"I'm getting my eyes lasered" is a remark becoming almost as common as "I'm getting my hair done," and it's often uttered with the same mixture of excitement and nonchalance. Millions of Americans are undergoing the laser in the hope of kissing their glasses goodbye — and giving themselves a more youthful appearance to boot.

The popularity of LASIK, short for Laser-Assisted In-Situ Keratomileusis, is no mystery. Promising almost instant, nearly painless vision correction for a few thousand dollars, it's nearly irresistible. And to those for whom the technique has been successful, the results seem nothing short of miraculous. But before exposing your only set of eyes to the procedure, know that Lasik isn't foolproof, and isn't for everyone.

Even Ken Moadel, a Manhattan ophthalmologist, will say as much, and he loves LASIK. He'd better; he's performed more than 7,000 LASIK operations since the technique was approved by the FDA in 1995, and he anticipates doing thousands more this year alone. "The wonderful part about this procedure is you're taking a lifelong ailment — having spent 20 or 30 years waking up and putting on a pair of glasses every morning — and then with a five-minute procedure, and results within 12 to 24 hours, you cure them of that ailment."

Who's a Candidate for LASIK?
"You should be at least 18 years of age, but there really is no upper age limit, as long as the eye is otherwise healthy," says Moadel. "There are certain conditions — types of eye diseases or eye shapes — which would exclude a patient as a candidate."

Limitations on who can undergo LASIK successfully stem from the fact that the technique does one thing, and does it well, unless there are confounding conditions in the eye before or after surgery. To understand how LASIK works, a brief refresher on eye anatomy will help.

We've all seen the textbook line diagrams illustrating how light beams entering the eye from different angles meet at a single point, creating a focused image. If that focal point happens to be right on the retina, a sort of projection screen in the back of the eye, the image is clear. If, however, it falls short when looking at faraway objects (a condition called myopia or "shortsightedness"), the image projected onto the retina will be blurry. If the focal point is somewhere beyond the retina when viewing nearby objects (a condition called hyperopia or farsightedness), the effect is also a blur.

Bending or refracting incoming light so the focal point is right where you want it is accomplished in a camera by the lens. The human eye has a lens that does the same thing, but incoming light is also bent (before it ever hits that lens) by the cornea, a dome-shaped outside surface of the eyeball covering the pupil. In fact, the lens provides less than half the eye's focusing ability; the cornea does the bulk of the work.

Made mostly of collagen and water, a healthy cornea is circular and perfectly clear. However, one that protrudes too far will result in a too-short focal length, or myopia. If the cornea is too flat, the opposite condition of farsightedness results. And if the cornea's shape or surface is uneven, called astigmatism, incoming light will also be distorted.

Improving Nature
Contact lenses work by sitting atop the cornea and compensating for its flaws to achieve focus on the retina. Lasik permanently sculpts the cornea into the proper shape. "We use a machine to lift a thin membrane, or flap, which is a tenth-of-a-millimeter thick," Moadel explains. "We then just lift the flap and use the laser to reshape the surface underneath by a few thousandths of a millimeter. Then we simply put this flap back, moisten it, and it seals in place by itself, within a 12 to 24 hour period."

During office visits prior to the day of surgery, Moadel makes precise measurements of a patient's eyes, and feeds that information into a computer to determine how much corneal material to remove. "We actually measure patients two to three times for consistency," he says.

Once a flap in one eye has been peeled back, Moadel is ready to use the excimer laser, which destroys corneal material with ultraviolet light. "The laser is not burning, it's not heat," Moadel explains. "It's basically vaporizing. Every pulse of that laser removes a quarter-micron of cornea." In cases of myopia, microthin layers of cornea are eliminated to flatten its shape.

For hyperopia, a doughnut-shaped hole is made to create a more conical shape. On average, the laser procedure itself takes less than two minutes per eye. When it's done, patients spend a half-hour resting in a darkened lounge, then following one more examination, they're free to go home.

Know the Limits of LASIK
For most patients, LASIK is a vast improvement over previously available corneal surgery techniques in terms of pain, recovery time, and results. Overall, LASIK also has impressive rates of safety and effectiveness. In a 1999 study of LASIK performed on 1,013 eyes, 92 percent of patients gained 20/40 vision or better, and 47 percent were corrected to 20/20 or better. However, there are risks, limits to what Lasik can accomplish, and some important drawbacks: As with any invasive procedure, there is always a possibility of infection. According to the American Academy of Ophthalmology, complication rates vary from 0 to 5 percent in clinical studies, and 5 to 15 percent of patients return to their surgeon for enhancements to improve results.

• Short-term side effects following surgery can include pain, light sensitivity, and corneal swelling. In some patients these can last for several weeks. One of the most common complications of LASIK is wrinkling of the flap, causing distorted vision. It can usually be repositioned or treated with drugs, though in rare cases the flap may have to be re-cut.
• The majority of LASIK patients also report seeing glare or halos at night. This usually occurs in low light because the pupils widen beyond the area of the cornea corrected by the surgery. Most, though not all, find the problem subsides within six months, but the British Aviation Authority has discouraged pilots from undergoing LASIK for fear of permanently damaging their night vision. The U.S. military is still reviewing the long-term effects of Lasik.

• Those who require the greatest vision correction, measured in diopters, are most likely to have ongoing problems with glare and halos because of the steep drop-off between the altered and unaltered areas of the cornea. For that reason, people with myopia exceeding minus-12 diopters, or farsightedness of more than plus-6 diopters, are often discouraged from undergoing Lasik. Or, they can opt for having one or more Lasik treatments that will improve their prescription, but don't attempt to achieve 20/20 vision.
• At present, surgeons' calculations of how much corneal material to remove are based on statistical averages of all patients with a given set of eye characteristics. They yield very good educated guesses, but can't take into account minor idiosyncrasies in individual eyes, and 5 to 10 percent of LASIK patients require a second procedure to refine their vision correction. However, a computer scanner that actually maps a patient's eyes to plan the corneal correction is in development, so a more precise, custom Lasik procedure could soon become the standard.
• Finally, LASIK can't prevent the inevitable. As we age, the lenses in our eyes lose elasticity, just like our skin, and their ability to focus declines, a condition called presbyopia. So, most patients over 40 are still likely to need reading glasses after LASIK, and even if a younger person achieves perfect vision, it won't last forever.

Choosing LASIK
A good doctor will offer all the above caveats to a patient seeking LASIK. "When patients come for an examination in our office," Moadel explains, "they're basically given a very thorough examination, however, the most important thing we do for our patients is educate them. That's critical to a patient, to understand what this procedure can do for them, the limitations of the procedure, and we determine whether or not they should have it done. About 20 percent of people that come to our office are not good candidates for this procedure."

LASIK cannot correct vision problems caused by defects in parts of the eye other than the cornea. Among the other conditions that would preclude LASIK are pregnancy, nursing, pre-existing eye diseases such as herpes, as well as diabetes or immune disorders that would hamper the healing process. Just as a qualified ophthalmologist will screen patients, you should screen doctors carefully if you're considering LASIK.

Find out what kind of training the doctor has had — a year-long fellowship is preferable to a weekend seminar. Ask how many of the procedures the doctor has performed. One study showed considerable drop-offs in surgeon errors after 600 operations, so the more experience your doctor has the better. And don't be afraid to ask how many of that doctor's patients have worse vision following the operation — for the best Lasik surgeons, the figure is 5 in 1,000 or fewer.