The thumb flexor pulley system consists of two annular pulleys (A1 at the MCP joint and A2 at the IP joint level) and one oblique pulley (the primary mechanical pulley of the thumb, running obliquely from the ulnar proximal phalanx to the radial distal phalanx region). The oblique pulley is biomechanically the most critical, providing the greatest mechanical efficiency for FPL during IP joint flexion. The A1 pulley at the MCP is the site of trigger thumb.
| Location | Palmar side of the thumb, from the MCP joint to the distal phalanx |
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Keeps the flexor pollicis longus tendon close to the bones so it can flex the thumb efficiently; the oblique pulley is the most important, and the A1 pulley is the site of trigger thumb.
The A1 pulley is the site of trigger thumb from constrictive tenosynovitis: the FPL nodule catching at the A1 pulley opening produces a painful trigger or locked IP joint. A1 pulley release (percutaneous or open) is the treatment. The oblique pulley is preserved during pulley reconstruction procedures to maintain FPL mechanical efficiency. Rupture of the A2 pulley (annular pulley at the IP joint level) from forced extension produces bowstringing of the FPL visible as a palmar cord at the IP joint level. In rock climbers, the equivalent phalangeal pulleys of the fingers (A2 being critical) are most commonly ruptured.
Constrictive tenosynovitis of the FPL tendon at the thumb A1 pulley opening produces a nodule that catches on the pulley edge causing locking of the IP joint in flexion; percutaneous A1 pulley release with a 25-gauge needle through the palmar thumb skin base provides immediate triggering resolution with minimal surgical trauma.
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