IOLs with an aspheric optic design have been shown to provide better contrast sensitivity than comparable IOLs with a biconvex or equibiconvex optic design.1,2,3 This optical enhancement can improve functional vision for patients after cataract surgery, especially under mesopic and scotopic conditions.
Since the introduction of the first aspheric IOL in the early years of this century, virtually every major IOL manufacturer has added an aspheric model to its portfolio of IOLs. These lenses have been widely adopted by ophthalmologists. In the 2007 survey of practice styles and preferences among US members of the American Society of Cataract and Refractive Surgery (ASCRS) by David V. Leaming, MD, of California, only 13% of respondents said they never use an aspheric IOL, and more than half reported that they use an aspheric IOL almost all the time.4
But are aspheric IOLs truly benefiting most patients? After initial enthusiasm at the introduction of aspheric IOLs, during which I implanted aspheric IOLs routinely in my patients, I have now become more selective. I now implant aspheric IOLs in patients who have little or no astigmatism and whose corneal profile indicates that they would benefit from an aspheric IOL.
This article explores some of the reasons for this change in my IOL selection criteria.
SPHERICAL ABERRATION
The impetus for the incorporation of asphericity into IOL optics was to compensate for the positive spherical aberration of the average human cornea.5 Traditional biconvex spherical IOLs increase the total positive spherical aberration of the eye. The first aspheric IOL introduced, the Tecnis (originally Pharmacia, now Advanced Medical Optics, Inc., Santa Ana, California), incorporated -0.27 µm of spherical aberration to compensate for the mean positive corneal spherical aberration measured in a cohort of patients.5
Subsequently, other IOL models were introduced with less negative spherical aberration. The AcrySof IQ (Alcon, Fort Worth, Texas) incorporates -0.2 µm of spherical aberration. The SofPort AO and Akreos Adapt IOLs (both manufactured by Bausch & Lomb, Rochester, New York) and the C-flex Aspheric (Rayner Intraocular Lenses Ltd., Hove, England) are neutral in terms of spherical aberration (sometimes called aberration-free)—neither adding to nor subtracting from the asphericity of the cornea.
With this available range of almost 0.3 µm for correction of spherical aberration, the clinician is confronted with the question, "How much total postop spherical aberration is desirable?" Holladay and colleagues concluded that 90% of their measured cohort would experience a reduction in total-eye spherical aberration with the -0.27 µm compensation incorporated into the Tecnis IOL.5
However, because the asphericity of the human cornea varies between eyes and even more between patients and is distributed following a bell-shaped curve, there are some patients who would experience an increase in negative spherical aberration with implantation of the prolate Tecnis or AcrySof IQ lenses.
George Beiko, BM, BCh, FRCS(C), of Canada, proposed a personalized approach to correction of spherical aberration, measuring patients' corneal spherical aberration preoperatively using topography and targeting a postoperative residual spherical aberration of 0.1 µm.6 Using this approach, selected patients achieved better contrast sensitivity outcomes than a control group of consecutive patients.
When prolate aspheric IOLs became available, I began to use them nearly routinely without calculating the cornea's spherical aberration from corneal topography. Most patients were happy postoperatively, as would be expected after cataract surgery, with good visual acuity and good contrast sensitivity; however, a few patients were less happy with their results. They demonstrated good postoperative acuity, but they were not as euphoric about their visual results as other patients. When I looked at corneal topography postoperatively in some of these patients, it became apparent that the aspheric IOL they received was not appropriate for the shape of their cornea.
After realizing that, I have become more selective in my use of aspheric IOLs.
OTHER CONCERNS
In addition to the possibility of increasing spherical aberration in certain eyes with the use of aspheric IOLs, there are other issues to consider.
One concern is that, although aspheric IOLs provide improved contrast sensitivity, they decrease the patient's depth of focus.7 A little bit of astigmatism, especially against-the-rule astigmatism, combined with higher-order aberrations, such as spherical aberration and coma, can often provide patients who have good distance vision with enough pseudoaccommodation to see objects in the intermediate range or even to read large print under good lighting conditions. If, however, we are correcting astigmatism at cataract surgery and reducing patients' spherical aberration to zero with an aspheric IOL, we may be taking away some of their pseudoaccommodative ability and potentially increasing their postoperative spectacle dependence for near work.
A related concern comes from the correction of astigmatism at cataract surgery. In patients with less than 2.00 D of astigmatism, surgeons employ a number of incisional approaches to reduce cylinder at the time of surgery. Performing limbal relaxing incisions, operating on the steep axis, and creating a second clear corneal incision opposite the cataract incision are popular approaches to astigmatism reduction. We know that standard small-incision cataract surgery has little effect on the spherical aberration of the cornea postoperatively, but we do not have equivalent knowledge about the effect of these larger incisions used for astigmatism correction. Clinicians who use them know that the response of the cornea to these incisions varies from patient to patient. If we are creating variable effects on corneal shape with these incisions, possibly also changing the spherical aberration, then introducing a specific amount of correction of spherical aberration with an IOL may not make much sense.
There is also the possibility of IOL decentration or tilt occuring as the result of a complication; but in a small number of patients in whom the capsular bag is not aligned with the visual axis of the eye, even uncomplicated surgery can result in significant IOL decentration (Figure 1). In a model eye, Altmann and colleagues found that optical quality degraded rapidly with decentration of a prolate aspheric IOL by 0.5 mm or less, although this did not occur with an aberration-free IOL.8 Spherical IOLs are more forgiving of decentration and tilt than prolate aspheric IOLs.
If decentration is more than 0.5 mm, the benefit of an aspheric IOL is lost. With decentration greater than that, the optical quality decreases, and the patient would be better off with a standard spherical lens because it would be more forgiving.
Adding asphericity to multifocal IOLs probably makes a lot of sense. Multifocal IOLs reduce contrast sensitivity because they split up the incoming light intensity into two or more focal points, so the increase in contrast sensitivity provided by an aspheric optic design may help to offset some of this loss. However, the same principles discussed previously concerning customizing IOL choice to the corneal shape in the individual patient apply here as well.
CONCLUSIONS
For all these reasons, I currently prefer to implant aspheric IOLs primarily in patients who have little or no astigmatism, who are active, play sports, spend a lot of time outdoors, or are nighttime drivers. Also good candidates for an aspheric IOL are hyperopic patients with the need for a high-powered IOL.
Additionally, I perform corneal topography to measure the existing spherical aberration of the cornea before deciding whether an aspheric IOL makes sense or not for that patient. I do so preferably with a Placido-disc–based corneal topography device. Unfortunately, routine corneal topography is not always possible in a busy preop clinic.
We are currently planning a study with an IOL manufacturer in which we will custom-fit aspheric IOLs for each patient. We will perform topography on the patients; the data will be sent directly to the company, which will design and produce the lens for that eye; and 6 weeks later we will receive the IOL and implant it. The other eye will receive a standard aberration-free IOL, and we will compare the eyes to see if the custom-fit IOL improves visual function.
I call this the lock-and-key approach. First we examine the lock—the cornea—and then we make the key—the IOL—to fit it (Figure 2).
This pilot trial in 40 patients will examine not only whether this approach improves patients' visual function, but also whether it is feasible logistically, first on this small scale, and then possibly on a larger scale in an ongoing fashion.
Oliver Findl, MD, MBA, is a Consultant Ophthalmic Surgeon at Moorfields Eye Hospital, London, England. He is also on the faculty of the Medical University of Vienna, Austria. Dr. Findl states that he has no financial interests in the products and companies mentioned. He may be reached at e-mail: oliver@findl.at.
