|Photo-Refractive Keratectomy (PRK): Basic Information|
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What is Refractive Surgery?
Refractive surgery refers to a family of new surgical procedures designed to produce better eye focus with less dependence upon glasses or contact lenses for near-sighted, far-sighted, and astigmatic individuals. This field is one of the most exciting and intensively studied within modern ophthalmology, offering patients the realistic prospect of safe, effective and significant visual improvement.
Historically, ophthalmologists have long performed refractive surgery. Procedures which have an influence on either of the eye's natural focusing elements--the cornea or the internal crystalline lens, will have an impact on how well the eye can focus. Cataract surgery and corneal transplantation are therefore also members of the family of refractive surgical procedures performed routinely by ophthalmologists every day.
What is an excimer laser?
The excimer laser is a system that produces an intense coherent beam of ultraviolet light through the electrical stimulation of a gas mixture containing Argon and Fluorine. Pulses of high voltage electricity cause atoms of Argon and Fluorine to bind ( Excited Dimer= Excimer). When the newly formed molecules of argon fluoride break apart they release packets of light energy with a wavelength of 193 nanometers which go on to stimulate other argon fluoride molecules to break up and release more light energy. This light energy exits the system in a continuous beam and is delivered to the eye in a series of pulses.
When this intense beam of ultraviolet laser energy is directed upon the cornea a precise tissue ablation of tissue is produced at a rate of 0.25 microns per pulse. This photo-ablation occurs through the actual breakdown of molecular bonds in a precise controlled manner which produces layer by layer removal of material with minimal damage to the adjacent corneal tissue. Thus, using the excimer laser, the refractive surgeon is able to reshape the cornea by removing small amounts of tissue with each laser pulse, reducing myopia, hyperopia, and astigmatism. This system of precision tissue removal represents a new and distinct manner of using laser energy as a tool which will be used for multiple medical applications in the future.
What is the expected outcome - the prognosis - of PRK surgery?
Clinical trials have indicated that PRK is a safe and highly effective treatment for mild to high ( -1.00 to -12.00) myopia, hyperopia (+1.00 to +6.00, and astigmatism (1.00 to 4.00). It should be noted that the laser has the capacity to correct higher amounts of myopia and astigmatism with the use of modified software programming of the system. The FDA, however, has given approval for PRK treatment only in the ranges listed pending further evaluation. Long term risks of PRK beyond three years in this country have not been studied, although long term evaluation of excimer treated patients in Europe and Canada have provided no indication of unexpected side effects.
FDA studies using a 6.00 mm zone of photoablation have shown that of 341 studied eyes at 6 months following surgery 95% were corrected to 20/40 or better without glasses or contact lenses and 66% to 20/20 or better without glasses or contact lenses.
Who is a good candidate for Excimer PRK refractive surgery?
Under current FDA guidlines candidates for this type of refractive surgery should be 21 years of age or older with a stable refraction of myopia (-1.00 to -12.00), hyperopia (+1.00 to +6.00), or astigmatism (1.00 to 4.00). FDA guidelines allow treatment of the combination of myopic astigmatism, but not hyperopic astigmatism, although we anticipate approval for this early in the year 2000.
PRK should not be performed upon people with medical history of auto immune disease (such as lupus erythematosis) as it has been shown that these patients may heal poorly following treatment and have an uncontrolled reaction to the surgery. It is not recommended that this procedure be performed upon pregnant women who often have unstable myopia nor should it be offered to individuals with a history of corneal dystrophy ( inherited degenerations of the cornea) or past viral infection by herpes simplex or herpes zoster( shingles).
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