The thumb metacarpophalangeal joint is a condyloid articulation between the convex head of the first metacarpal and the slightly concave base of the thumb proximal phalanx. Unlike the finger MCP joints, the thumb MCP is reinforced by two sesamoid bones on the palmar surface and has a larger and more prominent volar plate. The range of motion is primarily flexion-extension (up to 90 degrees), with less abduction-adduction than finger MCPs.
The thumb MCP joint is the site of the most clinically important collateral ligament injury in the hand: gamekeeper thumb (now skier thumb), which is a rupture of the ulnar collateral ligament (UCL) from forced thumb abduction. The Stener lesion, where the adductor aponeurosis interposes between the torn UCL and its phalanx insertion, prevents healing and requires surgical repair. Thumb MCP arthrosis produces painful pinch that is managed with joint injection, fusion, or arthroplasty. The dorsal capsule and extensor mechanism must be protected during MCP surgery.
Forced thumb abduction from a fall with a ski pole or tackle tears the ulnar collateral ligament from the proximal phalanx, producing radial instability at the MCP with the Stener lesion (adductor aponeurosis interposition) blocking spontaneous healing in complete tears requiring surgical UCL repair.
Degenerative arthrosis of the thumb MCP produces pain with pinch and key activities, managed with splinting and injection, with fusion being the surgical gold standard given its superior pain relief over arthroplasty for this high-stress joint.
