Extensor carpi ulnaris

Origin

The extensor carpi ulnaris lies on the ulnar side of the forearm. It arises via its humeral and ulnar heads.

• The humeral head of extensor carpi ulnaris arises from the lateral epicondyle of the humerus via the common extensor tendon. Other muscles, including the extensor carpi ulnaris, extensor digitorum, extensor carpi radialis brevis and extensor digiti minimi muscles share this common origin.

• The ulnar (or medial) head of the extensor carpi ulnaris muscle originates from the medial margin of the olecranon and the upper two-thirds of the posterior border of the ulna, via an aponeurosis. Other muscles, including the flexor carpi ulnaris and the flexor digitorum profundus, share this origin.

Course and insertion

As the tendon of extensor carpi ulnaris crosses the dorsum (back) of the wrist, this tendon passes through a specialized groove located between the head and styloid process of the ulna.

Within this groove, the tendon is encased in a fibro-osseous tunnel, which is formed by the sub-sheath specific to the extensor carpi ulnaris tendon and the adjacent ulnar bones. As the tendon continues distally, it receives further stabilization from the extensor retinaculum and fibrous septa that anchor it to the triquetrum bone. This intricate arrangement forms the sixth extensor compartment of the forearm, providing a strong support system crucial for maintaining tendon integrity, especially during forearm rotation and wrist movements.

These retinacular structures, including the extensor carpi ulnaris subsheath and the extensor retinaculum, are vital. They keep the tendon securely positioned within its groove, preventing it from slipping out (subluxation or dislocation) during powerful wrist extension, ulnar deviation, and pronation-supination movements of the forearm.

Ultimately, the extensor carpi ulnaris tendon inserts onto the dorsal aspect of the base of the fifth metacarpal bone.