Just about everyone knows somebody who has had laser vision surgery, which is not surprising considering over nine million people have had LASIK surgery in the United States alone (Kornmehl et al 2006). Developed in 1990, LASIK (laser in-situ keratomlieusis) is a highly popular and successful surgery to correct refractive errors such as near sightedness (myopia), farsightedness (hyperopia), and astigmatism (blurred or double vision). While LASIK surgery may seem to be quite new, it is, in fact, the culmination of years of collaborative work in two areas: refractive surgery (or surgery that corrects refractive errors by reshaping the curvature of the cornea) and developments in LASER technology.

Precursor to Refractive Surgery and LASIK

While Leonardo da Vinci studied the eye and possible sources of refractive errors in the sixteenth century, the study of the eye’s curvature as a source of visual problems really didn’t occur until the mid 1800s. During the Civil War, the development of eye drop anesthesia led to the keratometer (an instrument for measuring the curvature of the “kerato” or cornea). Around the same time, advertisements appeared for an eye cup with a spring mounted mallet that could flatten the cornea by striking it through the eyelid, but this technique failed to produce any significant degree of visual correction (Brint et al 2000). In 1869, Dutch ophthalmologist Herman Snellen (who created modern-day vision charts) theorized that astigmatisms could be treated by flattening the cornea with small incisions. But no one would dare try cutting into the eye for another two decades when Dutch physician L.J. Lans described the basic principles of keratotomy (the Greek keratos means cornea and totomy means incision or removal). He argued that an incision to reshape the cornea would change the way light is bent or refracted, redirecting images onto the retina rather than behind or in front of it. By varying the incisions, he could manipulate and tailor the type of visual correction. Though Lans’s research was careful and methodical, refractive surgery was still hit-and-miss from 1885 through 1939 (Brint et al 2000).

In the 1930s, Japanese ophthalmologist Tsutomu Sato conducted pioneering work in corneal incisions that established the value of radial keratotomy (RK), a form of refractive surgery that attempted to treat myopia and astigmatism by making spoke-like incisions around the cornea (Kornmehl et al 2006). Russian scientist, Svyatoslav Fyodorov, first practicing on rabbit corneas, perfected RK by placing the multiple incisions on the anterior surface of the eye and leaving a clear central optical zone. Soon RK was introduced into the U.S. in 1978 by ophthalmologist Leo Bores. The National Eye Institute authorized the national PERK (Prospective Evaluation of Radial Keratotomy) study, which concluded that the surgery was actually unstable with people who were once nearsighted sometimes becoming farsighted months or years after undergoing the procedure. In addition, it noted that the greater the needs for correction, the more incisions were required, creating a greater possibility that the structural stability of the eye would be compromised. Even with improvements in RK surgical techniques, it soon became largely obsolete (Ehrlich 1999).

Rather than working on refining the corneal incision techniques of RK, Jose Barraquer of Bogotá, Colombia, was interested in developing surgical techniques involving the actual removal of corneal tissue. His work in 1949 would eventually lead to LASIK surgery. Barraquer, now known as the father of modern eye surgery, developed lamellar (layered) surgery, or surgery that corrects focusing errors by removing or reshaping corneal layers with a microkeratome (a small blade). He used the microkeratome to cut corneal tissue to correct high levels of myopia in a procedure called myopic keratomileusis (MKM) and to correct hyperopia in a process called hyperopic keratomileusis (HKM). The microkeratome enabled Barraquer to cut a 300-micron layer of anterior corneal tissue (today’s LASIK corneal flap) which he removed, froze, and placed on a cryolathe, a device used to shape contact lenses. After reshaping the tissue, it was then thawed, repositioned on the eye, and sutured in place. Later, physicians abandoned stitches when they realized the corneal cap (or flap) naturally adheres to the surface of the eye (Brint et al 2000).

