Upper limb

Tendon sheath of sixth extensor compartment (ECU)

Vagina tendinis musculi extensoris carpi ulnaris

Origin

Sixth dorsal compartment residing in the ulnar groove on the distal ulna with its own distinct subsheath

Insertion

Encloses the extensor carpi ulnaris (ECU) tendon to its insertion at the base of the fifth metacarpal

Function

Provides dynamic stabilization to the ulnar side of the wrist while guiding wrist extension and ulnar deviation

Innervation

Posterior interosseous nerve (C7–C8)

Movement & chain

Exposed to rotational and shear loads during forearm pronosupination and wrist deviation

Palpation

Palpable in the groove on the dorsomedial aspect of the ulnar head

Clinical

ECU tendinopathy, ECU tendon subluxation/dislocation (due to subsheath rupture during sudden forearm supination/wrist flexion in tennis/golf athletes)

Segmental innervation

C7–C8 spinal segments

In clinical practice

ECU synergy test for assessment; conservative immobilization in slight wrist extension/pronation or surgical subsheath reconstruction

Region in the model

Upper limb