Flap-induced aberration

Anterior corneal optical irregularity measured by higher-order aberrations induced by a broad beam excimer laser

BACKGROUND

The aim was to analyse anterior corneal optical irregularity in patients receiving photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK) with a conventional broad beam laser.

METHODS

Pre- and post-operative anterior corneal aberrations including Zernike coefficients and root mean squares of higher-order aberrations were measured and derived with a Placido-ring-based corneal topographer and wavefront software, respectively, from 768 right eyes of patients receiving either PRK or LASIK for myopia with a conventional broad beam excimer laser in National Taiwan University Hospital, Taipei, Taiwan. Multiple linear regression models selected by Mallows' Cp criteria were used to evaluate predictability of surgically induced aberrations and relevant factors.

RESULTS

Mean attempted spherical equivalent correction was -7.37 ± 2.33 D (range: -16.5 to -1.95). Root mean squares of higher-order aberrations and variances of Zernike coefficients all increased significantly after surgery (p < 0.01). All post-operative higher-order aberrations increased with the amount of attempted myopic correction and secondary astigmatism increased with attempted cylindrical correction. LASIK surgery induced less coma and third-order aberrations but more fifth- and sixth-order aberrations than PRK, although squared partial correlation coefficients were low. Larger optic and transitional zones meant fewer higher-order aberrations after surgery. Younger patients tended to have more surgically induced aberrations in the 3.0 mm pupil zone.

CONCLUSIONS

Anterior corneal optical irregularity increased tremendously after refractive surgery by conventional broad beam lasers and correlated with greater attempted myopic correction, younger age and a smaller treatment zone. The effect of corneal flaps on post-operative corneal irregularity differed trivially.