Traumatic Brain Injuries (TBIs) account for about half of the 1 300 000 annual traffic related deaths and the 50 000 000 injuries worldwide. The burden of TBI is ethically unacceptable and economically unsustainable. Recognising the efforts and achievements in reducing TBI conducted by the vehicle industry, research institutions and academy worldwide, the problem still call for additional research that lead to prevention of TBI. The main aim of this thesis is to develop a brain injury criterion and associated injury thresholds that can be used with crash test dummies in the design of safer cars.

The craniocervical motion that produces diffuse brain injuries in experimental settings with animals was investigated by introducing finite element (FE) models of the animals. One rat and one monkey brain FE model were developed from medical images of the animals and validated using experimental data. The validated rat model was applied to simulate sagittal head rotational acceleration experiments with rats. Sequential analysis of the trauma progression indicated that acute subdural haematoma occurred at an early stage of the trauma, while diffuse axonal injury likely occurred at a later stage. The validated monkey model was applied to simulate past head impact experiments with primates that typically produced concussion symptoms. The analysis revealed large brainstem strains supporting the hypothesis that concussions are produced due to mechanical loading of the brainstem. These results also indicate the need to incorporate the craniocervical motion in human FE models and physical test devices in the development of countermeasures for concussive injury prevention.

A method to make primate brain injury experimental data applicable for humans was also investigated. The monkey FE model was used to simulate 43 primate head impact experiments. Brain tissue injury risk curves that relate probability of injury, obtained in the experiments, with brain strains estimated in the simulations were developed. By assuming comparable mechanical properties of the brain tissues in monkeys and humans, these risk curves were applied to estimate injury risk in 76 impacts simulated with a human head-neck FE model which was also developed and validated for the purpose of this investigation. Overall, the investigated method proved to be technically feasible and to provide biomechanically justifiable means to related craniocervical kinematics and brain strains. This method accounts for contact phenomena typical from vehicle crash like head impacts, which past scaling techniques did not.

Finally, new conceptual global brain injury criterion and injury risk functions that have the potential to predict the risk of diffuse brain injuries, were developed. The concept, denoted as Brain Injury Threshold Surface (BITS), establishes equal brain injury risk surfaces as a function of time-dependent and combined translational and rotational head kinematics typical in head impacts in car crashes. BITS appeared to explain the variance seen in both concussion from the monkey experiments and brain strains levels from the simulations with the monkey and the human brain FE models. Although evaluations of the new criteria and associated risk surfaces are pending, these have the potential to guide the development of superior restraints which would reduce the number and severity of brain injuries in future traffic accidents.