NONHOMOGENEOUS PERMEABILITY OF CANINE ANULUS FIBROSUS

Study Design. This report examines the permeability coefficient and ag gregate modulus of slices of anulus cut from canine lumbar interverteb ral 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 fib rosus 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 mo deling of axial compression shows that this time course of radial bulg e can not be obtained using a uniform permeability coefficient accordi ng to values in the literature. Methods. Confined consolidation experi ments (controlled load) were designed to measure the time course of un iaxial 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 t he radial direction of the disc. The radial permeability coefficient a nd 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 ed ge 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 sig nificantly 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 anulu s 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.