A two-dimensional analysis of the biomechanics of frontal and occipital head impact injuries

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Abstract
A two-dimensional plane strain finite element representation of the head was constructed and incorporated a single layered skull, cerebrospinal fluid layer, and the brain. The model that was taken in an off-centre, mid-sagittal plane in an anterior-posterior direction was used to investigate the dynamic response of the human head when subject to direct translational impact events. The physiological consequences of modelling a viscoelastic brain instead of an elastic brain were established. The influence of the stiffness of the brain matter upon the response of the head and the effect of different impact sites were also modelled. The foramen magnum was shown to have a direct influence upon the magnitude of the strains predicted to develop within the intracranial contents. The first resonant natural frequency of the head was predicted by the model as being 126 Hz. The mode shape associated with this first resonant frequency indicated that the brain mass would rotate around a point that was located at the centre of a circle which circumscribed the boundary of the skull in the sagittal plane. Compressive and tensile strains were predicted at the coup and contrecoup sites for both frontal and occipital impact events by both elastic and viscoelastic analyses. These correspond directly to coup and contrecoup injuries similar to those witnessed in clinical studies that involve translational deceleration. The effects of damping and the absorption of impact energy are more accurately modelled by using viscoelastic constitutive properties for the brain material. The initial response is no different from that predicted using elastic properties, namely, compression (negative strain or positive pressure) develops at the site of coup contusion with tension (positive strain or negative pressure) developing at the contrecoup site immediately following impact. Elastic properties fail to account for the transient response by which strain levels attenuate with time and tensile and compressive waves reverberate between the contrecoup and coup sites within the brain