Changes in corneal aberration structure after photorefractive keratectomy

Purpose. To determine, using videokeratography, the changes in shape and aberration structure of the cornea induced by photorefractive keratectomy (PRK). Methods. Videokeratographs of 112 eyes from 89 PRK patients were measured preoperatively and then at 1 (N=99), 3 (N=103), 6 (N=86), 12 (N=71), 18 (N=41), and 24 (N=30) months postoperatively using the TMS-1 Corneal Modeling System. We calculated higher order aberrations as described by Applegate and Howland (IOVS 1995; 36:1437-385 . For a 3-mm pupil, the coefficients of an orthogonal Zernike polynomial were computed, as well as Taylor coefficients. Zernike polynomials contain terms representative of fundamental corneal shapes such as the sphere and cylinder. In addition, from the Zernike representation, the mean square deviation of the error lens from a perfect spherocylindrical lens was found and factored into its third order (S3, coma-like) and fourth order (S4, spherical aberration-like) components. Results. The wavefront variance after PRK increased immediately following surgery (p< 0.0001), but recovered after 12 months. After PRK, all components of the wavefront variance increased but the third-order term became dominant. Conclusions. PEK increases the wavefront variance of the cornea. After PRK for a 3-mm pupil, the third-order term becomes dominant. Videokeratography can be used to quantify and characterize the wavefront aberrations of the cornea.