What causes ankylosing spondylitis?
The specific cause of ankylosing spondylitis is unknown, although genetic factors appear to play a role. In about nine out of 10 people with ankylosing spondylitis, the presence of a gene called HLA-B27 has been tentatively implicated. The condition begins with inflammation of the sacroiliac and facet joints. Over time, bony growths can begin to form and cause fusion of vertebrae, causing limited movement and pain in the lower back.