The anulus fibrosus (AF) of the intervertebral disc is a highly structured material made up of alternating tissue layers (lamella) with collagen fibers oriented +30° and −30° to the circumferential axis (Figure 1). Collagen confers a high tensile stiffness to the AF which contributes to the load-bearing function of the intervertebral disc in vivo. In addition, the specialized organization of the AF gives rise to strongly anisotropic behaviors which may contribute to its mechanical function. Numerous models have been proposed for the material behavior of the AF including a fiber-reinforced composite or membrane, a layered transversely isotropic material, an orthotropic material, and a poroelastic isotropic material [6,8,9,12]. Fiber-reinforced models are complicated by the need to quantify the collagen content, the collagen and ground substance material properties, and associated fiber-matrix interactions. Continuum models of the AF are complicated by the need to determine a unique set of material properties which can require as many as 9 independent constants for a linear orthotropic material.

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