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Modal and Zonal Analysis of Wavefront Aberrations After Lasik


Purpose: To study the magnitude and nature of the wavefront aberrations induced after standard LASIK refractive surgery. To compare the results provided by a modal Zernike polynomial analysis to those obtained with a zonal fitting of the wavefront aberration in eyes after LASIK. 

Methods: Ocular wavefront aberrations were measured using our own research prototype near–infrared Hartmann–Shack sensor for a 6 mm pupil diameter, sampled with around 300 microlenses. Measurements were done before and two months after standard LASIK surgery. Three groups of patients with different pre–op refractive errors were included: moderate myopia (n=10), low myopia (n=10) and low hyperopia (n=4). For each eye and condition, wavefront aberrations were analyzed both by using modal fitting to Zernike polynomials up to the 8th order and Fourier–analysis zonal fitting of the wavefront slopes. 

Results: The root mean square of aberrations after LASIK was found to be correlated to the pre–op refractive error (r2=0.15). Coma induced by surgery was well correlated (r2=0.39). However, we found a somehow no expected behavior of 4th–order spherical aberration (SA). While subjects with low refractive errors showed a clear correlation of induced SA with pre–op refraction (r2=0.68), this tendency did not hold for the larger myopic subjects (above 5 D). For these subjects, lower than expected values of induced SA were found, while coma consistently increased. The wavefront zonal fitting confirmed these results, excluding the possibility of these results being an artifact of the Zernike modal fitting algorithm for this type of eyes. 

Conclusions: We found an increase of aberrations after standard LASIK as previously reported. Induced SA and coma are correlated with pre–op refractive error for a limited range, the SA. However, induced SA in patients with larger myopia was quite small, failing to follow the general tendency. These results were confirmed by Fourier–analysis zonal fitting. The presence of induced coma not secondary to SA can be explained by the overlapping of a portion of the transition zone with the subject’s pupil, caused by ablation decentration.

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