American Academy of Ophthalmology Web Site: www.aao.org
Clinical Update: Cataract
Inside the Calhoun Light-Adjustable Lens
A new intraocular lens whose power can be adjusted in situ by ultraviolet light may one day solve the problem of IOLs that offer disappointing refractive power after implantation.
New UV application. The adjustability function required a new digital light delivery (DLD) device, which was developed jointly by Calhoun and Carl Zeiss Meditec. The DLD delivers a cool, low-intensity beam of near-ultraviolet light. It is slit-lamp mounted, produces a customized light pattern for each patient, and centers the light treatment on the visual axis.
Migration of the macromers. When a portion of the lens is irradiated with near-UV light, it polymerizes the macromers in that portion. That creates an excess concentration of macromers in the nonirradiated portion and sets up a diffusion gradient over which the free macromers move from the concentrated area to the less concentrated area, explained I. Howard Fine, MD, clinical professor of ophthalmology, Oregon Health & Science University, Eugene.
Consequently, Dr. Fine explained, if you irradiate the central portion of the light adjustable lens, the unpolymerized macromers in the periphery of the lens will move into that central portion. As a result, the central portion of the lens will swell. This would correct hyperopia. If you irradiate the periphery of the IOL, the centrally located macromers will migrate to the periphery of the lens causing that to swell. “This gives you a more minus-powered, myopic lens,” Dr. Fine said.
The Moment of Adjustment
After you have implanted the LAL, Dr. Schwartz said, “You do the power adjustment at the time that you would normally prescribe glasses for the patient, about two to four weeks after surgery. By this time, the eye has healed and the refraction has stabilized.”
The patient’s refraction is tested, and he or she sits at the DLD device. A contact lens is used to provide a smooth corneal surface and stabilize the eye during treatment. The patient’s refractive error is entered onto a display screen, and the DLD shines the specified profile of light onto the lens for about two minutes. “We can program in any kind of light pattern that we want to shine onto the lens, and the lens captures that pattern,” Dr. Schwartz said. “When the light hits the surface of the lens, the lens changes shape, thereby changing the power. The patient goes home, and when they wake up the next morning, their refractive error has been corrected.”
Adjustment can provide 2 D of correction in hyperopic, myopic or astigmatic eyes. “With the current version of the lens, we can adjust the lens more than once. If we do a 2-diopter adjustment one day, then we could do an approximate 1.5-diopter additional adjustment on the next day,” Dr. Schwartz said.
Correcting for astigmatism. In addition to spherical correction for myopia or hyperopia, Dr. Schwartz emphasized that the surgeon can do customized cylinder adjustment along any axis. “We can make a toric IOL to correct the patient’s astigmatism,” he said. “We have successfully adjusted for spherical error in 16 consecutive patients,” Dr. Schwartz reported, noting that all were within one-quarter of a diopter of the intended refraction. “Using the new digital light delivery device, we have adjusted two patients with astigmatic error, both around 0.75 to 1.0 diopters of toric correction, and we will be able to correct larger amounts of astigmatism in the future,” he said.
“Performing the adjustment a few weeks after surgery also negates any astigmatic unpredictability introduced by the incision,” Dr. Chang added.
The surgeon can also write multifocal patterns onto the lens. “We can confer multifocality on a lens that is emmetropic and centered on the visual axis,” Dr. Schwartz said.
Locking in the results. After the LAL is adjusted for all the components of
the patient’s refractive error, and both surgeon and patient are happy with the refraction, the lens power is locked in by providing a second dose of light that irradiates the entire optic. Dr. Schwartz noted that “you cannot leave the lens unlocked. Once you lock in the lens power, you are not able to adjust it again.”
The lens then becomes just like other currently used foldable silicone or acrylic IOLs, Dr. Chang added. “There should be no differences in long-term performance with respect to centration, safety or PCO,” he said.
Randall J. Olson, MD, professor and chairman of ophthalmology and visual sciences and director of the John A. Moran Eye Center, University of Utah, hopes to see LAL technology evolve into a lens that will remain adjustable over the lifetime of the lens.
Candidates, Contraindication and Cost
According to Dr. Schwartz, almost all patients having cataract surgery would be candidates for the LAL. There is one known contraindication, according to Dr. Chang. “The patient’s pupil will need to dilate widely enough, 6 to 6.5 millimeters, to be able to irradiate the entire optic.” But Dr. Schwartz added, “We have developed a special contact lens that will enable adjustment and lock-in to be performed with pupils that dilate less than 6 mm.”
There will also be a cost factor. Some patients may not be able to afford this lens. Also related to cost, some surgeons may not want to invest in the light delivery device and may choose to refer the patient elsewhere for the adjustment procedure, Dr. Olson said.
Trials proceeding. Nick Mamalis, MD, professor of ophthalmology, University of Utah, Salt Lake City, performed the animal studies for this lens. “The light-adjustable lens showed excellent biocompatibility up to six months postoperatively. It was equivalent to that of a standard three-piece silicone IOL,” he said.
All the clinical trials in humans to date have been done in Mexico by Arturo Chayet, MD. According to Dr. Schwartz, trials will begin soon at a second site in Barcelona, Spain, under the direction of Jose L. Guell, MD, professor of ophthalmology and cornea at Universidad Autónoma de Barcelona. FDA trials in the United States are expected to be under way soon. Dr. Olson will be the medical monitor for these trials.
According to Dr. Schwartz, the technology will work with both acrylic and silicone lenses, but the first commercial version of the lens will be in silicone. Manufacturers are developing numerous lens designs for cataract surgery and refractive lens procedures, and Dr. Schwartz noted that all of these lenses will benefit from the new adjustability technology, whether they are made of acrylic or silicone. Adjustability would also be an advantage for patients who are receiving phakic IOL implantation, he said.
“Our goal is to make our adjustable lens material available for these various lens designs,” he said, “so we can achieve better outcomes for all of our patients.”