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Extensor carpi ulnaris muscle

The extensor carpi ulnaris (ECU) is one of the superficial muscles of the posterior compartment of the forearm. It acts as a powerful wrist extensor and ulnar deviator, playing a crucial role in stabilizing the wrist during gripping and rotational movements.

The muscle originates from both the lateral epicondyle of the humerus and the posterior border of the ulna, forming a fusiform belly that continues as a long tendon passing through a distinct fibro-osseous tunnel at the dorsal ulnar aspect of the wrist. It inserts on the base of the fifth metacarpal bone.

The ECU tendon lies within a separate compartment of the extensor retinaculum (sixth dorsal compartment), making it vulnerable to tenosynovitis, subluxation, and tendon tears due to repetitive wrist rotation or forceful extension — commonly seen in racquet sports and golf.

Synonyms

  • Ulnar wrist extensor

  • Dorsal ulnar extensor muscle

Origin, Course, and Insertion

  • Origin:

    • Common extensor tendon from the lateral epicondyle of the humerus

    • Posterior border of the ulna via an aponeurosis shared with flexor carpi ulnaris and extensor digitorum

  • Course:

    • Runs distally on the posterior forearm along the ulnar side

    • Forms a tendon in the distal third of the forearm, passing through the sixth dorsal compartment of the extensor retinaculum, within a fibrous sheath

  • Insertion:

    • Dorsal base of the fifth metacarpal bone

Relations

  • Anteriorly: Ulna and interosseous membrane

  • Posteriorly: Skin and superficial fascia of dorsal wrist

  • Medially: Flexor carpi ulnaris and ulnar neurovascular bundle

  • Laterally: Extensor digiti minimi and extensor digitorum tendons

  • Distally: Ulnar styloid process and dorsal capsule of wrist joint

Tendon Attachments

  • Crosses the wrist joint within the sixth dorsal compartment beneath the extensor retinaculum

  • Held in place by the ECU subsheath, preventing bowstringing during wrist motion

  • Inserts into the base of the fifth metacarpal, providing leverage for wrist extension and ulnar deviation

Nerve Supply

  • Posterior interosseous nerve (deep branch of radial nerve), roots C7–C8

Arterial Supply

  • Posterior interosseous artery (branch of ulnar artery via the common interosseous trunk)

  • Additional branches from ulnar artery and radial recurrent artery contribute

Venous Drainage

  • Venae comitantes of the posterior interosseous artery

  • Drains into the brachial and ulnar veins through deep venous channels

Function

  • Wrist extension: Acts synergistically with extensor carpi radialis longus and brevis

  • Ulnar deviation: Moves the wrist medially in conjunction with flexor carpi ulnaris

  • Wrist stabilization: Provides dynamic support to the ulnar side of the wrist during grip and forearm rotation

  • Rotational assistance: Aids in pronation and supination stabilization during power movements

Clinical Significance

  • Tendinopathy: Common in athletes (tennis, golf) due to repetitive ulnar deviation and wrist extension

  • Tendon subluxation: ECU subsheath rupture leads to snapping or painful clicking over ulnar wrist

  • Tenosynovitis: Chronic inflammation causes swelling and dorsal wrist pain

  • Tears or partial rupture: Present with weakness in extension and tenderness at ulnar styloid

  • Surgical relevance: Important in wrist stabilization procedures and ECU sheath repair

  • Imaging importance: MRI and CT define tendon integrity, sheath injury, and inflammation

MRI Appearance

  • T1-weighted images:

    • Muscle belly: intermediate signal intensity with clear fascicular pattern

    • Tendon: low signal (dark), continuous from distal forearm to fifth metacarpal base

    • Peritendinous fat: bright, outlining the tendon sheath

    • Chronic tendinopathy: may show mild intermediate signal within tendon substance

  • T2-weighted images:

    • Muscle: intermediate-to-low signal, slightly darker than on T1

    • Tendon: low signal; hyperintense foci may represent degeneration or small tears

    • Peritendinous fluid and inflammation: bright hyperintense signal around tendon sheath

    • ECU subsheath injury: focal fluid collection or discontinuity posterior to ulna

  • STIR:

    • Normal muscle: intermediate-to-dark signal

    • Pathologic tendon or sheath: bright hyperintense due to edema, tenosynovitis, or tear

    • Distinguishes active inflammation from chronic fibrosis

  • Proton Density Fat-Saturated (PD FS):

    • Normal tendon: dark uniform signal

    • Pathology: focal bright signal areas in tendon or sheath indicating tendinitis or partial tear

    • Excellent for detecting subsheath disruption, peritendinous fluid, or low-grade tears

  • T1 Fat-Sat Post-Contrast:

    • Normal muscle: mild uniform enhancement

    • Inflamed ECU sheath or tendon: shows prominent enhancement outlining sheath

    • Chronic tendinopathy: peripheral rim enhancement with central low-signal fibrosis

    • Useful for differentiating tenosynovitis (enhancing) from degenerative tendinosis (non-enhancing core)

CT Appearance

Non-Contrast CT:

  • Muscle: soft-tissue density along posterior forearm

  • Tendon: dense linear band posterior to ulna within ECU groove

  • ECU groove: shallow sulcus along dorsal ulna, clearly visualized

  • Pathology: tendon calcification or subluxation, subsheath avulsion fractures at ulnar styloid

Post-Contrast CT (standard):

  • Muscle: uniform enhancement

  • Inflammatory changes in ECU sheath or peritendinous tissue: focal enhancement

  • Helpful for assessing bony abnormalities, calcific tendinitis, or ECU sheath lesions when MRI is unavailable

MRI images

Extensor carpi ulnaris muscle axial cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000

MRI images

Extensor carpi ulnaris muscle axial cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000_00001

MRI images

Extensor carpi ulnaris muscle axial cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000_00002

MRI images

Extensor carpi ulnaris muscle axial cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000_00003

MRI images

Extensor carpi ulnaris muscle axial cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000_00004

MRI images

Extensor carpi ulnaris muscle coronal cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000_00005

CT images

Extensor carpi ulnaris ct axial image