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    Toric IOLs versus incisions

    Take-home

    While manual limbal relaxing incisions and femtosecond astigmatic keratotomies can address mild to moderate astigmatism, toric IOLs are quite effective at all ranges of astigmatism, relates one surgeon.

     

    The goal of modern cataract surgery is not just to remove the cataractous lens, but also to provide the patient with good uncorrected vision. When evaluating patients for surgery, it is necessary to consider not only the spherical equivalent, but also the expected residual astigmatism.

    Even if the spherical equivalent is exactly on target, quality of vision is compromised by residual astigmatism. With approximately 86% of patients with cataracts needing some kind of astigmatism correction,1 it is not surprising that a number of different treatment options exist.

    Here are some of the considerations when deciding how best to address astigmatism in patients with cataracts.

    Incisions

    About 64% of patients with cataracts are estimated to have corneal astigmatism between 0.25 and 1.24 D,1 and studies have shown that correction of even low levels of postoperative astigmatism yields better visual acuity and reading performance.2

    Limbal relaxing incisions (LRI) are perhaps the most common technique to correct small amounts of astigmatism at the time of cataract surgery, and I have been performing them for many years. Providing this treatment is relatively inexpensive, and the procedure is quite reliable for reducing astigmatism of 1.25 D of less, which is sufficient for the majority of patients.

    Until the introduction of the toric IOL, the only option for a patient with presbyopia was a presbyopic lens combined with a LRI. This continues to be my preferred method of correction in patients with small amounts of astigmatism.

    However, LRIs with a diamond blade do have limitations. Since the incision is made manually, skip lesions can occur and there is always a potential for perforation. In addition, there is more variability in results when attempting to treat higher levels of astigmatism.

    With femtosecond lasers becoming more common, femtosecond astigmatic keratotomy is a newly developing surgical procedure that has enormous potential. The ability to program the laser power, length of the incision arc, depth, and even optical zone makes the cuts highly precise and repeatable. There are no skip lesions and a minimized risk of perforation. Centration, angulation, and pairing of incisions can be achieved; and the full potential is yet to be discovered. While more studies need to be conducted and the nomograms refined, initial experience suggests that these non-penetrating arcuate incisions will be very effective at eliminating both small and moderate levels of astigmatism.

    Toric IOLs

    I have been using toric IOLs for the correction of astigmatism since they were first approved by the FDA. They have had a great track record, with results very much on target. In a review of 700 toric IOL procedures performed in my practice by six surgeons, only 7 eyes (1%) underwent laser vision enhancement postoperatively due to residual myopia, hyperopia, or astigmatism. Intraoperative aberrometry appears to be a promising technology that can further enhance the success rate for patients receiving toric IOLs.

    The recent approval of a one-piece toric IOL (Tecnis Toric 1-piece IOL, Abbott Medical Optics) is expanding our options. The lens was launched in Europe in 2011 and studies thus far show it to have very high rotational stability.3 In addition, it has –0.27 µm of spherical aberration (SA), fully compensating for the average amount of SA in the cornea.4 This lens is showing excellent centration and fixation, has very crisp optics, and has not been associated with glistenings.5

    Careful preoperative evaluation

    Preoperative evaluation is critical for optimizing the success of astigmatism management. Both preoperative topography and keratometry measurement (typically with the IOLMaster [Carl Zeiss Meditec] or Lenstar [Haag-Streit]) are essential. In most cases, topography and keratometry produce relatively close results in both the axis and magnitude of astigmatism.

    However, in the presence of significant dry eye or meibomian gland disease, the topography and keratometry will be unreliable. In these cases, the ocular surface condition should be treated, and then the patient can be brought back for repeat testing.

    I also consider repeat testing if the topography and keratometry demonstrate significant differences. One important evaluation is to ensure that the astigmatism on topography is regular and not asymmetric. Patients with irregular astigmatism—such as with keratoconus, pellucid, or Salzmann’s nodular degeneration, for example—are not the best candidates for a toric IOL.

    Summary

    Patients undergoing cataract surgery have high expectations, and surgeons can deliver excellent visual results by ensuring that astigmatism present preoperatively is addressed. While manual LRIs and femtosecond astigmatic keratotomies can address mild to moderate astigmatism, toric IOLs are quite effective at all ranges of astigmatism. Careful preoperative evaluation with topography and keratometry can help surgeons identify appropriate candidates for toric implants, and help increase the success rate for all techniques for astigmatism management.

    References

    1. Ferrer-Blasco T, Montés-Micó R, Peixoto-de-Matos SC, González-Méijome JM, Cerviño A. Prevalence of corneal astigmatism before cataract surgery. J Cataract Refract Surg. 2009;35:70-75.

    2. Lehmann RP, Houtman DM. Visual performance in cataract patients with low levels of postoperative astigmatism; full correction versus spherical equivalent correction. Clin Ophthalmol. 2012;6:333–338.

    3. Nixon DR et al. Rotational stability of new 1-piece hydrophobic acrylic toric IOL. Presented at ASCRS 2013, San Francisco.

    4. Smith G, Cox MJ, Calver R, Garner LF. The spherical aberration of the crystalline lens of the human eye. Vision Res. 2001;41:235-243.

    5. Miyata A, Yaguchi S. Equilibrium water content and glistening in acrylic intraocular lenses. J Cataract Refract Surg. 2004;30:1768-1772.


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    William B. Trattler, MD
    Dr. Trattler, a corneal specialist at Miami's Center for Excellence in Eye Care, Miami.

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