Correlating material properties with tissue composition in enzymatically digested bovine annulus fibrosus and nucleus pulposus tissue

Aging and degeneration of the intervertebral disk are accompanied by decreases in water and proteoglycan contents, and structural alterations. The aim of this study was to determine the impact of compositional changes on the material properties of intervertebral disk tissues. Confined compression stress-relaxation experiments were applied to bovine caudal annulus fibrosus and nucleus pulposus tissue specimens that were separated into three experimental groups: in situ, free-swelling control (PBS), and digestion (chondroitinase-ABC). Measurements of glycosaminoglycan (GAG) and water content, as well as nonlinear finite deformation biphasic theory and multiple linear regression analyses were performed. The compressive modulus H (A0) and permeability k (0) of in situ specimens were 0.37 +/- 0.06 MPa and 0.49 +/- 0.08x10(-15) m(4) N-1 s(-1) for nucleus, and 0.74 +/- 0.13 MPa and 0.42 +/- 0.05x10(-15) m(4) N-1 s(-1) for annulus, respectively. There was a larger effect of swelling and digestion on the material properties and biochemical composition of nucleus pulposus than for annulus fibrosus. Alterations in proteoglycan and water content affected the compressive modulus and permeability, although the permeability was somewhat more strongly affected by water content than by proteoglycan content. Correlation coefficients r <= 0.75 for the multiple regression indicated water and GAG content can moderately predict material properties, however other compositional and structural factors must be considered.