Annulus Fibrosus Shear Properties Are Consistent With Motion Segment Mechanics When Fibers Are Loaded

The annulus fibrosus (AF) is a highly organized structure made up of concentric lamellae of fibers embedded in a hydrated extrafibrillar matrix; the collagen fibers are oriented at alternating angles in each lamella. The AF undergoes multidirectional loading through combinations of compression, bending, torsion and shear of the motion segment. The composition and structure of the AF leads to mechanical stress-strain nonlinearity and anisotropy. Previous tissue-based studies of shear have tested the AF tissue under compressive simple shear and torsion, producing shear modulus on the order of 0.06–0.4 MPa [1, 2]. However, structural testing and mathematical models of the IVD have reported the shear modulus to be between 3–20 MPa [3–6]. We hypothesize that when the fibers of the AF are loaded the shear modulus will be on the same order as structural tests and mathematical models of the IVD. The objectives of this study are to measure the shear mechanical properties of the bovine outer AF and compare the regional variances between anterior and posterior AF.