Effects of white, grey, and pia mater properties on tissue level stresses and strains in the compressed spinal cord.

Recent demographics demonstrate an increase in the number of elderly spinal cord injury patients, motivating the desire for a better understanding of age effects on injury susceptibility. Knowing that age and disease affect neurological tissue, there is a need to better understand the sensitivity of spinal cord injury mechanics to variations in tissue behavior. To address this issue, a plane-strain, geometrically nonlinear, finite element model of a section of a generic human thoracic spinal cord was constructed to model the response to dorsal compression. The material models and stiffness responses for the grey and white matter and pia mater were varied across a range of reported values to observe the sensitivity of model outcomes to the assigned properties. Outcome measures were evaluated for percent change in magnitude and alterations in spatial distribution. In general, principal stresses (114-244% change) and pressure (75-119% change) were the outcomes most sensitive to material variation. Strain outcome measures were less sensitive (7-27% change) than stresses (74-244% change) to variations in material tangent modulus. The pia mater characteristics had limited (<4% change) effects on outcomes. Using linear elastic models to represent non-linear behavior had variable effects on outcome measures, and resulted in highly concentrated areas of elevated stresses and strains. Pressure measurements in both the grey and white matter were particularly sensitive to white matter properties, suggesting that degenerative changes in white matter may influence perfusion in a compressed spinal cord. Our results suggest that the mechanics of spinal cord compression are likely to be affected by changes in tissue resulting from aging and disease, indicating a need to study the biomechanical aspects of spinal cord injury in these specific populations.