In vivo quantification of the functional characteristics of the rearfoot complex
During normal weight bearing the ankle, sub talar and mid tarsal joints function as a kinematic chain and their motions are interdependent. This chain has three important characteristics. Firstly, the motion in one of the components (joints) produces motion at the other components (joints). Secondly, the three joints are interdependent because the function of each is dependent on the position and motion at the other joints. Thirdly, the pattern of motion between the joints during weight bearing motion is fixed. The aim of this investigation was to determine the functional characteristics of the combined ankle, sub talar and mid tarsal joints (the rearfoot complex). A non invasive in vivo kinematic assessment was conducted to determine the relative rotations at the ankle/sub talar complex, the mid tarsal joint and the rearfoot complex, during weight bearing internal and external rotation of the leg in 25 subjects. The results confirm that the rearfoot joints operate as a kinematic chain. The motion at the ankle/sub talar complex suggests that the ankle is capable of a considerable range of transverse plane motion. The axes of rotation for the mid tarsal joint described in this thesis are the first for this joint quantified from a kinematic assessment and thus supersede the theoretical axes for this joint described in the literature. The predominant motion in the overall rearfoot complex is transverse plane motion. This would suggest that the primary function of the foot is to permit transverse plane rotations of the leg and proximal structures whilst maintaining the foot in a stable position of the floor. In contrast to some of the literature, the ability of the foot to accommodate the transverse plane motion of the leg is a function of all three rearfoot joints.