DEGENERATION AND AGING AFFECT THE TENSILE BEHAVIOR OF HUMAN LUMBAR ANULUS FIBROSUS

Study Design. Samples of human lumbar (L3-L4) anulus fibrosus from fou r different anatomic sites (anterior outer, posterolateral outer, ante rior inner, posterolateral inner), ranging from normal to severely deg enerate, were studied in uniaxial tension and measured for water conte nt. Objectives. To evaluate the effects of aging and degeneration on t he tensile properties and hydration of the anulus fibrosus in a site-s pecific manner. The relationship between hydration and parameters of t he tensile behavior were investigated. Summary of Background Data. Deg eneration and aging have been shown to be related to dramatic changes in the composition and structure of the anulus fibrosus. The associate d changes in the tensile, compressive, and shear properties of the anu lus fibrosus have not been documented. Numerical studies using finite element models have attempted to simulate the degenerative process by incorporating estimated mechanical properties meant to represent the d egenerate anulus fibrosus. Their results present findings that suggest that altered material properties of the anulus fibrosus affect the me chanics of the entire intervertebral disc. Methods. Samples of human l umbar anulus fibrosus were classified by grade of degeneration based o n a morphologic grading scheme. Multiple layer anulus specimens from f our sites in the disc were tested in uniaxial tension under quasistati c conditions in a physiologic saline bath. The tensile modulus, Poisso n's ratio, failure stress and strain, the strain energy density to fai lure, and the corresponding hydration were determined for each test sa mple. Results. The Poisson's ratio, failure stress, and strain energy density of the anulus fibrosus were found to be affected significantly by degeneration, with some evidence of a sensitivity of the tensile m odulus to grade of degeneration. All material properties were found to exhibit a significant and greater dependence on site within the disc than on degenerative grade. Weak correlations between aging and the Po isson's ratio and strain energy density were observed. Water content o f anulus fibrosus tissue was not affected by degeneration or aging, al though correlations with tensile properties were observed. Conclusions . The dramatic changes in morphology, composition, and structure that occur in anulus fibrosus with aging and degeneration are accompanied b y specific variations in the tensile properties, which were generally small in magnitude. Position of the anulus fibrosus within the interve rtebral disc, particularly in the radial direction, appeared to be the most important variable affecting anulus fibrosus tensile properties. This dependence on position did not change with either aging or degen eration. Results from the present study may be useful in future finite element models to assess how altered material properties of the anulu s fibrosus during degeneration and aging may affect the mechanics of t he entire intervertebral disc.