Refractive Surgery And The Excimer Laser

Timothy L. Schneider, M.D.

Timothy L. Schneider, M.D. is with the
Department of Ophthalmology at Mayo Clinic Jacksonville.

There are estimated to be more than 70 million people in the United States who are nearsighted (myopic), and many others who are farsighted (hyperopic) or have astigmatism. For hundreds of years the only way to correct these refractive errors was eyeglasses. Contact lenses were introduced in the 1940's and their popularity has grown into a multimillion dollar industry. Radial keratotomy (RK) was introduced in Russia in 1973 and imported into the United States in the early 1980's as a way to correct nearsightedness and lessen ones dependence on glasses or contact lenses. RK involved making small (3-4 mm in length) incisions, deep into the cornea, (90% depth) in a radial fashion. This weakening of the peripheral cornea caused the central cornea to flatten, thereby reducing nearsightedness. While RK is still performed in the U.S., its popularity is decreasing.

One of the most exciting new technologies to occur in ophthalmology in recent years is the excimer laser. The excimer laser has the ability to reshape the anterior corneal surface thereby changing the focal length of the human eye. The process of "sculpting" the cornea is called photorefractive keratectomy or PRK. New technology combines PRK with an initial cut made with a micro-keratome to make a 160 micron-thick flap of cornea first, and under this flap the refractive ablation is made with the laser. This procedure is called Laser Assisted In-situ Keratomileusis or LASIK.

How The Excimer Works

The excimer laser generates ultraviolet light measuring 193 nm when high voltage energy is discharged into a cavity containing a mixture of a rare gas i.e., (argon, xenon or krypton) and a halogen (i.e., fluoride, chloride or bromide). This gas mixture is pre-ionized by a set of electrodes before a high-voltage current (about 30,000 electron volts [eV] is applied, resulting in formation of highly unstable rare gas-halide molecules, which rapidly dissociate, emitting UV light whose wavelength is determined by the particular gas mixture chosen. Excimer lasers today use a mixture of argon and fluoride gas. The term excimer is derived from the two words "excited" and "dimer" which are used to describe the reaction in which the laser transfers energy through an ultraviolet beam of light. The pulses of light with controlled and specific wavelength are the basis for the excimer laser's ability to sculpt the surface of the cornea.

The excimer laser was first developed in 1976 by IBM and has been used in industry for years. It was developed as a means of intricately etching a variety of materials including the patterns on computer chips. Trokel reported in 1983 that UV light at 193 nm ablated corneal tissue at a predicable rate and more importantly, that it produced minimal damage to the surrounding structures. The photons of far UV light emitted by the excimer laser are of sufficient energy to break intramolecular bonds and the material decomposes ablatively, hence the name photo ablation. No heat is produced in this process and therefore no shrinkage of collagen and damage to surrounding tissues. Each pulse of laser light removes a constant amount of corneal tissue between 0.21 and 0.27 microns per pulse. Most excimer lasers work at a pulse frequency of 6-10 Hz (pulses per second). Using this information, we know that it would take about 200 pulses to etch through a human hair which is 50 microns in diameter, and take 20 seconds to do so. (Figure 1.)

Figure 1. Photo of a human hair etched
by the excimer laser. (Photo courtesy of IBM.)

What Are Refractive Errors?

Myopia (nearsightedness) is a condition caused by an eye being too long in axial length from cornea to retina or the cornea being too steep (Figure 2B) Hyperopia (farsightedness) is when the eye is too short in axial length or the cornea is too flat. (Figure 2C) Astigmatism is when the cornea instead of being round or spherical for 360 degrees is longer in one meridian than the other, i.e. shaped like a football. Presbyopia occurs universally after the age of 40 when the lens loses its ability to accommodate as it once did and is corrected by reading glasses or bifocals.

At this time we have no means of shortening or lengthening the axial length of the eye, so refractive surgery changes the shape of the cornea, either flattening it in myopia, or steepening it in hyperopia.

Figure 2. Three drawings of normal (top), myopic (middle) and hyperopic eye (bottom)

2A. Normal
2B. Nearsightedness (myopia)
2C. Farsightedness (hyperopia)

Who Is A Candidate For Excimer Surgery?

