The eye
and vision errors
The cornea is a part of the eye that helps focus light
to create an image on the retina. It works in much the same
way that the
lens of a camera focuses light to create an image on film. The bending
and focusing of light is also known as refraction. Usually the shape of
the cornea and the eye are not perfect and the image on the retina is
out-of-focus (blurred) or distorted. These imperfections in
the focusing
power of the eye are called refractive errors. There are three primary
types of refractive errors: myopia, hyperopia and astigmatism. Persons
with myopia, or nearsightedness, have more difficulty seeing
distant objects
as clearly as near objects. Persons with hyperopia, or
farsightedness, have
more difficulty seeing near objects as clearly as distant
objects. Astigmatism is a distortion
of the image on the retina caused by irregularities in the cornea or
lens of the eye.
Combinations of myopia and astigmatism or hyperopia and astigmatism are
common. Glasses or contact lenses are designed to compensate
for the eye's
imperfections. Surgical procedures aimed at improving the
focusing power
of the eye are called refractive surgery. In LASIK surgery, precise and
controlled removal of corneal tissue by a special laser
reshapes the cornea
changing its focusing power.
Other types of refractive surgery
Radial Keratotomy or RK and Photorefractive
Keratectomy or PRK are other refractive surgeries used to reshape the
cornea. In RK, a very sharp knife is used to cut slits in the
cornea changing its shape. PRK was the first surgical
procedure developed
to reshape the cornea, by sculpting, using a laser. Later,
LASIK was developed. The same
type of laser is used for LASIK and PRK. Often the exact same laser is
used for the two types of surgery. The major difference between the two
surgeries is the way that the stroma, the middle layer of the cornea,
is exposed before it is vaporized with the laser. In PRK, the top layer
of the cornea, called the epithelium, is scraped away to
expose the stromal
layer underneath. In LASIK, a flap is cut in the stromal layer and the
flap is folded back.
Another type of refractive surgery is thermokeratoplasty in which heat
is used to reshape the cornea. The source of the heat can be a laser,
but it is a different kind of laser than is used for LASIK
and PRK. Other
refractive devices include corneal ring segments that are inserted into
the stroma and special contact lenses that temporarily
reshape the cornea
(orthokeratology).
What the FDA regulates
In the United States, the Food and Drug Administration (FDA) regulates
the sale of medical devices such as the lasers used for LASIK. Before
a medical device can be legally sold in the U.S., the person or company
that wants to sell the device must seek approval from the FDA. To gain
approval, they must present evidence that the device is reasonably safe
and effective for a particular use, the "indication." Once the FDA has
approved a medical device, a doctor may decide to use that device for
other indications if the doctor feels it is in the best interest of a
patient. The use of an approved device for other than its FDA-approved
indication is called "off-label use." The FDA does not regulate off-label
use or the practice of medicine.
The FDA does not have the authority to:
- Regulate a doctor's practice. In other words, FDA does not
tell doctors what to do when running their business or what they can
or cannot tell their patients.
- Set the amount a doctor can charge for LASIK eye surgery.
- "Insist" the patient information booklet from the laser
manufacturer be provided to the potential patient.
- Make recommendations for individual doctors, clinics, or eye
centers. FDA does not maintain nor have access to any such list
of doctors performing LASIK eye surgery.
- Conduct or provide a rating system on any medical device it
regulates.
The first refractive laser systems approved by FDA
were excimer lasers for use in PRK to treat myopia
and later to treat astigmatism. However, doctors began using
these lasers for LASIK (not just PRK),
and to treat other refractive errors (not just myopia). Over
the last several years, LASIK has become the main surgery
doctors use to treat myopia in the United States. More
recently, some laser manufacturers
have gained FDA approval for laser systems for LASIK to treat myopia,
hyperopia and astigmatism and for PRK to treat hyperopia and
astigmatism.
See the section on
FDA-approved lasers for more details on which lasers have
received FDA approval and the specific indications and treatment
ranges for which they were approved.