What is Refractive Surgery?
Clear vision is the result of light rays passing through the cornea, pupil and lens and
focusing directly upon the retina. If the cornea is not round or is too steep or too flat in
relation to the length of the eye, light rays focus either in front of or behind the retina,
resulting in "refractive errors" such as nearsightedness, farsightedness and astigmatism.
Refractive Surgery is a term for several procedures designed to treat these vision
abnormalities by altering the way your eye focuses light by changing the shape of your
cornea
Function Of The Eye
In a normally structured eye, the cornea is a rounded curve. Light rays pass through the
cornea and the pupil to the lens, which further "refracts" or focuses the light directly onto
the retina creating a sharp and clear image.
Myopia
Myopia is the medical term for nearsightedness. This is a condition in which light rays
entering the eye are focused in front of the retina instead of directly on it as in the normal
eye. Myopia is an
inherited condition which usually begins in childhood and stabilizes inthe late teens or early twenties. This term implies that you have
trouble seeing in the distance without correction but have no
trouble seeing objects that are near in your visual field.
A myopic eye has too much focusing power. The extra focusing
power is a result of an eyeball that is too long or a cornea that is
too steep. The result is that the image we see falls short of the retina and is therefore
improperly transmitted to the brain for processing.
Hyperopia
Hyperopia is the medical term for farsightedness. This is a condition in which light rays
entering the eye are focused behind the retina instead of directly on it as in the normal
eye
. Hyperopia is an inherited condition which is present in childhood but does notbecome apparent to most people until their 30's, when their eye
muscles no longer can overcome it. This term implies that near
objects are blurred and those in the distance are clear.
A hyperopic eye is one with too little focusing power. This results
from an eyeball that is too short or a cornea that lacks curvature -
is too flat. The result is that the image being focused wants to focus beyond the retina and
is therefore improperly transmitted to the brain for processing.
Astigmatism
When an eye is astigmatic it lacks a uniform surface and is thus unable to have one focal
point. This asymmetry is due to the cornea being more curved or steeper at one axis thanall the others. The astigmatic eye is an irregular curvature of the
eye resulting in more than one focal point, preventing a sharp
point focus on the retina. The concept is clearer as one considers
a normal eye to be shaped like a basketball, while an astigmatic
eye is shaped more like a football.
Astigmatism is usually found in combination with myopia and
hyperopia such that objects may appear blurry in the distance as well as up close.
Presbyopia
Presbyopia is a natural part of the aging process whereby special muscles within the eye
weaken and the lens hardens. The result is that the accommodation process (the ability of
the eye to alter the shape of the lens thereby intensifying the focusing effect) is inadequate
for those objects that are near. Between the ages of 40 and 50 nearsighted individuals
move into bifocals and most people who did not need corrective lenses prior to this age
move into reading glasses.
There is a technique that allows for the fitting of contact lenses to compensate for bifocals.
This unique procedure is known as monovision and it serves to provide you with one eye
that is dominant for distance vision and one eye that is dominant for near vision. The brain
adapts to this setup and many patients have chosen to be free of bifocals and glasses by
opting for this procedure. Your candidacy for monovision can be discussed with your
Ophthalmologist or contact lens specialist.
Diopters
Diopters are the unit of measurement for refractive disorders in the eye. They represent the
amount of correction (your prescription) needed in corrective lenses to normalize your
vision. The higher the number of diopters in your prescription, the more nearsighted or
farsighted you are.
What is an Excimer Laser?
Until now, in the U.S., refractive surgery to correct nearsightedness was limited to RK
(Radial Keratotomy), but with the introduction of the Excimer laser a new technological
era in refractive surgery has begun. Although just recently approved in the U.S., the
Excimer laser has been widely used around the world with over 500,000 procedures
performed. It is the most advanced technology available to reduce dependency on glasses
and contact lenses.
Excimer laser correction of nearsightedness is a precise method of reshaping the cornea to
correct Nearsightedness. During the treatment, the computerized laser removes thin layers
of tissue from the cornea. As a result, the cornea flattens to the desired correction so that
the eye can focus properly. The goal is to eliminate or reduce a person's reliance on
contact lenses and glasses.
The excimer laser was developed at IBM in 1976. Due to its extreme precision, this type of
laser was developed to etch computer microchips. Several years later researchers began
investigating its use in medicine and surgery. The excimer laser was coupled with a
computer and now allows the refractive surgeon to reshape the cornea thereby altering
the refractive focal length of the eye. The process of sculpting the cornea is called
photorefractive keratectomy, or PRK.
How does the Excimer Laser work?
The excimer laser uses controlled pulses of light to remove only the very outermost layer
of the cornea. The laser’s computer determines the location, number of pulses, and
surface area that is to be removed. This changes the curvature of cornea and the location
where the image is focused. Now light rays entering the eye are focused on the retina
resulting in improved vision.
When the "cool" laser beam touches the cornea, the intermolecular bonds are broken and
the molecules are dispersed into the air. This leaves a clear and smooth underlying
corneal surface. In order to induce a refractive change to correct myopia, more tissue
must be removed from the thicker center of the cornea than the thinner periphery. Since
there is no cutting or burning, there is absolutely no damage done to surrounding tissue;
thus the excimer laser is an ideal technology for reshaping the cornea. While the Excimer
laser surgically alters only the cornea, it can also compensate for presbyopia. The
ophthalmologist who performs the screening can provide a full explanation.
What is an Excimer Laser procedure?
