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The arthritis of Crohn's disease usually is considered to be of two types--an arthritis that affects the spine, called ankylosing spondylitis, and an arthritis that affects other large joints, such as the knees.
Usually the arthritis (either type) begins after the intestinal manifestations of the Crohn's disease. In a small percent of cases, however, it begins before the Crohn's disease.
Once ankylosing spondylitis begins, it progresses no matter what the activity of the Crohn's disease and can result in a fused, immobile spine. The arthritis that affects other large joints is frequently active when the Crohn's disease is active; however, unlike ankylosing spondylitis, permanent deformity of the joints is uncommon. It is because of this tendency toward progression of the arthritis and potential joint damage that medications should be used as part of the treatment program.