Extensor carpi ulnaris muscle

Origin and insertion

The extensor carpi ulnaris muscle originates from the lateral epicondyle of humerus, via the common extensor tendon, and adjacent fascia. The lateral epicondyle is a common origin for extensor carpi ulnaris, extensor digitorum, extensor carpi radialis brevis and extensor digiti minimi muscles.

Extensor carpi ulnaris comprises its most medial part. The muscle also has attachment to the posterior border of the ulna, via an aponeurosis which it shares with the flexor carpi ulnaris and flexor digitorum profundus muscles.

From its origin, the extensor carpi ulnaris muscle fibers curve inferomedially towards the ulnar side of the hand. Just proximal to the wrist, the muscle gives off a tendon that passes beneath the extensor retinaculum, through a groove on the posterior surface of the head and styloid process of ulna.

Within the retinacular space, the tendon has its own synovial sheath which presents the sixth and the most medial dorsal (extensor) compartment of the wrist. Shortly after entering the hand, the tendon passes over the dorsal surface of triquetral bone and inserts to the medial aspect of the base of metacarpal bone 5.

Relations

Extensor carpi ulnaris is the most medial muscle of the posterior forearm. At its superior attachment, the originating fibers of extensor carpi ulnaris form the most medial part of the common extensor tendon, sitting lateral only to the elbow’s anconeus muscle. As it descends down the forearm the muscle maintains this medial position. It is bounded by its counterpart in the anterior forearm, the flexor carpi ulnaris. Just above the level of the wrist, the dorsal cutaneous branch of the ulnar artery arises from between the extensor and flexor carpi ulnaris muscles.

Innervation

Extensor carpi ulnaris is innervated by the posterior interosseous nerve (C7, C8), a branch of the deep division of the radial nerve. The radial nerve stems from the posterior cord of the brachial plexus.

Blood supply

Blood supply to the extensor carpi ulnaris muscle is provided by branches of the radial recurrent and posterior interosseous arteries, which stem from the radial and ulnar arteries, respectively.

Function

The oblique course of extensor carpi ulnaris orientates its direction of pull posterolaterally, meaning that its contraction results with a combined extension and adduction (ulnar deviation) of the hand.

Working together with extensor carpi radialis brevis and extensor carpi radialis longus, the extensor carpi ulnaris muscle contributes to a balanced extension of wrist without deviating the hand in the transverse plane. This action is important for activities of the hand, such as strongly clenching a fist or making a power grip. In this process, extension of the wrist blocks the forearm flexors from on acting upon it. Instead, these flexors remain able to act solely on the digits, flexing them to produce an effective grip such as that seen in a tennis backhand.

Similarly, synergistic action of flexor carpi ulnaris and extensor carpi ulnaris results in balanced adduction (ulnar deviation) of the hand at the wrist. That is, adduction without added flexion or extension. This action is necessary for processes like hammering, throwing, golfing and swinging a baseball bat. Extensor carpi ulnaris also contributes to the stability of the distal radioulnar joint.

Test your knowledge on the extensors of the forearm with this quiz.