A fiber-reinforced composite model of the viscoelastic behavior of the brainstem in shear

Brainstem trauma occurs frequently in severe head injury, often resulting i n fatal lesions due to importance of brainstem in crucial neural functions. Structurally, the brainstem is composed of bundles of axonal fibers distin ctly oriented in a longitudinal direction surrounded by an extracellular ma trix. We hypothesize that the oriented structure and architecture of the br ainstem dictates this mechanical response and results in its selective vuln erability in rotational loading. In order to understand the relationship be tween the biologic architecture and the mechanical response and provide fur ther insight into the high vulnerability of this region, a structural and m athematical model was created. A fiber-reinforced composite model composed of viscoelastic fibers surrounded by a viscoelastic matrix was used to rela te the biological architecture of the brainstem to its anisotropic mechanic al response. Relevant model parameters measured include the brainstem's com posite complex moduli and relative fraction of matrix and fiber. The model predicted that the fiber component is three times stiffer and more Viscous than the matrix. The fiber modulus predictions were compared with experimen tal tissue measurements. The optic nerve, a bundle of tightly packed longit udinally arranged myelinated fibers with little matrix, served as a surroga te for the brainstem fiber component. Model predictions agreed with experim ental measures, offering a validation of the model. This approach provided an understanding of the relationship between the specific biologic architec ture of the brainstem and the anisotropic mechanical response and allowed i nsight into reasons for the selective vulnerability of this region in rotat ional head injury. (C) 1999 Elsevier Science Ltd. All rights reserved.