Biofidelity of Test Devices and Validity of Injury Criteria for Evaluating Knee Injuries to Pedestrians

In the current test procedure proposed by the European Enhanced Vehicle-safety Committee (EEVC) /WG17 for evaluating leg injuries to pedestrians, a legform impactor with a rigid bony structure is used. The risk of damages to knee ligaments is evaluated with the shearing displacement and the bending angle at the knee joint. A recent study has focused on evaluating biofidelity of the legform. However, it was not possible to obtain a local deformation at the knee joint from published experiments with Post Mortem Human Subjects (PMHSs). In addition, past PMHS experiments have suggested that the height of a bumper significantly affects the risk of ligamentous damages. In this study, three kinds of finite element models were used in order to investigate the relationship between the bumper height and the shearing displacement / bending angle of the knee; 1) a legform impactor with a rigid bony structure, 2) a recently developed pedestrian dummy (Polar) with both a flexible tibia and a biofidelic knee joint structure, 3) a human lower limb. By utilizing the human model, a local deformation at the knee joint could be obtained. The model for the legform impactor and the pedestrian dummy have been validated against experiments with an actual car and in component level, respectively. The human lower limb model has been validated against published PMHS experiments. The result of a parameter study with these models in a range of bumper heights showed that the dynamic response of the dummy model is quite similar to that of the human model. In addition, it was found that the mass of the upper body significantly affects the bending angle of the knee. A geometric analysis of the knee joint was also performed to obtain tensile strains of four principal knee ligaments as a function of both the shearing displacement and the bending angle. The result suggested that the shearing displacement and the bending angle should be considered in combination when developing an injury criteria for knee ligaments.