Reanalysis of primate head impact experiments to clarify mild traumatic brain injury kinematics and thresholds

Licentiatavhandling, 2013

Fatalities and severe injuries due to road traffic accidents still represent a serious health and economic issue in today’s society. Brain injuries are the most common severe injuries and these injuries account for more than half of the 1.3 million traffic related deaths annually worldwide. However, improved accident avoidance systems are predicted to mitigate and reduce future accident severity. Thus, the number of traffic related deaths and severe injuries would be reduced and therefore a focus shift towards long-term disabling injuries is expected. One of the most common of such injuries is mild traumatic brain injury. To develop effective countermeasures aimed at reducing traumatic brain injuries, it is essential to understand head-neck and brain kinematics at impacts to be taken into account in the development of injury criteria and the establishment of thresholds. With this purpose, experiments comprising animals, used as human surrogates, are deemed essential and historically, head impact experiments on non-human primates have been carried out. Some of these experimental results have been scaled to suit humans and were used in the development of the head injury criterion, currently in use in the FMVSS 208 US regulation for motor vehicles. This head injury criterion has been used for decades, but is still criticised for not considering many factors that are important to brain injury. Such factors include the impact direction and area of contact, stiffness of the impacting surface, and rotational accelerations induced by oblique impacts or when the torso is restrained. Therefore, alternative or complementary criteria that consider rotational acceleration of the head have been proposed in combination with brain tissue injury criteria, for human head finite element models. The finite element models are currently undergoing validation with reconstructions of real-life sports and traffic accidents, as well as scaled animal injury data. Unfortunately, the accuracy of the methods used to capture head kinematics and detailed brain injury location and severity from real-life events has limitations. The existing criteria also fail to capture the head-neck kinematics that causes the brain injuries. Moreover, the methods used to scale animal data to humans are not reliable. The ultimate goal of this study is to generate knowledge that contributes to the development of MTBI criteria and associated limits that will, when properly applied, reduce the number of moderate brain injuries due to closed head impacts. This thesis aims at proposing improved criteria that account for the head-neck and brain kinematics that occur during brain trauma and to provide thresholds for concussions. This will be facilitated by numerical reconstruction of past head trauma experiments using primates. By re-analysing the existing primate trauma experiments, using a finite element model of these specimens, the reliability of global head injury criteria available in literature was evaluated. This was done by simulating and analysing sub-sets of frontal and occipital primate head impacts; selected from large series of trauma experiments previously conducted at the Japan Automobile Research Institute. Based on the simulation results, the brain injury kinematics hypothesised when the former experiments were analysed is supported: concussions occur due to physical stress to the brainstem. Based on these findings, brain tissue injury thresholds were also proposed. Assuming that tissue thresholds are the same for non-human primates and humans, these results can be used to interpret results obtained with human finite element models without scaling. In the future, these results are expected to be used as references for virtual safety assessment and to provide the basis for further development of protective strategies for humans.