Diameter Of Clear Zone Research Articles

Three-year Results of the Prospective Evaluation of Radial Keratotomy (PERK) Study

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). The authors 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 3 years after surgery, 58% of eyes had refractive error within 1.00 D of emmetropia; 26% were undercorrected and 16% were overcorrected by more than 1.00 D. 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 D. Between 1 and 3 years after surgery, the refractive error changed by 1.00 D or more in 12% of eyes, indicating a lack of stability in some eyes. In the 435 eyes, there was a small number of complications including six eyes that lost two or three lines of best-corrected acuity, 16 that experienced vascularization of the incisions, 2 that had delayed bacterial keratitis, and 4 that had recurrent epithelial erosions.

Unwanted optical effects caused by intraocular lens positioning holes

The positioning holes in posterior chamber intraocular lenses (IOLs) are the source of many complaints from postoperative cataract patients with well-placed posterior chamber IOLs and otherwise excellent surgical results. Factors influencing which patients are most bothered are (1) size and reactivity of the pupil, (2) distance between the posterior surface of the iris and the anterior surface of the IOL, and (3) diameter of the IOL's clear optical zone. Eliminating the positioning holes is the best way to reduce or prevent the unwanted effects of these lenses.

Factors affecting outcome and predictability of radial keratotomy in the PERK Study.

The Prospective Evaluation of Radial Keratotomy (PERK) study used a standardized surgical technique that included a central clear zone with a diameter of 4.0, 3.5, or 3.0 mm. Multiple regression analysis of the outcome in one eye from each of 411 patients disclosed that the diameter of the central clear zone, patient age, and depth of the incision scar were the major factors affecting the change in refraction one year after surgery. Preoperative factors examined that did not have a significant influence on the outcome were sex, average central keratometric power, corneal thickness, corneal diameter, intraocular pressure, and ocular rigidity. The predictability of radial keratotomy, ie, the precision with which the outcome can be estimated, was measured by the 90% confidence interval for the change in refraction based on the regression equation. The width of this interval within each clear zone group was as follows: 4.0 mm, 2.49 diopters; 3.5 mm, 3.38 D; 3.0 mm, 4.12 D. For all 411 eyes, the 90% confidence interval was approximately 3.50 D wide. Thus, the surgeon could be 90% certain that an individual patient's refraction would be within 1.75 D of the predicted value one year after surgery.

Radial keratotomy: Analysis of efficacy and predictability in 1,058 consecutive cases Part I: Efficacy

Effectiveness and predictability of anterior radial keratotomy to correct myopia and astigmatism or astigmatism alone were evaluated in 1,058 consecutive surgeries. The surgical technique consisted of retrobulbar anesthetic injection, topical anesthetic, from four to 32 radial incisions with a diamond blade, cutting from the optic zone to the limbus, optical clear zone diameter from 2.75 mm to 5.00 mm, longitudinal or tangential incisions, if necessary, to correct astigmatism, and bilateral surgery at the same sitting in 72% of cases. There were 816 of 1,018 subjects (80.2%) between ± 1 diopter (D) of emmetropia following surgery; 86.1% of 1,003 eyes were 20/40 or better uncorrected postoperatively; mean astigmatism was reduced from 0.97 D preoperatively to 0.66 D postoperatively. Sixty-seven eyes (6.3%) had microperforations; seven (0.7%) had macroperforations requiring a suture. The mean change in refraction for patients with and without perforations was 5.43 D and 4.20 D, respectively.

Review: The Changing Status of Radial Keratotomy for Myopia. Part II

ABSTRACT: The basic principles of radial keratotomy for myopia were described in 1898: radial incisions flatten the cornea in the direction of the incision, deep incisions were more effective, and some of the result diminished as the cornea healed. Most surgeons now agree on clear zone diameters between 3.0 and 4.0 mm, four to eight incisions, and the use of a diamond micrometer knife, but consensus has not been reached on the value of deepening incisions and on how to use preoperative variables in designing the operative technique.

Results of the Prospective Evaluation of Radial Keratotomy (PERK) Study One Year After Surgery

The Prospective Evaluation of Radial Keratotomy (PERK) study is a nine-center, self-controlled clinical trial of a standardized technique of radial keratotomy in 435 patients who had physiologic myopia with a preoperative refraction between −2.00 and −8.00 diopters. The surgical technique consisted of eight incisions using a diamond micrometer knife with blade length determined by intraoperative ultrasonic pachymetry and the diameter of central clear zone determined by preoperative refraction. At one year after surgery, myopia was reduced in all eyes; 60% were within ±1.00 diopter of emmetropia; 30% were undercorrected and 10% were overcorrected by more than 1.00 diopter (range of refraction, −4.25 to +3.38 D). Uncorrected visual acuity was 20/40 or better in 78% of eyes. The operation was most effective in eyes with a refraction between −2.00 and −4.25 diopters.Thirteen percent of patients lost one or two Snellen lines of best corrected visual acuity. However, all but three eyes could be corrected to 20/20. Ten percent of patients increased astigmatism more than 1.00 diopter. Disabling glare was not detected with a clinical glare tester, but three patients reduced their driving at night because of glare. Between six months and one year, the refraction changed by >0.50 diopters in 19% of eyes.

Rationale for and Design of the National Eye Institute Prospective Evaluation of Radial Keratotomy (PERK) Study

A five-year, multicenter, collaborative clinical trial of radial keratotomy for myopia is being carried out at nine clinical centers. The study, funded by the National Eye Institute, is recruiting approximately 500 patients aged 21 years or older with −2.00 to −8.00 diopters of physiologic myopia who also meet other clearly defined eligibility criteria. Surgeons use a diamond-bladed micrometer knife to make eight radial incisions in the anterior cornea. The diameter of the central clear zone is determined by the amount of myopia and the depth of the incisions by intraoperative ultrasonic pachymetry. Investigators other than the surgeon gather all pre- and post-operative data. Examinations include measurement of visual acuity with standardized charts, verification of refractions by a second observer, measurement of corneal curvature by keratometry and photokeratoscopy, quantitation of glare with a glare tester, and measurement of the size of individual endothelial cells from specular photomicrographs with a computerized digitizer. A formal psychometric questionnaire is used to evaluate the patients' subjective response. Physician monitors insure adherence to the protocol during site visits and members of the Data and Safety Monitoring Board oversee the progress of the study. Biostatisticians at the Coordinating Center insure complete data collection, process the data, and assist in the interpretation of results.