On the conditional equivalence of chemical loading and mechanical loading on articular cartilage

Osmotic pressure loading of articular cartilage has been customarily invoked to be equivalent to mechanical loading. In the literature, this equivalence is defined by the amount of water squeezed from the tissue, i.e. if the amount of water content lost by these two modes of loading are the same, it has been generally regarded that the two loadings are equivalent. This assumption has never been proven. Using the water content lost concept, in this paper, we derived the exact conditions under which an osmotic pressure loading of cartilage can be considered to be equivalent to a mechanical loading. However, the mechanical loading condition satisfying this equivalancy criterion, i.e. an isotropic loading delivered via a porous–permeable rigid platen uniformily applied all around the specimen, is not practically achievable. Moreover, even if this were achieved experimentally, the interstitial fluid pressure caused by the two loading conditions are not the same. This result has important ramifications for interpretation of experimental data from mechanical stimulations of cartilage explant studies

Proteoglycans and Mechanical Behavior of Condylar Cartilage

Mandibular condylar cartilage functions as the load-bearing, shock-absorbing, lubricating material in temporomandibular joints. Little is known about the precise nature of the biomechanical characteristics of this fibro-cartilaginous tissue. We hypothesized that the fixed charge density associated with proteoglycans that introduces an osmotic pressure inside condylar cartilage will significantly increase the tissue’s apparent stiffness. Micro-indentation creep tests were performed on porcine TMJ condylar cartilage at 5 different regions—anterior, posterior, medial, lateral, and central—in physiologic and hypertonic solutions. The intrinsic and apparent mechanical properties, including aggregate modulus, shear modulus, and permeability, were calculated by indentation test data and the biphasic theory. The apparent properties (with osmotic effect) were statistically higher than those of the intrinsic solid matrix (without osmotic effect). Regional variations in fixed charge density, permeability, and mechanical modulus were also calculated for condylar surface. The present results provide important quantitative data on the biomechanical properties of TMJ condylar cartilage