A special theory of biphasic mixtures and experimental results for human annulus fibrosus tested in confined compression

A finite deformation mixture theory is used to quantify the mechanical prop erties of the annulus fibrosus using experimental data obtained from a conf ined compression protocol. Certain constitutive assumptions are introduced to derive a special mixture of an elastic solid and and inviscid fluid, and the constraint of intrinsic incompressibility is introduced in a manner th at is consistent with results obtained for the special theory. Thirty-two a nnulus fibrosus specimens oriented in axial (n=16) and radial (n=16) direct ions were obtained from the middle-lateral portion of intact intervertebral discs from human lumbar spines and tested in a stress-relaxation protocol. Material constants are determined by fitting the theory to experimental da ta representing the equilibrium stress versus stretch and the surface stres s time history curves. No significant differences in material constants due to orientation existed, but significant differences existed due to the cho ice of theory used to fit the data. In comparison with earlier studies with healthy annular tissue, we report a lower aggregate modulus and a higher i nitial permeability constant. These differences are explained by the choice of reference configuration for the experimental studies. [S0148-0731(00)01 002-5].