An accurate constitutive law for the annulus fibrosus (AF) is critical to modeling the complex loads and deformations of the intervertebral disc. In the healthy disc, the loads applied to the AF are primarily a combination of tension and torsion. Several researchers [1–5] have developed nonlinear strain energy functions to model the AF using the fiber-reinforced composite theory of Spencer [6]. However, a limitation to these previous studies is that they did not include shear data in calculating the model coefficients. Although many of these models have non-physical representations of the stress contribution of the collagen fibers [1, 2, 4], (i.e. collagen fibers only support tensile loads, but in the models they also support compressive axial loads), others have implemented a tension-only restriction for collagen fibers [3, 5]. The objective of this study was to model shear behavior of the AF in addition to previously modeled loading conditions [2]. The currently proposed model includes shear data in determining model coefficients. Additionally, we examined the effect of a tension-only representation of collagen fibers on the model’s fit to the experimental data.

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