spotlight on technology & technique
If At First You Don't Succeed . . .
A new technology makes it possible to revise IOL power until it's perfect -- after the IOL is in the eye. And that may be just the beginning.
By Christopher Kent, Senior Associate Editor

As any golfer knows, you can't always hit a hole-in-one. But given several shots, a good golfer will always sink the ball.

Today, cataract surgeons are expected to score the equivalent of a hole-in-one on every shot. Unfortunately, thanks to imperfect technology, the best you can hope for with many patients is to get the ball on the green. And even if you score a hole-in-one, some patients may not like the option they selected, such as a multifocal lens or monovision. For all these reasons, a large number of cataract patients still need glasses for optimal post-op vision, and a significant number of IOL exchanges are performed every year.

Now, researchers have developed a foldable silicone IOL whose refractive power can be adjusted multiple times after it's inserted in the eye, without performing any further surgery. Using this technology, it may be possible to reach that "perfect" refraction -- and have a happy patient -- every time.

Shape-Shifter

The "light-adjustable" silicone IOL, or LAL (being developed by Calhoun Vision in Pasadena, Calif.) can be made to change shape, and hence refraction, by exposing it to a specific wavelength of light. Adjustments can be made over a range of several diopters with remarkable precision. Animal studies conducted by Nick Mamalis, M.D., at the Moran Eye Center at the Universtiy of Utah, have shown that changes in refractive power can be controlled to ±0.2 D or less. And because the adjustment can be precisely customized, it's possible to adjust (or create) not only cylindrical correction, but multifocal correction, aspheric correction and higher-order correction.

Even more remarkable, the shape can be adjusted several times before the final refraction is "locked in." This not only means you have several opportunities to get the desired refraction, it also means that patients who don't like the refractive option they chose can try a different option (or two) without undergoing further surgery.

Chemistry/Physics 101

Here's how the process works. The silicone polymer is in a partially liquid state at body temperature. The polymerized strands of silicone that make the IOL "solid" are too long to move freely, but not-yet-polymerized monomers mixed into the substance can still move within the lens.

Exposure to the appropriate wavelength of light (365 nm) causes some of the free-roaming mono-mers to bind together into longer polymer chains. If light exposure is limited to discrete sections of the lens, the concentration of mono-mers in those areas is reduced as a result. Then, diffusion -- the principle of physics that says freely moving particles tend to disperse evenly in a solution -- takes over. This tendency is so strong that it occurs even if it increases pressure in some areas of the lens.

And that's exactly what happens. The areas in which monomers have been polymerized now draw more monomers in to equalize the monomer distribution throughout the lens. This packs this part of the lens with more molecules, since the recently polymerized monomers never left, and swelling results. The diffusion and resulting change in shape take 12 to 15 hours.

How much adjustment is possible? A single exposure consumes up to 20% of the free monomers, resulting in a refractive change of up to 2D. (Ninety-eight percent of IOL outcome errors fall within this range.) Because 80% of the free monomers are still available, the process can be repeated. In fact, irradiating a different area of the lens will cause the swelling to reduce in the previous area as monomers migrate to the new location. That means that early adjustments can be either augmented or undone, and you have several opportunities to do so before the monomers are all polymerized.

"Locking in" a satisfactory outcome is simple: Irradiating the entire lens for the appropriate length of time polymerizes all the monomers in their current locations, and the existing refraction becomes permanent.

Practical Issues

Implanting this lens is identical to implanting a standard foldable silicone lens. Following surgery, the patient returns after the eye has stabilized refractively (2 to 4 weeks) for examination and adjustment of the lens, if needed.

Once the surgeon determines that a refinement is needed and irradiates the lens, the patient goes home and returns the following day (giving the diffusion process time to reshape the lens). Further adjustments can be made as needed.

During the period before the lens is "locked in," patients must wear sunglasses if they're outside to prevent unintended changes in the lens. (The lens does contain a UV blocker that provides protection for about 2 hours, but if more UV blocker were added, a toxic level of light would have to be used by the surgeon to adjust the lens.)

Once the patient is satisfied, the surgeon "locks in" the refraction. Then the lens behaves like any standard 3-piece foldable silicone lens.

So far, biocompatibility studies using rabbits have found no inflammation or other problems, and leaving a lens in solution for a year produced no leaching. Early in vivo studies are demonstrating excellent biocompatibility and reproducible light-induced changes in the lens.

The Future Looks Bright

Like refractive surgery, the LAL should offer practices a significant economic benefit. Implanting the lens will be covered by Medicare, but the post-op adjustment will have to be paid for by the patient.

To date, only a few human patients have received the LAL; trials are being conducted in Mexico under the auspices of Arturo Chayet, M.D. So far, everything has gone as expected in vivo. According to the company, 25 implants are scheduled; these will become the basis for an investigational device exemption filing in the United States later this year. Up-dated results will be reported at ASCRS in San Francisco.

Randall J. Olson, M.D., chairman of the Department of Ophthalmology at the University of Utah School of Medicine, and director of the John A. Moran Eye Center in Salt Lake City, is serving as medical monitor for the clinical studies of the new lens. "This is the most exciting development I've seen in my career," he says. "Being able to guarantee refractive precision will dramatically enhance cataract surgery; it will make this the most accurate refractive procedure available. It's just a matter of time before this becomes the standard for refractive surgery precision."

For more information about the LAL, visit info@calhounvision.com on the Web, or contact Calhoun Vision at (626) 685-2000.