The extensor carpi ulnaris occupies the sixth and most medial extensor compartment at the wrist, running from the lateral epicondyle and posterior ulna to the fifth metacarpal base. Its tendon is held in a fibro-osseous groove on the ulnar head by a dedicated subsheath, and instability of this subsheath is a significant source of ulnar-sided wrist pain and snapping in racket sport athletes.
| Origin | Lateral epicondyle via the common extensor tendon; Posterior border of the ulna |
|---|---|
| Insertion | Base of the fifth metacarpal on its medial surface |
| Nerve Supply | Posterior interosseous nerve (C7, C8) |
| Blood Supply | Posterior interosseous artery |
| Actions | Extension of the wrist; Ulnar deviation of the wrist; Stabilisation of the ulnar wrist during forearm pronation |
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It works with the flexor carpi ulnaris to produce pure ulnar deviation without a flexion or extension component, and stabilises the ulnar carpus during power gripping in the pronated forearm position, which is critical in ball sports and any tool-based activity.
ECU tendinopathy and subsheath instability are leading causes of ulnar-sided wrist pain in tennis and squash players and are provoked by the ECU synergy test. Subsheath instability is confirmed by a palpable or audible snap as the tendon dislocates during active forearm supination with ulnar deviation. The ECU tendon is also a common incidental finding on wrist MRI, showing signal change without symptoms in up to 30 percent of recreational athletes.
The ECU tendon is the most medial dorsal wrist tendon, palpable between the ulnar styloid and the fifth metacarpal base and becoming prominent during resisted wrist extension with ulnar deviation.
Degenerative overuse producing ulnar-sided wrist pain worsening with gripping and forearm rotation, managed with splinting, load modification, and progressive tendon loading.
Tear or laxity of the retaining tunnel at the ulnar head producing a painful snap as the tendon dislocates with forearm supination, sometimes requiring surgical subsheath reconstruction in athletes.
