Osteoporosis and ankylosing spondylitis.

Ankylosing spondylitis is a chronic inflammatory joint disease in which inflammation of the entheses gradually causes local ossification and ankylosis. Diffuse osteoporosis responsible for bone fragility is another feature established by recent data. The bone loss predominates at the spine and can cause vertebral fractures that are rare but nonnegligible sources of mortality and morbidity late in the course of the disease.