Biomechanics of load-bearing of the intervertebral disc: an experimental and finite element model

This paper presents an experimental and finite element study of the biomechanical response of the intervertebral disc to static-axial loading in which classical consolidation theory was used to analyse its time-dependent response. A newly developed experimental technique was employed to load the disc in compression and measure simultaneously the matrix internal pressure and creep strain for the full consolidation process. It is shown that, upon loading, the disc develops a maximum hydrostatic excess pore pressure which gradually decays as water is exuded from the matrix. During this decay process, the applied load is progressively transferred to the solid components of the matrix until the load is borne in full by the solid at the end of consolidation. Material properties for the tissue were then obtained from the experimental stress-strain data and used in the finite element study in the development of a finite element solution based on Biot's theory of coupled solid-fluid interaction. An axisymmetric formulation was employed and the disc modelled as an anisotropic, non-linear poroelastic solid. A sensitivity analysis of the material properties for the structural components of the disc was carried out and the biomechanical response to compressive loading evaluated and compared to experimental data. The results show that the matrix permeability plays a significant role in determining the transient response of the tissue. Annualr disruptions of the disc were shown to result in an increase in the nuclear principal stresses suggesting that disrupted regions of the annulus fibrosus play a reduced role in load bearing.