FOURIER-ANALYSIS OF CORNEAL TOPOGRAPHY DATA AFTER PHOTOREFRACTIVE KERATECTOMY

Purpose: To evaluate the relation between measures of corneal power an d refractive error after photorefractive keratectomy (PRK) by applying fast Fourier transform (FFT) to computerized corneal topography data. Setting: Corneal Diseases and Excimer Laser Clinic, Sunderland Eye In firmary, Sunderland, England, and University of Dundee Department of O pthalmology, Dundee,Scotland. Methods: Twenty-six left eyes of consecu tive patients treated by PRK with a VISX Twenty-Twenty excimer laser w ere retrospectively anlayzed. preoperative and 3, 6, and 12 month post operative data were studied. Changes in corneal parameter derived from corneal topography data using the FFT were compared with changes in r efractive status vectors (spherical equivalent and astigmatic cosine a nd sine values). Results: Although highly correlated (r(2) = 0.8839), the change in FFT-derived corneal spherical equivalent underestimated the change in refractive spherical equivalent by 25.5% over the 12 mon ths follow-up. Decentration, measured by the 1-cycle FFT harmonic, inc reased significantly from a mean preoperative value of 0.12 mm +/- 0.0 7 (SD) to 0.51 +/- 0.35 mm 12 months postoperatively. Conclusions: The FFT is a powerful method for extracting clinically meaningful descrip tors for corneal topography data; however, care must be taken when int erpreting refractive changes from corneal data.