Four Procedures Commonly Used in Refractive Eye Surgery
One of the first things put on in the morning and one of the last things taken off in the evening, glasses and contacts play a large part in the everyday lives of those who depend upon them to see clearly. These two types of corrective lenses allow for clearer vision, but glasses and contacts are also inconvenient. Sports, especially contact sports, are more difficult to play when glasses get in the way or break. For those who can wear them, contacts are an improvement over glasses; wearing contacts for 18 hours a day, however, is not healthy for the contacts or the eyes. Therefore, other methods are desired to correct vision.
Myopia, hyperopia, and astigmatism are three common vision problems.
Myopia. Myopia , or nearsightedness, is part of more than 70 million people's lives in the United States [Raeburn, 1996]. The term "nearsightedness" means that objects "near" are seen more clearly than objects in the distance. Myopia occurs when the eye is too long relative to the curvature of the cornea, which causes light rays to focus in front of the retina and produce blurry images. By flattening the central cornea with micro-surgical refractive procedures, myopia can be corrected.
Hyperopia. The medical term for farsightedness is hyperopia. The term "farsightedness" means that objects "far" are seen more clearly than objects close. This refractive vision problem is associated with an eye too short relative to the curvature of the cornea. Therefore, the light rays entering the eye are focused behind the retina. To correct hyperopia, the central cornea is steepened.
Astigmatism. When the cornea is no longer spherical but rather has some degree of ovalness to its shape, astigmatism is present. As a result, the light rays are bent unequally and focused at more than one point over a large area on the retina. Astigmatism often occurs in conjunction with myopia and hyperopia but it can also be present by itself. Only a portion of the central cornea is reshaped, or rounded, to correct for astigmatism.
Four Refractive Eye Surgery Procedures
More than half of the world's population has inherently poor visual acuity sufficient to require some form of corrective lenses. Since the 1980s, refractive eye surgery has become a popular alternative to corrective lenses. More than a million people have chosen laser vision correction over glasses and contacts in the past ten years and millions more are predicted to undergo refractive surgery ["Discover," 1997]. The objective of refractive eye surgery is to focus light rays directly on the retina by flattening the curvature of the cornea (in myopia), steepening the curvature of the cornea (in hyperopia), or reshaping the cornea into a sphere (in astigmatism).
Radial Keratotomy. First studied in Japan by Dr. Sato, radial keratotomy (RK) has been researched for several decades. The basic procedure of RK is to flatten the curvature of the cornea, which indirectly corrects myopia, by manually making incisions under the epithelium (the outer protective tissue layer) with a scalpel. Prior to surgery, the surgeon marks the optical zone on the cornea with a surgical marker. In the 1970s, Dr. Fyodorov found that a smaller clear zone produced a greater refractive effect; by changing the optical zone size and the depth of the cuts, the degree of correction could be adjusted with RK [Sanders, 1984]. This parameter, along with the number of incisions, directly relates to the amount of correction obtainable.
RK has a narrow range of correction due to the risk associated with increasing the number of incisions to correct higher...