A finite element analysis employing linear biphasic theory is developed to study the dynamic response of the human head under impact. Intracranial tissues are modeled as a binary mixture, i.e. the fluid and solid phases. The head is modeled as an axisymmetric configuration consisting of skull, CSF(cerebral spinal fluid) in the subarachnoid space, and brain. The loading function is assumed to be distributed uniformly through polar angle and varied as a sine squared function with time.
The permeabilities of the subarachnoid space and brain which may reproduce the same coup and contre-coup CSF pressures from the monophasic model are searched in the specified range of skull permeability. Another numerical simulation is conducted to characterize the effect of the volume ratios between two phases in the skull and subarachnoid space. The variation of the volume ratio in the subarachnoid space affects the intracranial pressure distribution of the lateral part while the variation in skull does not.
Once the model is established, the finite element analyses of the head in frontal and occipital impact are performed. When the head is directly impacted, a positive(compression) pressure occurs at the impact side while the negative(tension) pressure occurs at the opposite side. The contre-coup pressure in the frontal lobe due to the occipital impact is a little higher than the one in the occipital lobe due to the frontal impact. Since the brain injury are resulting from the rupture of the blood vessesls or neurons, we may estimate the dangerous site of brain injury by the strain of the solid phase in addition to the hydrostatic pressure. At the impact side, the maximum tensile strain occurs beneath the subarachnoid space in radial direction. At the opposite side, the maximum tensile strain along with the large negative pressure drop occurs in the subarachnoid space in anterior-posterior direction. The pressure drop at this opposite side is significant enough that the blood vessels may rupture by the pressure difference. However, the blood vessels at the impact side as well as the neurons and meninges have no such adverse pressure differences and are speculated that they may be destroyed by the axial tension. These results are in good agreement with Nahum's autopsy data showing hemorrhage at the frontal and occipital lobes.
