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What is PRK?
The second form of vision correction to come into use is the procedure known
as Photorefractive Keratectomy or PRK. Although the approach is similar to
RK, in
that the cornea is modified to correct vision, the process is vastly different
with remarkable improvements in patient risk and correction capabilities.
Rather than making cuts in the cornea, the PRK process uses an excimer laser
to sculpt an area 5 to 9 millimeters in diameter on the surface of the eye.
As
you can see from the diagram, this process removes only 5-10% of the thickness
of the cornea for mild to moderate myopia and up to 30% for extreme myopia -
about the thickness of 1 to 3 human hairs. The major benefit of this procedure
is that the integrity and the strength of the corneal dome is retained. The
excimer laser is set at a wavelength of 193nm, which can remove a microscopic
corneal cell layer without damaging any adjoining cells. This allows the
practitioner to make extremely accurate and specific modifications to the cornea
with little trauma to the eye.
This ability to sculpt, rather than cut, opens up the arena for treating
additional vision conditions. At this stage, there are excimer laser machines
that with a combination of masks and computer controls, can reliably treat
myopia, hyperopia, and now astigmatism.
PRK- Predictability and Safety
Although PRK sculpts only a tiny amount of tissue from the cornea, it is a
surgical procedure and thus the outcome cannot be guaranteed. Any surgical
procedure should be undertaken only after careful consideration of the
likelihood of success and consequences of any possible risks or side effects.
Thorough professional advice from a qualified eye surgeon or surgeons is
required before any eye treatment is undertaken. Predictability can be defined
in several ways- we favor a percentage approach to achievement of visual
goals, with 20/20 uncorrected vision being ideal and 20/40 uncorrected vision
being okay or acceptable. Uncorrected vision of 20/40 still allows driving
without glasses. Most PRK facilities and machines report that 65-70% of
patients with correction up to -6.00 diopters can expect 20/20 uncorrected
vision post operatively. The percentage with 20/40 uncorrected acuity is
90-95%. Corrections less than -6.00 diopters will have better odds and
corrections greater than -6.00 will have lower odds. The safety of the
procedure is judged on the basis of the chance of a possible complication.
Serious complications are extremely rare. Infection is the most worrisome
complication and fortunately it can usually be eliminated with antibiotic
medications. Other possible problems include delayed surface healing, corneal
haze and or scarring, over or under correction, and the development of
astigmatism. Some individuals can have a poor or excessive healing response.
Again most complications remain treatable with medications or further surgery.
It is also important to separate the normally expected side effects of surgery
and healing from real complications. Immediately after surgery some people
have discomfort, although the use of bandage contact lenses and medications
usually control this nicely. Light sensitivity is almost universal and halos
and other unusual light effects can occur. Vision can be reduced while healing
and from the normally planned over correction. You must discuss and fully understand all of these
possible side effects and problems prior to surgery. Hopefully, the
information here will assist you in that process.
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