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.
This
light-adjustable lens, or LAL, developed by Calhoun Vision, is now
in clinical trials and, if successful, will allow the surgeon to
adjust IOL power after cataract surgery, said Daniel M. Schwartz,
MD, associate professor of ophthalmology and director of retina
service at the University of California, San Francisco, and founder
and chairman of Calhoun.
The lens could address the
phenomenon of postoperative dissatisfaction with the refractive
correction. “There is always some risk that the IOL power selected
preoperatively will not achieve the patient’s refractive target,”
explained David F. Chang, MD, clinical professor of ophthalmology at
the University of California, San Francisco. Postoperative spherical
error is more likely to occur in certain patients, Dr. Chang noted,
such as those who have undergone refractive surgery, those with a
staphyloma, those with very long or short eyes and those needing a
simultaneous penetrating keratoplasty. In most patients, the
residual spherical error is small, he said, but in some patients it
may be significantly off and warrant explantation. LAL technology
could preempt this problem.
How the LAL
Works
Dr. Schwartz originated the idea
for a light-adjustable lens and implemented the concept by
collaborating with two polymer chemists, Robert Grubbs, PhD,
professor of chemistry, and Julia Kornfield, PhD, professor of
chemical engineering, both from the California Institute of
Technology.
Lens design not unfamiliar. The
LAL is based on a standard foldable, three-piece silicone or acrylic
platform with a 6-millimeter, square-edged optic. It has
modified C-loop, blue PMMA haptics. Dr. Schwartz noted that the lens
looks and feels very much like a standard foldable three-piece lens.
For implantation, there are no differences in surgical technique,
incision size or capsulorhexis size.
The lens material,
however, is a flexible silicone polymer matrix. In addition to the
regular silicone polymers, it contains mobile, photosensitive
silicone subunits, called macromers, Dr. Schwartz explained.
I Brake for
UV |
|
Caption: Near-ultraviolet light is
used after implantation to adjust and then lock in the power
of this lens. |
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.
Broad
Applications
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.”
______________________________
Dr. Schwartz is founder
and chairman of Calhoun Vision. He holds stock in the company. Dr.
Chang is a consultant for AMO, consultant and U.S. medical monitor
for Visiogen, and has received educational travel support from
Alcon. He has a financial interest in Calhoun. Dr. Olson is a
consultant for AMO and is head of the medical advisory board for
Calhoun. He will be the medical monitor for the FDA studies of the
LAL. Dr. Mamalis performs contract studies for many lens
manufacturers, including Calhoun, but has no financial interest in
the LAL. Dr. Fine is a member of the Scientific Advisory Board for
Calhoun Vision; he receives no compensation as a member of that
board and has no financial interest in the company.