Tissue-Level Tolerance Criteria for Crash-Related Head Injuries: A Combined Experimental and Numerical Approach

Head injury (HI) is the fourth leading cause of death in the Western world and a major source of post-traumatic disability. It is the result of a fierce acceleration or an impact to the head, which can be caused by falls, vehicle accidents, accidental hits in sport and recreation, and assaults. Over the last decade, approximately 4 million people suffered yearly from Traumatic Brain Injury (TBI) in Europe. The epidemiology, social and economical burden of HI proves that we need research in this area. Only with complete understanding of the head injury mechanisms and mechanical ethiogenesis, we will be able to prevent and protect against these injuries.

Controversy still exists within the biomechanics community regarding head injury mechanisms. Nevertheless, we widely accept that rotational and translational acceleration and velocity can cause injury if sufficiently large in absolute value and / or impulse duration. However, we need further research on experimental and numerical models to validate these hypotheses. It is a demanding challenge to combine subject-specific mechanical properties of the human head and evaluate tissue-level thresholds for head injuries.

This dissertation focuses on the study of tissue tolerances in crash-related head injuries combining experimental and numerical approaches. The challenge in the identification of these tissue-level tolerance criteria comes from, among other things, the anatomical complexity of the human head. This thesis focused on the lower tissue-level threshold for cerebral contusions. For this purpose, we developed an in vivo porcine model of cerebral contusion. It evaluated the minimal levels of strain resulting in cerebral contusion and related the mechanical loading input to contusion characteristics. These results reveal a lower tissue threshold for cerebral contusion development at an impact velocity of 0.5 - 2 m/s and impact depth of 1 - 2 mm. In combination with a Finite Element (FE) model, we evaluated the stress and strain patterns during induced cerebral contusions. The internal response of the brain tissue can be analysed in-depth and related to the ethiogenesis of cerebral contusions. Results affirm the feasibility to evaluate the internal brain response with the proposed methodology.

Additionally, this thesis evaluated the implication of subject-specificity of soft and hard tissues in human head FE models. Doing so, this thesis analysed case-specific head impacts, as well as provided subject-specific soft tissue material models. First, we characterised human cranial vault dura mater in vitro using planar biaxial tensile tests. We obtained stress-stretch curves and a constitutive model succesfully captured the tissue's material behaviour. Results reveal that the Gasser-Ogden-Holzapfel model most successfully captures the behaviour of the dura mater. Secondly, we implemented experimental data of skull fractures in case-specific FE models, combining both a subject-specific skull geometry and a subject-specific material model for cranial bone based on Hounsfield Units. We analysed the influence of the local geometry at the impact site and the material model on the internal skull strain energy. These subject-specific models predict the fracture lines with high precision. The results reveal influencing factors on the skull strain energy such as contact area, scalp thickness and impactor positioning.

Finally, this PhD presents engineering contributions to the mechanopathogenesis of head injuries, particularly in fronto-temporal cerebral contusions and skull fractures. This thesis objectively demonstrates the challenges, limitations and opportunities in head injury research, hopefully leading to an improved design of protective headgear.