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.