Measurement reproducibility of two commercial knee test devices

Objective evaluation of patients' knee motion using mechanical devices, whether for diagnostic purposes or for assessing rehabilitative procedures, requires that these devices be reproducible, in order to avoid errors independent of the patients' condition. This study prospectively evaluates the reproducibility of two commercial knee test systems, the KT 1000 Knee Ligament Arthrometer and the Genucom Knee Analysis System, by performing repeated measurements on twenty normal men. Average knee motion, between-subject variance, and within-subject variance were determined by performing repeat tests on the same day and one week later. No significant difference was found between measurements taken on the two different test days, but the within-subject variation was high. We found that larger applied forces or moments resulted in a larger variation in mean displacements. However, the percent variation about the mean (coefficient of variation) decreased with increasing applied loads. The coefficient of variation for the KT 1000 varied from 8 to 33% and for the Genucom, from 13 to 87%. On an individual basis, large variations were found in repeated measures for both devices. To minimize errors, we recommend that repeated tests be performed, higher forces and moments utilized, and specific flexion angles be used for each device and test. Even under these conditions, caution must be exercised when evaluating individual subjects.