Modeling and Simulation of the Foot and Ankle to Predict Ankle and Subtalar Joint Motion

The classical geometric model of the ankle complex has described the ankle and subtalar joints of the foot as revolute joints. This presents a general and intuitive description of the motion of the two joints. However, the motion of the subtalar joint cannot be described accurately by a single fixed axis. Knowledge of subtalar motion would help in the diagnosis and treatment of gait abnormalities. A new geometric model of the ankle complex was developed to be used with video-based motion analysis systems to predict subtalar motion based on the motion of other segments of the foot. The spatial ankle model (SAM) incorporated a revolute joint for the ankle joint and a unique spatial linkage mechanism for the subtalar joint that was based on key anatomical features and ligaments of the ankle complex. The SAM used the hypotheses that (1) the tibiocalcaneal and calcaneofibular ligaments remain relatively isometric through the normal range of motion of the foot and ankle and can be modeled as links of the system and (2) the articular surfaces between the calcaneus and talus can be modeled using a spherical joint. A cadaveric study was done to gather data to test the model in two configurations: plantarflexion and dorsiflexion (P/D) to compare ankle joint motion; inversion and eversion (I/E) to compare subtalar motion. The actual motions of the bone segments of the ankle and subtalar joints were compared with the marker-based motion of the model segments. The correlations during subtalar joint motion between the bone and skin marker results were between .94 and .99. The patterns of motion in all the clinical planes matched well. The peak differences exhibited in the sagittal and coronal plane data were due to repositioning of the specimen during changeover from skin markers to implanted bone markers. The ankle joint motion during P/D testing of the skin and bone markers nearly matched each other with a correlation of 1.00 and a maximum peak difference in the sagittal plane of 1.5°