In 1985, Casimir Swinger developed a method of changing the shape of the cornea without freezing it and, in 1987, Barraquer’s protégé Luis Ruiz modified the principles of microkeratome corneal surgery by using an automated form of the blade directly on the eye, leading to what is called automated lamellar keratoplasty (ALK). With the motorized microkeratome, or automated corneal shaper, it became possible to cut and lift the corneal cap without removing it from the eye. This advancement led to fewer incidences of what was called “free caps” or lost corneal tissues. ALK would treat high levels of myopia and hyperopia, but it is now no longer performed because LASIK is much less risky and more precise (Ehrlich 1999).

Development of the Excimer Laser

While ophthalmologists were busy refining refractive surgery, scientists were also developing excimer laser technology that would revolutionize refractive surgery and become key in LASIK surgery. Lasers (light amplification by stimulated emission of radiation) were first observed in 1960 when the first beam of laser light was emitted from a ruby crystal. While ophthalmologists have used laser since the early 1970s to treat glaucoma, diabetic retinopathy, post-cataract surgery, clouding, and retinal tears, it wasn’t until the development of the excimer laser that they would be useful for refractive surgeries (Weiss 2002).

The excimer laser (derived from the words excite and dimers) was created in 1975 by Stuart Searles who bombarded a medium of xenon-bromide with an electron beam gun. In 1983, Dr. Srinivsan at IBM’s Thomas J. Watson Research in Yorktown Heights, New York, initially used the excimer laser to etch circuits into computer chips because it would not melt the silicon that the chips were made of. During his work with the ultraviolet excimer laser, Srinivasan also noted the effect of it on biological materials and was impressed with the clean cuts it made with no thermal damage. Also in 1983, Dr. Stephen Trokel, a research ophthalmologist at New York’s Columbia-Presbyterian Medical Center, read about a physicist with the military who tested the excimer laser on animals to see what effect the beam had on tissue, including corneal tissue. He also visited Srivinasan at the center and applied the excimer to cow eyes he brought along (Kornmehl et al 2006). That same year, Trokel published the landmark paper in the American Journal of Ophthalmology outlining the potential use of excimer laser in refractive surgery. And in 1986, a company named Cooper Surgical began constructing the first complete excimer system designed for potential use in the treatment of human eyes (Brint et al 2000).

PRK: "Flapless" LASIK

Working under less strict regulations than U.S. doctors, the first clinical application of the excimer laser was by German physician Theo Seiler. In April 1985, Seiler used the excimer laser on blind eyes to determine whether astigmatic correction would be possible; shortly afterward, he performed the first excimer refractive surgery in a sighted eye. Six months later, in June 1987, Dr. Marguerite McDonald, an ophthalmologist working with Dr. Trokel at Louisiana State University, performed the first PRK (photorefractive keratectomy) surgery on a sighted eye within an FDA trial in 198l. This was the beginning of the laser portion of laser eye surgery. The FDA first approved the excimer laser for PRK in 1995, prior its approval of the same laser for LASIK (Brint et al 2006).

Although PRK is similar to RK, it is very different in terms of patient risk and corrective capabilities. Rather than making cuts or incisions in the cornea, PRK uses an excimer laser to sculpt an area 5 to 9 mm in diameter on the surface of the eye. In PRK, the surgeon first removes the outermost corneal cell layer (the epithelium) and then applies the laser to the surface of the eye. This creates a corneal abrasion and, like any abrasion, it can be painful. Surgeons manage any discomfort with anti-inflammatory eye drops and low-powered contact lenses that act as a bandage over the wound until the epithelium grows back, usually within three to five days (Ehrlich 1999). Patients may be sensitive to light (photophobia) and most experience fluctuations in vision for the first few weeks. A few patients experience extreme discomfort and light sensitivity that lasts for more than a few days and their vision can take months to stabilize. However, because PRK sculpts rather than cuts, the strength of the corneal dome is better retained in PRK than RK (Brint et al 2006).

