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Intrastromal incisions provide advantages for astigmatism management
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Astigmatism comes in different types, almost all of which will usually degrade a patient’s final visual experience. The astigmatic spectrum broadly includes regular and irregular astigmatism.
Regular astigmatism is based on axis of principal meridians — with-the-rule-astigmatism, against-the-rule-astigmatism and oblique astigmatism — and focus of principal meridians — simple astigmatism (myopic, hyperopic), compound astigmatism (myopic, hyperopic) and mixed astigmatism. It is also based on the optics of the entire eye, including the anterior and posterior corneal surface, lens, intraocular fluids and retina.
In one study evaluating corneal astigmatism in 4,540 eyes of 2,415 cataract surgery candidates, 86.6% had corneal astigmatism, of which 64.4% had corneal astigmatism between 0.25 D and 1.25 D and 22.2% had corneal astigmatism of 1.5 D or more. Thus, corneal astigmatism forms an integral part of the majority of cataract surgery patients, and correction of astigmatism at the time of cataract surgery will usually augment the final visual quality of the individual patient.
Lower degrees of corneal astigmatism are usually addressed by an incisional procedure at the steep axis using manual or laser-assisted techniques, and for higher amounts of astigmatism, based on surgeon preference, toric IOLs are often chosen to correct regular astigmatism. In the former, incision-related issues need to be addressed, and in the latter, proper IOL alignment becomes important to offer optimal postoperative result. Laser-assisted incisions are reproducible and precise, and they can be placed intrastromally or communicate to the corneal surface. Because intrastromal incisions do not disrupt or focally alter the corneal surface, they offer the added advantage of tear film stability, patient comfort and elimination of potential infection. Additionally, laser-assisted incisional procedures that communicate to the ocular surface can be titrated to provide graded astigmatic correction based on whether the surgeon fully separates the lips of the incisional corneal wound.
Presently, cataract surgeons have an opportunity to correct corneal astigmatism from a menu list of techniques at the time of cataract surgery, and every attempt should be made to take advantage of the current technological advances in this arena, thus providing our patients with an overall better visual quality after cataract surgery that enhances their quality of life.
In this column, Dr. Silverman describes his techniques to address and correct corneal astigmatism at the time of cataract surgery.
Thomas “TJ” John, MD
OSN Surgical Maneuvers Editor
Cary M. Silverman
Precision is everything when it comes to refractive cataract surgery, especially in the correction of astigmatism. As little as 0.5 D of residual astigmatism can reduce visual acuity by one line, with an even greater impact on functional visual acuity and low-contrast acuity.
We can reduce astigmatism at the time of cataract surgery by, first, taking care to minimize induced astigmatism. After that, the amount of astigmatism typically guides my decision to implant a toric IOL (1.5 D or greater) or perform corneal relaxing incisions. In either case, using a femtosecond laser improves the procedure.
Toric IOL implantation
After one has identified the correct axis of astigmatism, it is critical to mark the steep axis on the cornea so that the IOL can be correctly positioned. Each degree of rotational misalignment reduces the toric effect by 3.3%. In the past, I typically marked with ink on the table.
Femtosecond lasers have brought new challenges for axis marking — the laser interface or lighting can make it impossible to see the ink marks or corneal indentations — but they have also given us new options that may be superior.
My current technique in any toric IOL case in which I am also using the femtosecond laser is to mark the cornea with tiny intrastromal incisions during the laser portion of the procedure with my Catalys laser system (Abbott Medical Optics). This eliminates time spent marking on the table. I create nonpenetrating 10° arcs at a depth of 60% at the 9-mm optical zone. These are large enough to see clearly but small enough that they do not affect the astigmatism. The laser arcs are more precise than a manual ink mark, and they will not fade or disappear during surgery.
These marks are permanent, so a major advantage over any form of impermanent corneal marking is that the laser arcs are still there days or months later and can serve as a postoperative reference point for determining whether the IOL is correctly aligned or malpositioned.
In addition to marking, Catalys laser-assisted cataract surgery affects the refractive outcome in a toric IOL case in two other ways. First, the lens softening, which I consider the most important overall contribution of laser-assisted cataract surgery, significantly reduces phaco time. This makes complicated or hard cataracts much more routine and allows me to often simply aspirate softer nuclei with zero phaco power, making for quiet eyes with excellent visual acuity on the first postoperative day. Secondly, the laser creates round, perfectly sized, uniform capsulotomies. I find it easier to rotate the toric lens in the bag and align it properly, with just the right amount of capsular overlap, with a laser capsulotomy compared with a manual capsulotomy. The tension on the capsular bag and zonules during capsulotomy creation is minimal and consistent, which also reduces the chance of inducing astigmatism with surgery.
Figure 1. On the Catalys laser planning screen, the planned capsulotomy and lens softening pattern, cataract incision and axis mark at 180° are all clearly identified.
Images: Silverman CM
Figure 2. During the femtosecond procedure, the location of the toric axis mark and other laser cuts are overlaid on the eye image.
Figure 3. The toric axis mark is visible on the right side of the photo after the femtosecond laser procedure.
Arcuate incisions
In eyes with lower amounts of astigmatism, I opt for nonpenetrating arcuate incisions. I make intrastromal incisions, using the Donnenfeld nomogram for arc length, although I make them at the 8.5-mm optical zone rather than 9 mm. The laser depth, arc length and positioning are superior to those made by hand, and I think it will eventually be shown that intrastromal incisions are even more predictable than penetrating ones. Intrastromal arcs also avoid potential discomfort or wound complications.
Regardless of the magnitude of astigmatism, femtosecond laser-assisted cataract surgery offers us new options for efficient and effective management of astigmatism at the time of cataract surgery.
References:
Abell RG, et al. Ophthalmology. 2013;doi:10.1016/j.ophtha.2012.11.045.Dick HB, et al. J Cataract Refract Surg. 2013;doi:10.1016/j.jcrs.2013.07.002.
Ferrer-Blasco T, et al. J Cataract Refract Surg. 2009;doi:10.1016/j.jcrs.2008.09.027.
Ma JJ, et al. J Cataract Refract Surg. 2008;doi:10.1016/j.jcrs.2008.04.041.
Watanabe K, et al. J Refract Surg. 2013;doi:10.3928/1081597X-20121211-01.
For more information:
Cary M. Silverman, MD, can be reached at EyeCare 20/20, 46 Eagle Rock Ave., East Hanover, NJ 07936; 973-560-1500; email: csilverman@eyecare2020.com.Edited by Thomas “TJ” John, MD, a clinical associate professor at Loyola University at Chicago and in private practice in Oak Brook, Tinley Park and Oak Lawn, Ill. He can be reached at 708-429-2223; fax: 708-429-2226; email: tjcornea@gmail.com.
Disclosure: Silverman and John have no relevant financial disclosures.