The A1 pulley is the first and most proximal annular pulley of the flexor tendon sheath of each finger, positioned at the level of the metacarpophalangeal joint on the palmar surface. It arises from the palmar plate and the metacarpal head, and forms the entrance to the fibro-osseous flexor tendon tunnel. The A1 pulley of each finger is approximately 5-10 mm in length. The corresponding thumb A1 pulley overlies the metacarpophalangeal joint.
| Origin | Radial side of the palmar plate of the MCP joint and base of the proximal phalanx |
|---|---|
| Insertion | Ulnar side of the palmar plate of the MCP joint and base of the proximal phalanx, arching over the flexor tendons |
Prevents bowstringing of the flexor tendon at the MCP joint during finger flexion, holds the tendons close to the metacarpal head, and maintains mechanical efficiency of the flexor mechanism at the proximal finger level.
Stenosing tenosynovitis of the A1 pulley (trigger finger) is caused by hypertrophy and fibrocartilaginous metaplasia of the A1 pulley rim, trapping the flexor tendon nodule beneath the pulley during flexion and producing a painful catching or locking that requires forced extension to unlock. Corticosteroid injection into the tendon sheath at the A1 pulley level resolves symptoms in 50-70% of cases; surgical A1 pulley release is definitive for failures. Trigger thumb involves the thumb A1 pulley and is common in babies (congenital trigger thumb).
Hypertrophy of the A1 pulley rim impedes passage of a flexor tendon nodule through the pulley, causing a painful clicking catch during flexion and extension with a palpable nodule at the distal palm; treated by corticosteroid injection into the sheath or surgical A1 pulley release.
Failure of the flexor pollicis longus tendon to pass freely under the thumb A1 pulley produces a fixed interphalangeal flexion deformity in infants from a Notta node; managed by surgical A1 pulley release after the age of one year.
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