Mechanisms of pathologic new bone formation.

Pathologic new bone formation occurs in response to a variety of stimuli. Heterotopic and orthotopic bone formation can interfere with the normal function of the joint and can contribute to disability in inflammatory joint diseases. Syndesmophyte formation and progressive ankylosis are characteristic features of spondyloarthropathies, including psoriatic arthritis and ankylosing spondylitis, and they can be regarded as abnormal bone remodeling. Successful blocking of inflammation in patients with spondyloarthropathy apparently fails to halt progression of ankylosis in cohort studies. This suggests that though they may be linked in some way, bone formation and inflammation are largely independent phenomena. Indeed, new bone formation also occurs in diseases such as osteoarthritis and diffuse idiopathic skeletal hyperostosis. Therefore, therapeutic strategies in spondyloarthropathy ideally should control both inflammation and bone formation.