Clinical effects of pure cyclotorsional errors during refractive surgery.

PURPOSE To describe the theoretical effects of cyclotorted ablations on induced aberrations and determine the limits of tolerance of cyclotorsional accuracy. METHODS A method was developed to determine the average cyclotorsion during refractive surgery without a cyclotorsion tracker. Mathematical conditions were simulated to determine the optical, visual, and absolute benefits in 76 consecutive treatments performed on right eyes. The results were evaluated as Zernike expansion of residual wavefront aberrations. RESULTS Ablations based purely on Zernike decomposition but with cyclotorsion applied resulted in residual aberrations of the same Zernike modes of different magnitudes and orientations, indicating that the effect of cyclotorted compensation can be analyzed by single Zernike modes in magnitude and orientation. The effect on single Zernike modes depends on angular frequency, and not on radial order. A mean of 4.39 degrees of cyclotorsion was obtained. A theoretical optical benefit was achieved in 95% of treatments, a theoretical visual benefit in 95%, and an absolute benefit in 93% compared with 89%, 87%, and 96% of treatments achieving actual benefits, respectively. CONCLUSIONS Residual aberrations resulting from cyclotorsion depend on aberrations included in the ablation and cyclotorsional error. The theoretical impact of cyclotorted ablations is smaller than that of decentered ablations or edge effects in coma and spherical aberrations. The results are valid within a single-failure condition of pure cyclotorsional errors, because no other sources of aberrations are considered. The leap from the mathematical model to the real-world outcome cannot be extrapolated without further study.