Articular Cartilage is a load bearing tissue whose microarchitecture, electrochemical composition, and fluid interactions afford it unique mechanical properties. It consists of an extracellular matrix (ECM) interspersed with a sparse population of chondrocytes, varying in density by depth. The structure and mechanical properties of this highly specialized tissue also vary depending on depth from the articular surface; with three specialized zones, each with unique material properties. Typically this tissue is mechanically modeled as a biphasic material, consisting of a solid phase and a fluid phase, which can redistribute itself under loading, altering hydrostatic pressure within the material. Thus, articular cartilage exhibits a time-dependent viscoelastic behavior when subjected to constant loading or deformation, and will reach an equilibrium via stress relaxation and creep behavior. The objective of this study was to test a custom designed confined compression chamber. We characterize the ability of the test chamber to generate curves capable of quantifying the stress relaxation level and equilibrium state in bovine articular cartilage, and to show the preliminary results of a comparison between the equilibrium aggregate modulus (HA) obtained from pre- conditioned and non-conditioned tissues. Using fresh bovine articular cartilage samples, stress relaxation tests were conducted in compression, obtaining equilibrium stress and HA through a linear relation between the initial strain and the equilibrium stress. The test specimens were divided into two groups, one with a pre-conditioning load and the other without. The tests resulted in equilibrium stresses of 0.015 ± 0.0067 MPa for the non-conditioned and 0.067 ± 0.012 MPA for the pre-conditioned, and HA values of 0.205 ± 0.100 MPa for the unconditioned group and 0.878 ± 0.160 MPa in the pre-conditioned group. Our confined compression chamber successfully produced the stress relaxation curve characterizing the mechanical behavior of articular cartilage, defining both the equilibrium stress and HA. Our results suggest that pre-conditioning correlates with a higher equilibrium stress and aggregate modulus based on the fact that pre-loading the specimens reduces the effects of viscoelasticity.
