Are Aspheric IOLs the Answer?
With proven advantages over sphericals, the question is just how popular they will become.
BY DESIREE IFFT, EDITORIAL DIRECTOR

Cataract surgery with IOLs has so brilliantly restored vision for so many millions of patients in recent decades, it hardly seemed necessary to scrutinize the exact optical effects at work in the pseudophakic eye. However, now that wavefront technology has been applied to this area, the benefits of such scrutiny are being realized. The ability to measure higher-order aberrations has led to a much greater understanding of the eye as an optical system and how that system is altered by an IOL. The result is a new category of IOLs -- aspherics -- which are capable of providing better quality of vision than any of their spherical predecessors.

Three aspheric IOLs are currently FDA-approved: the Tecnis (AMO), the AcrySof HOA (aspheric optic SN60WF, Alcon), and the SofPort AO (Bausch & Lomb). This article reviews the theories behind them and how they're performing so far for surgeons and their patients.

Two Schools of Thought

The main advantage of these new IOLs compared with standard spherical IOLs is their ability to improve image contrast on the retina by correcting for the vision-degrading spherical aberration that exists in the aging eye. Modulation Transfer Function curves, also known as spatial frequency responses, measured in laboratories have illustrated this. In young eyes, the positive spherical aberration of the cornea is mostly balanced by the negative spherical aberration in the crystalline lens, which results in optimal vision. With age, that negative spherical aberration shifts toward positive, creating an imbalance that reduces contrast sensitivity and thus visual function. Standard spherical IOLs are positive spherical aberration lenses, so when they are implanted, they add to the positive spherical aberration of the cornea, leaving the patient with visual function below what it could be.

The three aspherics on the market today are based on two different theories. The Tecnis and the AcrySof aspheric optic SN60WF are both negative spherical aberration lenses. The goal of both designs is to counteract the mean amount of spherical aberration that is found in the general population of cataract patients so that the resulting total spherical aberration of the eye is zero. The Tecnis accomplishes this via its modified anterior prolate surface, while the AcrySof SN60WF has an aspheric posterior surface.

So far, the only studies of aspheric IOLs in human eyes to be published in peer-reviewed journals are those that pertain to the Tecnis. Key studies include those published in 2002 and 2003 by Packer et al., Mester et al., and Kershner, which all arrived at the same general conclusion: The Tecnis significantly enhances patients' contrast sensitivity and functional vision performance compared with standard IOLs, especially in mesopic conditions. Last year, the FDA approved a new label for the Tecnis, which now states that the lens has been shown to reduce spherical aberration and, based on simulator tests, make driving at night safer.

Those claims are likely appealing to cataract-age patients. In a November 2004 A.C. Neilsen survey, 91% of patients between the ages of 55 and 75 said it is "extremely" or "very" important to have a lens implant that makes driving safer, especially at night.

Stephen S. Lane, M.D., has performed studies with his patients, implanting an SN60WF in one eye and an SN60AT in the other. "Significant decreases are seen in spherical aberration in the WF eye during aberrometry testing as compared to the spherical lenses implanted in the opposite eye," he says. "Mesopic contrast sensitivity clearly improved in some patients. While the clinical significance of this is at this point uncertain, theoretically this may translate to improved night vision and decreased glare."

The third aspheric IOL, the SofPort AO, is based on a different theory. Both its anterior and posterior surfaces are aspheric, with uniform power from center to edge. Rather than adding negative spherical aberration to the eye as the other two lenses do, it is aberration-free. According to its designers, laboratory studies, and initial field studies, this means the SofPort AO can improve vision over standard IOLs for all patients, including the estimated 5% to 7% who do not have an average amount of positive spherical aberration in their corneas either naturally or because of previous hyperopic corneal refractive surgery. Furthermore, if the SofPort AO decenters in the eye, it would not induce other aberrations, such as coma and astigmatism, as negative-aberration and spherical lenses do if they decenter.

"I have implanted about 500 SofPort AO IOLs; my results have been excellent, with no issues to date," says Uday Devgan, M.D. "Patients have been very pleased, particularly those who have a SofPort AO in one eye and a SofPort SE in the other. They tend to notice the difference in low-light conditions. A few have even inquired about IOL exchange so that they may have the AO in both eyes. The advantages of the AO is amplified when patients have larger pupils or have less-than-perfect IOL centration."

Comparing Theories in the Real World

Further study will determine how much the differences between these two approaches matter in the real world. Expect these questions in particular to be explored:

If it were possible to bring every pseudophakic eye to the level of zero spherical aberration, would that be ideal? Some optical experts say no. They argue that some positive spherical aberration in the eye is beneficial because it provides some depth of field and also helps to "smooth" the visual effects of other aberrations. In addition, they point out that human vision is at its best in young eyes, and young eyes do contain positive spherical aberration.

