Finite Element Modeling of the Human Head

Abstract: The main objectives of the present thesis were to define the dimension of head injuries in Sweden over a longer period and to present a Finite Element (FE) model of the human head which can be used for preventive strategies in the future. The annual incidence of head injuries in Sweden between 1987 and 2000 was defined at over 22 000, cases most of which were mild head injuries. In contrast to traffic accidents, head injuriy due to fall was the most important etiology. Of special interest was that the number of hematoma cases has increased.A detailed and parameterized FE model of the human head was developed and used to evaluate the effects of head size, brain size and impact directions. The maximal effective stresses in the brain increased more than a fourfold, from 3.6 kPa for the smallest head size to 16.3 kPa for the largest head size using the same acceleration impulse. The size dependence of the intracranial stresses associated with injury is not predicted by the Head Injury Criterion (HIC). Simulations with various brain sizes indicated that the increased risk of Subdural Hematoma (SDH) in elderly people may to a part be explained by the reduced brain size resulting in a larger relative motion between the skull and the brain with distension of bridging veins. The consequences of this increased relative motion due to brain atrophy cannot be predicted by existing injury criteria.From studies of the influence of impact directions to the human head, the highest shear strain in the brain stem is found for a Superior-Inferior (SI) translational impulse, and in the corpus callosum for a lateral rotational impulse when imposing acceleration pulses corresponding to the same impact power. It was concluded that HIC is unable to predict consequences of a pure rotational impulse, while the Head Impact Power (HIP) criterion needs individual scaling coefficients for the different terms to account for differences in intracranial response due to a variation in load direction. It is also suggested that a further evaluation of synergic effects of the directional terms of the HIP is necessary to include combined terms and to improve the injuryprediction.Comparison of the model with experiments on localized motion of the brain shows that the magnitude and characteristics of the deformation are highly sensitive to the shear properties of the brain tissue. The results suggest that significantly lower values of these properties of the human brain than utilized in most 3D FE models today must be used to be able to predict the localised brain response of an impact to the human head. There is a symmetry in the motion of the superior and inferior markers for both the model and the experiments following a sagittal and a coronal impact. This can possibly be explained by the nearly incompressible properties of brain tissue. Larger relative motion between the skull and the brain is more apparent for an occipital impact than for a frontal one in both experiments and FE model. This correlates with clinical findings. Moreover, smaller relative motion between the skull and the brain is more apparent for a lateral impact than for a frontal one for both experiments and FE model. This is thought to be due to the supporting structure of the falx cerebri.Such a parametrized and detailed 3D model of the human head has not, to the best knowledge of the author, previously been developed. This 3D model is thought to be of significant value for looking into the effects of geometrical variations of the human head.