Chapter 170 - Incisional Keratotomy

First conceptualized by Lans, incisional keratotomy is widely used for treating astigmatism, although it is now only infrequently used for treating myopia. The current methods of refractive keratotomy derive from the work of Fyodorov in Russia in the mid-twentieth century. Radial incisions cause the peripheral cornea to bulge outward, producing central flattening. Astigmatic keratotomy (AK) and peripheral corneal relaxing incisions (PCRIs) cause flattening of the cornea in the incisional meridian and steepening 90 degrees away. Somewhat arbitrarily, ‘AK’ generally refers to incisions made in an 8-mm or smaller zone, whereas ‘PCRI’ is used if the zone is 9 mm or greater. The Prospective Evaluation of Radial Keratotomy (PERK) study was the first scientific investigation of incisional keratotomy. In this study, 435 eyes were treated with a standardized testing procedure, surgical technique, and instrumentation. With a remarkable 10 years of follow-up after the surgery, major findings of the PERK study included the effect of age on treatment, the approach to titrating and enhancing the outcome, and the long-term complications, the most prevalent of which is progressive hyperopic shift. A number of surgeons in the early 1980s, among them Fenzl, Lindstrom, Martin, Neumann, Nordan, Tate, Terry, and Thornton, began investigating surgical techniques to correct naturally occurring astigmatism. In 1983, Osher began a study that addressed the correction of pre-existing astigmatism by combining transverse relaxing incisions with cataract surgery. He presented preliminary results at general meetings as early as 1984. Osher’s original technique consisted of placing paired straight corneal relaxing incisions centered on the steep meridian at a 7–10.5-mm diameter optical zone at the end of surgery. Other surgeons have tried to amplify the effect by varying incision length, number of incisions, optical zone size, and incision depth. Merlin introduced arcuate incisions, and Thornton and Lindstrom became leading advocates while refining diamond blade technology. Lindstrom found that the coupling ratio, the amount of flattening in the incised meridian divided by the amount of steepening in the opposite meridian, was approximately 1:1 when a straight 3-mm keratotomy or a 45–90-degree arcuate keratotomy was used at the 5–7-mm diameter optical zones. The maximal effect of either straight or arcuate incisions occurred when incisions were placed around a 5–7-mmdiameter optical zone. Although most of the effect was achieved with the first pair of incisions, a 20–30% additional effect could be attained with a second pair of incisions. The effect could not be increased by placing more than four relaxing incisions in the cornea. Thornton described what he believed was the geometric advantage of arcuate incisions, which is the most common method now performed. He stated that true 1:1 coupling can occur only when the corneal circumference is unchanged, which is achieved only with short, concentric arcuate incisions. A straight transverse incision increases the overall corneal circumference, creating a flatter cornea and necessitating a compensatory addition of power to the intraocular lens (IOL). Furthermore, a shorter arcuate incision achieves the same result as a longer straight incision. As mentioned previously, the term AK had been often used when describing treatments planned within an 8-mm or small zone. While this is a somewhat arbitrary distinction, PCRIs will be discussed as a separate topic since they are often planned as a treatment in a 9-mm or greater zone. While radial keratotomy has been replaced by laser refractive surgery for the treatment of myopia, it is still occasionally used to treat small amounts of myopia. Astigmatic keratotomy and peripheral relaxing incisions, on the other hand, remain mainstream methods of treating astigmatism, either in virgin eyes or eyes that have undergone prior surgery. They are an intrinsic component of refractive cataract surgery as surgeons strive to provide optimal uncorrected vision to patients with pre-existing astigmatism.