TENSILE PROPERTIES OF NONDEGENERATE HUMAN LUMBAR ANULUS FIBROSUS

Design. The in vitro tensile behavior of multiple-layer samples of anu lus fibrosus were investigated from nondegenerate intervertebral discs . Objectives. To quantify the intrinsic tensile behavior of nondegener ate anulus fibrosus and the variations with position and age in the in tervertebral disc.Summary of Background Data. Tension is an important loading mode in the anulus fibrosus, the tensile behavior of single-an d multiple-layer samples of anulus fibrosus has been shown to vary wit h specimen orientation, position in the disc, and environmental condit ions. Little is known of the changes in these site-specific tensile pr operties of the anulus with aging or degeneration of the intervertebra l disc. Methods. Multiple-layer specimens of anulus fibrosus were harv ested with an orientation parallel to the circumference of the disc. C onstant 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 wer e determined. Statistical analyses were done to evaluate regional and age-related differences in these properties. Results. Significant radi al and circumferential variations in the intrinsic tensile properties of anular samples were detected. The anterior anulus fibrosus had larg er values for tensile moduli and failure stresses than the posterolate ral anulus. Also, the outer regions of the anulus had greater moduli a nd failure stresses and lower failure strains than the inner regions. Strain energy density did not vary significantly with region. Signific ant, but very weak, correlations were detected between tensile propert ies and age of the intervertebral disc. Conclusions. The observd varia tions in tensile behavior of multiple-layer anulus samples indicate th at large variations in tensile modulus and failure properties occur wi th radial position in the disc than from anterior to posterolateral re gions. This pattern is likely related to site-specific variations in t he tensile properties of the single-layer samples of anulus fibrosus l amellae and the organization of successive lamellae and their interati ons. 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 fibr osus.