Word: corneas
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Dates: during 1990-1999
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...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...
Nearly everyone who undergoes LASIK experiences at least some glare and halos, usually at night or under fluorescent lights. This occurs because the pupil widens in dim light, allowing incoming light to pass through both the corrected and uncorrected sections of the cornea, creating either a blinding or a hazy image. The problems usually diminish within six months. The best guess is that 5% of patients continue to be substantially bothered by glare and halos over the long term...
...dark often have the biggest problems. An equally critical factor, however, is the amount of correction you need, measured in negative (-) diopters for nearsightedness and positive (+) diopters for farsightedness. The greater your correction, the more abrupt the transition zone between the sculpted and unsculpted portion of the cornea, and the greater the risk of troublesome glare and halos...
Another big advance in the works could provide nearly all patients with better than 20/20 vision. Today's lasers, and the computer programs that run them, assume all corneas are more or less spherical. Scientists are developing instruments that will map the entire surface of the cornea and make point-by-point alterations to smooth out individual aberrations. Such carefully customized reshaping of the cornea could make astounding improvements in vision more of a sure thing...