Multiple regression analysis between preoperative sphere, preoperative cylinder and surgical astigmatic correction in Photorefractive Keratectomy (PRK)

Purpose. To evaluate the strength and statistical significance of the association between two preoperative refractive variables (sphere and cylinder), and postoperative astigmatic correction. Methods. Ninety five eyes from 72 patients, underwent PRK for myopic astigmatism, and were included in this study. All cases were performed with an excimer laser VISX 20/20. The magnitude of both the preoperative sphere and the preoperative refractive cylinder were correlated (multiple regression) with the index of success, which was defined as the ratio of target induced astigmatism to the surgically induced astigmatism by vectorial analysis (Taylor HR, et al. J Cataract Refract Surg 1994;20:243-51). Results. The mean age was 29.4 ± 5.91 (SD) years. The mean preoperative sphere was -5.94 D (range, -0.75 to -17.50 D), and the mean preoperative cylinder was -2.54 D (range, -1.50 to -5.00 D). All eyes were evaluated at 6 months postoperatively. The partial and multiple regression analysis results are shown in the following table. Partial r p Beta Multiple r p Sphere -0.08 0.54 -0.01 0.14 0.48 Cylinder 0.14 0.23 0.14 Conclusions. As this analysis shows, the preoperative sphere and cylinder fail to predict the index of success. These results suggest that there is not contribution from any of these two variables to the obtained success of corrected astigmatism.