The thumb A1 pulley is the first annular pulley of the flexor pollicis longus tendon sheath, positioned at the metacarpophalangeal joint of the thumb on the palmar surface. It arises from the palmar plate and the metacarpal head, forming the entrance to the thumb flexor sheath. The thumb has three pulleys (A1, oblique, and A2) rather than the five found in the fingers.
Prevents bowstringing of the FPL tendon at the thumb MCP joint and holds the tendon close to the bone during thumb flexion, maintaining mechanical efficiency.
Trigger thumb (stenosing tenosynovitis of the thumb FPL at the A1 pulley) is both the most common digital trigger disorder in adults and the most common congenital digital problem in infants. In congenital trigger thumb, a palpable nodule (Notta node) prevents IP joint extension, typically presenting after age one as a fixed IP joint flexion deformity. Surgical A1 pulley release is the definitive treatment and is performed through a transverse palmar crease incision at the thumb MCP level.
Hypertrophy of the thumb A1 pulley or the FPL tendon nodule (Notta node) causes painful triggering or locking of the thumb IP joint in adults, and fixed IP flexion in congenital cases, managed by steroid injection or surgical A1 pulley release at the thumb MCP level.
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