Surgical Vision Correction (Refractive Surgery with the Excimer Laser)

Introduction
How Does the Laser Work?
How the Excimer Laser Corrects Vision
Laser In Situ Keratomileusis (LASIK)
Photorefractive Keratectomy (PRK)
Correction of Nearsightedness, Farsightedness, and Astigmatism
How Much Tissue Should be Removed?
Differences Between LASIK and PRK
Summary
 

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 lasers— thermal (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).
 

Laser In Situ Keratomileusis (LASIK)

The LASIK procedure also uses the excimer laser to reshape the cornea, but this is done under a thin corneal flap, which preservers the surface epithelial cells. Rather than scraping away the epithelial cells to expose the corneal stroma as in PRK, a specialized surgical instrument known as a microkeratome (which works somewhat like a carpenter’s plane) creates a flap of corneal tissue that is attached by a hinge. This flap is gently pulled back like a clear, hinged pancake and the corneal stroma is exposed. The laser part of the LASIK procedure takes place in the exposed corneal bed (corneal stroma). The laser application itself lasts about 30 to 90 seconds.

After the exposed corneal bed is treated by the laser and minute amounts of cells are vaporized, the flap is replaced in its original position. The flap is held in position by the eye’s natural suction facility and natural sticky sugars, and provides increased comfort and decreased recovery time for the patient.
 

Photorefractive Keratectomy (PRK)

This is a procedure in which the front surface of the cornea is directly sculpted by the excimer laser. The surgeon prepares the eye by gently removing the surface layer known as the corneal epithelium. This layer regenerates itself within a few days. As in the LASIK procedure, computer-controlled pulses are directed at the exposed surface (the corneal stroma) to reshape the cornea. Less than ten percent of the cornea is affected, with the deeper layers remaining untouched.
 

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.
 

Differences Between LASIK and PRK

Refractive vision correction performed under a flap (LASIK) offers numerous advantages over refractive vision correction performed on the cornea’s surface (PRK). Notably, there is a more rapid improvement in vision and decreased discomfort with LASIK since the surface epithelial cells have been preserved and do not need to regrow. This is undoubtedly the reason that LASIK has become the corrective surgery of choice for both doctors and patients. However, in some patients, PRK is a safer alternative that may promise better outcomes. Patients who might benefit from PRK include those in whom the cornea might too thin for LASIK, or in some patients with corneal irregularities or scars.

A brief comparison of PRK and LASIK is outlined in the following chart.
 

Item PRKLASIK
Initial examination Similar to LASIKSimilar to PRK
Procedural differenceSurface epithelium removedCorneal flap made with microkeratome
Excimer laser procedureSimilar to LASIKSimilar to PRK
Postoperative discomfort VariableUsually minimal
Visual recoveryStarts improving at three daysStarts improving at one day
Possible side effectsGlare/halo/ghosting
Corneal haze
Glare/halo/ghosting
Corneal flap problems

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, the 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.
 

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