Overview of Refractive Surgery

Author: Kraig Scot Bower, Thomas J. Kim
Date: Oct 1, 2001

Patients with myopia, hyperopia and astigmatism can now reduce or eliminate their dependence on contact lenses and eyeglasses through refractive surgery that includes radial keratotomy (RK), photorefractive keratectomy (PRK), laser-assisted in situ keratomileusis (LASIK), laser thermal keratoplasty (LTK) and intrastromal corneal rings (ICR). Since the approval of the excimer laser in 1995, the popularity of RK has declined because of the superior outcomes from PRK and LASIK. In patients with low-to-moderate myopia, PRK produces stable and predictable results with an excellent safety profile. LASIK is also efficacious, predictable and safe, with the additional advantages of rapid vision recovery and minimal pain. LASIK has rapidly become the most widely performed refractive surgery, with high patient and surgeon satisfaction. Noncontact Holium: YAG LTK provides satisfactory correction in patients with low hyperopia. ICR offers patients with low myopia the potential advantage of removal if the vision outcome is unsatisfactory. Despite the current widespread advertising and media attention about laser refractive surgery, not all patients are good candidates for this surgery. Family physicians should be familiar with the different refractive surgeries and their potential complications. (Am Fam Physician 2001;64:1183-90,1193-4.)

Extensive television, radio and newspaper advertising have promoted laser vision correction, and the public has a variety of refractive procedures from which to choose. Because of this media attention, many patients will be asking their family physician about these procedures. Family physicians should be familiar with the indications and contraindications of the refractive procedures, as well as the results, follow-up courses and potential complications.

Anatomy of the Cornea

The transparent cornea is about 0.5 mm thick, with five distinct layers: the epithelium, Bowman's membrane, stroma, endothelium and Descemet's membrane. The epithelium is the most exterior layer, providing the smooth refractive surface and serving as a barrier against infection. The function of Bowman's membrane, which lies beneath the epithelium and its basement membrane, is unclear. The stroma, made up of intertwining lamellae of collagen fibrils, provides structure to the cornea and accounts for 90 percent of the corneal thickness. The endothelium and its basement membrane (Descemet's membrane) form the innermost layers. Endothelial cells, via an active sodium potassium-adenosine triphosphatase pump, are responsible for the relative corneal dehydration necessary for corneal clarity.

Optics, Refraction and Refractive Error

Refraction is the bending of light rays as they pass from one transparent medium to another medium of a different density; it is measured in diopters. The refractive power of a lens is the reciprocal of its focal length measured in meters (e.g., a one-diopter lens has a focal point of 1 m; a two-diopter lens has a focal length of 0.5 m). The cornea and the crystalline lens refract light that enters the eye. The cornea is responsible for two thirds of the eye's total focusing power, while the crystalline lens accounts for the remaining one third. The focusing power of the cornea is fixed, whereas the focusing power of the crystalline lens is not. Through a process called accommodation, the lens changes its shape to bring objects into focus.

In emmetropia (an eye with normal vision), the focusing powers of the cornea and the lens are perfectly matched to the length of the globe. When a person with normal vision acuity views an object, the cornea and the lens focus the parallel light rays emitted from the object precisely on the retina (Figure 1a), and a clear image is perceived. The eye's focal point is at infinity.

Refractive errors occur when the cornea and the lens do not properly focus the light rays on the retina. In myopia (nearsightedness), the most common type of refractive error, the cornea is too curved or the lens too powerful for the length of the globe. Distant objects cannot be seen clearly because light rays are focused in front of the retina (Figure 1b); however, near objects appear clear. Concave lenses with minus or divergent power correct this refractive error and refocus the light rays on the correct point on the retina.

In hyperopia (farsightedness), the cornea is too flat or the lens too weak for the length of the globe. As a result, the cornea and lens focus the light rays behind the retina (Figure 1c). The process of accommodation may bring a distant object into focus; however, near vision is unclear. Convex lenses with plus or convergent power correct this refractive error and refocus the light rays to the correct point on the retina.

In astigmatism, the refractive power of the eye is different in different meridians. The cornea and the lens cannot bring the light rays to the precise point on the retina to provide clear vision; thus, objects will appear blurry at any distance. Astigmatism may occur with myopia or hyperopia.

Refractive Surgery

Radial keratotomy (RK), photorefractive keratectomy (PRK), laser-assisted in situ keratomileusis (LASIK), laser thermal keratoplasty (LTK) and intrastromal corneal rings (ICR) are the most common refractive surgeries presently performed in the United States in the treatment of patients with myopia, hyperopia and astigmatism. The object of these procedures is to change the refractive state of the eye by changing the shape of the cornea.


In the past, RK was performed to treat patients with myopia. The surgeon makes a number of microscopic corneal incisions in a radial or spoke-like pattern. This allows the outer cornea to relax so that the central cornea flattens (Figure 2). The new shape of the cornea is permanently retained as the cornea heals.

