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Introduction
Refractive Cataract Surgery: Toric Intraocular Lenses
Charles A. Barsam, MD · Peter J. McDonnell, MD
Before the advent of small-incision cataract surgery, a primary goal of cataract surgery was the prevention and postoperative management of unwanted corneal astigmatism. With the introduction of astigmatically neutral, small-incision cataract surgery, surgeons have now focused on the reduction or elimination of preexisting corneal astigmatism. Between 15% and 20% of cataract patients present with more than 1.50 D of corneal astigmatism.1
The options for correcting preexisting corneal astigmatism in the cataract patient primarily include the use of various cataract incisions, relaxing corneal incisions, excimer laser ablation, and toric intraocular lenses (IOLs). The widespread use of the toric IOL has been aided by the acceptance of clear corneal small-incision cataract surgery, as well as the availability of foldable toric IOLs that pass through small incisions. Unlike the other methods currently available for correcting preexisting astigmatism in the cataract population, the toric IOL is particularly appealing to the cataract surgeon. It does not require the alteration of current surgical technique or the acquisition of new instrumentation or skills for successful outcomes. Additionally, should the outcome of surgery prove unacceptable, the toric IOL implantation procedure is reversible. The toric IOL is available with a preset level of astigmatic correction or it can be customized to meet the specific needs of a patient. The surgery requires matching the axis of the toric IOL with the steeper anterior corneal cylinder meridian as determined by preoperative keratometry. This cylinder meridian is marked preoperatively with the patient in an upright position. By avoiding the natural torsional eye movements occurring when patients are placed in the supine position, incorrect assessment of the axis of astigmatism is avoided.
In 1994, Shimizu and colleagues published results of the first clinical trial to evaluate toric IOLs.2 The investigators reported the results of 47 patients who underwent implantation of a nonfoldable toric IOL inserted through a 5.7-mm scleral incision after phacoemulsification.2 The combination of a nonfoldable lens inserted through a potentially astigmatically inducing incision was likely to yield suboptimal outcomes in this series when compared to the latter studies that involve use of smaller incisions with folding IOLs. Although the study makes no mention of postsurgical uncorrected visual acuity, it demonstrates the effective reduction of corneal astigmatism after toric IOL implantation.
As patients were evaluated after toric IOL insertion, the authors demonstrated a reduction of astigmatism in all but one of the eyes evaluated provided that IOL rotation was less than 30% of its intended axis. It was noted that the closer the IOL came to its target axis, the greater was the reduction in astigmatism. The toric IOLs used in this study were of two astigmatic powers of 2 D and 3 D. Of those lenses that remained within 10° of the preset axis, the average reduction of astigmatism for the IOL with 2 D and 3 D of astigmatic correction was approximately 1 D and 1.5 D, respectively.
A toric IOL's long-term rotational stability contributes to its success; off-axis rotation has a deleterious effect on visual acuity. As a toric IOL rotates off-axis, there is a diminution of the astigmatic correction until the maximum rotation of 30° where the effective astigmatic correction is entirely negated.3 IOL rotations of more than 30° actually increase the amount of preexisting cylinder. Therefore, it is suggested that a toric IOL with postimplantation rotation of more than 30° be rotated back to its intended axis to optimize refractive outcomes. This pioneering study by Shimizu and associates reported approximately a 20% incidence of IOL rotation greater than or equal to 30°.3 However, this paper helped advance this then novel technology by demonstrating the safety and efficacy of toric IOLs.
The initial American experience with the foldable toric IOL was published by Grabow.4 The foldable toric IOL used in his study had an advantage of being able to be placed through an astigmatically neutral incision. Grabow examined the incidence of off-axis rotation in 81 cases 6 months after implantation with a foldable toric IOL.
