Numerical Accident Reconstructions A Biomechanical Tool to Understand and Prevent Head Injuries

Traumatic brain injuries (TBIs) are a major health and socioeconomic problem throughout the world, with an estimated 10 million deaths and instances of hospitalization annually. Numerical methods such as finite element (FE) methods can be used to study head injuries and optimize the protection, which can lead to a decrease in the number of injuries. The FE head models were initially evaluated for biofidelity by comparing with donated corpses experiments. However, there are some limitations in experiments of corpses, including material degradation after death. One feasible alternative to evaluating head models with living human tissue is to use reconstruction of real accidents. However, the process of accident reconstruction entails some uncertainties since it is not a controlled experiment. Therefore, a deeper understanding of the accident reconstruction process is needed in order to be able to improve the FE human models. Thus, the aim of this thesis was to evaluate and further develop more advanced strategies for accident reconstructions involving head injuries. A FE head model was used to study head injuries in accidents. Existing bicycle accident data was used, as were hypothetical accident situations for cyclists and pedestrians. A FE bicycle helmet model having different designs was developed to study the protective effect. An objective method was developed based on the Overlap Index (OI) and Location Index (LI) to facilitate the comparison of FE model responses with injuries visible in medical images. Three bicycle accident reconstructions were performed and the proposed method evaluated. The method showed to have potential to be an objective method to compare FE model response with medical images and could be a step towards improving the evaluation of results from injury reconstructions. The simulations demonstrated the protective effect of a bicycle helmet. A decrease was seen in the injurious effect on both the brain tissue and the skull. However, the results also showed that the brain tissue strain could be further decreased by modifying the helmet design. Two different numerical pedestrian models were compared to evaluate whether the more timeefficient rigid body model could be used, instead of a FE pedestrian model, to roughly determine the initial conditions as an accident reconstruction involves some uncertainties. The difference, in terms of the head impact location, rotation and velocity, attributable to the two models was in the same range as differences due to uncertainties in some of the initial parameters, such as vehicle impact velocity.