Anisotropic and inhomogeneous tensile behavior of the human anulus fibrosus: Experimental measurement and material model predictions

The anulus fibrosus (AF) of the intervertebral disc exhibits spatial variat ions in structure and composition that give rise to both anisotropy and inh omogeneity in its material behaviors in tension. In this stud,, the tensile moduli and Poisson's ratios were measured in samples of human AF along cir cumferential, axial. and radial directions at inner and outer sires. There was evidence of significant inhomogeneity in the linear-region circumferent ial tensile modulus (17.4 +/- 14.3 MPa versus 5.6 +/-4.7 MPa, outer versus inner sites) and the Poisson's ratio nu (21) (0.67 +/-0.22 versus 1.6 +/-0. 7, outer versus inner), but not in the axial modulus (0.8 +/-0.9 MPa) or th e Poisson's ratios nu (12) (1.8 +/-1.4) or nu (13) (0.6 P +/-0.7). These pr operties were implemented in a linens anisotropic material model of the AF to determine a complete set of model properties and to predict material beh aviors for the AF under idealized kinematic states. These predictions demon strate that interactions between fiber populations in the multilamellae AF significantly contribute To the material behavior, suggesting that a model for the AF as concentric and physically isolated lamellae may not be approp riate.