It is thought that Uc is an auto immune disease. With any AI, there is a higher chance that your immune system will turn and attack another part of the body. There are a couple of AI arthritis forms that Uc'ers can get such as RA (rheumatoid arthritis) and AS (Ankylosing spondylitis). There is also Peripheral arthritis which is a form of arthritis that wax and wanes with an IBD'ers flare.
http://www.ccfa.org/info/about/complications/arthritis <--copy and paste to your web browser