Risk factors for postoperative cylindrical prediction error after laser in situ keratomileusis for myopia and myopic astigmatism

Purpose: To study the risk factors for the occurrence of cylindrical prediction error (PE) after laser in situkeratomileusis (LASIK) for myopia and myopic astigmatism. Methods: The study was a nested case-control study. Five hundred eyes of 252 consecutive patients who underwent LASIK for myopia and myopic astigmatism on the Chiron Technolas 217C laser and completed 6 months of follow-up. There were 435 controls and 65 cases based on the postoperative refractive cylindrical PE. The probable risk factors studied included preoperative sphere and cylinder, keratometry, pachymetry, suction ring used, flap thickness, hinge centeration, optic zone, ablation depth, and intraoperative complications. Results: By univariate analysis, the cylindrical PE was found to be associated with preoperative spherical equivalent higher than -6D (χ²=10.83; P=0.001), preoperative sphere higher than -6D (χ²=6.15, P=0.013), preoperative cylinder more than -0.75D (χ²=6.61; P-value=0.010), and an optic zone less than 5.5 mm (χ²=19.3; P=0.001). Risk factors for postoperative astigmatism by stepwise multivariate logistic regression analysis were an optic zone of less than 5.5 mm with an odds ratio (OR) of 2.81 (95% confidence interval (CI)=1.62-4.86) and preoperative cylinder more than -0.75D with an OR of 1.60 (95% CI=0.92-2.77). Conclusion: Postoperative astigmatism (as indicated by the cylindrical PE) is more likely to occur with an optic zone of less than 5.5 mm and a higher preoperative cylindrical error.