Price DA, et al. - Researchers examined how measurement error affect cylinder treatment planning in corneal refractive surgery. In addition, they compared the reliability of two indices of cylinder change: the Correction Index, based on the surgically induced astigmatism, and the Flattening Index, based on the flattening effect. A Monte Carlo simulation was used to evaluate preoperative refractions and surgical outcomes. Vector analysis was performed to calculate cylinder change. They matched the initial distribution of cylinder magnitudes to a population sample of 1,000 eyes prior to LASIK. Outcomes suggest superiority of the Flattening Index vs the Correction Index when errors in preoperative and postoperative cylinder measurement were considered, especially when preoperative cylinder is 1.00 diopter or less. Postoperative cylinder minimized with targeting undercorrection, vs targeting full correction, with the optimal target dependent on the amount of random error in refraction measurement. It was optimal to partially treat 0.25 D of cylinder, even when there is relatively high presumed level of measurement error. Based on findings, they emphasize adjusting treatment nomograms to use the Flattening Index to assess cylinder change, and updating reporting guidelines to include the Flattening Index.