The extensor carpi ulnaris muscle is an elongated fusiform muscle located in the posterior compartment of the forearm and primarily functions to extend and adduct the wrist. It spans between the elbow and base of the little finger. The extensor carpi ulnaris muscle belongs to the superficial group of extensors of the forearm along with brachioradialis, anconeus, extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorum, and extensor digiti minimi.

Its origin lies in the lateral epicondyle of the distal humerus and the posterior aspect of the ulna. It inserts onto the dorsal base of the fifth metacarpal after passing through the sixth compartment of the extensor retinaculum. It receives its innervation from the posterior interosseous nerve and gets its blood supply from the ulnar artery.[1]

Structure:

The extensor carpi ulnaris muscle has two heads, i.e., a humeral head and an ulnar head. The humeral head originates from the common extensor tendon and is attached to the lateral epicondyle of the humerus. The ulnar head derives from the common aponeurosis attached to the posterior border of the ulna. This common extensor tendon is shared by the flexor carpi ulnaris, flexor digitorum profundus, and deep fascia of the forearm. Distally, just proximal to the wrist, the extensor carpi ulnaris tendon, within the tendinous sheath of the extensor carpi ulnaris, runs beneath the extensor retinaculum in a groove between the posterior surface of the head and the styloid process of the ulna, which forms a separate compartment of the extensor retinaculum.

Relations:

The extensor carpi ulnaris is the most medial muscle of the extensor compartment of the forearm; however, only a small triangular shape muscle called anconeus lies medially to the superior fibers of the extensor carpi ulnaris. Laterally, the extensor carpi ulnaris is related to the extensor digiti minimi and the extensor digitorum muscles. Distally, the cutaneous branch of the ulnar artery arises from between the tendons of the extensor carpi ulnaris and the flexor carpi ulnaris.

Functions:

The extensor carpi ulnaris serves two functions. It tends to extend the hand at the wrist joint by acting along extensor carpi radialis longus and extensor carpi radialis brevis. Along with the flexor carpi ulnaris, the extensor carpi ulnaris adducts the hand at the wrist. The simultaneous action of extensor carpi radialis longus and extensor carpi radialis brevis along with extensor carpi ulnaris helps to provide medial stability to the wrist while clenching the fist (tight flexion of the medial four digits).

The muscles of the forearm have their origin in the dorsolateral somite cells that form muscles after they migrate to the limb at approximately the fourth week of development. As the limb buds lengthen, the muscles begin to divide into separate functional groups, such as the flexors and extensors. The anterior and posterior pattern of the limb is developed by a sonic hedgehog protein, which gets secreted from the zone of polarizing activity.[2]

The extensor carpi ulnaris gets its vascular supply primarily from the ulnar artery which branches off of the brachial artery near the antecubital fossa and supplies the medial aspect of the forearm. Due to the muscle's location in the posterior compartment of the forearm, it also receives some blood supply from the posterior interosseous artery, a posterior branch of the radial artery, that runs between the superficial and deep extensor muscle groups and supplies them both.

The lymphatic drainage of the upper limb consists of both superficial and deep lymphatic vessels. The superficial lymphatic vessels of the hand and forearm ascend the arm along with the superficial veins, such as the basilic vein, and enter the cubital lymph nodes, near the medial epicondyle of the humerus. Superficial lymphatic vessels traveling more proximally to the cephalic vein primarily drain into the apical axillary lymph nodes, though the deltopectoral lymph nodes are another potential site of drainage. The deep lymphatic vessels of the arm and forearm follow the deep veins and end up in the humeral axillary lymph nodes and eventually the subclavian lymphatic trunk.[3]

The muscle receives its nerve supply from the posterior interosseous nerve (C7-C8), which is a motor branch of the radial nerve. The radial nerve dives posteriorly through the heads of the supinator muscle in the antecubital fossa to form the posterior interosseous nerve. It innervates the extensor carpi ulnaris muscle in addition to the other muscles in the posterior compartment of the forearm. The radial nerve arises from the brachial plexus by way of the posterior cord, which has contributions from the spinal nerve roots of C5 to T1.[4]