Bone Wasn't Built in a Day: Destruction and Formation of Bone in the Rheumatic Diseases

The hallmark of rheumatoid arthritis is synovitis, or inflammation of synovial tissues lining joints. Synovitis in rheumatoid arthritis promotes destruction of articular bone by inducing the differentiation and function of osteoclasts, leading to significant patient morbidity. The cell types and pathways mediating articular bone destruction have now been elucidated and the critical role of receptor activator of nuclear factor-kappa B ligand has been recognized, leading to the identification of new targets for the protection of articular bone. Synovitis not only promotes bone destruction, but also inhibits the ability of bone-forming osteoblasts to repair bone. In stark contrast, inflammation in spondyloarthritis, including ankylosing spondylitis, promotes bone formation at periosteal sites, resulting in pain and decreased motion of the spine and joints. Local anatomic factors contribute to these distinct outcomes for bone and anabolic pathways regulating bone formation are now being investigated to identify novel targets for prevention of abnormal bone formation.