Fourier analysis of corneal topography data after photorefractive keratectomy

To evaluate the relation between measures of corneal power and refractive error after photorefractive keratectomy (PRK) by applying fast Fourier transform (FFT) to computerized corneal topography data.

Corneal Diseases and Excimer Laser Clinic, Sunderland Eye Infirmary, Sunderland, England, and University of Dundee Department of Ophthalmology, Dundee, Scotland.

Twenty-six left eyes of consecutive patients treated by PRK with a VISX Twenty-Twenty excimer laser were retrospectively analyzed. Preoperative and 3, 6, and 12 month postoperative data were studied. Changes in corneal parameters derived from corneal topography data using the FFT were compared with changes in refractive status vectors (spherical equivalent and astigmatic cosine and sine values).

Although highly correlated (r2 = 0.8839), the change in FFT-derived corneal spherical equivalent underestimated the change in refractive spherical equivalent by 25.5% over the 12 month follow-up. Decentration, measured by the 1-cycle FFT harmonic, increased significantly from a mean preoperative value of 0.12 mm +/- 0.07 (SD) to 0.51 +/- 0.35 mm 12 months postoperatively.

The FFT is a powerful method for extracting clinically meaningful descriptors from corneal topography data; however, care must be taken when interpreting refractive changes from corneal data.