Rheumatoid arthritis, on the other hand, is completely different, and is caused by the immune system rather than wear and tear. The immune system mistakes parts of your joints as being foreign, and attacks the synovial membrane, or lining of the joint. It might also attack the sheath around the tendons. This eventually causes the cartilage to thin and the joints to wear, and the inflammatory response can cause painful inflammations. There is no apparent cause, though heredity, lifestyle and hormones might all be connected.