd., Hove, United Kingdom) and the Acri.Comfort and the multifocal AT.LISA Toric (Carl Zeiss Meditec). Both the Rayner and Zeiss toric IOLs are made with hydrophilic acrylic materials. (See the sidebars throughout this article for more information on these and other toric IOLs.)
Initial clinical experiences with the Rayner toric IOL, presented at the ESCRS in 2006, were promising.10 These results have not yet been published.
A randomized, controlled trial was performed to assess the safety and efficacy of AcrySof toric IOLs and was used in the application for US Food and Drug Administration (FDA) approval.11 Approximately 250 patients were implanted with a toric IOL (models SA60T3-T5), and 250 were implanted with a control lens (model SA60AT). The mean misalignment after 6 months follow-up was 3.4 ±3.0° with a maximum misalignment of 14°. Two eyes (0.8%) required a surgical intervention to realign the IOL.
Several other noncomparative studies have examined the misalignment rate of AcrySof toric IOLs. All report a mean postoperative IOL misalignment of less than 4° and surgical repositioning required due to IOL rotation of 0% to 1.8% of implanted eyes.12-16
Weinand et al17 determined the rotational stability of AcrySof IOLs using digital photographs obtained immediately postoperatively and 6 months after surgery. The mean IOL rotation was 0.9° (range, 0.1–1.8°). These results indicate that IOL misalignment is largely due to causes other than IOL rotation, such as errors during pre- and intraoperative reference markings, errors related to the surgery itself, and postoperative axis readings. Based on current marking techniques, we believe a mean misalignment of less than 4° is close to optimal.
Other outcome parameters used to determine the efficacy of toric IOLs include uncorrected distance visual acuity (UCDVA), spectacle independence for distance vision, and residual refractive cylinder. Results from the literature can be seen in Table 2.
UCDVA of 20/40 or better has been reported in 91% to 95%, and of 20/25 or better in and 66% to 79% of eyes implanted with the AcrySof Toric IOL.11,12,14,16 Additionally, 97% of those who received bilateral AcrySof Toric IOL implantation achieved spectacle freedom for distance vision, compared with 50% of the control patients receiving a nontoric IOL.11 A postoperative residual refractive cylinder of 1.00 D or less has been reported in 88% to 100% of eyes implanted with AcrySof Toric IOLs.11,12,16 In 74% of eyes implanted with the AcrySof Toric IOL, a residual refractive cylinder of 0.50 D or less was achieved.12 Taken together, these results show that AcrySof toric IOL implantation is a safe, efficient, and predictable method of managing corneal astigmatism in cataract patients.
We began using the AT.LISA toric IOL in our center in 2008 (Figure 3). We have implanted this IOL in 22 eyes of 12 patients (mean age, 57.2 ±12.3 years). Mean preoperative K value as measured with the IOLMaster was 3.1 ±1.2. After a follow-up of 1.5 ±0.7 months, the postoperative Snellen UCDVA and best corrected distance visual acuity (BCDVA) were 0.9 ±0.2 and 1.0 ±0.2 respectively. Binocular uncorrected near visual acuity at 40 cm, measured with the Early Treatment Diabetic Retinopathy Study chart, was 0.8 ±0.1. Mean absolute misalignment was 2.5 ±2.4°, and no eye required surgical realignment. These initial experiences indicate excellent visual outcomes and good stability of the AT.LISA toric IOL. (Personal communication, N. Bauer.)
To date, three AcrySof Toric IOL models (SA60T3-T5) are available, with cylinder powers of 1.50, 2.25, and 3.00 D at the IOL plane, corresponding to 1.00, 1.50, and 2.00 D at the corneal plane. Beginning this summer, four additional models (SA60T6-T9) will become available, with cylinder powers from 3.75 to 6.00 D at the IOL plane, corresponding to 2.50 to 4.00 D at the corneal plane. Spherical powers from 6.00 to 30.00 D will be available.
Rayner toric IOLs are available standard in cylinder powers ranging from 1.00 to 6.00 D, but they can be custom ordered with cylinder powers up to 11.00 D. The Acri.Comfort and AT.LISA Toric are available in cylinder powers up to 12.00 D.
