To better understand the link between spinal cord impact and the resulting tissue damage, computational models are often used. These models typically simulate the spinal cord as a homogeneous and isotropic material. Recent research suggests that grey and white matter tissue differences and directional differences, i.e. anisotropy, are important to predict spinal cord damage. The objective of this research was to characterize the mechanical properties of spinal cord grey and white matter tissue in confined compression.

Spinal cords (n=11) from the thoracic and cervical regions of pigs (Yorkshire and Yucatan) were harvested immediately following euthanasia. The spinal cords were flash frozen (60 secs at -80 oC) and prepared into four types of test samples:​ grey matter axial, grey matter transverse, white matter axial, white matter transverse. For each sample type, 2 mm diameter biopsy samples were collected, thawed, and subsequently tested with a custom confined compression apparatus. This was performed within 6 hours of euthanasia, minimizing time post-mortem effects. All samples were compressed to 10% strain at a quasi-static strain rate (0.001/sec) and allowed to relax for 120 secs. A quasi-linear viscoelastic model combining a first-order exponential with a 1-term Prony series was used to characterize the loading and relaxation responses respectively. The effect of tissue type (grey matter vs. white matter), direction (axial vs. transverse), and their interaction were evaluated with a two-way ANOVA (p<0.05) with peak stress, aggregate modulus, and relaxation time as dependent variables.

The mechanical properties of spinal cord grey and white matter were found to be heterogeneous and slightly anisotropic. For peak stress, the effect of tissue type showed that grey matter was 1.6 times stiffer than white matter. For aggregate modulus, the effect of tissue type showed that grey matter was 2 times stiffer than white matter. The effect of direction showed that the transverse direction was 1.3 times stiffer than the axial direction. For relaxation time, grey matter took 1.6 times longer to relax than white matter in the transverse direction. These findings emphasize the importance of tissue type and to a lesser extent direction when studying SCI biomechanics using computational models.