Measurement of Knee Laxity and Pivot-Shift Kinematics With Magnetic Sensors

This article describes work to develop a system to measure translational and rotational knee laxity and complex “dynamic” tests, such as the pivot-shift test, by the use of electromagnetic sensors mounted onto the limb segments with specially developed splints and clamps. We have measured in vitro and in vivo the motion of bone-mounted sensors versus skin-mounted sensors, and thus shown that there was an overall accuracy of approximately ±1 mm in translation and ±1° in rotation when the movements were relatively slow. It was found that, for clinical pivot-shift testing, the dynamic motion led to inaccuracy because of the inertia of the sensor mounting system, which led to the peaks of the sudden reduction motions not being measured; this system requires further development. We show that it also is possible to use this system in the clinic to record both rotational and translational knee laxity motions, both of which are integral to the pivot-shift instability. The force and torque were applied to the limb with sets of constant-force springs, which allowed the knee to be flexed while maintaining a constant load across the knee. This maneuver led to graphs showing the limits of translation and rotation laxity, across the range of knee flexion, for a series of 60 patients. It was then shown that the pivot-shift motion occurred within these laxity limits. The level of measurement accuracy means that the system described should be clinically useful.