spotlight on technology & technique
Keepin' Away the Blues
Attenuating high-energy visible light could be the next important advance in IOL technology.
By Christopher Kent, Senior Associate Editor

Scientists often spend their time trying to duplicate wonders that Mother Nature has already created, whether it be a cavity-preventing chemical as good as the one in our saliva, or a fiber that comes close to the relative strength of a spider's thread. (So far, science hasn't managed either of those feats.) This is certainly the case in vision care: Duplicating the clear, flexible vision of youth has always been the Holy Grail.

In terms of IOLs, much energy has been devoted to developing artificial lenses that provide focusing flexibility. However, the natural crystalline lens has other characteristics worth duplicating, including providing protection from ultraviolet (UV) light. Not surprisingly, most IOLs now duplicate this protective characteristic of the natural lens.

But UV light may not be the only light that can damage the retina. Considerable evidence suggests that blue wavelength light -- the visible light closest to the frequency of UV light -- can also be harmful, producing the kind of retinal changes seen in macular degeneration. And Mother Nature would seem to agree: During the aging process, the natural crystalline lens becomes increasingly yellow, filtering out more UV and blue light as we get older.

Although duplicating this characteristic seems a promising avenue to pursue, most currently available UV-filtering IOLs don't filter out a significant amount of blue light. A new IOL from Alcon, however, is designed to do just that.

Alcon's Acrys of Natural IOL approximates the light transmission properties of the human lens.

Creating the Lens

The Acrysof Natural IOL is specially formulated to approximate the light-filtering characteristics of the natural adult crystalline lens, including the slight yellow tint that reduces the amount of blue light reaching the retina. Alcon's intention was to add a filter for blue-wavelength light to the currently available Acrysof Single-Piece IOL without altering any of its existing performance advantages.

To test the feasibility of this premise, an initial study was conducted by Oscar Cuzzani, M.D., at the Gimbel Eye Centre in Calgary, Alberta, Canada. Patients who had been implanted with standard (non-blue blocking) IOLs wore clip-on eyeglasses tinted with different intensities of yellow. The filtering level closest to that of the natural adult crystalline lens (in terms of the amount of yellow) had no negative effect on color vision, contrast sensitivity or night driving, and it had a positive effect on daytime driving. Consequently, the concentration of yellow chromophore in the Acrysof Natural lens was designed to duplicate this amount of blue filtering.

To create the lens, researchers at Alcon began with the same material used in the Single-Piece lens: a highly cross-linked acrylate/ methacrylate copolymer that has advantageous mechanical properties (foldability and strength), optical properties (high refractive index) and physical properties (biocompatibility and stability). The blue-filtering chromophore was then bound to the copolymer.

The resulting material attenuates the transmission of light between 400 and 500 nm, making the lens' light transmission properties more closely resemble that of an adult's natural crystalline lens. (See chart, page 82.)

Comparing Old and New

Testing of optical, physical and mechanical characteristics has found no significant differences between the Acrysof Natural IOL and the currently marketed Acrysof Single-Piece IOL -- other than filtering out more blue light. Likewise, clinical performance has matched that of the Single-Piece.

In fact, testing found a slight improvement in color perception with the addition of the blue filtering, although the difference isn't statistically significant. (See chart, above, right.) This is supported by reports from surgeons who have implanted the Acrysof Natural IOL. Patients sometimes complain that whites seem bluish when implanted with a standard lens (especially if the other eye still has its natural -- slightly yellow -- crystalline lens). The Acrysof Natural lens seems to eliminate this complaint, without causing any other color perception problem. (For example, patients don't complain that everything looks yellow.)

Feedback from the Field

Richard Braunstein, M.D., assistant professor of ophthalmology at Columbia University College of Physicians and Surgeons in New York City, says his experience with the Acrysof Natural IOL has been very positive. "Several patients have noted that they prefer the way they see with this lens. In fact, I've implanted the lens in a number of pseudophakic patients, and all of them preferred the Natural IOL eye. They didn't note any overall shift in color; they just said that colors seemed very bright and clear with it.

"As the lens becomes available, I expect to implant it exclusively, partly because patients report im-proved color perception, but more importantly because of the protective nature of the lens. Work done in our department, along with other clinical evidence, makes it pretty clear that blue light plays an important role in macular degeneration. This implant offers some protection. Patients also appreciate that possibility: If it might help, why not use it?

"There's doesn't appear to be any downside to using the Acrysof Natural IOL," adds Dr. Braunstein, "so I think these are reasons to change what you're doing in practice and implant this lens."

M. Bowes Hamill, M.D., associate professor of ophthalmology at Baylor College of Medicine in Houston, and past president of the Texas Ophthalmological Associa-tion, agrees. "I liken this to the days when we had non-UV blocker lenses, and then the UV blockers came out. There was a pretty strong sense that UV light was probably harmful over the long term, although we didn't have conclusive evidence at the time.

"Whether attenuating blue light will, in fact, turn out to be important is yet to be determined. But as long as there's no documented evidence of harm, or any evidence of a clinical disadvantage, I don't think there's any downside to using it.

"I'm implanting more and more of these lenses," Dr. Hamill concludes, "because they don't seem to hurt -- and they might help."

A Big Potential Payoff

It will take many years to determine conclusively whether attenuating blue-wavelength light really does protect the eye. But if it does, the benefits of having implanted blue-blocking IOLs could be enormous. A reduction in the occurrence of and/or severity of macular degeneration would be a major step forward. Also, the lens could be particularly valuable for replacing childhood cataracts; without the natural yellowing of the lens over time, a child's eyes would be exposed to far higher levels of blue-wavelength light over the course of his or her lifetime.

For more information about the Acrysof Natural IOL, contact your Alcon representative or call (800) TO-ALCON [(800) 862-5266].