The equilibrium stiffness of articular cartilage is controlled by flow-independent elastie properties (Young's modulus, Es, and Poisson's ratio, νs) of the hydrated tissue matrix. In the current study, an optical (microscopic) method has been developed for the visualization of the boundaries of cylindrical bovine humeral head articular cartilage disks (n = 9), immersed in physiological solution, and compressed in unconfined geometry. This method allowed a direct, model-independent estimation of Poisson's ratio of the tissue at equilibrium, as well as characterization of the shape changes of the sample during the nonequilibrium dynamic phase. In addition to optical analyses, the equilibrium behavior of cartilage disks in unconfined and confined ramp-stress relaxation tests provided a direct estimation of the aggregate modulus, Ha, and Young's modulus and, indirectly. Poisson's ratio for the articular cartilage. The mean value for Poisson's ratio obtained from the optical analysis was 0.185 ± 0.065 (mean ± S.D., n = 9). Values of elastic parameters obtained from the mechanical tests were 0.754 ± 0.198 MPa, 0.677 ± 0.223 MPa, and 0.174 ± 0.106 for Ha, Es, and νs, respectively (mean ± S.D., n = 7). The similar νa-values obtained with optical and mechanical techniques imply that, at equilibrium, for these two tests, the isotropic model is acceptable for mechanical analysis. However, the microscopic techniques revealed that the lateral expansion, especially during the initial phase of relaxation, was inhomogeneous through the tissue depth. The superficial cartilage zone expanded less than the radial zone. The zonal differences in expansion were attributed to the known zonal differences in the fibrillar collagen architecture and proteoglycan concentration.
Articular cartilage; Cartilage mechanics; Cartilage material properties; Mechanical testing; Video microscopy