Anisotropic shear behavior of the annulus fibrosus: effect of harvest site and tissue prestrain

We used a shear protocol to investigate the time-independent, anisotropic structural behavior of 30 cube and 56 sheet specimens of human annulus fibrosus from six non-degenerate lumbar spines. The specimen sides were aligned with the natural collagen lamellar architecture. For the cube specimens, simple shear was applied sequentially in three orthogonal directions defined by the cube axes. The sheets were also tested in simple shear, with the stress applied in either the anatomic axial or circumferential directions. With the sheet specimens we also investigated the contribution of annular collagen to the shear modulus by applying a tensile prestrain (0, 5 or 10 percent) perpendicular to the direction of applied shear stress. Our cube data indicated that the shear modulus was anisotropic, being 56.04±36.3 kPa in the plane of the lamellae and approximately half that in the orthogonal directions. The sheet specimens demonstrated that shear modulus increased progressively by a factor of between 3 and 5 from the inner to outer annulus. The ratio of the axial to circumferential shear modulus for the sheets increased from being near unity for the inner annulus to near 2 for the outer annulus. Finally, the addition of a 10 percent tensile prestrain increased the shear modulus by between 1.5 and 2.5 times for the middle and outer annulus. The shear anisotropy we report is consistent with prior anatomical observations of this tissue and appears to develop through separate contributions from the matrix, the collagen fibers, and collagen fiber interactions.