Foot-Ankle Joints Responses. Epidemiology, Biomechanics and Mathematical Modeling

Abstract: The purpose of this thesis is to acquire better knowledge of foot-ankle injuries because of their increasing importance in car crashes. Three approaches were used: epidemiology, biomechanics and mathematical modeling. The knowledge is intended to serve as a basis development of dummies and protective systems in order to reduce the frequency and cost of foot-ankle injury.

In the epidemiological approach, the influence on foot-ankle injuries of impact location, seating position and age were investigated using Swedish insurance data. Impact location and seating position were of greater significance than age. The results also indicated that both intrusion and local deceleration in the footwell area need to be considered as causal factors for foot-ankle injuries in frontal impacts. However, the role of foot-controls on lower leg biomechanics and kinematics remains unclear.

In the biomechanical approach, a new method was developed to determine the center of rotation (CR) and moment-angle characteristics of the ankle and subtalar joints quasi-statically under four basic movements: dorsiflexion, plantarflexion, inversion and eversion. Human subtalar and ankle-subtalar joints were tested in their natural range of motion (ROM) and to the first sign of joint failure. The results showed that the CR of ankle-subtalar joints was not fixed but moved with calcaneal rotation. CR was near the ankle joint in the dorsiflexion/plantarflexion ROM. In inversion/eversion ROM, the CR of the ankle-subtalar joints coincided with the CR of the subtalar joint. The moment-angle characteristics were determined at a fixed CR. From these results, the stiffness of the ankle-subtalar and the subtalar joints were calculated, and their contribution was estimated. Average moment and angle failure levels were determined.

As another part of the biomechanical approach, the stress-strain characteristics of 7 foot-ankle ligaments were determined quasi-statically. Using the results from isolated ligament tests and ankle-subtalar joint biomechanics, a physical model of the human foot-ankle was developed to investigate ligament injury mechanisms in 2-D loading. The physical model was used as a first step for the development of a mathematical model.

In the mathematical approach, MADYMO models of the Hybrid III, advanced Hybrid III (GM/FTSS), and a human lower leg were developed. The models were validated in quasi-static loading. A parameter investigation was carried out to evaluate the effect of crash acceleration, toepan intrusion and toepan rotation. Acceleration and intrusion influenced dorsiflexion responses, while toepan rotation affected inversion responses. The mathematical models were found useful to investigate crash configurations, safety-countermeasures and injury parameters since the models provide additional information not otherwise obtainable with the current Hybrid III lower leg.