The
most common impairment of vision is caused by errors
in refraction (or focus). Refraction occurs when light
enters the eye through the cornea and lens. Refractive
errors result when light rays do not
come to a focus on the retina, causing the conditions
of myopia (nearsightedness), hyperopia (farsightedness),
or astigmatism.
There are three available methods to help correct refractive
errors. The first two use lenses to redirect the incoming
light to focus and include glasses and contact lenses.
The third is an excimer
laser refractive procedure such as PRK
or LASIK
which can reshape the cornea to better focus incoming
light.
Photorefractive keratectomy (PRK) uses the excimer
laser to correct nearsightedness or astigmatism by removing
a small amount of tissue from the corneal surface. This
reshapes the corneal surface, allowing it to better
focus the image on the retina, with the goal of reducing
a patient's reliance on eyeglasses or contact lenses.
With laser in situ keratomileusis (LASIK), the laser
treatment is performed under a thin "flap" of the cornea.
This differs from PRK in which the laser treatment is
performed on the surface of the cornea after the surface
epithelial cells are removed. The laser treatment procedure,
itself, is the same as in PRK. Similarly, the goal of
the treatment is to reduce a patient's reliance on glasses
and contact lenses.
The phototherapeutic keratectomy procedure (PTK)
uses the excimer laser to treat patients with corneal
disorders, such as scarring, dystrophies, degenerations,
and surface irregularities.
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