Refractive Surgery - LASIK - Laser assisted in situ keratomileusis

LASIK — Laser assisted in situ keratomileusis

For people who are too nearsighted for excimer laser PRK

he FDA approved upper limit for treatment of nearsightedness in the United States with PRK is -7.00 diopters. Why is this? Well, remember that PRK works by removing the front layers of the cornea, thus flattening it. The more nearsighted an eye is, the more corneal tissue needs to be removed. There is some evidence that if one treats beyond -7.00, the cornea reacts by producing too much haze, or actually forms a scar. Personally, I feel that the upper limit of PRK should be -5.00D and no more.

LASIK was developed to get around this problem. It was discovered that if the surgeon can remove corneal tissue from the deeper corneal layers without disrupting the smooth corneal surface, the whole cornea will be thinner, and thus flatter, with essentially no scarring at all. The figure below demonstrates how this is accomplished. You can see that a flap of cornea is cut and folded back. The excimer laser beam is then applied to the bed of remaining corneal tissue to remove part of it. Then the flap is folded back with the result that the cornea is a little thinner and quite a bit flatter.

Figure 1. Laser Augmented In-Situ Keratomileusis

LASIK was initially used for those patients who were very nearsighted, but because it has some advantages over standard PRK, some surgeons are using it for patients with a correction as little as -1.00 diopters, which is akin to swatting flies with a cannon. See why later on.

LASIK's advantages over PRK are that it leaves no corneal abrasion like PRK. PRK patients take several days to heal their abrasion and have some discomfort from it. There is a degree of corneal haze in most cases, which takes some weeks to clear up. Also, PRK patients are on eye drops for several months. These are usually minor inconveniences, but still, they are there. LASIK patients are visually rehabilitated much earlier.

There are some disadvantages to LASIK, however, as contrasted with PRK (see LASIK — the future of refractive surgery?). LASIK requires the use of a “microkeratome” — a cutting instrument used to produce the corneal flap. Because of this there is a chance of surgical problems related to the flap. The cut can be too short such that the procedure must be cancelled and delayed several months. The cut can be too long such that the flap can be totally removed from the cornea, possibly requiring sutures, and there are a few reported cases where the flap was totally lost. The flap can become wrinkled or dislocated over several days after surgery. These problems can be solved, but it means more corneal surgery and certainly more than a bother for the patient. Also, the predictability of LASIK is somewhat less than for PRK. If you are very nearsighted, LASIK probably will not free you entirely from glasses, but you could expect only thin lenses afterwards.

If you are nearsighted to a degree more than -7.00 diopters, LASIK may be the best refractive surgical option for you. The risks are reasonable and the potential benefits are very great.