DEGENERATION AFFECTS THE ANISOTROPIC AND NONLINEAR BEHAVIORS OF HUMANANULUS FIBROSUS IN COMPRESSION

Axial and radial specimens of non-degenerate and degenerate human anul us fibrosus (AF) were tested in confined compression to test the hypot hesis that degeneration significantly affects the compressive properti es of AF. Due to the highly oriented structure of AF, a secondary obje ctive was to investigate anisotropic behaviors of AF in compression. U niaxial swelling and stress-relaxation experiments were performed on s ite-matched samples of anulus from the anterior outer region of L2-3 i ntervertebral discs. The experimental stress-relaxation behavior was m odeled using the finite deformation biphasic theory and a finite-diffe rence approximation scheme. Significant effects of degeneration but no t orientation were detected for the reference stress offset, sigma(off set), and parameters describing the compressive stiffness (i.e. refere nce aggregate modulus, H-Ao, and nonlinear stiffening coefficient, bet a). Average values were 0.13 +/- 0.06 and 0.05 +/- 0.05 MPa for sigma( offset), 0.56 +/- 0.21 and 1.10 +/- 0.53 MPa for H-Ao and 2.13 +/- 1.4 8 and 0.44 +/- 0.61 for beta for all normal and degenerate specimens, respectively. No significant effect of degeneration or orientation wer e detected for either of the parameters describing the strain-dependen t permeability (i,e. reference permeability, k(o), and strain-dependen t permeability coefficient, M) with average values for all specimens o f 0.20 +/- 0.10 x 10(-15) m(4)/N-s and 1.18 +/- 1.30 for k(o) and M, r espectively. The loss of sigma(offset) was compensated with an elastic stiffening and change in the shape of the equilibrium stress-strain c urve with H-Ao for degenerate tissues almost twice that of normal tiss ues and beta less than one sixth. The increase in reference elastic mo dulus with degeneration is likely related to an increase in tissue den sity resulting from the loss of water content. The significant effects of degeneration reported in this study suggested a shift in load carr iage from fluid pressurization and swelling pressure to deformation of the solid matrix of the AF. The results also suggest that the highly organized and layered network of the anulus fibrosus, which gives rise to significant anisotropic effects in tension, does not play a major role in contributing to the magnitude of compressive stiffness or the mechanisms of fluid flow of the anulus in the confined compression con figuration. (C) 1998 Elsevier Science Ltd. All rights reserved.