A transversely isotropic constitutive model of excised guinea pig spinal cord white matter

Compression behavior of porcine spinal cord white matter

“…The compressive properties of spinal cord white matter have been extrapolated from computational studies of spinal cord injury (Galle et al, 2007(Galle et al, , 2010Maikos et al, 2008), tensile experiments (Hung et al, 1981;Bilston and Thibault, 1996;Ichihara et al, 2001Ichihara et al, , 2003Fiford and Bilston, 2005) and compression testing of brain white matter (Miller and Chinzei, 1997;Cheng and Bilston, 2007;Tamura et al, 2007). Inverse finite element methods showed the spinal cord compressive response was less stiff than the tensile properties of the cord (Galle et al, 2007(Galle et al, , 2010Maikos et al, 2008) and provided the basis for hyperelastic and transversely isotropic constitutive models.…”

“…Inverse finite element methods showed the spinal cord compressive response was less stiff than the tensile properties of the cord (Galle et al, 2007(Galle et al, , 2010Maikos et al, 2008) and provided the basis for hyperelastic and transversely isotropic constitutive models. Inverse finite element methods assume a constitutive model, a priori; however, neurological tissues have been represented by a spectrum of models including: linear elastic (Ichihara et al, 2001;Oakland et al, 2006), hyperelastic (Bilston and Thibault, 1996) viscoelastic (Chang et al, 1988;Miller and Chinzei, 2002;Fiford and Bilston, 2005), poroviscoelastic (Cheng and Bilston, 2007), and transversely isotropic (Meaney, 2003;Ning et al, 2006;Galle et al, 2010). It is unclear which model best captures spinal cord white matter compressive behavior.…”

Mechanical indicators of injury severity are decreased with increased thecal sac dimension in a bench-top model of contusion type spinal cord injury

“…We did not model the spinal canal geometry; using a flat plate or an anatomic model did not affect maximum cord compression or internal pressure in an in vitro model (Hall et al, 2006); however, pressures were higher in surrogate cords residing in narrower cadaver spines (Pintar et al, 1993). The synthetic materials did not completely replicate the viscoelastic nature of biological tissues; while the surrogate cord is the most rigorously validated that we are aware of, the model may benefit from more complete characterization of the mechanical behavior of the synthetic cord under uniaxial compression, particularly in light of recent constitutive models developed for spinal cord white matter (Galle et al, 2007(Galle et al, , 2010. The synthetic constructs described in Table 3 Regression model coefficients for cord compression and impactor, base and tether loads.…”

“…The model's biofidelity may be improved using a synthetic dura with direction-dependent properties-a composite material that mimics the anisotropic arrangement of collagen and elastin fibers in the native spinal dura (Haupt and Stofft, 1978;Fink and Walker, 1989;Patin et al, 1993;Runza et al, 1999) may be successful. Similarly, the cord did not mimic the transverse isotropy which results from the longitudinal alignment of axons in the real spinal cord (Galle et al, 2010) and these directional mechanical properties may affect the propagation of pressure waves within the cord and CSF. Furthermore, the cord did not incorporate a fluid phase; however, this may not have affected the results significantly as the spinal cord is largely incompressible in vivo when the surrounding pial membrane is intact (Galle et al, 2010) and at the relatively high impact speeds associated with traumatic SCI the movement of water into or out of this viscoelastic tissue may be limited.…”

“…Similarly, the cord did not mimic the transverse isotropy which results from the longitudinal alignment of axons in the real spinal cord (Galle et al, 2010) and these directional mechanical properties may affect the propagation of pressure waves within the cord and CSF. Furthermore, the cord did not incorporate a fluid phase; however, this may not have affected the results significantly as the spinal cord is largely incompressible in vivo when the surrounding pial membrane is intact (Galle et al, 2010) and at the relatively high impact speeds associated with traumatic SCI the movement of water into or out of this viscoelastic tissue may be limited. The sensitivity of the model to cord and dura strain, and baseline pressure, was not assessed.…”