Trigger finger (stenosing flexor tenosynovitis) is a mismatch between a flexor tendon and the A1 pulley at the metacarpophalangeal (MCP) joint, causing painful catching, clicking or locking of the digit.
It has a reported lifetime risk of around 2.6%, is more common in middle-aged women and in people with diabetes, and most often affects the thumb, ring and middle fingers. Paediatric trigger thumb is a separate condition.
The flexor tendons of each digit run through a fibro-osseous sheath with 5 annular (A1 to A5) and 3 cruciate pulleys. The A1 pulley, at the MCP joint, is the entrance to the sheath and the commonest site of stenosis, because power grip loads its distal edge as the tendon angles over the metacarpal head.
The A1 pulley thickens, with fibrocartilaginous metaplasia, and the tendon may develop a nodule. The thickened tendon catches as it passes the narrowed pulley, then releases with a snap, sometimes only with help from the other hand. Despite the name, histology shows no inflammation in primary trigger finger.
Associations include diabetes (particularly insulin-treated), rheumatoid arthritis, hypothyroidism, amyloidosis and carpal tunnel syndrome.
Paediatric trigger thumb is a different condition:
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