Experimental and model determination of human intervertebral disc osmoviscoelasticity.

Finite element (FE) models have become an important tool to study load distribution in the healthy and degenerated disc. However, model predictions require accurate constitutive laws and material properties. As the mechanical properties of the intervertebral disc are regulated by its biochemical composition and fiber-reinforced structure, the relationship between the constitutive behavior of the tissue and its composition requires careful consideration. While numerous studies have investigated the annulus fibrosus compressive and tensile properties, specific conditions required to determine model parameters for the osmoviscoelastic model are unavailable. Therefore, the objectives of this study were (1) to complement the existing material testing in the literature with confined compression and tensile tests on human annulus fibrosus and (2) to use these data, together with existing nucleus pulposus compression data to tune a composition-based, osmoviscoelastic material constitutive law. The osmoviscoelastic material constitutive law and the experimental data were used to describe the fiber and nonfiber properties of the human disc. The compressive material properties of normal disc tissue were G(m) = 1.23 MPa, M = 1.57, and alpha = 1.964 x 10(-16) m(4)/Ns; the tensile fiber material parameters were E(0) = 77.0 MPa; E(epsilon) = 500 MPa, and eta = 1.8 x 10(3) MPa(-s). The goodness of fit ranged from 0.88 to 0.96 for the four experimental conditions evaluated. The constitutive law emphasized the interdependency of the strong swelling ability of the tissue and the viscoelastic nature of the collagen fibers. This is especially important for numerical models to further study the load sharing behavior with regard to disc degeneration and regeneration.