Physiologically-based spatial mechanisms for rehabilitation of the human knee and ankle joints

In virtually unloaded conditions, the tibiofemoral (knee) and tibiotalar (ankle) joints behave as single degree-of-freedom systems. In these conditions, fibres within the ligaments remain nearly isometric throughout the flexion arc and articular surfaces do not deform considerably. Relevant theoretical models show that ligaments and articular surfaces act together as mechanisms to control passive joint kinematics. In the knee, isometric fibres were identified within the ACL, PCL, MCL ligaments, and rigid contacts were associated to the two condylar articular surfaces. In the ankle, isometric fibres were identified within the calcaneal-fibular and tibio-calcaneal ligaments, and rigid contacts were associated to the articular surfaces between the tibio-fibular mortise and the talus. Based on these assumptions, equivalent mechanisms were defined which accurately replicate the knee and ankle motion. These mechanisms are useful for a more physiology-based comprehension of human diarthrodial joint motion and for devising efficient rehabilitation techniques. In particular, based on experimental observations, mechanisms were found that model the knee and ankle joint motion. These mechanisms feature two members (i.e. the rigid bones - cartilages) in mutual contact (at the articular surfaces) and interconnected by rigid links (i.e. the ligaments’ isometric fibres). Different models having contact surfaces with increasing approximation were analyzed. A further series of mechanisms feature two members interconnected by one spherical joint and two rigid links: the lower number of members makes this model geometrically and mathematically much simpler. Geometrical configuration of these models and validation in terms of comparison between instrumental measurements and model predictions were obtained from experiments in fresh frozen amputated lower limbs, free from anatomical defects. For each model, a bounded optimization procedure was used to find the optimal geometric parameters which allow the different models to best-fit the experimental motion. Joint kinematics predicted by these models replicated very well the corresponding experimental measurements. The difference between the experimentally determined and optimally defined ligament attachment points varied between 0.2 and 10 mm. Mechanisms with a spherical pair replicated motion with a good, though lower, precision, but with much smaller computational costs. The proposed spatial mechanisms are important tools for devising more physiological mechanical models of both the knee and the ankle joints and of the entire lower limb, that make it possible to improve rehabilitation procedures.