Potential serious complications include loss of best-corrected vision acuity, perforation of the cornea, infection and rupture of the globe. Some of the major concerns with this procedure relate to the significant corneal instability induced by the surgery, including diurnal fluctuation of refractive error, overcorrection, hyperopic shift and potential rupture of the globe with blunt trauma.(1) This procedure has declined in popularity since 1995, when the U.S. Food and Drug Administration (FDA) approved the use of the excimer laser, and because of the superior results of the other commonly performed refractive surgeries.


The excimer laser is used to perform PRK and LASIK procedures and works by changing the shape of the cornea. The excimer laser emits an ultraviolet beam that has sufficient energy to break intermolecular bonds within the cornea (photoablation). Because little or no thermal damage occurs to adjacent tissue, this is often referred to as a "cool" laser beam. A computer, programmed with the patient's refraction and corneal topography, controls the laser beam to precisely remove corneal tissue.(2) With improving technology, the width of the laser beam has continued to decrease to less than 100 m. In addition, laser eye-tracking systems are now available that allow precision corneal ablation during eye movements.(3)

In myopia, the laser flattens the central cornea to decrease its focusing power. In hyperopia, the laser indirectly steepens the central cornea by removing tissue from the periphery, thus increasing the cornea's focusing power. Astigmatism is treated with an elliptic or cylindrical beam that flattens the steepest corneal meridian (Figure 3).

Not every patient is a candidate for treatment using the excimer laser. Age, high refractive error, and ocular and medical disease may prevent a patient from obtaining a predictable refractive outcome. Table 1 reviews patient selection criteria for PRK and LASIK procedures.

TABLE 1Patient Selection Criteriafor LASIK and PRK[*]Age 18 years or olderStable refraction of at least one year's durationMyopia between -0.50 and -12.00 dioptersAstigmatism [less than or equal] 5.00 dioptersHyperopia [less than] +6.00 dioptersAbsence of ocular contraindications: Keratoconus Herpetic keratitis Progressive myopia Corneal disease[] Glaucoma[] Cataract[] Any other preexisting pathology of the cornea or anterior segment, including scarring, agophthalmos, dry eye and blepharitis[]Absence of medical contraindications: Uncontrolled vascular disease Autoimmune disease Immunosuppressed/immunocompromised Pregnant or nursing History of keloids Diabetes mellitus[]LASIK = laser-assisted in situ keratomileusis;PRK = photorefractive keratectomy.[*]--Correction with the excimer laser.[]--Determined by an ophthalmologist.


With any procedure using the excimer laser, the refractive outcome may not always result in an uncorrected vision acuity or best-spectacle corrected vision acuity of 20/20 or better. Some patients may develop a worsening of vision clarity and acuity secondary to scarring, glare, halos, monocular diplopia and reduced contrast sensitivity.(4,5,7,29,30) Patients may have a postoperative overcorrection, undercorrection and astigmatism that may need an enhancement to correct the residual refractive error.(6,9,10,17,32) Furthermore, following excimer laser refractive surgery, most patients will complain of dry-eye symptoms secondary to disruption of the corneal nerve innervation. These patients are most effectively treated with nonpreserved artificial tears or punctual plugs, as needed; the dry-eye symptoms usually resolve within three months.

Finally, patients should understand that there is a possibility they may still require correction with eyeglasses or contact lenses in order to obtain the best-vision acuity and that over time, postoperative refractive error regression may require additional laser treatment.(31)

The opinions and assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the U.S. Army Medical Department or the U.S. Army Service at large.

The authors indicate that they do not have any conflicts of interest. Sources of funding: none reported.


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KRAIG SCOT BOWER, LTC, MC, USA, is director of refractive surgery, cornea and external disease at Walter Reed Army Medical Center, Washington, D.C., and assistant professor of surgery at the Uniformed Services University of the Health Sciences F. Edward Hebert School of Medicine, Bethesda, Md. Dr. Bower received his medical degree from the University of Texas Southwestern Medical School, Dallas. He completed a residency in ophthalmology and a fellowship in cornea and external disease at the University of Pittsburgh Medical Center, Pittsburgh, Pa.

ERIC D. WEICHEL, CPT, MC, USA, is the chief resident of ophthalmology at Walter Reed Army Medical Center. He received his medical degree from the Northeastern Ohio Universities College of Medicine, Akron. Dr. Weichel completed an internship in internal medicine at Tripler Army Medical Center, Honolulu, Hawaii. Before beginning his residency in ophthalmology, Dr. Weichel served as a general medical officer and flight surgeon.

THOMAS J. KIM, CPT, MC, USA, is an ophthalmology resident at Walter Reed Army Medical Center. He received his medical degree from the Finch University of Health Sciences/Chicago Medical School, North Chicago, Ill. Dr. Kim completed a transitional internship at Walter Reed Army Medical Center.

Address correspondence to Eric D. Weichel, M.D., 2007 Westchester Dr., Silver Spring, MD 20902 (e-mail: eweichel@hotmail.com). Reprints are not available from the authors.

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