JAN 08, 2013
This prospective study evaluated the comparability and repeatability of corneal astigmatism measurements obtained with four different devices: automated keratometers, manual keratometer, corneal topographer and Scheimpflug imager. The authors also evaluated the interobserver variability of a new automated keratometer (SMI Reference Unit, Sensomotoric Instruments GmbH).
They note that their study differs from previous studies comparing corneal astigmatism measurements obtained with different keratometers because they used astigmatism magnitudes only, with no regard to the astigmatism meridian. Because astigmatism is a vector with a magnitude and a direction, they say vector analysis must be used to compare astigmatism data.
Using an astigmatism vector analysis, they showed that corneal astigmatism measurements obtained by automated, manual, and simulated keratometry were comparable. However, equivalent K values obtained with the Pentacam device were not comparable with those of the other keratometers. While the repeatability of the astigmatism magnitude was acceptable for all devices, the repeatability of the astigmatism meridian was moderate. This may be clinically relevant when calculating the toric IOL power and implantation axis, as repeated measurements with the same device may be required to improve the accuracy of toric IOL calculations. Finally, the SMI Reference Unit was found to have a good interobserver variability.
It is interesting that the magnitude for all devices evaluated is similar except for the Pentacam, which reportedly measures the back curvature of the cornea. Also, this confirms that the ability to nail the axis is inconsistent, which is no surprise to those of us in the trenches.