The extensor carpi ulnaris (ECU) tendon occupies the sixth dorsal compartment at the wrist within its own fibro-osseous tunnel on the ulnar side of the dorsal wrist, stabilised by the ECU subsheath (a thickening of the sixth compartment retinaculum). The ECU acts as a wrist extensor and ulnar deviator, and its subsheath maintains the tendon in the ulnar groove of the distal ulna during forearm rotation.
ECU tendinopathy and subsheath instability are common in racquet sport athletes and gymnasts. ECU instability (subluxation) occurs when the subsheath tears, allowing the ECU to snap over the ulnar styloid during forearm supination, producing the characteristic snapping wrist. MRI or dynamic ultrasound confirms subluxation. Acute subsheath tears are splinted in pronation; chronic instability requires subsheath reconstruction. ECU tendinopathy at the DRUJ level is a component of TFCC pathology and ulnar-sided wrist pain, and its integrity is important in assessing DRUJ stability.
Forced supination and ulnar deviation tears the ECU subsheath, allowing the tendon to sublux over the ulnar styloid during forearm rotation with an audible and palpable snap; dynamic ultrasound confirms the subluxation and acute cases are managed with 6 weeks of pronation splinting, while chronic instability requires subsheath reconstruction.
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