Extensor Carpi Ulnaris (Humeral Head): Anatomy, Function, Imaging

Topic

Extensor carpi ulnaris muscle (Humeral Head)

The humeral head of the extensor carpi ulnaris (ECU) is the superficial portion of the ECU muscle, located in the posterior compartment of the forearm. It arises from the common extensor origin on the lateral epicondyle of the humerus, and joins the ulnar head to form a common muscle belly. The ECU primarily extends and adducts (ulnar deviates) the wrist.

The humeral head plays a greater role in wrist extension, while the ulnar head contributes more to ulnar deviation. Together, they act synergistically during gripping and wrist stabilization, especially during forceful extension and rotation.

This portion of the ECU is clinically significant because it is a frequent site of tendinopathy, partial tears, and overuse strain, especially in athletes and individuals with repetitive wrist motion (e.g., tennis, golf, or manual labor).

Synonyms

Humeral portion of extensor carpi ulnaris
Common extensor origin component of ECU
Lateral epicondylar head of ECU

Origin, Course, and Insertion

Origin: Lateral epicondyle of the humerus via the common extensor tendon and deep antebrachial fascia.
Course: Muscle fibers descend obliquely along the posterior forearm, merging with fibers from the ulnar head. The tendon passes deep to the extensor retinaculum through the sixth dorsal compartment of the wrist.
Insertion: Dorsal base of the fifth metacarpal bone.

Relations

Anteriorly: Supinator and extensor digitorum muscles.
Posteriorly: Deep fascia and skin of the posterior forearm.
Laterally: Extensor digiti minimi and extensor digitorum.
Medially: Ulnar head of ECU and flexor carpi ulnaris.
Distally: Ulnar groove on distal ulna; ECU tendon enclosed by synovial sheath under the extensor retinaculum.

Nerve Supply

Posterior interosseous nerve (deep branch of the radial nerve), root values C7–C8.

Arterial Supply

Posterior interosseous artery (branch of the ulnar artery via the common interosseous trunk).
Radial recurrent artery contributes small muscular branches proximally.
Ulnar artery perforators may supply distal regions near the ulnar groove.

Venous Drainage

Venae comitantes accompanying the posterior interosseous artery.
Drain into ulnar veins, which ultimately empty into the brachial veins.
Superficial drainage via posterior forearm cutaneous veins into the basilic system.

Function

Extension of wrist: Acts with extensor carpi radialis longus and brevis.
Ulnar deviation (adduction): Works synergistically with flexor carpi ulnaris.
Wrist stabilization: Prevents radial deviation during grasping or lifting.
Dynamic joint control: Provides stability during pronation-supination and load-bearing wrist motion.

Clinical Significance

Tendinopathy: Repetitive stress or degeneration of ECU tendon at ulnar groove.
Subluxation: ECU tendon instability, especially in racquet sports or golfers.
Lateral epicondylitis: Inflammation at the common extensor origin involving the humeral head of ECU.
Tear or strain: Partial fiber disruption detected on MRI, often associated with pain on resisted wrist extension.
Post-traumatic fibrosis or adhesion: May restrict tendon gliding under extensor retinaculum.
Surgical importance: ECU tendon is sometimes used for reconstructive tendon transfers.

MRI Appearance

T1-weighted images:
- Muscle belly: intermediate signal intensity with distinct fascicular texture.
- Tendon: low signal (dark) along its course under the extensor retinaculum.
- Surrounding fat planes: bright, clearly outlining the muscle margins.
- Chronic tendinopathy: focal thickening and intermediate signal in tendon substance.
T2-weighted images:
- Normal muscle: intermediate-to-low signal, darker than on T1.
- Tendon: low signal, homogeneous.
- Pathology: bright hyperintense foci in tendon or myotendinous junction indicate edema, strain, or tear.
- Peritendinous fluid or sheath inflammation appears hyperintense around distal tendon.
STIR:
- Normal muscle: intermediate-to-dark signal intensity.
- Pathology: bright hyperintense signal representing edema, myositis, or tenosynovitis.
- Sensitive for detecting subtle strains or enthesopathy at the lateral epicondyle.
Proton Density Fat-Saturated (PD FS):
- Normal muscle: intermediate-to-dark, uniform signal.
- Pathologic tendon: focal bright linear hyperintensity indicating partial tear or tendinopathy.
- Excellent for identifying tendon sheath fluid, peritendinous inflammation, or lateral epicondylar pathology.
T1 Fat-Sat Post-Contrast:
- Normal muscle: homogeneous mild enhancement.
- Tendinitis or tenosynovitis: shows peritendinous and sheath enhancement.
- Chronic tendinopathy: focal, irregular enhancement in degenerated tendon fibers.
- Scar or adhesion: low-signal bands with minimal or delayed enhancement.

CT Appearance

Non-Contrast CT:

Muscle belly: soft-tissue density with uniform attenuation.
Tendon: linear soft-tissue density along posterior ulna, visible within the ulnar groove.
Calcific tendinopathy: may appear as small dense foci near the lateral epicondyle or distal insertion.
Muscle injury: subtle fascial stranding or focal hypoattenuation in acute strain.

Post-Contrast CT (standard):

Normal muscle: homogeneous mild enhancement.
Inflammation or strain: focal or diffuse enhancement along muscle or tendon.
Tendon sheath enhancement: indicates tenosynovitis or peritendinous inflammation.
Excellent for evaluating chronic calcification, fibrosis, or postoperative scar formation.

MRI images

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