Compression behavior of porcine spinal cord white matter

Spinal cord injury often results from a compressive load; however, the compression behavior of spinal cord white matter has not been clearly established. Quantifying the compression behavior is important for advancing our understanding of spinal cord injury mechanics and facilitating the use of finite element models to study injury. The objective of this study was to characterize the unconfined compression behavior of isolated white matter segments and determine the constitutive model which best captured the stress-strain behavior. Spinal cord white matter samples were harvested immediately following sacrifice from juvenile Yorkshire pigs (n=104). The samples were compressed to 40% strain at four strain rates (0.005, 0.05, 0.5, and 5.0/s) and allowed to relax for 60 s. The effects of preload, peak strain, sample aspect ratio, and time post mortem on peak stress, and constitutive model parameters were also examined. Strain rate had a significant effect on peak stress (p<0.001). A first-order Ogden model best captured the loading response of spinal cord white matter (R2=0.99) and a viscoelastic material model combining a first-order Ogden model with a 3-term Prony series effectively captured the effect of strain rate and the relaxation response. This study showed spinal cord white matter to be less stiff than previously estimated by inverse finite element methods, which will have a significant effect on finite element model predictions of the magnitude and distribution of stresses and strains in the spinal cord. This study is the first to quantify the unconfined compression response of spinal cord white matter.