Comparison of the equilibrium response of articular cartilage in unconfined compression, confined compression and indentation

At mechanical equilibrium, articular cartilage is usually characterized as an isotropic elastic material with no interstitial fluid flow. In this study, the equilibrium properties (Youngʹs modulus, aggregate modulus and Poissonʹs ratio) of bovine humeral, patellar and femoral cartilage specimens (n=26) were investigated using unconfined compression, confined compression, and indentation tests. Optical measurements of the Poissonʹs ratio of cartilage were also carried out. Mean values of the Youngʹs modulus (assessed from the unconfined compression test) were 0.80±0.33, 0.57±0.17 and 0.31±0.18 MPa and of the Poissonʹs ratio (assessed from the optical test) 0.15±0.06, 0.16±0.05 and 0.21±0.05 for humeral, patellar, and femoral cartilages, respectively. The indentation tests showed 30–79% (p<0.01) higher Youngʹs modulus values than the unconfined compression tests. In indentation, values of the Youngʹs modulus were independent of the indenter diameter only in the humeral cartilage. The mean values of the Poissonʹs ratio, obtained indirectly using the mathematical relation between the Youngʹs modulus and the aggregate modulus in isotropic material, were 0.16±0.06, 0.21±0.05, and 0.26±0.08 for humeral, patellar, and femoral cartilages, respectively. We conclude that the values of the elastic parameters of the cartilage are dependent on the measurement technique in use. Based on the similar values of Poissonʹs ratios, as determined directly or indirectly, the equilibrium response of articular cartilage under unconfined and confined compression is satisfactorily described by the isotropic elastic model. However, values of the isotropic Youngʹs modulus obtained from the in situ indentation tests are higher than those obtained from the in vitro unconfined or confined compression tests and may depend on the indenter size in use.