Laser Eye Surgery
Quick History:
Corrective eye surgery has been in
practice since the 1940’s. Radial
keratotomy (RK) and photorefractive keratectomy (PRK) have had the greatest
impact on current methods of refractive eye surgery. The most common refractive surgery used today
is Laser in Situ Keratomileusis (LASIK).
RK uses a diamond tipped knife to flatten the central cornea by making a
series of incisions radiating from the cornea.
RK can only treat low degrees of myopia and astigmatism, unlike
LASIK. PRK uses a laser to correct the
refractive area by removing corneal tissue.
PRK can be discomforting or even painful because it removes nerve fibers
that run through the corneal epithelium.
After this surgery, it can take up to six months for vision to
stabilize. Immediately following the
surgery, vision is blurry until the corneal epithelium healed, and the bandage
lens removed. PRK is similar to LASIK
because it uses a laser to correct the corneal defects. LASIK has the same benefits, but with fewer,
less-sever side effects than PRK. Also,
LASIK is less painful, and recovery time is shortened because the corneal
epithelium is not removed. The LASIK
procedure folds back the corneal epithelium and then a laser is used to reshape
the cornea.
The first lasers used were broad beam
lasers (about 6 mm) that acted similar to a showerhead, splitting the main
laser into smaller beams. Current laser
technology implements the “flying spot laser”, named to describe the technique
of creating a pattern with the laser, so that the full width of the flying spot
beam hits the cornea with each pulse.
Advantages of this type of laser include a smoother corneal shape, and
increased accuracy and efficiency of the surgery.
Reference: Pictures taken from Bausch & Lomb http://www.bausch.com/us/vision/concerns/surgery/history.jsp
The Role of Physics in Laser Eye Surgery:
The deficiency of the eye can be examined using the physics of
refraction. Refraction occurs mostly at
the cornea due to the substantial difference between the index of refraction of
the cornea, and that of air. The cornea
and the lens combine to focus the light at the retina. The eye adjusts the curvature of the lens in
order to alter the focal length, enabling it to focus objects at varying
distances. A normal eye is able to
converge light properly by changing the lens shape so that objects both near
and far appear clearly.
In nearsightedness,
the cornea is overly curved resulting in a greater index of refraction. The light converges, and the image forms in
front of the retina. In order to correct
this problem with laser eye surgery the procedure begins with a surgeon temporarily
peeling back the epithelium. A laser is
then trained on the eye to remove small amounts of the cornea. The cornea is flattened in order to correct
nearsightedness. The flattened cornea
has a lower index of refraction than previously, focusing the image on the
retina instead of in front of the retina.
Reference: Pictures taken from Physics at work http://kineticbooks.com/physics/17372/17405/sp.html
Another dysfunction of the eye is farsightedness. In farsightedness, the cornea is not curved
enough, and thus, the refractive index is lower. This causes the image’s focal point to be
located behind the retina. The surgeon
corrects this condition by using a laser to remove the outer edge of the
cornea, thus steepening it. As a result
of this surgery, the eye is able to focus the image directly on the retina, due
to the greater index of refraction resulting from the curvature of the eye
being increased.
Reference: Pictures taken from Physics at work http://kineticbooks.com/physics/17372/17405/sp.html
The type of laser used for these procedures is a
ultraviolet excimer laser, this type of laser uses
and unstable inert gas with halogen, argon and fluorine to create an intense UV
beam. A list of approved lasers used for
LASIK surgery is linked below.
http://www.lasikeyesurgery.com/approved-lasik.asp
Advantages, Disadvantages, and Risks:
Laser eye surgery is one of the fastest methods to correct visual
impairments. It has the ability to correct even severe refractive errors, but
minimize the recovery time. Most patients are able to return to work within a
week! Many patients even return to normal activities the day after surgery. The
treatment is permanent according to studies done so far, but as eyes change
with age re-treatment may be necessary over time. Eye drop medication is
necessary for the first week, but discomfort wears off after 5-6 hours. The
medication is used to prevent infection, not as a pain suppressant. The main
disadvantage of LASIK surgery is that it requires the creation of a flap in the
cornea. This step takes less than a minute to complete, but there are risks of
complications.
As with any surgery, there are risks associated with the
procedure. The best way to avoid certain risks is to have a thorough
examination with your doctor pre-surgery. Some of these risks of surgery
include:
· Corneal infection (less than 0.1% with
LASIK)
· Undercorrection/overcorrection
· Decrease in best corrected vision
· Corneal haze, though risk is less with
LASIK than PRK
· Regression, can be re-treated to correct
· Halo effect in dim light
· Flap damage or loss, when hinged part of
the cornea is removed completely
· Distorted flap when the corneal flap heals
irregularly
What to Expect:
The first step to
having laser eye surgery is a medical examination with your doctor to determine
your prescription, look for any retinal problems or unusual intraocular
pressure, and look for signs of eye disease. A corneal topographer is used to
make a map of your cornea to help the surgeon determine how to correct the
irregularities. Before you go in for
surgery, be sure to not wear any contact lenses for a week because that changes
the shape of the cornea. Also don’t use makeup or lotions around the eyes.
During surgery,
which lasts approximately 30 minutes, you will lie on your back and numbing
drops are placed in your eye. A lid speculum holds the eye open as it is
cleaned and high pressures are applied to create suction around the cornea. The
doctor uses a microkeratome, a blade, to cut a flap
into the cornea. Then the suction ring is removed. The flap is folded back and
a laser positioned over your eye. The doctor will ask you to fixate on a point
of light, then use a laser to remove the corneal
tissue. Pulses are heard as the computer programmed laser removes a fixed
amount of tissue determined by pre-operation measurements. A shield is placed
over the eye to prevent damage to the eye or movement of the flap, as no
stitches are used. The pressure from the eye will cause the flap to reseal
against the eye and it will heal naturally.
Reference: Eye Clinic, P.C. LASIK http://www.eyeclinicpc.com/lasik/lasik.htm
After surgery,
there may be slight discomfort, which can typically be removed by a mild over
the counter pain reliever. These symptoms should disappear within the first few
days. A follow up appointment is absolutely necessary the day after surgery to
ensure the flap has not been dislodged and is in the appropriate place. After that, check-ups on regular intervals for only six months.
Vision may fluctuate during the months after surgery but should stabilize
within three to six months.
Who should get laser eye surgery?:
Laser eye surgery is ideal for anyone who wants the freedom that
improved eyesight can give. The hassles of glasses and corrective lenses can be
removed, often permanently, by the procedure. But, laser eye surgery is not for
everyone. Before undergoing surgery, all patients should have a complete
medical examination with their doctor. People with persistent medical
conditions, have difficulties healing, or have vascular, autoimmune, or eye
diseases may not be
suitable for the procedure. Pregnant women should also refrain from surgery due
to changes in the eye’s refraction during pregnancy.
Typical patients
for eye surgery are at least over 18, though the age preference changes with
the type of laser used. Patients have mild to moderate nearsightedness,
farsightedness, and astigmatisms, within the range of correction for the
treatment. Each laser has different ranges for the procedure performed.
Other References:
Bausch & Lomb http://www.bausch.com
Kinetic Books http://kineticbooks.com
Lasik Eye Surgery http://www.lasikeyesurgery.com
By: Eva McElligott, Lila Farrag
& Teri Dombrowski