The Excimer laser is used primarily for two types of refractive procedures, Photorefractive
Keratectomy (PRK) and Laser Assisted in-Situ Keratomileusis (LASIK).
What is PRK?
PRK is a laser vision correction procedure that uses pulses from an excimer laser to gently
reshape the outside surface cells, which is less than the width of a human hair. On VISX
Laser Systems (an innovator of ophthalmic excimer lasers), control of the laser beam is
achieved by an aperture system that progressively enlarges the diameter of the laser beam
in several steps until it reaches its maximum diameter of approximately 6mm. A slit
aperture operating in connection with the circular aperture provides the capability to treat
astigmatism. This reshaping of the cornea allows the light rays entering the eye to be
focused on the retina, resulting in clearer vision. Since the layer of cornea removed is so
thin the cornea is not weakened. Usually one eye is done at a time, as usable vision may
not return to normal for several days to several weeks, however occasionally both eyes
are done at the same time.
What is LASIK?
The newest, most advanced Excimer laser procedure is LASIK (Laser Assisted in-Situ
Keratomileusis) and is the procedure of choice among refractive surgeons worldwide.
LASIK combines the use of an excimer laser with a procedure known as ALK to treat
mild to extreme nearsightedness with incredible accuracy and precision. LASIK may also
be used to correct farsightedness.
The LASIK procedure begins by the ophthalmologist lifting a very thin, outer layer of the
cornea (thinner than a contact lens) and gently folding it back. This creates a "protective
flap" that stays connected on one side (Step 1). Then, in seconds, the Excimer laser
reshapes the underlying corneal tissue (Step 2). The protective flap is then replaced to its
original position and naturally bonds requiring no stitches (Step 3). Just as in PRK, this
reshaping of the cornea allow light rays to bend or refract onto the retina, allowing for
clearer vision. LASIK has proven to provide patients with severe myopia a greater chance
for successful results. Both eyes are typically done at the same time as visual recovery is
quite rapid.
What is the difference between PRK and LASIK?
Both PRK and LASIK use the same excimer laser. The difference is in the surface
preparation of the eye. In PRK the surface tissue is removed, in LASIK it is undercut to
create a protective cap.
Pain (i.e. scratchiness)
Average pain resulting from any refractive surgery
Minimal
Treatment of Astigmatism
Yes
Yes
Number of surgical visits
One per eye - usually
Both at the same visit
Time of Recovery
Seven days per eye
24 hours
Medication Use (drops)
Varies
5 days
Time until visual stabilization
By one year
By three months
Corneal Scarring
Yes (common if sphere over -6.00)
No
Destruction of corneal layer
Yes
No
Surface Preparation
Tissue removed
Tissue undercut
What other options besides PRK and LASIK are there for correcting nearsightedness?
Glasses, contact lenses or radial keratotomy (RK) are other options available to correct
nearsightedness. However, for people with active lifestyles or physically demanding jobs,
PRK and LASIK provide freedom and convenience far superior to that offered with either
glasses or contact lenses.
What is Radial Keratotomy?
Radial Keratotomy (RK) is a surgical technique for treating mild to moderate levels of
nearsightedness that involves the ophthalmologist making a pattern of freehand spoke-
like micro-incisions on the cornea with a diamond-edged knife. Variations in the pattern
of the micro-incisions can be used to treat astigmatism. RK has been refined and
improved over the past 20 years in the U.S., but it is not as precise as the laser
procedures. There is approximately a 30-35% chance of needing an enhancement
or touch-up procedure to achieve desirable results. Visual recovery is less rapid than
with the LASIK procedure, but more rapid than PRK. There is less post-operative
discomfort than PRK, but more discomfort than LASIK. Radial Karatotomy however
is rarely being done nowadays since the introduction of LASIK.
What determines whether a person is a candidate or not?
Whether you are a candidate or not depends upon a number of factors such as how
nearsighted you are, how stable your vision is, whether you have any disqualifying
conditions, and what your expectations are. A nearsighted candidate must meet certain
visual and medical criteria. For example, people who wear glasses only for reading are
not good candidates. In the addition to the visual and medical requirements, the best
candidates tend to be people who are dissatisfied with their contact lenses or glasses
and are motivated to make a change, whether it's due to occupational or lifestyle reasons.
However, only an eye doctor who is specially trained can evaluate whether a patient is
medically suitable for treatment utilizing the excimer laser. A thorough ophthalmological
eye exam must be performed to determine if you qualify for surgery. This exam requires
that you remove hard contacts at least three weeks prior to surgical evaluation and soft
lenses at least one week before.
For qualified candidates, PRK can result in improved vision. Though the success of PRK
depends mainly on your eyes, other important considerations include that you:
- Be at least 18 years of age for treatment of mild nearsightedness or 21 years of age
for treatment of high nearsightedness or astigmatism. - Have healthy eyes that are free from disease or corneal abnormality (i.e. scar,
infection, etc.) - Have nearsightedness (myopia) up to -12.0 diopters and/or between 0.75 and 4.0
diopters of astigmatism - Have a stable vision in which your refraction hasn’t changed by more than 0.5
diopters during the preceding year
Will I ever become nearsighted again?
The procedure is a permanent correction, once the cornea is reshaped tissue will not
regenerate. Studies have shown there is little if any regression once healing is completed,
three to six months after the procedure. A prerequisite to be considered a good candidate
is a stable prescription, i.e. less than one diopter change over the course of a year. If you
undergo the procedure when your prescription is unstable you may regress from your
desired PRK result.
[Laser Vision Correction] [Frequently Asked Questions] [World Wide Web Links]