Word: corneas
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...eyes are surrounded by a tough, protective layer called the sclera. Only at the front of the eyeball does the sclera give way to the cornea, which is transparent. Light passes through the cornea to the pupil, the hole in the middle of the iris, or colored part of your eye. Depending on how bright the incoming light is, the pupil grows wider or narrower, much like the adjustable aperture of a camera. The light then passes through the lens, which lies directly behind the iris and changes shape as needed--curving or flattening--to help focus the image onto...
...happens, the eye's lens provides just a third of the eye's focusing power. The rest comes from the cornea, which acts like a second lens to help focus light onto the retina. If you're nearsighted, or myopic, your eye produces clear images of nearby objects or people. But light from distant sources is focused on a point somewhere in front of your retina--either because the curve of your cornea is too steep relative to the length of your eyeball, or the eyeball is too long relative to the corneal curve. If you're farsighted, or hyperopic...
Attempts to change the way the cornea focuses light by surgically altering its surface began as early as the 1950s. By the 1970s Soviet doctors routinely used scalpels to reshape the corneas of nearsighted patients in an operation called radial keratotomy. But the surgery, involving a spokelike ring of incisions, never really caught on in the U.S., because the results were so difficult to predict and the healing process was often slow and painful...
...surgeons first tried using the excimer laser to correct vision in a procedure called photorefractive keratectomy. They scaled off the cornea's outermost protective layer, or epithelium. Then they vaporized some of the underlying tissue with the laser, forcing the cornea to flatten or steepen, depending on the correction. Although the epithelium always grew back, the cornea retained its new shape. It was a big improvement over radial keratotomy, although the healing of the epithelium remained painful...
LASIK solved this problem. Using a delicate cutting instrument called a microkeratome, surgeons made a sideways slice through the cornea's outermost layers, leaving one side attached, and carefully lifted the flap of tissue out of the way. In nearsighted patients, an invisible beam of laser light then trimmed away layers of tissue from the center of the cornea, producing a flatter curve. In farsighted patients, the beam scooped out a doughnut-shaped ring that resulted in a steeper curve. Then the doctors lifted the flap back into place. After a few minutes of drying, it rebonded with the rest...