Seventy-seven of the 81 (95%) implanted IOLs remained within 30° on the intended axis. Of the four IOLs that required repositioning due to excessive rotation, one rotated 47° and the other three rotated 40° degrees off-axis.4 No other adverse events related to the insertion of these IOLs were reported. Grabow intimates that this form of surgical correction of astigmatism is superior to limbal relaxing incisions (LRIs) or laser refractive corneal surgery which are the other commonly implemented means of reducing astigmatism. It is felt that the cataract surgeon could easily adopt the toric IOL into their surgical repertoire without any alteration of their current technique. Furthermore, unlike the other modalities of surgical astigmatic correction there is little additional risk of delayed visual recovery, infection or added expense to the patient with the toric IOL.4
More recent studies have evaluated the long-term effects of toric IOL implantation. The first of these papers retrospectively examined 37 eyes, without a control group, that underwent toric IOL implantation. The mean follow-up was 20.3 months.5 These toric IOLs were folded and placed through a 3.0-mm to 3.2-mm incision. The cylindrical powers of the toric IOLs used in this study were 2.00 D (n = 29), 3.50 D (n = 7), and 4.00 D (n = 1). The investigators determined that the mean reduction for the 2.00 D IOL was 1.59 D (+/- 0.50 D) and 2.49 D (+/- 0.84 D) for the 3.50 D IOLs. The mean reduction of astigmatism with these toric IOL implants was consistent with the anticipated 60% to 80% reduction expected postoperatively at the spectacle plane.4 At the last point of follow-up within the study, the IOL-induced reduction of both spherical and astigmatic aberration resulted in 67.6% of eyes achieving an uncorrected visual acuity of 20/40 or better and 83.8% of patients achieving a vision of 20/40 or better with the aid of spherical correction alone.5 Only one implanted toric IOL rotated significantly enough to require repositioning. This IOL rotated degrees off-axis within the early postoperative period but after repositioning remained stable. Other than the aforementioned IOL, the rotational stability of the IOLs was deemed adequate in both the early and late postoperative period with all of the IOLs remaining within 30° of the target axis. However, 18.9% of these IOLs rotated off-axis within this 30° target. The authors were able to identify risk factors for lens rotation and demonstrated that both increased axial length and increased capsular bag diameter significantly correlated with an increased risk for IOL rotation.5
In another study evaluating toric IOLs, the traditional IOL implantation technique was compared with the spherical IOL implantation technique using adjunctive LRIs that were performed to diminish preexisting corneal astigmatism.6 The authors performed astigmatically neutral, small-incision cataract surgery combined with implantation of foldable toric IOLs. At the average follow-up of 6.9 months, 84% of the 130 patients who underwent toric IOL insertion versus 76% of the 51 patients in the spherical IOL/LRI group had 20/40 or better uncorrected visual acuity. Residual refractive cylinder of less than 0.75 D was appreciated in 55.4% of the toric IOL group and 21.5% of the spherical IOL/LRI group.6 Additionally, refractive cylinder outcomes of less than 1.25 D were achieved in 85.4% of the toric IOL group and 49% of the spherical IOL/LRI group. The authors conclude that these data demonstrate that the toric IOL readily corrects preexisting corneal astigmatism more effectively than that of the comparative study group. This was believed to be due in part to the fact that toric IOLs correct the full amount of corneal astigmatism whereas there is an inherent undercorrection built into the LRI nomograms. The investigators also encountered the complication of postimplantation IOL rotation. Of the 130 eyes implanted with the toric IOLs, 12 eyes (9.2%) underwent a secondary procedure for IOL realignment for IOLs that had rotated greater than 30 degrees off axis, thereby causing increased astigmatism or patient dissatisfaction, or both. Each IOL was rotated into its correct orientation within 1 month of the initial procedure. Rotating toric IOLs within 1 month of surgery minimizes the risk for anterior-posterior capsular contraction, which has the potential to result in resistance to IOL rotation and the potential for loss of capsular integrity.1
Cataract surgery has made a quantum leap as it has transformed from the realm of therapeutic to refractive. The advent and success of the toric IOL in treating not only spherical aberration but also corneal astigmatism is evident by its efficacy demonstrated by the studies discussed above. IOL rotation and its diminution of visual acuity appear to be the greatest drawback to this lens. The toric IOL equips the cataract surgeon with yet another tool to use in the quest for improved visual outcomes after cataract surgery.
- Novis C. Astigmatism and toric intraocular lenses. Curr Opin Ophthalmol. 2000;11(1):47-50.
- Shimizu K, Misawa A, Suzuki Y. Toric intraocular lenses: Correcting astigmatism while controlling axis shift. J Cataract Refract Surg. 1994;20:523-526.
- Gillis JP. Treating astigmatism at the time of cataract surgery. Curr Opin Ophthalmol. 2002;13(1):2-6.
- Grabow HB. Toric intraocular lens report. Ann Ophthalmology. 1997;29:161-163.
- Ruhswurm I, Scholz U, Zehetmayer M, et al. Astigmatism correction with a foldable toric intraocular lens in cataract patients. J Cataract Refract Surg. 2000;26:1022-1027.
- Xiao-Yi S, Vicary D, Montgomery P, Griffths M. Toric intraocular lenses for correcting astigmatism in 130 Eyes. Ophthalmology. 2000;107:1776-1782.