Flexor Carpi Ulnaris

What is the Flexor Carpi Ulnaris

The flexor carpi ulnaris (FCU) is one of the five superficial muscles in the anterior compartment of the forearm, along with the pronator teres, palmaris longus, flexor carpi radialis, and flexor digitorum superficialis. It is one of the primary flexors of the wrist joint. It is also a spindle-shaped fusiform muscle like the others in this region.

Anatomy

Location and Attachments

The muscle originates in two heads that come together as a tendinous arch. Here are the attachment points of both heads.

Origin	Humeral head: Medial epicondyle of the humerus (the common flexor tendon) Ulnar head: Olecranon of ulna
Insertion	Pisiform, the hook of hamate, the base of the 5th metacarpal

The humeral head is smaller, originating via the common flexor tendon, along with the other 4 anterior forearm muscles, at the medial epicondyle.

The ulnar head is more extensive, originating from the olecranon and the superior two-thirds of the ulna’s posterior border. It rises through an aponeurosis, which it shares with the originating tendons of the extensor carpi ulnaris and flexor digitorum profundus.

Since the flexor carpi ulnaris is a fusiform muscle, its fibers run straight along the muscle belly, ending in a long tendon that accounts for almost the entire lower half. This tendon then inserts into the pisiform bone and blends with the pisohamate and pisometacarpal ligaments to extend to the hook of hamate and the 5th metacarpal (of the small finger) bones. A few muscle fibers also attach to the flexor retinaculum, the fibrous band of connective tissue that forms the anterior roof of the carpal tunnel.

Relations With Surrounding Muscles and Structures

The flexor carpi ulnaris tendon can easily be seen and palpated underneath the skin layer, just below the wrist joint. The muscle runs from the medial epicondyle towards the ulnar side of the hand. Its inserting ligament is the most medial out of those detectable in the wrist area, with the palmaris longus (when present) lying lateral to it.

The flexor digitorum superficialis is located deep to the flexor carpi ulnaris.

Around its point of origin near the elbow, the posterior ulnar recurrent artery and the ulnar nerve pass from underneath the tendinous arch, the connecting point of the two heads of the muscle. At the distal end, the ulnar artery and nerve run laterally alongside the flexor carpi ulnaris tendon.

Functions

Action

Flexion and adduction of the hand at the wrist

Flexing the Wrist: Contraction of the muscle, along with the flexor carpi radialis and the palmaris longus, results in flexion of the wrist joint, which is bending the wrist to bring the hand inwards or downwards.

Adducting the Wrist: When the flexor carpi ulnaris contracts with its antagonist muscle, the extensor carpi ulnaris from the posterior compartment, the movement causes the hand to adduct at the wrist, which is bending it towards the side of the small finger (ulnar flexion).

Flexing the elbow: With its attachment at the medial epicondyle, the muscle weakly assists in elbow flexion as well.

Flexing and adducting the hands at the wrist is instrumental for almost everything you do with your hands, like writing, typing, eating, brushing your teeth, and driving.

Innervation

Nerve

Ulnar nerve (C7, C8, and T1)

The ulnar nerve (C7, C8, and T1) that innervates the flexor carpi ulnaris is a branch of the brachial plexus.

Blood Supply

Artery

The posterior ulnar recurrent artery, and other ulnar artery branches

The posterior ulnar recurrent artery that passes through the tendinous arch supplies the proximal part of the muscle. The middle and distal parts receive blood supply from additional branches of the ulnar artery. There is a third, accessory arterial supply from the inferior ulnar collateral artery on the distal side of the muscle.