In October of 1995 the FDA approved the use of the excimer laser for performing PRK in the United States. Currently we are limited to treat only myopia and myopic astigmatism up to 7.0 diopters (mild to moderated nearsightedness). Good candidates must be:

• 21 years of age or older
• no more than -7.0 diopters of myopia with or without astigmatism
• stable refraction for at least one year, i.e. no change in glasses rx for one year
• realistic expectations and goals for the surgery

Each patient must have realistic expectations of the surgery. "Perfect" uncorrected visual acuity cannot be promised. Recent PRK data submitted to the FDA found 94% of qualified patients achieved 20/40 visual acuity or better, which is good enough to drive a car without corrective lenses. 60-65% achieved 20/20 vision without correction and 4-10% required re-treatment to achieve the desired result. The goal of refractive surgery is to lessen ones dependence on eyeglasses or contact lenses. If 20/20 vision is not achieved, a thin pair of glasses may be necessary for night driving or to achieve absolute best visual acuity or to improve reading vision. If a patient's absolute goal is 20/20 uncorrected vision, they may not be classified as a "good candidate". If their goal is to perform most of his or her activities of daily living without glasses and use them only when absolute best vision is required, they will likely be happier with their results. Patient education and realistic expectations are key to a patient's satisfaction.

How Is The Surgery Performed?

Currently there are two methods for excimer laser vision correction: PRK and Laser Assisted In-situ Keratomileusis (LASIK).

PRK

PRK, while only approved in the U.S. since 1995 has been performed over 500,000 times in over 48 countries worldwide over the past 7 years. The purpose of the surgery is to flatten the central cornea thereby reducing nearsightedness. The laser does this by ablating a circular area of the central cornea 6-8 mm in diameter. The excimer removes only 50 microns of corneal tissue, about the thickness of one human hair or 5-10% of the corneal thickness. (Average corneal thickness is 500 microns). During the surgery the patient lies flat on a reclining chair. (Figure 3) Topical anesthetic drops are administered to numb the surgical eye. A lid speculum (separator) is used to keep the eye open during surgery. The surface epithelium is brushed away using a manual scraping technique or with the aid of the laser. Once the epithelium has been removed down to Bowman's layer of the cornea, the patient is asked to fixate on a red flashing fixation light under the head of the laser. Once the patient is looking at the fixation light and holding perfectly still, the laser is applied. A typical ablation lasts 30-60 seconds with most of the surgeons efforts making sure the patients' treatment is centered and coaching the patient to hold still by concentrating on the fixation light. When the procedure is completed, an antibiotic drop is placed on the eye and in some cases, a bandage contact lens until the surface epithelium has healed back again. This usually takes about 3-4 days. While the surface cells are healing, the patient may experience some discomfort and blurred vision.

Figure 3. Chiron Technolas 217 Clasik excimer laser.

LASIK

Laser Assisted In-situ Keratomileusis (LASIK) is a promising procedure which differs from PRK in that first a microkeratome "planes" a hinged flap of corneal tissue 8mm in diameter which is 160 microns thick. This flap is then lifted from the corneal stromal bed exposing the corneal stromal surface. The laser is then applied. After the ablation is completed, the corneal flap is repositioned and allowed to seal in place. Potential advantages of LASIK are that vision recovers more quickly and with less discomfort since the surface cells are left in place on the surface of the corneal flap. LASIK is still being evaluated by the FDA for safety and efficacy at clinical trial sites and is not approved.

Summary

Many innovations continue to be developed to correct refractive errors and improve the quality of ones vision and the quality of ones life. Just a few of the advances that are currently being investigated include:

• intraocular contact lenses for correction of high myopia;
• intracorneal lenses for the correction of presbyopia;
• intrastromal corneal rings for correction of myopia;
• water-jet technology for sculpting and cutting the cornea;
• solid state lasers and computer programs for the excimer which allow the correction of both hyperopia and presbyopia;
• thermokeratoplasty for the correction of hyperopia.

Refractive surgery has made great strides in the past 10 years. The results continue to improve as the technology advances.

February, 1998/ Jacksonville Medicine

What's New · Northeast Florida Medicine Journal · Know Your Physician · Legal & Legislative
· DCMS Alliance · Academy of Medicine · Member Websites · Community Health
About the DCMS · Meetings Calendar · Member Benefits · Employment Connection · Home

Duval County Medical Society · 555 Bishopgate Lane · Jacksonville, FL 32204
Phone: (904) 355-6561 · FAX: (904) 353-5848
General Email: dcms@dcmsonline.org · Webmaster's Email: mdoran@dcmsonline.org
Privacy Policy and Disclaimers