The thumb interphalangeal joint (IPJ) is the single interphalangeal joint of the thumb between the proximal and distal phalanges, stabilised by bilateral collateral ligaments, a volar plate, and the extensor pollicis longus (dorsal) and FPL (palmar) tendons. It allows 0-80 degrees of flexion. The thumb IPJ differs from all other digital IP joints by having two sesamoid bones in the FPB tendon on the palmar aspect at the MCP level (not the IPJ).
Thumb IPJ dislocation (usually dorsal) is reduced by traction and gentle manipulation, followed by buddy-strapping or splinting. Gamekeeper's thumb refers to chronic ulnar collateral ligament (UCL) insufficiency at the MCP joint (not the IPJ) from repetitive abduction stress. Trigger thumb involves the FPL at the A1 pulley at the MCP level, not the IPJ itself. Arthritis of the thumb IPJ is common in rheumatoid arthritis and in patients with long-standing carpal tunnel syndrome (median nerve territory intrinsic wasting). IPJ arthrodesis in extension provides durable pain relief and pinch stability.
Hyperextension injury to the thumb IPJ disrupts the volar plate and may avulse a collateral ligament, producing dorsal IPJ dislocation; reduction by traction and flexion is usually straightforward and subsequent dorsal extension block splinting for 4-6 weeks allows volar plate healing while preventing volar plate contracture.
