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LASIK Eye Surgery FAQs...

Surgical Vision Correction (Refractive Surgery with the Excimer Laser)
Peter S. Hersh, MD

Introduction

Laser vision correction (LASIK and PRK) is a relatively new procedure used to reduce or eliminate a person's dependence on glasses and contact lenses. The procedure is performed on an outpatient basis and is effective for treating nearsightedness, farsightedness, and astigmatism. To date, over two million procedures have been performed worldwide.

How Does the Laser Work?

The goal of excimer laser LASIK and PRK is to reshape the cornea so that the rays of light that enter the eye are focused clearly onto the retina. The laser produces an ultraviolet beam of light that meticulously removes tissue. Tissue is removed in a precise fashion on a microscopic level, without harming or heating adjacent corneal tissue. This process of tissue removal is called photoablation.

The excimer laser is unique amongst the many types of lasers used in eye surgery. There are three basic categories of medical lasersthermal (heats tissue), mechanical (cuts tissue), and photochemical (interacts directly with molecules). For example, argon lasers heat tissue and have been used for years to treat disorders such as glaucoma and diabetic eye disease. YAG lasers break tissue bonds by creating a shock wave and are generally used following cataract surgery and to treat certain types of glaucoma. The excimer laser is the only laser properly suited to the task of refractive corneal surgery since it does not heat or mechanically damage tissue. Rather, it directly interacts with chemical bonds, neither heating nor disrupting the tissue, thus minimizing possible scarring.

In addition, the precision of the excimer laser makes it uniquely suited to the task of refractive corneal surgery. Each pulse of the laser removes only 0.25 microns of tissue (1/28 of a red blood cell) in four-billionths of a second. This allows the surgeon to safely sculpt the cornea into a more optically desirable shape, ultimately allowing the rays of light to focus properly on the retina.

How the Excimer Laser Corrects Vision

To correct nearsightedness, the excimer laser removes a small amount of tissue from the center of the cornea to flatten the curvature of the cornea. In the case of farsightedness, tissue is removed from the periphery of the cornea to steepen its curvature. In the case of astigmatism, an elliptical, or football-shaped corneal surface shape, is made more spherical. In all cases, the laser reshapes the cornea's front surface to change its optical characteristics to focus light better.

In actually applying the laser to the eye, the corneal stroma (the tissue beneath the corneal epithelium) must be exposed. This can be accomplished by directly removing the surface corneal epithelial cells (photorefractive keratectomy or PRK) or by creating a corneal flap with a microkeratome (laser in situ keratomileusis or LASIK).

Correction of Nearsightedness, Farsightedness, and Astigmatism

Patients with nearsightedness have corneas that are too steep for the length of their eye. The excimer laser is used to flatten the cornea so that the light rays that pass through it come to a point of focus on the retina, rather than in front of it.

Patients with farsightedness have corneas that are too flat for the length of their eye. The excimer laser is used to steepen the cornea so that the light rays are focused on the retina, rather than behind it.

In astigmatism, the cornea is elliptical (or football shaped) rather than being round. In this case, the laser reshapes the front surface of the cornea into a rounder shape, again improving the eye's focus.

How Much Tissue Should be Removed?

The amount of tissue removed in each of these procedures is determined by the patient's degree of refractive error. Before the laser is used, the degree of refractive error is translated into numbers that are entered into the laser's computer. The quantity and pattern of tissue removal unique to each patient are then calculated. Both PRK and LASIK are refractive procedures that utilize the precision of the excimer laser to reshape the optical surface of the eye.

Summary

Which procedure you should have is best determined by consulting with your surgeon. Although the surgeon cannot promise 20/20 vision without correction, approximately 95 percent of typical myopic patients achieve vision within two or three lines of 20/20 without correction. In fact, many patients can drive without glasses the day after their LASIK surgery. Future articles will deal with some of the complications and side effects, which are possible with these procedures.

(c) 2001 Healthology, Inc.

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