The photokeratoscope has increased the understanding of the aspheric nature of the cornea as well as a better understanding of normal corneal topography. This has significantly affected the development of newer and more predictable models of surgical astigmatic correction. Relaxing incisions effectively flatten the steeper meridian an equivalent amount as they steepen the flatter meridian. The net change in spherical equivalent is, therefore, negligible. Poor predictability is the major limitation of relaxing incisions. Wedge resection can correct large degrees of postkeratoplasty astigmatism, Resection of 0.10 mm of tissue results in approximately 2 diopters of astigmatic correction. Prolonged postoperative rehabilitation and induced irregular astigmatism are limitations of the procedure. Transverse incisions flatten the steeper meridian an equivalent amount as they steepen the flatter meridian. Semiradial incisions result in two times the amount of flattening in the meridian of the incision compared to the meridian 90 degrees away. Combination of transverse incisions with semiradial incisions describes the trapezoidal astigmatic keratotomy. This procedure may correct from 5.5 to 11.0 diopters dependent upon the age of the patient. The use of the surgical keratometer is helpful in assessing a proper endpoint during surgical correction of astigmatism.

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The presently employed procedure of radial keratotomy is essentially unchanged from that developed by Fyodorov and introduced into the United States by Bores in 1978. The surgical procedure essentially consists of six basic steps: 1) application of appropriate anesthesia; 2) marking the visual axis; 3) marking the optical zone; 4) measuring the corneal thickness; 5) setting the depth of the blade; and 6) marking the corneal incisions. This review will carefully consider the pros and cons of many potential variations associated with each of these steps.

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Radial keratotomy has always produced ambivalent feelings in the Ophthalmologist. The severe complications produced by the Sato procedure has been an unforgettable episode. The work of Fyodorov and Durney (1979) and their American counterparts, Bores (1981) and Hoffer (1981) have done a great service to radial keratotomy in having it established as an acceptable procedure. The controversy really only abated with the work of the National Institute of Health Funded Prospective Evaluation of Radial Keratotomy (PERK) study which suggested that the procedure has a low initial risk and was effective in decreasing myopic correction. Over the last 3 years a series of cases where evaluated and followed up in detail in an effort to gain a perspective on the one problem which has bugged the radial keratotomy programme from the beginning-its predict-ability.

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R.K. is a surgery of tomorrow. It is still in the stage of evolution. It should be under-taken only for precise indications. Case evaluation and honest advice to the patient is important. The patient should be explained that the worst can happen. The surgery should be limited to moderate degrees of myopia, not less than -3.0 D sphere and not more than 8 to 10 D. It has no place for myopia with degenerative retinal changes. Surgery should be interrupted at the first sign of a significant operative complication. It will be unfortunate if a healthy eye is lost due to complications of R.K. done with indiscriminate approach and ignorance of possible post-operative consequences.

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The Prospective Evaluation of Radial Keratotomy (PERK) study is a nine-center clinical trial of a standardized technique of radial keratotomy in 435 patients who had simple myopia with a preoperative refractive error between -2.00 and -8.00 diopters (D). We report results for one eye of each patient. The surgical technique consisted of eight incisions using a diamond micrometer knife with the blade length determined by intraoperative ultrasonic pachymetry and the diameter of the central clear zone determined by the preoperative refractive error. At three years after surgery, 58% of eyes had refractive error within one diopter of emmetropia; 26% were undercorrected, and 16% were overcorrected by more than one diopter. Uncorrected visual acuity was 20/40 or better in 76% of eyes. The operation was more effective in eyes with a preoperative refractive error between -2.00 and -4.37 diopters. Between one and three years after surgery, the refractive error changed by 1.00 diopter or more in 12% of eyes, indicating a lack of stability in some eyes.

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