Material characterization of the brainstem from oscillatory shear tests

Area of Science:

• Biomechanics
• Neuroscience
• Trauma Research

Background:

• Traumatic brain injury (TBI) frequently involves diffuse axonal injury (DAI).
• The brainstem's vulnerability in TBI is linked to load direction and tissue deformation from rotational inertial loads.
• Hypotheses for brainstem vulnerability include CNS geometry, regional material stiffness, and anisotropic mechanical properties.

Purpose of the Study:

• To investigate the hypotheses that regional stiffness and anisotropic mechanical properties contribute to brainstem vulnerability in TBI.
• To quantify the biomechanical response of the brainstem under shear loading.
• To compare the mechanical properties of the brainstem to cerebral tissue.

Main Methods:

• Oscillatory shear tests were performed on adult porcine brainstem samples.
• Tests were conducted in three mutually perpendicular directions across a range of frequencies (20-200 Hz) and peak engineering strains (2.5%, 5.0%, 7.5%).
• Complex shear moduli were calculated to assess material properties.

Main Results:

• The brainstem demonstrated significant transversely isotropic behavior, indicating direction-dependent mechanical properties.
• Mechanical responses were significantly higher when axonal fibers were oriented parallel to the shear plane but transverse to the shear direction.
• The brainstem exhibited a stiffer biomechanical response compared to cerebral tissue, with differences increasing at higher strain magnitudes.

Conclusions:

• The brainstem's regional stiffness and anisotropic mechanical properties contribute to its selective vulnerability in rotational loading scenarios.
• These biomechanical characteristics, combined with its anatomical location, explain its susceptibility during traumatic events.
• Understanding these properties is crucial for developing protective strategies against TBI.