How much of an issue is decentration? As stated previously, it's known that when negative spherical aberration IOLs decenter, vision may be compromised. But how often and by how much IOLs in general tend to decenter is not well established. Also not established is how much decentration can occur before a negative spherical aberration lens degrades vision. It has been estimated that most lenses do not move more than .4 mm, and that the threshold for detrimental visual effects is closer to .8 mm.

The question "decentered from where?" further complicates the issue. Ideally, IOLs would be centered on the visual axis, the "line" that "connects" the fovea to the object being visualized. But the only way to center an IOL during cataract surgery is based on the pupillary axis, which tends to be, on average, .37 mm away from the visual axis. Therefore, even when an IOL is centered to the pupil and to the capsular bag, it may not be centered on the visual axis.

Optical theories aside, Ralph Chu, M.D., says he believes the issue of decentration with the Tecnis has been overemphasized. He has implanted close to 2,000 Tecnis lenses and says he is "getting more uncorrected 20/20 and 20/25 visual acuities because of the better image contrast," and none of his patients has complained about vision issues. "The Tecnis is my lens of choice because of the quality of vision it produces, which can be directly tied to an added measure of safety, especially for our older patients."

Stephen Bylsma, M.D., has implanted more than 150 Tecnis lenses since the 2002 FDA approval and characterizes his results as "outstanding, reliable, and consistent."

Richard Tipperman, M.D., and Stephen Lichtenstein, M.D., have implanted more than 500 AcrySof SN60WF lenses in their practice. IOL decentration has not been an issue for their patients. "With regard to biomechanical properties," says Dr. Tipperman, "I believe the single-piece acrylic platform is far superior to a silicone or multipiece design in terms of ease of insertion, long-term centration, and long-term safety. However, this is not just my opinion. Worldwide, the single-piece AcrySof platform lens is the market-leading lens and used in a greater percentage of cataract surgeries than any other intraocular lens."

Dr. Lane also said the AcrySof single-piece platform has been shown to be very stable. He cites the study he and colleagues performed, "Comparison of the biomechanical behavior of foldable intraocular lenses," which was published in the November 2004 issue of the Journal of Cataract and Refractive Surgery. "The planar haptics of the single-piece AcrySof lens limit axial displacement," he explains. Also, "single-piece AcrySof lenses apply a (low) constant force to the capsular bag. This will lead to better maintenance of the capsular bag shape, resulting in less capsular striae, minimal external force that would extend a capsular tear, and excellent IOL centration."

Are other factors involved? This new category of lenses clearly impacts the spherical aberration status of the eye, but it remains to be seen whether other variables, such as IOL material, the posterior surface of the cornea, or even the vitreous, are also involved.

Aspherics in Every OR?

While some surgeons are already convinced that aspheric IOLs provide clinical benefits, others await firmer data on all three before bringing them into their practices. Cost may be a barrier to wider adoption as well. New Technology IOL status could somewhat alter the economic equation in the future, but currently, aspheric IOLs are priced in line with other premium monofocal lenses.

Dr. Devgan, for one, sees no reason to continue using spherical IOLs. "Our surgical microscopes, slit lamps, fundus lenses, retinal cameras, and even glasses and contact lenses use aspheric optics. Our IOLs will definitely follow suit," he says. "Within a few years, nearly every IOL design will incorporate aspheric optics, and the premium currently being charged for these IOLs will be eliminated."

Aspheric IOL Specifications

AcrySof aspheric optic (SN60WF) Lens style: single-piece Optic/haptic material: acrylate methacrylate copolymer with UV and proprietary blue light-filtering chromophore Haptic type: modified-L Haptic angle: 0 degrees Optic diameter: 6.0 mm Overall length: 13.0 mm Optic edge type: modified square edge Refractive index: 1.55 Special feature: posterior aspheric surface Available powers: +6D to +30D in 0.5D increments Injector: Monarch II IOL Delivery System Tecnis (Z-9000, Z-9001) Lens style: three-piece Optic material: silicone (polysiloxane) Haptic type/material: C/polyvinylidene fluoride Haptic angle: 6 degrees Edge type: square (anterior and posterior edges) Optic diameter: 6 mm Overall length: 12 mm (Z9000), 13 mm (Z9001) Refractive index: 1.46 Special feature: modified prolate anterior surface Available powers: +5 to +30D in .5D steps Injector: Silver Series Unfolders (Silver and Silver T) SofPort AO (LI61AO) Lens style: three-piece Optic material: silicone Haptic type/material: modified C-loop/PMMA Optic diameter: 6 mm Overall length: 13 mm Optic edge type: 360º anterior/posterior square edge Haptic angle: 5 degrees Refractive index: 1.43 Special feature: aspheric anterior and posterior surfaces; uniform power from center to edge Available powers: 10D to 30D in 0.5D increments (0D to 9.5D in July) Injector: disposable SofPort System inserter (will be available in May with the new Easy-Load inserter)