Indentation determined mechanoelectrochemical properties and fixed charge density of articular cartilage

As a nondestructive technique, the indentation test has been used, both in vitro and in vivo, to determine the in situ apparent mechanical properties of cartilage. In this study, a simple new algorithm was developed using the indentation creep test, combined with both biphasic and triphasic analyses to calculate simultaneously the apparent and intrinsic mechanical (aggregate modulus and Poisson's ratio) and an electrochemical properties, i.e., the fixed charge density (FCD) of the intact articular cartilage. The calculated FCD values were compared with those measured using the biochemical assay of the proteoglycan content in the tissue. It was found: (1) the FCDs obtained from this new indentation method (0.287 +/- 0.157 mEq/ml) were significantly correlated with the results from biochemical assay; (2) significantly positive linear relationships existed between the intrinsic and apparent mechanical moduli; (3) both the apparent and intrinsic mechanical properties correlated significantly with the proteoglycan content in the cartilage specimen. These results suggest two distinct interaction mechanisms between the collagen network and the proteoglycans in cartilage layer. The proteoglycans contribute to the mechanical properties of articular cartilage not only by the Donnan osmotic pressure induced by the fixed charges, but also by its bulk mass. Current study represents a first step toward developing a valid and effective method for the study of structure-function relationship in cartilage and possibly for future early stage OA detection in vivo.