Abstract
Abstract: :
Purpose: Autorefraction is a well–established method for measuring the refraction of the eye. However, the technique may have difficulties measuring highly aberrated eyes. We investigated this using wavefront sensing to measure keratoconic eyes. Methods: 40 keratoconus outpatients, a total of 80 eyes, at Osaka university hospital were measured using a Topcon KR9000PW Wavefront Analyzer (WFA) (Topcon Corporation). Written informed consent was obtained from every patient. 4 measurements were taken of each eye. The Wavefront Analyzer measures refraction with a Scheiner principle autorefractometer, then measures wavefront with a Shack–Hartmann wavefront sensor (WFS). Zernike analysis relative to the center of the pupil was performed for a 4mm pupil. Autorefractometer astigmatism data were converted to equivalent Zernike astigmatism coefficient values (Z2–2, Z22) following Atchison et al. (OSA Tops Vol. 35, 2000). Standard deviations of autorefractometer and WFS measurements were compared, on the assumption that WFS measurements are more accurate. Autorefractometer values were subtracted from the actual values measured by the WFS to determine the residual wavefront resulting from autorefractometer–based correction. Results: Mean high order wavefront error ± SD was 0.717 ± 0.042 microns RMS. Mean astigmatism ± SD in RMS microns of wavefront error was 1.09 ± .36 microns for the autorefractometer and 2.56 ± .50 microns for the WFS. Average SD of axis of astigmatism measurements was 2.04 degrees for the autorefractometer and 3.66 degrees for the WFS. Axis values as measured by the autorefractometer and WFS were correlated, but the fit line left high residuals (mean 12.11degrees, SD 11.8). Cylindrical dioptric values were correlated, but left high residuals (mean .83D, SD .77). Correcting astigmatism with autorefractometer parameters left residual wavefront error due to astigmatism terms of 1.451microns RMS for a 4mm pupil, only a 0.689micron improvement from uncorrected. Conclusions: WFS measures of astigmatism account for a larger amount of wavefront error than autorefractometer measurements, and so are slightly more variable. Though precise, autorefractometer measurements are not sufficiently well matched to WFS measurements of astigmatism. Correction along inappropriate axes leaves a large residual wavefront error due to astigmatism. Caution should be taken when using autorefractometer measurements to correct highly aberrated eyes, such as keratoconic eyes.
Keywords: astigmatism • refraction