For years the largest safety concern for vehicle manufacturers has focused on preventing life threatening injuries, particularly, those to the head, chest, and neck. Improved safety measures have reduced these injuries and focus may be directed to the protection of other body regions, notably the lower extremities. This research begins with an extensive review of all aspects of the lower extremities with regards to the automotive crash environment. This includes a detailed description of the anatomy of the lower limbs, discussion of injuries from a clinical standpoint, mechanisms for their causation during a car crash, and coverage of current regulations and injury criteria. Due in part to the limited number of tools available to the engineer, it was determined that relatively little information is factually known concerning the injuries to the lower limbs in auto crashes, more specifically to the foot and ankle.

To address this issue, a three-dimensional finite element model of the 5th and 50th percentile human lower extremity has been developed, providing a research tool for impact biomechanics studies. The model accurately represents the geometry and mass of all skeletal elements. The model consists of five rigid segments connected via four joints:​ the hip, knee, ankle, and sub-talar. Each joint contains a non-linear active and passive component of joint stiffness and velocity dependence, and is capable of representing different levels of muscle activation or tensioning. Both models were validated using experimental data published on the impact response of the lower limbs.

The FEM model has been implemented to investigate ankle injury in varying crash modes. A model of the Hybrid HI crash test dummy was modified to replace the dummy’s lower limbs with that of the human. The model was placed in the occupant compartment of a mid-size passenger car where, boundary accelerations were applied. Results have revealed that in an offset, car-to-car collision, the occupant is subject to significant longitudinal and lateral loads simultaneously. The consequence of this is that, at maximum axial loading, the lateral forces are enough to potentially cause injury, especially to the 5th percentile female. Another result of this study indicated that current measuring devices, such as the Hybrid III dummy, are not able to detect this mechanism of injury. The FEM model also demonstrated that a commonly used crash test, intended to mimic the car-to-car offset collision, does not properly represent the real world case.

To further investigate this problem, a case study was performed. The case study involved an occupant who suffered two major lower limb injuries, both of which have caused permanent disability. The impact scenario was a car-to-car, offset frontal collision where there was no intrusion of the vehicle’s structure into the occupant compartment. This analysis revealed that the design of the vehicle’s interior acted to magnify the injury mechanism previously identified. The case study has further validated the results of the previous simulations and clearly supports the theory that lateral ankle injuries are an issue to address in vehicle design. It is concluded from this study that a new ankle injury mechanism has been identified, FEM modeling is an effective tool for addressing vehicle safety issues, and current methods for assessing and regulating lower limb injury need to be modified.