R U Ready To Dump Your Glasses?

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As it 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, on the other hand, the focal point for distant objects is fine, while the one for close sources actually falls behind the retina. In this case, the cornea is too flat relative to the length, or vice versa. Astigmatism occurs because the cornea's curvature is not uniform, making both distant and nearby objects blurry.

Age adds another complication. Most people, as they get older, need reading glasses or bifocals for close work. This condition, called presbyopia, is different from farsightedness because it has nothing to do with the shape of the eye; it happens when the lenses in the eyes lose their ability to curve sufficiently to focus on nearby objects.

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.

Enter the excimer laser. Originally developed in the 1970s for the precise etching of computer chips, it is a so-called cool laser, meaning that it can cut through almost any material without generating a lot of heat damage. That's just the kind of exacting low-impact tool surgeons needed to rework the delicate tissues of the eye. So a company called Summit Technology, of Waltham, Mass., dedicated itself to figuring out how to adapt the excimer laser to eye surgery. Today, Summit and another company, Visx, of Santa Clara, Calif., dominate the eye-laser industry in the U.S.

Eye 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.