Topographical variation of the elastic properties of articular cartilage in the canine knee

Equilibrium response of articular cartilage to indentation loading is controlled by the thickness (h) and elastic properties (shear modulus, μ, and Poissonʹs ratio, ν) of the tissue. In this study, we characterized topographical variation of Poissonʹs ratio of the articular cartilage in the canine knee joint (N=6). Poissonʹs ratio was measured using a microscopic technique. In this technique, the shape change of the cartilage disk was visualized while the cartilage was immersed in physiological solution and compressed in unconfined geometry. After a constant 5% axial strain, the lateral strain was measured during stress relaxation. At equilibrium, the lateral-to-axial strain ratio indicates the Poissonʹs ratio of the tissue. Indentation (equilibrium) data from our prior study (Arokoski et al., 1994. International Journal of Sports Medicine 15, 254–260) was re-analyzed using the Poissonʹs ratio results at the test site to derive values for shear and aggregate moduli. The lowest Poissonʹs ratio (0.070±0.016) located at the patellar surface of femur (FPI) and the highest (0.236±0.026) at the medial tibial plateau (TMI). The stiffest cartilage was found at the patellar groove of femur (μ=0.964±0.189 MPa, Ha=2.084±0.409 MPa) and the softest at the tibial plateaus (μ=0.385±0.062 MPa, Ha=1.113±0.141 MPa). Comparison of the mechanical results and the biochemical composition of the tissue (Jurvelin et al., 1988. Engineering in Medicine 17, 157–162) at the matched sites of the canine knee joint indicated a negative correlation between the Poissonʹs ratio and collagen-to-PG content ratio. This is in harmony with our previous findings which suggested that, in unconfined compression, the degree of lateral expansion in different tissue zones is related to collagen-to-PG ratio of the zone.