The ECU tendon runs in its own fibro-osseous groove on the dorsal distal ulna within the sixth extensor compartment, stabilised by the subsheath — a separate restraint within the sixth compartment that maintains the tendon in its groove during forearm rotation. ECU tendon subluxation occurs when the subsheath tears during forceful forearm supination with wrist flexion and ulnar deviation (as in a backhand tennis stroke), allowing the tendon to displace ulnarward off its groove.
Wrist extension and ulnar deviation; ulnar wrist stabiliser
ECU tendon subluxation produces a painful ulnar wrist snap during forearm rotation, reproduced by active forearm supination from a pronated position while palpating the ECU groove. MRI or dynamic ultrasound confirms subluxation. Acute tears managed with immobilisation in pronation (which reduces the tendon into its groove); chronic subluxation requires surgical subsheath reconstruction.
Subsheath disruption producing ECU tendon displacement during forearm rotation with painful ulnar wrist snap, managed with pronation immobilisation acutely or surgical subsheath reconstruction for chronic instability.
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