Effects of degeneration on the biphasic material properties of human nucleus pulposus in confined compression

Study Design. The biphasic compressive material properties of normal and degenerate human nucleus pulposus tissue were measured in confined compression. Objectives. The objective of this study was to determine the effects of degeneration and age on the mechanical properties of human nucleus pulposus. Summary of Background Data. The nucleus pulposus exhibits swelling behavior in proportion to proteoglycan content. In shear, the nucleus exhibits both fluid-like and solid-like properties, suggesting a biphasic nature. To date, biphasic compressive properties of human nucleus pulpous have not been reported. Methods. Human nucleus pulposus samples were tested in confined compression. Isometric swelling stress and effective aggregate modulus were measured. Linear biphasic theory was used to determine the permeability of the tissue. Mechanical behavior was correlated with proteoglycan and water content. Results. Degeneration produced significant decreases in swelling stress (P-sw = 0.138 +/- 0.029 MPa nondegenerate, P-sw = 0.037 +/- 0.038 MPa degenerate) and effective aggregate modulus (H-A(eff) = 1.01 +/- 0.43 MPa nondegenerate, H-A(eff) = 0.44 +/- 0.19 MPa degenerate). Both properties were inversely correlated with proteoglycan content. Permeability increased with degeneration (k(a) = 0.9 +/- 0.43 x 10(-15) m(4)/N-s nondegenerate, k(a) = 1.4 +/- 0.58 x 10(-15) m(4)/ N-s degenerate). Conclusions. Swelling is the primary load-bearing mechanism in both nondegenerate and degenerate nucleus pulposus. Knowledge of the biphasic material properties of the nucleus pulposus will aid the development of new treatment strategies for disc degeneration aimed at restoring mechanical function of the intervertebral disc.