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

We used a shear protocol to investigate the time-independent, anisotropic s tructural behavior of 30 cube and 56 sheet specimens of human annulus fibro sus from six non-degenerate lumbar spines. The specimen sides were aligned with the natural collagen lamellar architecture. For the cube specimens, si mple shear was applied sequentially in three orthogonal directions defined by the cube axes. The sheets were also tested in simple shear, with the str ess applied in either the anatomic axial or circumferential directions. Wit h the sheet specimens we also investigated the contribution of annular coll agen to the shear modulus by applying a tensile prestrain (0, 5 or 10 perce nt) 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 annulu s. The ratio of the axial to circumferential shear modulus for the sheets i ncreased from being near unity for the inner annulus to near 2 for the oute r annulus. Finally, the addition of a 10 percent tensile prestrain increase d the shear modulus by between 1.5 and 2.5 times for the middle and outer a nnulus. The shear anisotropy we report is consistent with prior anatomical observations of this tissue and appears to develop through separate contrib utions from the matrix, the collagen fibers, and collagen fiber interaction s. (C) 2000 IPEM. Published by Elsevier Science Ltd. All rights reserved.