Linear Viscoelastic Properties of Cerbral Cortex at Thresholds for Axonal Damage

Traumatic brain injury (TBI) is caused by rapid deformation of the brain that leads to shearing of axons. While deformation below the limits of ultimate failure can activate pathophysiological cascades that cause neurodegeneration [1], bleeding does not always occur even after tearing of axons. Traditional imaging studies such as CT and MRI are designed to detect areas of bleeding but these can fail to detect the presence of multiple, widespread, microscopic axonal injuries that can result in devastating neurological deficits. A large knowledge gap still exists defining the relationship between axonal injury at a microscopic level (morphological injury) and the material properties of the corpus callosum, hippocampus and cerebral cortex on the macroscopic level, but at identical strain levels. This research investigates the linear viscoelastic properties of the cerebral cortex at known thresholds of axonal injury (0.14 - 0.34 strains [2]). During quasi static loading of tissue in creep tests, instantaneous strains were generated corresponding to axonal thresholds. A linear viscoelastic constitutive model was used to determine six Prony parameters suitable for finite element simulation in ABAQUS and ANSYS. Use of such properties at the levels of axonal damage will help to accurately predict injuries during numerical simulations, to design safety helmets and air bags, and also to refine existing injury criteria and to improve the precision in surgical procedures.