Kinematics of the ankle: a hinge axis model.

We used a mechanical method to locate the axis of rotation of the talocrural joint. A single constant axis of rotation was found just distal to the tips of the malleoli. The paths of light-emitting diodes mounted on a talar pin were recorded with time lapse photography during flexion-extension. These paths were analyzed both in the sagittal plane and in a plane perpendicular to the axis of rotation. Sagittal plane studies were distorted by perspective error. The plane used to analyze joint motion or shape must be the plane of motion. Models of the talocrural joint for gait analysis, reconstructive surgery and prosthetic design should have a single offset axis of rotation.