Extensor Carpi Ulnaris

The extensor carpi ulnaris is a long, thin muscle in the posterior compartment of the forearm. It plays a key role in wrist extension and ulnar deviation (adduction). Despite its name, it is innervated by the posterior interosseous nerve (a branch of the radial nerve), not the ulnar nerve. It is essential for stabilizing the wrist during powerful hand movements such as gripping or lifting.

Location

The extensor carpi ulnaris is located on the medial (ulnar) side of the posterior forearm. It originates from the lateral epicondyle and posterior border of the ulna and runs down the forearm to insert on the base of the fifth metacarpal.

Structure

The muscle has a fusiform belly and a long tendon that passes through a dedicated compartment beneath the extensor retinaculum.

• Origin:
  • Lateral epicondyle of the humerus (via the common extensor tendon)
  • Posterior border of the ulna
• Insertion: Dorsal surface of the base of the fifth metacarpal bone.
• Tendon Path: Passes through the sixth dorsal compartment of the wrist under the extensor retinaculum.

Function

The extensor carpi ulnaris contributes to:

• Wrist Extension: Extends the hand at the wrist joint.
• Ulnar Deviation (Adduction): Moves the wrist medially in coordination with the flexor carpi ulnaris.
• Wrist Stabilization: Active during forceful grips, especially in extension and ulnar deviation movements.

Innervation

Supplied by the posterior interosseous nerve (C7–C8), a continuation of the deep branch of the radial nerve after it passes through the supinator muscle.

Blood Supply

• Posterior interosseous artery: Primary vascular supply.

Relations

The extensor carpi ulnaris is the most medial of the wrist extensors. It lies lateral to the flexor carpi ulnaris (on the anterior side) and medial to the extensor digiti minimi and extensor digitorum on the dorsal side. Its tendon runs in a fibrosseous tunnel on the dorsal aspect of the ulna.

Surface Anatomy

The tendon of the extensor carpi ulnaris is visible and palpable on the dorsal-ulnar aspect of the wrist during wrist extension and ulnar deviation. It becomes more prominent with resisted movements.

Development

The extensor carpi ulnaris develops from the dorsal extensor muscle mass of the forearm, originating from paraxial mesoderm. Its posterior interosseous nerve innervation aligns with its derivation from the extensor group.

Anatomical Variations

• May present with a double tendon or split insertion.
• The tendon may be partially fused with the extensor digiti minimi.
• Accessory muscle slips to nearby carpal bones or fascia may be present.

Clinical Significance

• Tendon Subluxation: The ECU tendon may dislocate from its groove on the ulna during forceful wrist rotation, especially in athletes (e.g., tennis, golf).
• Tendinopathy: Overuse or repetitive motion can cause inflammation, pain, and reduced range of motion (common in racquet sports and construction workers).
• Sixth Compartment Syndrome: Inflammation of the fibrosseous tunnel can lead to restriction or pain during extension and ulnar deviation.
• Surgical Landmark: Used as a landmark in dorsal wrist surgeries or when accessing the distal radioulnar joint.

Comparative Anatomy

In some mammals, the extensor carpi ulnaris acts primarily as a flexor rather than an extensor. In humans, it retains its name due to historical convention, although it functions in wrist extension and adduction.