Residual higher-order aberrations caused by clinically measured cyclotorsional misalignment or decentration during wavefront-guided excimer laser corneal ablation

PURPOSE: To evaluate the impact of residual wavefront aberrations on optical image quality induced by cyclotorsional misalignment and centration error. SETTING: Cullen Eye Institute, Baylor College of Medicine, Houston, Texas, USA. METHODS: Cyclotorsional misalignment and decentration were measured in 58 eyes of 38 patients using an iris-registration system. To calculate residual wavefront aberrations if iris registration was not engaged, each eye's preoperative wavefront map was rotated or decentered by the amounts calculated by iris registration; this wavefront was subtracted from the preoperative wavefront (6.0 mm pupil). The impact of the residual wavefront aberrations on optical quality were analyzed by (1) the root mean square of the total aberrations (2nd to 6th orders), lower-order aberrations (LOAs) (2nd order), and higher-order aberrations (HOAs) (3rd to 6th orders); (2) normalized polar Zernike coefficients; (3) polychromatic modulation transfer function (PMTF) with the Stiles-Craw-ford effect. RESULTS: On average, centration error induced 4.9 times, 2.8 times, and 8.7 times higher values of total aberrations, LOAs, and HOAs, respectively, than cyclotorsional misalignment. For all Zernike terms except 2nd-order astigmatism, centration error produced significantly higher values than cyclorotational misalignment (all P < .05 with Bonferroni correction). Compared with the PMTF value with perfect alignment, cyclotorsional misalignment decreased the PMTF values slightly whereas centration error reduced the PMTF values by 31% to 66%. CONCLUSION: Theoretically, image quality was reduced more by clinically measured decentration than by cyclotorsional misalignment.