Joint deformities in rheumatoid arthritis occur as a result of many different mechanisms, all related to synovitis and pannus formation. Persistent synovitis can weaken ligaments and tendons and erode cartilage, creating instability in the joints. Characteristically, the metacarpophalangeal (MCP) joint can sublux, altering the lines of force of the tendons in the hand. This results in ulnar drift. In this slide, ulnar drift is present at the MCP joints. There is also swelling due to synovitis over the MCP joints. Additionally, rheumatoid nodules are present over the distal interphalangeal and proximal interphalangeal joints. Reference: * Wilder RL, Anderson RJ, Williams HJ. Rheumatoid arthritis. In: Schumacher HR Jr, Klippel JH, Koopman WJ, editors. Primer on the rheumatic diseases. 10th ed. Atlanta: Arthritis Foundation; 1993. p. 86-99.