Optimized Joint Coordinate System Achieves Clinically Meaningful Kinematics of the Tibiofemoral Joint as Compared to the International Society of Biomechanics (ISB) Recommendation.

Quantification of clinically meaningful tibiofemoral motions requires a coordinate system where motions are free from kinematic crosstalk errors. The objectives were to 1) develop an algorithm for assigning an optimized joint coordinate system (OPT JCS) that minimizes kinematic crosstalk errors based on a kinematic model of the tibiofemoral joint, 2) determine tibiofemoral kinematics of the native (i.e. healthy) knee during deep knee bend using OPT JCS and a coordinate system recommended by the International Society of Biomechanics (ISB JCS), and 3) determine whether OPT JCS significantly reduces kinematic crosstalk errors compared to ISB JCS. A novel algorithm was developed for assigning a patient-specific, OPT JCS based on a two rotational degree-of-freedom kinematic model, which included flexion-extension and internal-external tibial rotation axes. Single-plane fluoroscopic images of native knees of 13 subjects performing deep knee bend were analyzed to show that tibiofemoral kinematics using OPT-JCS fell within the physiological range of motion in all six degrees of freedom. Internal rotation of the tibia with respect to the femur averaged 11 deg and off-axis motions were minimal as expected based on biomechanical constraints. In contrast, off-axis motions for ISB JCS were non-physiologic; maximum valgus rotation was 12 deg, maximum posterior translation was 12 mm, and maximum distraction translation was 28 mm. Hence, an optimized JCS achieved clinically meaningful kinematics by significantly reducing kinematic crosstalk errors compared to the ISB recommendation and is the more suitable coordinate system for evaluating tibiofemoral joint function.