The Tibia Index: A Step in the Right Direction

Leg injuries are frequent occurrences for occupants involved in automobile crashes. This paper investigates the efficacy of a proposed injury criterion, the tibia index, to predict fractures of the leg. Using an interaction formula, the tibia index combines the applied compressive force and moment to predict mid-shaft fractures of the tibia and fibula. Quasistatic and dynamic test data of the leg are reviewed in an effort to establish critical threshold values of force and moment. The data indicate that there is minimal dependence of the fracture threshold on the direction of applied moment and suggest that a resultant moment is appropriate for the index. Meanwhile, axial loading of the leg results in bending of the tibia due to the curvature of the limb and eccentricity of the load through the ankle and knee. Since the distal and proximal ends of the fibula and tibia are weaker than the shaft, a supplemental compressive criterion is required for these regions. Validation of proposed indices relative to published experimental testing shows good correlation for dynamic results and verifies the dependence of the fracture threshold on an interaction between the applied moment and compressive force. Until additional testing can be conducted to generate injury risk functions, individual investigators must decide whether the static or dynamic criteria are most applicable for their test environment. Furthermore, implementation of the indices into dummy designs requires biofidelic response in order to provide accurate estimates of the injury risk.