Surgical Vision Correction

How does the eye function?
How does the eye surgery correct your vision?

Until about 25 years ago, most defects in vision were corrected with either eyeglasses or contact lenses. Then, in 1973, a Russian ophthalmologist developed the first successful method for surgically correcting nearsightedness. Called radial keratotomy (RK), the procedure involved making several microscopic incisions in a radial pattern around the edge of the cornea. As the incisions healed, the cornea flattened, correcting for nearsightedness. For nearly 20 years, the RK procedure was the only acceptable surgical procedure for the correction of vision defects, and it was limited to the correction of myopia (nearsightedness). However, with the advent of several new and vastly improved techniques, the surgical correction of vision problems is now a viable option for those who want to lessen their dependence on eyeglasses or contact lenses. It is estimated that more than 1½ million people in the United States will have some form of vision correction surgery in the year 2000.

How does the eye function?

To understand how modern refractive surgery works, it is first necessary to understand the visual function of the eye. The eye works like a camera with two lenses. The first lens is the cornea, a clear membrane that covers the front of the eye. The second lens is the eye's natural crystalline lens, which is located behind the pupil. The cornea is responsible for about 70 percent of the eye's focusing power, while the natural lens "fine-tunes" the image before it is focused on the retina at the back of the eye. The retina, like the film in a camera, receives light images and sends them through the optic nerve to the brain. When both lenses are working properly, the image is focused precisely on the surface of the retina, and you have perfect "20/20" vision.

Each person has eyes that are unique in form and proportion. A "perfect" eye has an evenly rounded cornea that allows light to fall exactly on the retina, resulting in perfect vision. If the cornea is too steep or if the eye is too long from front to back, light rays are focused in front of the retina, resulting in myopia (nearsightedness). If the cornea is too flat or the eye is too short from front to back, light rays are theoretically focused behind the retina, resulting in hyperopia (farsightedness). If the front of the cornea is unevenly curved, it causes the light rays to fall on different spots on the retina, resulting in astigmatism.

How does the eye surgery correct your vision?

Popular surgical vision correction procedures reshape the cornea in order to correct imperfections. Nearsightedness is corrected by flattening the cornea, farsightedness is corrected by steepening the cornea, and astigmatism is corrected by giving the cornea a more even curvature. Some new refractive surgery procedures use plastic lenses that are implanted to replace or supplement the eye's natural lens, rather than reshape the cornea.

LASIK
The most popular of the modern vision correction procedures is called Laser In-Situ Keratomileusis (LASIK). In this procedure, a thin flap in the central area of the cornea is raised, and an Excimer laser is used to sculpt the underlying corneal tissue. The flap is then replaced, and the eye heals. LASIK is successful in correcting many cases of nearsightedness, farsightedness, and astigmatism. Vision recovery is rapid, and there is little postoperative discomfort.

PRK
Photorefractive Keratectomy (PRK) was the immediate predecessor of LASIK and is still performed by some surgeons. As in LASIK, the surgeon uses an Excimer laser to sculpt corneal tissue. However, unlike LASIK, no flap of tissue is lifted. Instead, the protective outer layer of the cornea, called the epithelium, is removed before the laser is applied. This procedure is also very successful, but it can be accompanied by more postoperative discomfort.

ICRS
Another procedure that reshapes the cornea, but does not use a laser, involves the insertion of Intrastromal Corneal Ring Segments (ICRS -- trade name Intacs™). Two tiny plastic arcs are surgically inserted in the peripheral area of the cornea. This serves to flatten the cornea to the degree required to correct the myopic condition. The segments can be removed later or replaced, if desired.

LTK
Laser Thermal Keratoplasty (LTK) is a new procedure that uses a Holmium laser to shrink the peripheral area of the cornea. This procedure makes the shape of the cornea steeper and corrects mild to moderate cases of farsightedness. It is also being used to a limited degree to treat some cases of astigmatism.

Implantable Lenses
Several refractive eye surgery procedures now replace or supplement the eye's natural lens with an implantable contact lens (ICL). A small prescription lens, resembling a contact lens, is surgically inserted between the iris and the natural lens of the eye. Intracorneal lens implants are similar to the ICL except the lens is implanted in the corneal tissue. In a procedure called clear lens extraction (CLE), the natural lens of the eye is removed, just as in cataract surgery, and replaced with a plastic prescription lens.