The ECU tendon sheath and its subsheath within the sixth extensor compartment constrains the ECU tendon in the groove between the ulnar head and styloid. The ECU subsheath is a separate stabilising structure that holds the ECU in its groove during forearm rotation, the ECU normally displaces slightly dorsally in supination. Tears of the ECU subsheath from forced supination-ulnar deviation produce ECU tendon instability with painful subluxation.
| Location | Ulnar side of the back of the wrist, in the sixth extensor compartment |
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Holds the ECU tendon in its groove during forearm rotation and lets it glide; tears of the subsheath allow painful ECU subluxation.
ECU subsheath tears produce a painful snap as the ECU subluxes out of its groove during forearm rotation. Ultrasound confirms tendon displacement during dynamic provocative manoeuvre (forearm supination with active ECU resisted). Surgical repair of the sixth compartment floor and sheath wall restores stability after failure of conservative management.
ECU tendon instability from subsheath disruption producing painful tendon subluxation during forearm rotation managed with splinting in pronation and surgical repair.
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