Confined compression experiments on bovine nucleus pulposus and annulus fibrosus: sensitivity of the experiment in the determination of compressive modulus and hydraulic permeability

The biphasic material properties for nucleus pulposus tissue in confined compression have not been reported previously, and are required for a better understanding of intervertebral disc function and to provide material properties for use in finite-element models. The aims of this study were to determine linear and non-linear material properties for nucleus pulposus and annulus fibrosus tissues in confined compression, to define the influence of swelling conditions on these properties, and to determine the changes in the compressive modulus and hydraulic permeability induced by the repetition of the stress-relaxation experiment after a return to swelling pressure equilibrium. Specimens from caudal bovine nucleus and annulus were tested in confined compression stress-relaxation experiments and analyzed to quantify the compressive modulus and hydraulic permeability using linear and non-linear biphasic models. Our results suggested the use of confined swelling pre-test condition and non-linear biphasic model, which provided the material parameters with lowest relative variance and water content most representative of physiological conditions. Smaller compressive modulus and higher hydraulic permeability were obtained for the nucleus (H(A0)=0.31+/-0.04 MPa, k(0)=0.67+/-0.09 x 10(-15)m(4)/Ns) than for the annulus (H(A0)=0.74+/-0.13 MPa, k(0)=0.23+/-0.19 x 10(-15)m(4)/Ns), with relatively weak non-linearities. Strains up to 20% resulted in material properties that were significantly altered upon retesting. These altered material properties are an effort to quantify non-recoverable damage that occurs in disc tissue and suggest that in vivo exposure of disc tissues to low strain-rate and high-deformation loading conditions which outpace biological repair may result in altered mechanical behaviors.