Low back pain is a common reason for activity limitation in people younger than 45 years old, and was proved to be associated with heavy physical works, repetitive lifting, impact, stationary work postures and vibrations. The study of load transferring and the loading condition encountered in spinal column can be simulated by finite element models. The intervertebral disc is a structure composed of a porous material. Many physical models were developed to simulate this phenomenon. The confounding effects of poroelastic properties and loading conditions on disc mechanical responses are, nevertheless, not cleared yet. The objective of this study was to develop an axisymmetric poroelastic finite element model of intervertebral disc and use it to investigate the confounding effect of material properties and loading conditions on the disc deformation and pore pressure. An axisymmetric poroelastic model of human lumbar L4–L5 motion segment was developed. The model was validated by comparing the height loss and intradiscal pressure of the L4–L5 intervertebral disc with in vitro cadaveric studies. The effect of permeability, void ratio, elastic modulus, and Poisson's ratio on disc height and pore pressure was investigated for the following three loading conditions: (1) 1334 N creep loading, (2) peak-to-peak, 1000-to-1600 N, 1 Hz cyclic loading, and (3) same loading magnitude, but at 5 Hz loading frequency. The disc height loss and pore pressure of the three loading conditions were analyzed. The predictions of the disc height loss and intradiscal pressure of the current FE model are well comparable with the results of in vitro cadaveric studies. After model validation, the parametric study of disc poroelastic properties on the disc mechanical responses shows that the increase of permeability and void ratio increases the disc height loss and decreases the pore pressure, and these effects are sensitive to external loading frequency. Higher elastic modulus reduces the disc deformation and the pore pressure, but this reduction is not sensitive to the loading frequency. The effect of Poisson's ratio on disc height loss and pore pressure is negligible. In conclusion, the hydraulic permeability describes the fluid flow capability within tissue matrix which has a higher sensitivity on the saturation time for disc deformation and pore pressure. Void ratio directly affects the amount of mobile water within disc and changes time-dependent response of disc. Increase in loading frequency reduces time for fluid inflow and outflow, which fades out the role of permeability and void ratio. Values of elastic modulus and Poisson's ratio, which demonstrates stiffness and bulging capacity, respectively, do not affect the overall dynamic response of disc.
Micro-CT evaluation of asymmetrical ovine intervertebral disc height loss from surgical approach