LASIK: "Flap and Zap"

In 1990, researchers Ionnis Pallikaris (who coined the term LASIK) and Lucio Buratto found ways to avoid the possible side effects of PRK, such as anterior stromal haze and pain. This is because LASIK only applies the laser within the corneal substance rather than using it to remove a large area of the epithelium (the thin outer surface of the eye), which leaves the nerve endings painfully exposed. There are also more fibroblasts underneath the epithelium which can contribute to scarring and infection. LASIK, or laser (laser assisted) in-situ (laser sculpting is performed on the cornea) keratomileusis (process of carving the cornea), keeps the epithelium in tact, keeping the nerves covered and the eye less susceptible to infection and scarring (Brint et al 2006). Consequently, while some doctors still offer PRK to their patients, most eye doctors performing refractive surgery recommend LASIK, which usually can be performed in less than ten minutes and costs anywhere from $1000-$2500 per eye.

During a LASIK procedure, the surgeon uses a microkeratome (based on Barraquer’s pioneering work) to peel back the outermost layer of the cornea to create a very thin flap which is lifted back to expose the corneal tissue beneath. The computer-controlled excimer laser then sculpts (photoablates) the cornea into a new shape by “vaporizing” a thin layer of molecules to correct the refractory error (Kornmehl et al 2000). In other words, the laser transfers your current eye prescription onto the surface of your eye—permanently (Brint et al 2006).

For example, nearsighted patients have corneas with too much curvature in proportion to the length of their eyes or, in other words, the cornea is too steep. Once the corneal flap is made and lifted back, the excimer laser reshapes the underlying stroma to achieve a flatter cornea. Light rays now come to a point of focus on the retina rather than in front of it.

Farsighted patients have corneas that are proportionately too flat for the length of their eyes. For them, the excimer laser is programmed to remove tissue from just the periphery of the stroma, leaving the middle untouched and creating a more domed shape. This increased curvature allows light rays to focus on the retina rather than behind it.

If a patient has astigmatism, the surgeon removes tissue in an oval fashion, adjusting the shape of the cornea in one direction more than the other. The goal is to create a symmetrical surface so that light rays passing through the cornea at various places will meet at a single point of focus on the retina (Kornmehl et al 2000).

It is important to note that the tissue around and underneath (or behind) the point of LASIK surgery is not affected because the laser’s wavelength is at the cool end of the light spectrum. In addition, the laser removes only 0.25 microns of corneal tissue at a time, which is 1/500^th of the thickness of human hair (which is 125 microns thick). In fact, it would take 500-600 laser pulses to break through human hair. When using the laser to correct refractive errors, surgeons remove between 10 and 160 microns of tissue, always being mindful to preserve the cornea’s strength by ensuring that sufficient tissue remains after refractive surgery (Brint et al 2006).

LASIK surgery is very successful with very few complications. Approved by the FDA in 1999, LASIK is by far the most commonly performed refractive surgery today. Patients have a 95% chance of achieving 20/20 vision, and 99% of typical myopic patients can drive without glasses, usually as soon as the morning after surgery. Still, LASIK surgery does not mean your vision will remain the same throughout your life. While LASIK results are permanent, the eye still can change internally; consequently, some people can become more myopic or hyperopic as they age after surgery. In addition, people with autoimmune diseases, vascular disorders, connective tissue disorders, retinal tears, thin corneas, or large pupils, or those who are under 18 years of age, may need to look to alternatives to LASIK.

-- Posted April 2, 2008

References

Brint, Stephen F, MD, Dennis Kennedy, OD, and Orinne Kuypers-Denlonger. 2006. The Laser Vision Breakthrough. Roseville, CA: Prima Health.

Ehrlich, Matthew. 1999. How to See Like a Hawk When You’re Blind as a Bat. Venice, FL: Doctor’s Advice Press.

Kornmehl, Ernest W, MD, Robert K. Maloney, MD, Jonathan M. Davidorf, MD. 2000. LASIK: A Guide to Laser Vision Correction. 2^nd ed. Omaha, NE: Addicus Books, Inc.