Osteoarthritis: a tale of three tissues.

While research in osteoarthritis has focused on the events that lead to the destruction of articular cartilage, recent evidence suggests that two other components of the joints-bone and synovium-also play key roles in pathogenesis. All three tissues undergo alterations in concert at the structural levels in response to mechanical stress and joint malalignment. Advanced imaging studies such as MRI support this interdependence, revealing the classical changes of joint space narrowing and cartilage degeneration as well as the more recently appreciated bone marrow lesions and synovitis that may correlate with clinical symptoms. Molecular evidence also points to a coordinated release of cytokines and other inflammatory mediators from each of the three tissues together in progression of disease, although we are still in search of biochemical signatures that will predict the subset of patients who progress more quickly-and who will provide key clues to successful molecular targets in future therapies. At this time we lack definitive evidence pointing to which, if any, of the three tissues should serve as the main target for disease modification or structure protection, although most efforts have focused on cartilage. Thus current therapies focus on controlling symptoms, while research efforts search for reliable imaging and molecular biomarkers to help guide future trials of potential disease-modifying agents.