The bone–cartilage unit in osteoarthritis

Osteoarthritis (OA) refers to a group of mechanically-induced joint disorders to which both genetic and acquired factors contribute. Current pathophysiological concepts focus on OA as a disease of the whole joint. Within these models, the functional unit formed by the articular cartilage and the subchondral bone seems to be of particular interest. Cartilage and bone receive and dissipate the stress associated with movement and loading, and are therefore continuously challenged biomechanically. Recent data support the view that cartilage and bone can communicate over the calcified tissue barrier; vessels reach out from bone into the cartilage zone, patches of uncalcified cartilage are in contact with bone, and microcracks and fissures further facilitate transfer of molecules. Several molecular signaling pathways such as bone morphogenetic proteins and Wnts are hypothesized to have a role in OA and can activate cellular and molecular processes in both cartilage and bone cells. In addition, intracellular activation of different kinase cascades seems to be involved in the molecular crosstalk between cartilage and bone cells. Further research is required to integrate these different elements into a comprehensive approach that will increase our understanding of the disease processes in OA, and that could lead to the development of specific therapeutics or treatment strategies.