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Experimental tests using porcine brainstem samples were performed on a custom designed stress relaxation shear device. Tests were performed dynamically at strain rates >1 s−1, to three levels of peak strain (2.5%-7.5%). The directional dependence of the material properties was investigated by shearing both parallel and transverse to the predominant direction of the axonal fibers. Quasi-linear viscoelastic theory was used to describe the reduced relaxation response and the instantaneous elastic function. The time constants of the reduced relaxation function demonstrate no directional dependence; however, the relative magnitude of the exponential functions and the parameter representing the final limiting value are significantly different for each direction. The elastic function qualitatively demonstrates a dependence on direction. These results suggest that the brainstem is an anisotropic material.

Physical models of the skull-brain system have been subjected to controlled inertial loading experiments in which the deformation response of the surrogate brain was measured. The propose of this report is to present the results of these studies. Two types of models are examined herein; an idealized right circular cylinderical geometry and a baboon skull, sectioned in a midcoronal plane. The surrogate brain, consisting of an optically transparent silicone-gel, contains a painted grid of orthogonal lines with approximately 5mm spacing. The experimental data are presented in the form of nodal displacements and associated strains with one millisecond temporal resolution. The loading conditions are described by the rigid body accelerations of the skull or cylinder models. In each case the motion of the model is a noncentroidal rotation. The experimental results permit one to investigate the relations between the deformation and the acceleration magnitude and temporal characteristics.

DIFFUSE AXONAL INJURY (DAI) is a brain injury characterized by prolonged traumatic coma not due to mass lesions that has dysfunction or structural damage to brain axons. DAI can be produced by inertial loading of the head in a centroidal or non-centroidal manner. This paper compares the effect of varying the direction of head movement on the severity of DAI. Three groups of 13 monkeys are presented, each subjected to a single non-impact distributed inertial acceleration pulse with head motion constrained to a single plane. In groups 1 and 3, non-centroidal acceleration was produced in the sagittal (rotation about the y axis) and coronal (about the x axis) planes respectively, with the center of rotation in the lower cervical spine. Group 2 was subjected to centroidal acceleration in the horizontal plane (z axis). Deceleration pulse duration (6-8 msec), peak angular deceleration (1-2 × 105 rad/sec2) and angular velocity (475-510 rad/sec) were comparable in each group.

This report discusses the development of brain injury tolerance criteria based on the study of three model systems: the primate, inanimate physical surrogates, and isolated tissue elements. Although we are equally concerned with the neural and neurovascular tissue components of the brain, the report will focus on the former and, in particular, the axonal elements. Under conditions of distributed, impulsive, angularacceleration loading, the primate model exhibits a pathophysiological response ranging from mild cerebral concussion to massive, diffuse white matter damage with prolonged coma. When physical models are subjected to identical loading conditions it becomes possible to map the displacements and calculate the associated strains and stresses within the field simulating the brain. Correlating these experimental models leads to predictive levels of tissue element deformation that may be considered as a threshold for specific mechanisms of injury.