Articular pathology of ankylosing spondylitis.

The essential articular pathology of AS may be said to reflect the occurrence, severity, and overall bias of (a) synovitis which tends to produce articular erosion, and (b) an inflammatory enthesopathy which results in capsular ossification in diarthrodial joints and syndesmophyte formation in cartilaginous joints, both of which are primarily responsible for bony ankylosis. Nonspecific secondary mechanisms contribute to the final picture. These include enchondral ossification, which produces synostosis, osteoporosis and altered stress distribution which make the axial skeleton susceptible to trivial trauma, the destructive effects of which are sometimes described as spondylodiskitis.