Comparison of objective methods for quantifying the refractive effect of photo-astigmatic refractive keratectomy using the MEL-60 excimer laser.

PURPOSE

To investigate the accuracy and precision of automated keratometry, automated refractometry, and computerized corneal topography in estimating the subjective refractive outcome of photo-astigmatic refractive keratectomy six months postoperatively.

METHODS

Photo-astigmatic refractive keratectomy (Aesculap-Meditec, MEL-60 Excimer Laser) was performed on 26 eyes with a preoperative myopia ranging from -4.0 to 7.6 dioptres, and a naturally occurring astigmatism from 0.75 to 5.0 dioptres. Six months postoperatively refractive outcome was evaluated by automated keratometry, automated refractometry (Nikon NRK-8000), computerized topography (TMS-1), and subjective refraction. Estimate errors were computed as the difference between the change in subjective refraction and the change in automated keratometry, automated refractometry, and surface topography, respectively. Astigmatic changes were evaluated by the second harmonic component in the Fourier series analysis.

RESULTS

Subjective spherical as well as cylindrical values were reduced significantly six months postoperatively. The estimate error (mean +/- one standard deviation) for automated keratometry was -1.26 +/- 0.72 dioptres for the spherical equivalent and -1.36 +/- 1.02 dioptres for the cylinder; for automated refractometry it was -0.78 +/- 0.91 dioptres for the spherical equivalent and -0.66 +/- 0.92 dioptres for the cylinder. The best estimates of subjective changes were obtained when the average of ring 2 and 3 of the topographic data was used: -0.15 +/- 0.82 dioptres for the spherical equivalent and -0.78 +/- 0.80 dioptres for the cylinder.

CONCLUSIONS

The computerized topographer with the Fourier analysis was superior to automated keratometry and automated refractometry in estimating the subjective spherical refractive outcome and comparable to automated refractometry in estimating the subjective cylinder refractive outcome after photo-astigmatic refractive keratectomy.