than average. As the crystalline lenses of
hyperopic people thicken due to normal
aging, they may crowd the small eyes and
interfere with the trabecular meshwork
of the iris, which normally regulates
fluids in the eyeball.
In a laser iridotomy, developed by
the French physician Danièle Aron-Rosa about 30 years ago, the ophthalmologist uses an Nd:YAG laser to drill
a small hole in the iris to relieve some
of the fluid pressure inside the eyeball. Although some types of lasers aid
coagulation, the Nd:YAG laser does not,
so there is some risk of bleeding in the
procedure. For this reason, Joos often
starts the surgery by using an argon
laser to spur coagulation in the area of
the hole and then finishes up with the
Nd:YAG laser.
In some cases, the ophthalmologist
must perform an iridoplasty with an
argon laser and a larger spot size (200 to
400 µm), Joos said. That contracts the
iris tissue and cuts a wider hole in it.
Open-angle glaucoma—
characterized by a gradual decrease in aqueous
fluid outflow despite an open anterior
chamber—is a chronic, silent thief of
sight. Once the condition has passed
beyond the point at which the patient
32 | OPN Optics & Photonics News
Wellman Center for Photomedicine, Boston, Mass. (U.S. A.)
can control it with eyedrops or pills, an
ophthalmologist can perform an argon
laser trabeculoplasty (ALT), punching 50-µm-size holes in the angle (the
region where the cornea and iris meet).
According to Joos, it’s believed that the
procedure stimulates the tissue to drain
the intraocular fluid more effectively.
Within the past five years, researchers have refined this surgery into a
technique called selective laser trabeculoplasty (SLT), which uses 532-nm
Nd:YAG light with a spot size of roughly
400 µm to increase the drainage in the
trabecular mesh. SLT proponents say
that it causes less tissue damage to the
angle, although some studies have shown
that both procedures have equal success
rates with patients.
The latest on LASIK
Laser-assisted in situ keratomileusis, or
LASIK, is a popular elective surgery
with people whose natural vision is
worse than 20/20. The procedure for
reshaping the cornea to reduce refractive
error is now routine in the United States,
but researchers have continued to refine
the technology incrementally.
Early this decade, wavefront-guided
LASIK appeared in the U.S. market.
These systems use Shack-Hartmann sensors to measure the original aberrations
of the cornea and generate a customized
map of the amount of ablation needed
to correct the pre-operative refractive
error. Wavefront guidance was designed
to reduce the amount of glare and haloes
that some LASIK patients experience
under post-surgical low-light conditions.
Today, the wavefront-guided technology is still considered an upgrade to
basic LASIK, but customer demand for
it during the current economic recession
has been flat, said Jim Schwiegerling,
associate professor of ophthalmology at
the University of Arizona (U.S.A.). Nevertheless, the technology has expanded
the treatment range of LASIK to stronger levels of myopia and astigmatism
than what was available in the early days
of the surgery. A quarter of the patients
who get this procedure can end up with
20/15 or 20/10 vision. With wavefront
sensing, people who would otherwise
have gotten 20/40 vision after conventional LASIK treatment can now end
up with 20/20 vision.
Ultraviolet excimer lasers with an
output at 193 nm are the only ones
approved for LASIK, according to
Schwiegerling. Most ophthalmologists use narrow-beam LASIK systems,
with spot sizes of 1 to 2 mm, instead
of the early broad-beam systems with
mechanical apertures.
The manufacturers of LASIK lasers
have also made them easier to use by
adding eye tracking to the laser. Since the
patient is conscious during the procedure,
some small eye movements are inevitable.
Previously, the surgeon would use a joy-stick to keep the instrument centered on
the eye. However, most modern systems
use video-based eye tracking to follow
features on the iris to calculate where
to deliver the laser power. Usually the
patient’s eye motions are pretty small—
less than 0.5 mm in any direction—but
the surgeon must take out a precise
amount of tissue in order to avoid introducing higher-order aberrations such as
spherical aberration and coma.
Femtosecond lasers, operating in
the near-infrared, have begun to play a