Implantation of the latest generation of toric IOLs appears to be an efficient, safe, and predictable method for managing corneal astigmatism in cataract patients. The adoption of toric IOLs requires only minor modifications of standard IOL implantation techniques. Surgeons considering introducing toric IOLs to their practices should be encouraged to do so, as these lenses provide the opportunity for patients with astigmatism to achieve excellent uncorrected distance vision and resulting spectacle independence.
Rudy M.M.A. Nuijts, MD, PhD, is an Associate Professor of Ophthalmology in the Department of Ophthalmology at Academic Hospital, Maastricht, Netherlands. He is a member of the CRST Europe Editorial Board. Dr. Nuijts states that he has no financial interest in the products or companies mentioned. He may be reached at e-mail: firstname.lastname@example.org.
Noël J.C. Bauer, MD, PhD, practices in the Department of Ophthalmology at the Academic Hospital, Maastricht, Netherlands. Dr. Bauer states that he has no financial interest in the products or companies mentioned. He may be reached at e-mail: email@example.com.
Nienke Visser, MD, is a doctoral (PhD) student in the Department of Ophthalmology at the Academic Hospital, Maastricht, Netherlands. Dr. Visser states that she has no financial interest in the products or companies mentioned. She may be reached at tel: +31 43 3877133; e-mail: firstname.lastname@example.org.
- Ferrer-Blasco T, et al. Prevalence of corneal astigmatism before cataract surgery. J Cataract Refract Surg. 2009;35(1):70-75.
- Chang DF. Early rotational stability of the longer STAAR toric intraocular lens: fifty consecutive cases. J Cataract Refract Surg. 2003;29(5):935-940.
- De Silva DJ, Ramkissoon YD, Bloom PA. Evaluation of a toric intraocular lens with a Z-haptic. J Cataract Refract Surg. 2006;32(9):1492-1498.
- Dick HB, Krummenauer F, Trober L. Compensation of corneal astigmatism with toric intraocular lens: results of a multicentre study. Klin Monatsbl Augenheilkd. 2006;223(7):593-608.
- Leyland M, et al. Prospective evaluation of a plate haptic toric intraocular lens. Eye. 2001;15:202-205.
- Ruhswurm I, et al. Astigmatism correction with a foldable toric intraocular lens in cataract patients. J Cataract Refract Surg. 2000;26(7):1022-1027.
- Sun XY, et al. Toric intraocular lenses for correcting astigmatism in 130 eyes. Ophthalmology. 2000;107(9):1776-1781; discussion 1781-1782.
- Till JS, et al. Toric intraocular lens implantation: 100 consecutive cases. J Cataract Refract Surg. 2002;28(2):295-301.
- Linnola RJ, et al. Adhesion of soluble fibronectin, vitronectin, and collagen type IV to intraocular lens materials. J Cataract Refract Surg. 2003;29(1):146-152.
- Peckar C. Rayner Centreflex Toric IOL shows stability in astigmatic eyes. Paper presented at: European Society of Cataract and Refractive Surgeons annual meeting. September 8-13, 2006; London.
- AcrySof Single-Piece Posterior Chamber Intraocular Lenses with Toric Optic, models SA60T3, SA60T4 and SA60T5. 2005, US Food and Drug Administration. Document number P930014/S15.
- Bauer NJ, et al. Astigmatism management in cataract surgery with the AcrySof toric intraocular lens. J Cataract Refract Surg. 2008;34(9):1483-1488.
- Chang DF. Comparative rotational stability of single-piece open-loop acrylic and plate-haptic silicone toric intraocular lenses. J Cataract Refract Surg. 2008;34(11):1842-1847.
- Mendicute J, et al. Foldable toric intraocular lens for astigmatism correction in cataract patients. J Cataract Refract Surg. 2008;34(4):601-607.
- Zuberbuhler B, et al. Rotational stability of the AcrySof SA60TT toric intraocular lenses: a cohort study. BMC Ophthalmol. 2008;8:8.
- Dardzhikova A, Shah CR, Gimbel HV, Early experience with the AcrySof toric IOL for the correction of astigmatism in cataract surgery. Can J Ophthalmol. 2009;44(3):269-273.
- Weinand F, et al. Rotational stability of a single-piece hydrophobic acrylic intraocular lens: new method for high-precision rotation control. J Cataract Refract Surg. 2007;33(5):800-803.