Have you ever thought of surgery as experimental physiology? If so, you must agree that modern cataract surgery has become experimental physiological optics. As biometry has improved, we have moved from purely using spherical IOLs to using multifocal, pseudoaccommodating, aspheric, and toric lenses. All of these IOLs present challenges when the patient is sitting in front of you.
In mid-2009, we have a variety of IOLs to choose from. Standard spherical optic IOLs are well known, and most surgeons are within their comfort zone using them. If you think about it, however, absolute presbyopia is the most common complication of uncomplicated cataract surgery with implantation of a monofocal IOL. Despite this, multifocal and pseudoaccommodating IOLs have remained a minority interest. Aspheric IOLs with differing degrees of asphericity are available from various manufacturers, but their benefits are subtle. Most of us have seen patients with an aspheric IOL in one eye and a spherical IOL in the contralateral eye who cannot appreciate much difference. In many cases, the patient prefers the eye with the least ametropia, reinforcing the standard teaching that we need to correct lower-order aberrations as fully as possible.
What are the lower-order aberrations? They are spherical error—as opposed to spherical aberration—and astigmatism. Although we do our utmost to have repeatable, accurate biometry and use appropriate IOL regression formulas with personalized A-constants to correct spherical error, it seems we have bypassed astigmatism correction by adopting aspheric IOLs before toric IOLs.
Total-eye astigmatism is the vector sum of an eye's corneal astigmatism and lenticular astigmatism. The latter is important for two reasons. First, in some populations (including the United Kingdom), substantial numbers of patients have lenticular astigmatism. Second, if astigmatism is unrecognized, lens-based surgery may create, rather than reduce, astigmatism. When the lens is removed, so is any lenticular astigmatism. It therefore follows that the refractive cataract surgeon must assess his patients' corneal shape with care, if at all possible using quality corneal topography.
As we have moved from large- to small-incision cataract surgery, we have become obsessed with the concept of astigmatically neutral surgery, with ever-smaller incisions that are increasingly difficult to inject IOLs through. I am not criticizing the concept or the proponents of astigmatically neutral cataract surgery; however, I suggest that truly astigmatically neutral surgery is appropriate only for the astigmatically neutral cornea. In real life, the vast majority of corneas have some degree of astigmatism, and the modern refractive cataract surgeon must take this into account and plan surgery accordingly.
There are a variety of strategies for reducing preexisting corneal astigmatism at the time of cataract surgery. These can be divided into corneal strategies, including on-meridian incisions, limbal relaxing incisions (LRIs), opposite clear corneal incisions (CCIs), arcuate keratotomies (AKs), and lens strategies. Corneal strategies have one advantage: They usually involve only knives, which are immediately available. However, because we are acting on biological tissue, there is always some degree of unpredictability relating to the wound-healing response.
I have been a proponent of on-meridian incisions; a CCI seems to me to be identical to an unpaired LRI. Because an incision is necessary to effect cataract surgery, it seems logical to use the incision to do as much as possible, treating some astigmatism as well as reach into the eye. The on-meridian incision allows limited modulation of preexisting corneal toricity. A 2.5-mm incision in the vertical meridian gives 0.75 D of flattening, 0.25 D in the horizontal meridian and about 0.50 D obliquely. Opposite CCIs, which are essentially double on-meridian incisions, can manage about double this level of astigmatism correction. If there is greater corneal astigmatism, LRIs can correct about 2.50 D with reasonable predictability and up to 4.00 D with less predictability. Higher astigmatism requires AKs with smaller optical zones (typically 7 mm), and surgeons who were not trained in the corneal subspecialty may not feel comfortable with this surgery.
As a result of the unpredictability and lack of comfort with corneal approaches to astigmatism correction, many cataract surgeons have been pleased with the recent availability of toric IOLs. Reviewing all manufacturers of toric IOLs, it is possible to distinguish two strategies. The first is used by manufacturers such as Alcon Laboratories, Inc. (Fort Worth, Texas) and STAAR Surgical (Monrovia, California), which make a limited number of toric adds to correct low to medium astigmatism. The second is used by companies such as Carl Zeiss Meditec (Jena, Germany) and Rayner Intraocular Lenses Ltd. (Hove, United Kingdom), which produce customized IOLs capable of treating up to very high levels of astigmatism (typically 12.00 D). This means that a fully customized IOL can be obtained, with the expectation of a better refractive outcome.
These two strategies have quite different consequences for the surgeon. With the first, the surgeon finds the disadvantage that his IOL bank has doubled or tripled in size, with consequent storage issues, and that this cannot be restricted to common IOL spherical powers because astigmatism is more common in ametropic eyes. On the other hand, the first strategy has the advantage that the desired IOL may be immediately available, unless a higher toric correction is required. The second strategy requires preordering the customized IOL, with concomitant logistical planning.
It is interesting that the two main proponents of toric IOLs provide an online IOL calculator (respectively, Alcon's AcrySof Toric IOL Web Based Calculator and Rayner's Raytrace). This implies that both companies expect surgeons to use the online software to plan surgery in advance. It would seem unlikely that the surgeon will do this in the operating room immediately before surgery. More likely, it is done well in advance. If this is the case, it would seem more logical that the use of online software be combined with online ordering, with the opportunity of obtaining a fully customized IOL. With this argument, the second manufacturing strategy appears more logical.
The introduction of premium lenses, such as toric IOLs, to one's practice is not difficult. They are the first step toward customized IOLs, and patients deserve the best bespoke products available.
Charles Claoué, MA (Cantab), BChir, MD, DO, FRCS, FRCOphth, FEBO, MAE, is a Consultant Ophthalmic Surgeon and Honorary Lecturer in Ophthalmology at the Queen's University Hospital, London. Dr. Claoué states that he has received travel grants from Alcon Laboratories, Inc., Abbott Medical Optics Inc., and Medennium, and was previously a paid consultant to Bausch & Lomb. He is currently a paid consultant to Rayner Intraocular Lenses Ltd. He may be reached at e-mail: eyes@dbcg.co.uk.
