Chapter Four

Refractive Surgery with
the Excimer Laser

Laser vision correction is the most technologically advanced method available today for reducing dependence on glasses and contact lenses. The procedure is performed on an outpatient basis and is effective for treating nearsightedness, farsightedness, and astigmatism. The results are rapid and permanent. Over 1.7 million procedures have been performed throughout the world, and the number is growing daily.

How Does the Laser Work?

Many patients ask, "How can a laser correct my vision?" What the laser does is remove tissue from the center of the cornea (in the case of myopia) to flatten its curvature and correct nearsightedness; in the case of farsightedness, tissue is removed from the periphery of the cornea to steepen its curvature. The laser essentially reshapes the cornea's front surface. To do this, the corneal stroma (tissue beneath the corneal epithelium) must be exposed. This can be accomplished by directly removing the corneal epithelium with a laser (PRK) or by creating a corneal flap with a microkeratome (LASIK).

The Human Cornea. With PRK, treatment is performed on the surface after the epithelium has been removed. With LASIK, the treatment is performed in the stroma, and the anterior architecture is preserved.

Many types of lasers are used in eye surgery. Argon lasers heat tissue and have been used for years to treat disorders such as diabetic retinopathy and glaucoma. YAG lasers break tissue bonds by creating a shock wave and are used following cataract surgery and to treat certain types of glaucoma. The excimer laser is a gentle laser uniquely suited to the task of refractive corneal surgery.

The goal is to reshape the cornea so that rays of light that enter the eye are focused clearly onto the retina. The excimer laser produces a cool, ultraviolet beam of light (193 nanometers in length) that literally vaporizes tissue away as it breaks carbon-to-carbon bonds without harming adjacent tissue. Tissue is removed in a precise fashion on a microscopic level, leaving adjacent tissue unharmed. This vaporizing process is called photoablation.

The unparalleled precision of the excimer laser makes it uniquely suited to the task of refractive corneal surgery. Each pulse of the laser removes 0.25 microns of tissue. Think of it as slicing 1/200 of a human hair, 1/28 of a red blood cell, or 1/39 millionth of an inch in 4 billionths of a second. This allows the surgeon to literally sculpt the cornea into a more desirable shape, gently and precisely, and allows rays of light to focus properly on the retina.

Photo of human hair ablated

Diagram of myopic correction

Myopic Correction

As explained in the previous chapter, 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.

Hyperopic Correction

As explained in Chapter Two, patients with hyperopia have corneas that are too flat for the length of their eye. The excimer laser is used to steepen the cornea so that light rays are focused on the retina, rather than behind it.

Diagram of hyperopic correction

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. Computer-controlled pulses are directed at the exposed surface (corneal stroma) to reshape the cornea. Less than ten percent of the cornea is affected, with the deeper layers remaining untouched. The entire procedure takes approximately ten minutes per eye and is virtually pain free.

Laser In-Situ Keratomileusis (LASIK)

The LASIK process also uses the excimer laser to reshape the cornea, but this is done under a thin, protective, corneal flap. Rather than vaporizing the epithelial cells to expose the corneal stroma, a specialized instrument known as a microkeratome creates a flap of corneal tissue that is attached by a "hinge." This flap is gently pulled back like a tiny, clear, hinged lid and the corneal stroma is exposed. The laser part of the LASIK procedure takes place in the exposed corneal bed. The laser application itself lasts about thirty to ninety seconds.

After the exposed corneal stroma is treated by the laser and minute amounts of cells are vaporized, the flap is replaced in its original position. Amazingly, the flap is held in position by the eye's natural suction facility and provides increased comfort and decreased recovery time for the patient. The entire procedure takes approximately ten to fifteen minutes per eye and again is virtually pain free.

Diagram of LASIK

The amount of tissue removed in each of these procedures is determined by the patient's degree of refractive error. Before the laser is employed to vaporize the tissue, 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 surface of the eye by vaporizing corneal tissue.

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

As you can see in the table above, refractive vision correction performed in the interior of the cornea (LASIK) offers numerous advantages over refractive vision correction performed on the cornea's surface (PRK). This is undoubtedly the reason that LASIK has become the corrective surgery of choice for both doctors and patients. Which procedure you should have is best determined by consulting with your surgeon.

Although the surgeon cannot promise 20/20 vision without correction, historically ninety-five percent of typical myopic patients achieve vision within two or three lines of 20/20 without correction. In fact, the vast majority of patients can drive without glasses the day after their LASIK surgery. With current technology and more accurate LASIK nomograms, ninety-eight percent of patients see 20/20 or better, and the majority see some of the 20/15 letters on the Snellen chart. Patients are astounded with their new ability to see more clearly.

Because this book is dedicated to LASIK vision correction, nearly all of its chapters discuss the LASIK procedure and the important things you need to know concerning it. A brief explanation of the many other refractive procedures, past and present-- including PRK--can be found in Chapter Eleven.