COMPARISON BETWEEN STANDARD KERATOMETRY A ND COMPUTER-ASSISTED CORNEAL TOPOGRAPHY IN HIGH POSTOPERATIVE ASTIGMATISM FOLLOWING PENETRATING KERATOPLASTY

The standard keratometry measures the central corneal refractive power by only four sample points. There is a risk for poor or incorrect mea surements in high irregular postoperative astigmatism following penetr ating keratoplasty. Therefore we compared the standard keratometry wit h the computer assisted corneal topography. Method In the study 43 con secutive measurements after penetrating keratoplasty, with an astigmat ism >4 dpt measured with standard keratometry (Zeiss Ophthalmometer), were analyzed. A computer assisted corneal topography (Eye Sys) was pe rformed simultaneously. 22 measurements were received before, 21 measu rements after T-incisions. The astigmatism value and axis measured by keratometry and corneal topography were compared and the correlation c oefficient were calculated. As a simple index for the asymmetry of the resulting corneal cylinder measured by corneal topography, the freque ncy and the axis difference of both steep hemimeridians within the 3 m m zone were determined. Results The astigmatism values measured by sta ndard keratometry (8.1 +/- 2.7 dpt) were significantly higher than the va lues measured by the computer assisted corneal topography (6.1 +/- 2.2 dpt). The difference of the cylinder axis measured by standard ke ratometry and corneal topography was 19 +/- 23 degrees; 10 +/- 15 degr ees before and 29 degrees +/- 26 degrees after T-incision. The compari son 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 hemimeridian s (3-mm zone) was more than 30 degrees. After T-incisions this differe nce increased. Conclusions Based on the results of this study, it seem s reliable to identificate patients with high postoperative astigmatis m following penetrating keratoplasty by standard keratometry.