Effect of instrument rotation on handheld keratometry

To study the effect of instrument rotation on handheld automated keratometry. Department of Optometry and Radiography, The Hong Kong Polytechnic University, Hong Kong SAR, China. In 33 recruited subjects, corneal curvature in 1 eye (randomly selected) was measured using a Medmont topographic keratometer and a Nidek handheld keratometer. In handheld keratometry, measurements were obtained holding the instrument in the upright position and rotating it anticlockwise 5 degrees, 10 degrees, and 15 degrees and then clockwise 5 degrees, 10 degrees, and 15 degrees. The sequence of measurements was randomly assigned. The steepest and flattest corneal curvatures as well as the axis along the flattest meridian were compared in different conditions. The corneal power was converted to vector representation (M, J0, J45) for a second comparison. There was a significant difference in the steepest and flattest curvatures in different conditions (repeated-measures analysis of variation, P<.05). However, the mean difference between handheld and topographic keratometry was approximately 0.25 diopter (D) for the steepest curvature and 0.05 D for the flattest curvature regardless of the direction and amount of rotation. The intraclass correlation coefficients were nearly 1, which indicated good clinical reliability. The mean difference in axis determination followed the direction and amount of rotation. With vector representation, the difference between the devices increased with the amount of rotation, especially J0and J45. Handheld keratometry was in agreement with topographic keratometry. However, practitioners should adjust the axis manually according to the direction and amount of rotation. The difference between handheld and topographic keratometry increased with the rotational effect, which was seen with vector representation. Practitioners are advised to use the handheld keratometer in the upright position.