A Severe Ankle and Foot Injury in Frontal Crashes and Its Mechanism

In a frontal automotive crash, the driver's foot is usually stepping on the brake pedal as an instinctive response to avoid a collision. The tensile force generated in the Achilles tendon produces a compressive preload on the tibia. If there is intrusion of the toe board after the crash, an additional external force is applied to the driver's foot.

A series of dynamic impact tests using human cadaveric specimens was conducted to investigate the combined effect of muscle preloading and external force. A constant tendon force was applied to the calcaneus while an external impact force was applied to the forefoot by a rigid pendulum. Preloading the tibia significantly increased the tibial axial force and the combination of these forces resulted in five tibial pylon fractures out of sixteen specimens. This loading condition could be one of the mechanisms of tibial pylon fracture which is rated as one of the severest forms in lower leg injuries and which was very difficult to reproduce in the laboratory.

A finite element model was used to visualize the stress distribution in the foot and ankle complex. Deformable solid elements were used for some of the bones of the foot that were regarded as having a significant influence on the response. The boundary conditions in the model were carefully defined to simulate the cadaver test. The force-time history and stress distribution in the ankle were computed and the effect of the tendon force acting in concert with an external axial load was studied. In the presence of a tendon force, the stress was found to be higher in the tibia and lower in the calcaneus. This could explain the mechanism of tibial pylon fractures in frontal crashes and how such fractures were reproduced in the cadaver.