MATERIAL CHARACTERIZATION OF THE BRAIN-STEM FROM OSCILLATORY SHEAR TESTS

Traumatic damage to the brainstem occurs frequently when the brain-sku ll complex experiences injurious loading especially during those traum atic situations that produce diffuse axonal injury (DAI). DAI has been shown to be dependent on load direction and correlated with regional tissue deformation in response to rotational inertial loads. Possible mechanisms for the selective vulnerability of the brainstem are (1) th e geometry of the central nervous system is responsible for producing high tissue strains in these regions, (2) regional differences in over all material stiffness result in larger deformations at these sites, a nd (3) the anisotropic mechanical properties of these regions lead to a sensitivity to the rotational load direction and magnitude. This pap er investigates the latter two hypotheses by performing oscillatory sh ear tests on adult porcine brainstem in three mutually perpendicular d irections. The complex shear moduli were calculated over a range of fr equencies (20-200 Hz), for three levels of peak engineering strain (2. 5%, 5.0%, and 7.5%). The directional data demonstrated that the brains tem exhibits significant transversely isotropic behavior. Both compone nts of the complex modulus in which the axonal fibers are oriented par allel to the plane of shear but transverse to the shear direction were significantly higher than those of the other two, mutually indistingu ishable test cases across the range of strains tested. By comparison w ith similar tests on cerebral tissue, these data demonstrated that the brainstem displays a stiffer biomechanical response. These difference s were present for both components of the complex shear modulus and we re greater as the magnitude of the applied strain increased. The regio nal stiffness and anisotropic response of the brainstem coupled with i ts location as a narrow bridge between CNS regions interact to result in the selective vulnerability of this region in rotational loading. ( C) 1998 Elsevier Science Ltd. All rights reserved.