Rheumatoid arthritis of the feet. X-ray of a section through the feet of a patient affected by rheumatoid arthritis. Rheumatoid arthritis is a degenerative autoimmune disease that mainly affects the capsule (synovium) around the synovial (flexible) joints, causing chronic inflammation. It can be a debilitating and painful condition and can lead to deformities and loss of functioning and mobility. Here, it has caused severe deformity of the right foot (right) with moderate impairment of the left foot.