Ankle Kinematics Described By Means Of Stereophotogrammetry And Mathematical Modelling

The ankle is a complex structure allowing foot mobility while providing stability. In an attempt to improve the knowledge of the kinematics of the ankle, an approach incorporating both experimental and analytical techniques was developed. Stereophotogrammetry combined with the Direct Linear Transformation (DLT) technique, was used to quantify the spatial displacements of the foot. Four motorized cameras were fixed on a baseboard 0.62 m from a support frame so as to obtain two stereopairs, one medial and one lateral. For a pair, the cameras were 0.52 m apart and maintained a convergent angle of 21.5°. The support frame was designed to fix the tibia while allowing foot motion. A device comprised of 76 markers, 38 of which were visible to each pair of cameras was used for the calibration. The spatial position of each marker was measured to a precision of 0.05 mm whereas their computed spatial position using the DLT technique was accurate to 0.4 mm. For the experiment, two embalmed cadaver legs and feet, amputated at midshank and of normal appearance were used. After a partial dissection, three pin markers were embedded into each of the medial and lateral sides of the talus permitting the calculation of its center of rotation. Each foot was photographed in 5 positions at 10° intervals, ranging from 30A‚° of plantarflexion to 10° of dorsiflexion. An analytical model was developed to spatially describe the rotation of the foot about the ankle. The model calculates the plane of motion and the orientation of the axis of rotation relative to the sagittal, frontal and transverse planes. These were found respectively to be for foot one: 100°, 86°, 15° and for foot two: 91°, 69°, 21°.