The ECU is the ulnar wrist extensor, producing wrist extension combined with ulnar deviation. Its tendon runs in the sixth extensor compartment in a fibre-osseous groove at the ulnar head, stabilised by the ECU subsheath. ECU subsheath tears allow the tendon to sublux volarly during forearm supination, producing the snapping ECU syndrome. ECU contributes to DRUJ stability through its force couple with the pronator quadratus.
| Origin | Common extensor origin (lateral epicondyle) and posterior ulna |
|---|---|
| Insertion | Dorsal base of the fifth metacarpal |
| Nerve Supply | Posterior interosseous nerve (C7, C8) |
| Blood Supply | Posterior interosseous artery |
| Actions | Wrist extension; Ulnar deviation of the wrist; Stabilises the DRUJ during forearm rotation |
|---|
ECU and FCR together provide the radial-ulnar deviation couple for dart-throwing motion (radial extension to ulnar flexion) — the motion used in throwing, hammering, and golf swing — and are central to the neuromuscular control of wrist motion in sports.
ECU subsheath tears produce a painful snapping of the ECU tendon from its groove into the volar wrist during forearm supination and ulnar deviation. Ultrasound dynamically images the subluxation during provocative manoeuvres. Acute cases are managed with immobilisation in forearm pronation; chronic instability requires surgical subsheath reconstruction.
The ECU tendon is palpable in the groove on the ulnar dorsal wrist between the ulnar head and the fifth metacarpal base, becoming firm during resisted wrist extension and ulnar deviation.
Sixth compartment subsheath disruption producing painful ECU tendon subluxation during supination managed with immobilisation or surgical reconstruction.
