The foot and ankle complex as a four degrees-of-freedom system: Kinematic coupling among the foot bones.

Seventy-eight parameters are theoretically needed to describe the relative position and orientation of all the 14 bones in the foot and ankle with respect to a reference bone (foot posture). However, articular contacts and soft tissues introduce kinematic coupling, reducing the number of the foot degrees-of-freedom (DOF). This study aims at providing quantification and definition of these couplings. The foot posture was measured in vitro through a series of computed tomography scans, spanning the whole range of foot dorsi/plantar flexion and pronation/supination, also considering the effect of weightbearing. The envelope of foot postures was investigated by means of principal component analysis. The foot and ankle motion were well described with four principal sets of kinematic couplings, that is, synergies. One synergy covers the independent motion of the ankle, while three synergies describe the foot motion. The first foot synergy shows all the bones rotating approximatively about a common axis, mapping the foot abduction/adduction about the Chopart joint. The second foot synergy results in a spherical motion, whose center is located between lateral cuneiform and navicular bone, mapping the foot pronation/supination. The third foot synergy maps the opening of the foot arches during the load acceptance. The foot and ankle complex can thus be described as a four DOF system, whose motion is the result of the linear combination of four synergies. Significance: Synergies reveal the contribution of each bone to the three-dimensional foot posture, providing a compact representation of the motion of the foot and ankle complex, improving the comprehension of its physiology.