Cartilage self-heating contributes to chondrogenic expression

Articular cartilage is a soft tissue showing inelastic properties. Under cyclic loading, inelastic materials may dissipate mechanical energy into heat. In knee cartilage, due to an intrinsic reduced heat convection related to its avascularity, a local temperature increase could be observed. This phenomenon is referred to as self-heating. As cells are sensitive to temperature variation, the energy dissipation could influence their metabolism. The goal of this study was to evaluate the effect of cartilage self-heating on chondrogenic expression. In the first part of this study, using a custom-made deformation calorimeter, we quantified the heat generated in cartilage submitted to cyclic loading at different frequencies. We calculated the corresponding local increase of cartilage temperature. At the cellular level, we then assessed the effect of these temperature variations on chondro-progenitor cell metabolism by measuring the gene expression of transcription factors involved in chondrogenesis. An up-regulation in mRNA expression levels of Sox9 and its co-activator PGC-1α was observed with an increase of temperature. Taken together, the results of this study suggest a dissipation contribution to chondrogenic gene expression. Dissipation phenomena might then be considered as a new variable in mechanobiology.