The extensor carpi ulnaris (ECU) tendon occupies its own sixth extensor compartment but has a unique additional stabilising structure — the ECU subsheath, a separate fibrous sleeve between the ECU tendon and the floor of the sixth compartment. The subsheath anchors the ECU tendon within the ulnar groove during forearm rotation, preventing tendon displacement. It is distinct from the extensor retinaculum that forms the roof of the compartment.
ECU subsheath injury from acute wrist rotation in supination produces ECU tendon subluxation from the ulnar groove, the specific instability seen in tennis players (snapping ECU syndrome). The ECU subsheath tears from its ulnar groove attachment, allowing the tendon to snap over the ulnar head during forearm supination. Ultrasound is the optimal imaging modality, demonstrating dynamic subluxation during forearm rotation. Acute ECU subsheath repair prevents chronic subluxation. Chronic subsheath insufficiency is treated by subsheath reconstruction using retinacular tissue.
Sudden wrist injury in supination tears the ECU subsheath from the ulnar groove, producing a painful snap as the ECU tendon dislocates anteriorly over the ulnar head during forearm supination; ultrasound with dynamic supination manoeuvre demonstrates the tendon subluxation and acute subsheath repair within 3 weeks produces excellent outcomes.
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