1,2Büeler M., 1,2Mrochen M., 2Niederer P., 1Seiler T.

1Augenklinik, Universitätsspital Zürich; 2Institut für Biomed. Technik, ETH Zürich

Objective: Depending on the type and the magnitude of the wavefront aberrations preexisting in the subject's eye, slight centration differences between the wavefront measurement and the following treatment can induce considerable amounts of additional optical errors. The aim of this project was to investigate the effect of these induced aberrations on the optical outcome as a function of the decentration and the distribution of the different types of optical errors in the original wavefront.
Methods: Artificially generated wavefronts for 2nd, 3rd and 4th Zernike order aberrations, as well as measured wavefronts from 130 eyes, were used to calculate the optical errors induced by small differences in centration such as lateral shifts and rotations in three axes. To investigate the influence of the induced aberrations on the visual acuity, the so-called Marechalcriterion (MC = rms £ lambda/14) was used and correlated with the increments of decentration and the magnitude of the original aberrations.
Results: The required accuracy of centration to meet the Marechalcriterion strongly depends on the type and magnitude of the uncorrected aberrations. There was a linear relation between the rms-value and the torsion but a nonlinear relation to the lateral decentration. In case of an astigmatism as small as -0.27dpt (rms = 0.13) lateral shifts up to 0.15 mm and torsion up to 3 deg do not violate the MC. Small coma-like aberrations (rms = 0.06), as observed in normal eyes, can be decentered by up to 0.2 mm and rotated up to 12 deg without harming the MC. In contrast, a comalike aberrations of rms = 0.24 requires a much higher accuracy in lateral (0.02 mm) and rotational centration (3 deg) to meet MC.
Conclusions: The sensitivity of wavefront measurements and centration of the ablation zone with respect to the same reference axis is an important factor in refractive surgery, especially in customized ablations. Fixation errors by the examined subject, causing x-y-rotations, do not significantly affect the measurements. This work was supported by the Swiss National Science Foundation.

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