Rate of force application during knee arthrometer testing affects stiffness but not displacement measurements.

Abstract

Repeated-measures counterbalanced design. To determine the effect of rate of force application on anterior tibial displacement and anterior tibial stiffness when measured with the KT-2000 knee arthrometer. Clinicians and researchers frequently use the KT-2000 to quantify anterior tibial displacement and stiffness. While many factors (ie, tibial rotation, alignment of the arthrometer, etc) have been identified to affect KT-2000 measurements, the effect of the rate of force application has not been studied. Seventeen recreationally active males between the ages of 19 and 36 years (mean age +/- SD, 27.8 +/- 5.3 years) with no previous history of knee injury participated. With the knee in 25 degrees of flexion, the KT-2000 was applied to the participants' anterior tibia. While the distal femur was stabilized, the first author applied a posterior-to-anterior force at a fast and slow rate. Three trials for the slow and fast rates of force application were averaged and used for statistical analysis. Anterior tibial displacement (mm) was measured at 133 N and the related anterior tibial stiffness (N/mm) values were calculated between 89 and 133 N. Separate paired t tests with Bonferroni adjustment were used to determine if differences in displacement and stiffness between rates of force application were present. Stiffness was significantly greater in the slow (mean +/- SD, 58 +/- 22 N/mm) as compared to the fast trials (mean +/- SD, 47 +/- 19 N/mm) (P = .005). Differences in displacement, however, were small and not significant (P = .132) between the slow (mean +/- SD, 3.9 +/- 1.5 mm) and fast (mean +/- SD, 4.0 +/- 1.6 mm) trials. The rate of force application affects anterior tibial stiffness but not anterior tibial displacement when measured with the KT-2000. This suggests that to ensure reliable results when using the KT-2000, the rate of force application must be controlled when measuring stiffness between 89 and 133 N, but not when measuring anterior tibial displacement at 133 N.