Radial tensile properties of the lumbar annulus fibrosus are site and degeneration dependent

We conducted an in vitro study of the radial tensile properties of the annulus fibrosus. The stress-strain response was nonlinear, with a mean tangent modulus of 0.19 MPa at zero strain and 0.47 MPa at 70% of the yield strain. We also investigated whether these properties varied as a function of location within the disc and degree of degeneration. Specimens harvested from the middle layers of the annulus were stiffer and failed at smaller strain magnitudes than those from the inner or outer annulus (analysis of covariance, p < 0.05). Differences due to degeneration were evident; degenerated discs had a 30% decrease in yield and ultimate stress compared with normal discs. Similarity between our data and those reported for the annulus in compression suggests that these values reflect the material behavior of the interlaminar matrix and are an order of magnitude smaller than values used in previous analytical representations of this tissue. We expect that awareness of these data will result in improved understanding of the physical behavior and tolerance to injury of the annulus fibrosus.