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ON THE ROLE OF THE BRAIN'S GEOMETRY IN CLOSED HEAD INJURIES

INTRODUCTION The two most frequently used predictors for developing tolerance criteria for Closed Head Injuries (CHI) are (i) the distribution of the strain field in the brain matter [1] and (ii) the distribution of the intracranial pressure [2]. Although it is widely accepted that the fluidity of the brain tissue should be taken into account when modeling brain injuries (e.g., [3]), recent experiments with piglets show pressure to be an inferior predictor of brain injuries in comparison to strain [4]. Our study on the role of the brain's geometry in CHI provides an explanation why pressure might be an inadequate predictor. The results are based on a nonlinear, viscoelastic, (incompressible) fluid model that generalizes the linear, viscoelastic, solid Kelvin-Voigt model. To describe the fluidity of the brain, we replace the linear, temporal, partial derivative with the nonlinear, temporal (material) derivative and add pressure and hydrostatic compression terms [5]. The basic tenet of our approach is that nonlinear wave phenomena can explain the origin and features of CHI. Such phenomena arise when the material velocity V of the tissue is comparable to or exceeds the brain shear-wave velocity c. The nonlinear effects can induce large values of strain and consequently damage veins and/or neurons. We showed previously (see [5,6] and references therein) that when a head rotation induces V that exceeds c, a noncircular brain cross-section suffices for nonlinear phenomena to appear. We also showed that even if V induced in the gray matter is smaller than c in the gray matter, a deviation from circular symmetry combined with the differences in the physical properties of the gray and the white matter (or of the white matter and the cerebral fluid in the ventricles) can explain why Diffuse Axonal Injuries (DAI) are created in the white matter near the border with the gray matter (or the ventricles).