LAUSR

Quantification of clinically meaningful tibiofemoral motions requires a joint coordinate system (JCS) with motions free from kinematic crosstalk errors. Objectives were to use a JCS with literature-backed functional axes (FUNC) and a JCS recommended by the International Society of Biomechanics (ISB) to determine tibiofemoral kinematics of the native (i.e. healthy) knee, 2) determine variability associated with each JCS, and 3) determine whether FUNC JCS significantly reduces kinematic crosstalk errors compared to ISB JCS. Based on a kinematic model consisting of a three-cylindric joint chain, FUNC JCS included functional flexion-extension (F-E) and internal-external (I-E) tibial rotation axes. In contrast, ISB JCS included F-E and I-E axes defined using anatomic landmarks. Single-plane fluoroscopic images in 13 subjects performing a weighted deep knee bend were analyzed. Tibiofemoral kinematics using FUNC JCS fell within the physiological range of motion in all six degrees of freedom. Internal tibial rotation averaged 13° for FUNC JCS versus 10° for ISB JCS and motions in the other four degrees of freedom (collectively termed off-axis motions) were minimal as expected based on biomechanical constraints. Off-axis motions for ISB JCS were significantly greater; maximum valgus rotation was 4° and maximum anterior and distraction translations were 9 mm and 25 mm, respectively, which is not physiologic. Variability in off-axis motion was significantly greater with ISB JCS (p < 0.0002). FUNC JCS achieved clinically meaningful kinematics by significantly reducing kinematic crosstalk errors and is the more suitable coordinate system.