The two Poisson’s ratios in annulus fibrosus: relation with the osmo-inelastic features

Abstract

The annulus fibrosus of the intervertebral disc is a highly complex layered structure in which the inelastic features of the tangled extracellular matrix interact with the surrounding physiological fluid by osmotic effect. In this in vitro study, the time-dependent transversal behavior in the two planes (fibers and lamellae planes) of multi-lamellae annulus tissues is reported by means of an accurate optical strain measuring technique based upon digital image correlation. Fresh annulus specimens of square cross section, extracted from bovine cervical discs, are tested under quasi-static (cyclic uniaxial stretching) and relaxation (interrupted stretching) loading with variation in osmolarity and strain rate conditions. Significant osmotic and strain rate effects are found on the elastic stiffness and the apparent Poisson’s ratios (p < 0.05, ANOVA). Under quasi-static loading, the apparent Poisson’s ratio is found higher than 0.5 in fibers plane and negative (i.e., auxetic) in lamellae plane. This material property evolves progressively towards classical bounds with relaxation time, i.e., between 0 and 0.5. The strong dependence of the auxetic behavior on time and chemical environment provides valuable insights about internal fluid exchanges. An interpretation of the osmo-inelastic mechanisms is proposed in which mechanical-based and chemical-based fluid flow interact until chemo-mechanical equilibrium. The new information allows a better understanding of the disc functionality and must be considered in accurate modeling of the disc annulus.