Knee flexion angle estimation based on functionally instead of anatomically defined coordinate Systems

Abstract

Knee angles are commonly presented in gait analysis reports. They are typically calculated as the relative angles between coordinate systems (CSs) rigidly attached to the femur and the tibia. To give these angles a consistent meaning, the CSs are defined with respect to anatomical landmarks. Defining accurate anatomical CSs is not an easy task, because it requires skills in marker placement, landmark identification and definition of a biomechanical model. Therefore we present a novel method to (i) functionally define two CSs attached to femur and tibia and (ii) functionally calculate the knee angle based on the relative differential kinematics between those CSs. The method has the advantage of being independent (i.e. invariant) of the choice of the original CSs of the femur and tibia, removing the need for accurate marker placement. Consequently, (i) the markers may be placed on optimal landmarks, for example, minimizing the soft tissue artifacts or improving subject’s comfort, and (ii) there is no need for anatomical calibration when technical marker clusters/triads are used.