Reliable keratometry with a new hand held surgical keratometer: calibration of the keratoscopic astigmatic ruler

Some surgeons consider hand held surgical keratometers unreliable. This may be due to incorrect use through not realising that the distance that the keratometer is held from the cornea influences the shape of the image. When a keratometer is held closer to the astigmatic cornea, the elliptical image will appear more circular, particularly for larger degrees of astigmatism. However, the keratoscopic astigmatic ruler (KAR) has design features that correct the hitherto unrecognised problems with the use of a hand held keratometer. This study assesses the reliability and accuracy of measurement of astigmatism using the KAR.

The KAR and the Bausch & Lomb keratometer (B&L) were compared using six back surface toric cut contact lens blanks representing 1 to 6 dioptres of astigmatism. Two observers (one experienced in the use of the keratometers, the other a novice) took eight randomly repeated "masked" measurements of each lens blank with the KAR and four measurements with the B&L in a similar fashion.

There was no difference between the measurements with either instrument by each of the observers (p = 0.95, ANOVA). The standard error of measurement for the KAR was 0.59 D, for the B&L, 0.31 D. The intraclass correlation coefficient of reliability for the KAR was 0.90 and for the B&L it was 0.97. The coefficient of repeatability for the KAR was plus or minus 0.83 D, and for the B&L plus or minus 0.77 D. The interobserver reliability for the KAR was 0.898, and for the B&L, 0.975.

These results suggest that the KAR has good reliability and reproducibility and compares favourably with the B&L keratometer. Inexperience with use does not affect reliability.