Tensile properties of nondegenerate human lumbar anulus fibrosus

Design. The in vitro tensile behavior of multiple-layer samples of anulus fibrosus were investigated from nondegenerate intervertebral discs. Objectives. To quantify the intrinsic tensile behavior of nondegenerate anulus fibrosus and the variations with position and age in the intervertebral disc. Summary of Background Data. Tension is an important loading mode in the anulus fibrosus, the tensile behavior of single-and multiple-layer samples of anulus fibrosus has been shown to vary with specimen orientation, position in the disc, and environmental conditions. Little is known of the changes in these site-specific tensile properties of the anulus with aging or degeneration of the intervertebral disc. Methods. Multiple-layer specimens of anulus fibrosus were harvested with an orientation parallel to the circumference of the disc. Constant strain rate and uniaxial tensile tests were performed in 0.15 mol/l NaCl at slow strain rates to measure the intrinsic properties of the collagen-proteoglycan matrix of the anulus fibrosus. The tensile modulus, failure stress, failure strain, and strain energy density were determined. Statistical analyses were done to evaluate regional and age-related differences in these properties. Results. Significant radial and circumferential variations in the intrinsic tensile properties of anular samples were detected. The anterior anulus fibrosus had larger values for tensile moduli and failure stresses than the posterolateral anulus. Also, the outer regions of the anulus had greater moduli and failure stresses and lower failure strains than the inner regions. Strain energy density did not vary significantly with region. Significant, but very weak, correlations were detected between tensile properties and age of the intervertebral disc. Conclusions. The observd variations in tensile behavior of multiple-layer anulus samples indicate that large variations in tensile modulus and failure properties occur with radial position in the disc than from anterior to posterolateral regions. This pattern is likely related to site-specific variations in the tensile properties of the single-layer samples of anulus fibrosus lamellae and the organization of successive lamellae and their interations. The results of the present study suggest that factors other than age, such as compositional and structural variations in the disc, are the most important determinants of tensile behavior of the anulus fibrosus.