The mechanics of contusion spinal cord injury: Towards patient-specific assessments of mechanical loading and injury

Computational models are becoming an important tool for spinal cord injury (SCI) studies, specifically for transferring in-vivo preclinical achievements to clinical trials and injury prevention design. Despite this, spinal cord tissue properties, constitutive models, and the correlations between tissue mechanics and injury are unclear. Therefore, the anisotropic behaviour of spinal cord tissue was characterized in a human-like animal, constitutive models were employed in SCI computational models, and correlations between SCI model outcomes and tissue injury were evaluated in patient-specific models. Cervical spinal cords were harvested from nine non-human primates (NHPs). White matter samples were cut from lateral columns of the spinal cord. Samples were characterized under dynamic compression. The obtained model was combined with published in-vitro tensile response to capture the anisotropic behaviour of the spinal cord. The model was used to generate subject-specific finite element (FE) models of NHP in-vivo contusion SCI. Capability of several mechanical metrics in predicting tissue damage were evaluated using logistic analysis. NHP spinal cord white matter compressive response was sensitive to strain rate and showed substantial stress relaxation. An Ogden model best captured the white matter behaviour in a quasi-linear viscoelastic material model. Rapid relaxation and high strain rate sensitivity of the white matter indicate that incremental movement of the spinal cord during reparative interventions could substantially reduce the risk of ischemic injury. A fiber-reinforced conditional constitutive model best captured white matter anisotropy. Von-Mises and Tresca stresses showed the strongest correlations with damage in the gray matter. FE tissue damage thresholds were subject-specific except for white matter axonal strain and strain energy density. In summary, the work described herein indicate that measures of mechanical FE outputs correlate with tissue damage both in white and gray matters of spinal cord, and that subject-specific models of human-like animals that include spinal cord anisotropy are able to accurately mimic the biomechanics of contusion impacts.