[Comparison of classical keratometry and computer-assisted corneal topography in high grade postoperative astigmatism...

The standard keratometry measures the central corneal refractive power by only four sample points. there is a risk for poor or incorrect measurements in high irregular postoperative astigmatism following penetrating keratoplasty. Therefore we compared the standard keratometry with the computer assisted corneal topography.

In the study 43 consecutive measurements after penetrating keratoplasty, with an astigmatism > 4 dpt measured with standard keratometry (Zeiss Ophthalmometer), were analyzed. A computer assisted corneal topography (Eye Sys) was performed simultaneously. 22 measurements were received before, 21 measurements after T-incisions. The astigmatism value and axis measured by keratometry and corneal topography were compared and the correlation coefficient were calculated. As a simple index for the asymmetry of the resulting corneal cylinder measured by corneal topography, the frequency and the axis difference of both steep hemimeridians within the 3 mm zone were determined.

The astigmatism values measured by standard keratometry (8.1 +/- 2.7 dpt) were significantly higher than the values measured by the computer assisted corneal topography (6.1 +/- 2.2 dpt). The difference of the cylinder axis measured by standard keratometry and corneal topography was 19 +/- 23 degrees; 10 +/- 15 degrees before and 29 degrees +/- 26 degrees after T-incision. The comparison of the astigmatism values showed a correlation coefficient of 0.82 in the total group, of 0.84 before and of 0.83 after T-incisions. In 26% of all measurements the axis difference of both steep hemimeridians (3-mm zone) was more than 30 degrees. After T-incisions this difference increased.

Based on the results of this study, it seems reliable to identificate patients with high postoperative astigmatism following penetrating keratoplasty by standard keratometry.