RELIABLE KERATOMETRY WITH A NEW HAND-HELD SURGICAL KERATOMETER - CALIBRATION OF THE KERATOSCOPIC ASTIGMATIC RULER

Aim-Some surgeons consider hand held surgical keratometers unreliable. This may be due to incorrect use through not realising that the dista nce that the keratometer is held from the cornea influences the shape of the image. When a keratometer is held closer to the astigmatic corn ea, the elliptical image will appear more circular, particularly for l arger degrees of astigmatism. However, the keratoscopic astigmatic rul er (KAR) has design features that correct the hitherto unrecognised pr oblems with the use of a hand held keratometer. This study assesses th e reliability and accuracy of measurement of astigmatism using the KAR . Methods-The KAR and the Bausch & Lomb keratometer (B&L) were compare d using six back surface toric cut contact lens blanks representing 1 to 6 dioptres of astigmatism. Two observers (one experienced in the us e of the keratometers, the other a novice) took eight randomly repeate d ''masked'' measurements of each lens blank with the KAR and four mea surements with the B&L in a similar fashion. Results-There was no diff erence between the measurements with either instrument by each of the observers (p=0.95, ANOVA). The standard error of measurement for the K AR was 0.59 D, for the B&L, 0.31 D. The intraclass correlation coeffic ient 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 f or the KAR was 0.898, and for the B&L, 0.975. Conclusion-These results suggest that the KAR has good reliability and reproducibility and com pares favourably with the B&L keratometer. Inexperience with use does not affect reliability.