Ankylosing spondylosis (AS) is rheumatoid arthritis of the spine found predominately in persons 20 to 40 years of age. It produces pain and stiffness as a result of inflammation of the sacroiliac, intervertebral, and costovertebral joints. The AS disease process may progress to complete spinal and thoracic rigidity.

In the United States, the incidence of AS is approximately 0.1% to 0.2% of the population. Internationally, it accounts for 0.1% to 1.0% of the population. In persons with the HLA-B27 antigen, the incidence is 1% to 2%.

The cause of AS is not fully understood. There seems to be a strong genetic predisposition, specifically involving human leukocyte antigen (HLA). HLA-B27 is believed to resemble and/or act as a receptor for a causative antigen (e.g., a type of bacteria). Currently, spurious evidence points to a bacterial causative agent, such as Klebsiella pneumoniae.