Rheumatoid arthritis affects cells that line and normally lubricate the joints (synovium). This systemic condition can affect multiple joints all over your body. Your synovium becomes inflamed and erodes, or “eats away” cartilage and bone, stretching soft tissues, causing the joints to be unstable and your fingers or hands to appear deformed. Tendons also can rupture, or tear, with minimal trauma. Rheumatoid arthritis of the hand is most common in the wrist and finger knuckles (metacarpophalangeal and proximal interphalangeal joints).