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