Ocular aberrations before and after myopic corneal refractive surgery: LASIK-induced changes measured with laser ray tracing

PURPOSE. To determine objectively the changes in the ocular aberrations (3r d order and above) induced by myopic LASIK refractive surgery and its impac t on image quality. METHODS. The ocular aberrations of 22 normal myopic eyes (preoperative refr action ranged from -13 to -2 D) were measured before (2.9 +/- 4.3 weeks) an d after (7.7 +/- 3.2 weeks) LASIK refractive surgery using a laser ray trac ing technique. A set of laser pencils is sequentially delivered onto the ey e through different pupil locations. For each my, the corresponding retinal image is collected on a CCD camera. The displacement of the image centroid with respect to a reference provides direct information of the ocular aber rations. Root-mean-square (RMS) wavefront error was taken as image quality metric. RESULTS. RMS wavefront error increased significantly in all eyes but two af ter surgery. On average, LASIK induced a significant (P = 0.0003) 1.9-fold increase in the RMS error for a 6.5-mm pupil. The main contribution was due to the increase (fourfold, P < 0.0001) of spherical aberration. The increa se in the RMS for a 3-mm pupil (1.7-fold) was also significant (P = 0.02). The modulation transfer (computed for 6.5-mm pupil) decreased on average by a factor of 2 for middle-high spatial frequencies. CONCLUSION. (1) Laser ray tracing is a well-suited, robust, and reliable te chnique for the evaluation of the change of ocular aberrations with refract ive surgery. (2) Refractive surgery induces important amounts of 3rd and hi gher order aberrations. The largest increase occurs for spherical aberratio n. Decentration of the ablation pattern seems to generate 3rd order aberrat ions. (3) This result is important for the design of customized ablation al gorithms, which should cancel existing preoperative aberrations while avoid ing the generation of new aberrations.