Nonhomogeneous permeability of canine anulus fibrosus

Study Design. This report examines the permeability coefficient and aggregate modulus of slices of anulus cut from canine lumbar intervertebral discs. Objectives. To examine the influence of radial position on the properties of these materials, including outer samples with intact anulus edge. Summary of Background Data. The outer edge of anulus fibrosus shows radial bulge during axial compression of motion segments. The radial bulge increases monotonically when the axial compression is sustained for several hours, until a plateau is reached. Triphasic modeling of axial compression shows that this time course of radial bulge can not be obtained using a uniform permeability coefficient according to values in the literature. Methods. Confined consolidation experiments (controlled load) were designed to measure the time course of uniaxial deformation of samples of anulus that were 4 mm in diameter and 1 mm tall. The rotation symmetry axis of the samples was defined in the radial direction of the disc. The radial permeability coefficient and the aggregate modulus were determined using the consolidation data and the linear biphasic theory. Results. The permeability coefficient was lower al the periphery than in deeper layers of the anulus. Outer samples with outer surfaces that were 0.0-0.5 mm from the anulus edge had an average permeability coefficient of (1.02 +/- 0.57) x 10(-16) m(4)/Ns (n = 24). Inner samples that were 2.0-2.5 mm from the anulus edge had an average permeability coefficient of (2.81 +/- 0.98) x 10(-16) m(4)/Ns (n = 13). The aggregate modulus H-A of outer samples was significantly higher (H-A = 1.56 +/- 0.34 MPa) than that of inner samples (H-A = 1.31 +/- 0.47 MPa). Conclusions. The fact that the outer anulus is less permeable than the inner anulus may explain why radial bulge of anulus fibrosus increases monotonically in time to an equilibrium value during sustained axial compression of a motion segment.