Development of clinical measurement of the axes of rotation of the ankle and subtalar joints

For clinical assessment of kinematic foot function, it is of value to decouple ankle joint movement from subtalar joint movement. This cannot be done by simple gait analysis and requires a more complex mathematical analysis of the motion of the ankle-foot complex. Assuming the joints to be simple hinges it is possible to describe the parameters for the axes of these joints as part of a series of geometrical transformations from the shank via the ankle and the subtalar joints to the foot. Motion data for the shank and posterior foot were collected from a Vicon 510 8-camera gait analysis system. An optimization method was applied to obtain the best fit of the data to the model. Results show that a reproducibility of axes locations to within 2 mm and axes angles of about 2°. The system is being evaluated more fully in a series of clinical studies.