Flexor Carpi Ulnaris (Ulnar Head): Anatomy, Function, Imaging

Topic

Flexor carpi ulnaris (ulnar head)

The ulnar head of the flexor carpi ulnaris (FCU) is one of two proximal attachments of the FCU muscle, the other being the humeral head. The ulnar head is smaller and arises from the medial aspect of the olecranon and the proximal two-thirds of the posterior border of the ulna. It lies deep and medial to the humeral head and contributes significantly to the muscle’s overall strength and stability along the medial forearm.

The ulnar head assists in flexion and adduction of the wrist (ulnar deviation) and forms the medial boundary of the cubital tunnel, through which the ulnar nerve passes — a key anatomic consideration in entrapment syndromes.

Synonyms

Ulnar head of FCU
Deep head of flexor carpi ulnaris

Origin, Course, and Insertion

Origin: From the medial border of the olecranon of the ulna and the proximal two-thirds of the posterior border of the ulna, by an aponeurosis shared with the flexor digitorum profundus.
Course: Fibers descend anteromedially along the forearm, joining the humeral head. Together, they form a common belly that becomes tendinous in the distal forearm.
Insertion:
- The tendon passes superficial to the flexor retinaculum (not through the carpal tunnel).
- Inserts primarily on the pisiform bone, with extensions (via the pisohamate and pisometacarpal ligaments) to the hook of hamate and the base of the fifth metacarpal.

Relations

Anteriorly: Skin, fascia, and superficial veins (including the basilic vein).
Posteriorly: Ulna and flexor digitorum profundus.
Medially: Ulnar nerve (running between humeral and ulnar heads at origin).
Laterally: Flexor digitorum superficialis and palmaris longus.
Distally: Pisiform bone and ulnar artery near the wrist.

Tendon Attachments

The FCU tendon attaches to the pisiform, then continues via fibrous slips:
- Pisohamate ligament: To the hook of the hamate.
- Pisometacarpal ligament: To the base of the fifth metacarpal.
These attachments help transmit flexion and ulnar deviation forces to the wrist and hand.

Nerve Supply

Ulnar nerve (C7, C8, T1) — the only forearm flexor muscle entirely innervated by the ulnar nerve.

Arterial Supply

Ulnar artery: Provides the main vascular supply through muscular branches in the proximal forearm.
Posterior ulnar recurrent artery: Contributes to proximal arterial perfusion near the elbow.
Inferior ulnar collateral and dorsal ulnar carpal branches: Provide minor supplementary supply distally.

Venous Drainage

Companion veins of the ulnar artery (venae comitantes): Drain into the brachial veins proximally.
Superficial venous drainage: Via the basilic vein on the medial forearm.

Function

Flexion of the wrist: Acts strongly to bend the wrist joint.
Adduction (ulnar deviation): Pulls the wrist medially, working with extensor carpi ulnaris.
Elbow stabilization: Helps stabilize the medial side of the elbow joint during flexion.
Protective role: Its fibers and aponeurosis help form part of the roof of the cubital tunnel, shielding the ulnar nerve.

Clinical Significance

Cubital tunnel syndrome: The ulnar head forms part of the cubital tunnel; hypertrophy or fibrotic thickening may compress the ulnar nerve, causing paresthesia in the ring and little fingers.
Tendinopathy: Overuse or repetitive wrist flexion can cause insertional tendinitis near the pisiform.
Muscle strain: Can occur in athletes with excessive wrist loading or racket sports.
Surgical relevance: The ulnar head is an important anatomical landmark in ulnar nerve decompression and ulnar artery exposure procedures.
Imaging relevance: MRI helps differentiate FCU tendinopathy from ulnar nerve entrapment and pisiform-hamate joint pathology.

MRI Appearance

T1-weighted images:
- Muscle belly: intermediate signal intensity, uniform texture.
- Tendon: low signal (dark) along its course to pisiform.
- Perimuscular fat: bright, outlining muscle margins.
- Chronic fatty atrophy: bright intramuscular signal.
T2-weighted images:
- Normal muscle: intermediate signal, slightly darker than on T1.
- Tendon: low signal (black) continuous band.
- Pathology: bright hyperintense signal in tendon or myotendinous junction with tendinitis, tear, or inflammation.
- Fluid around tendon sheath (tenosynovitis): hyperintense fluid signal.
STIR:
- Normal muscle: intermediate-to-dark signal.
- Pathologic: bright hyperintense regions in edema, strain, or acute myositis.
- Enhances detection of cubital tunnel edema or perineural inflammation adjacent to ulnar nerve.
Proton Density Fat-Saturated (PD FS):
- Normal: intermediate-to-dark muscle, dark tendon.
- Pathologic: focal or diffuse bright signal indicating edema, myotendinous injury, or peritendinous fluid.
- Excellent for identifying partial tendon tears, tendinopathy, or myofascial strain.
T1 Fat-Sat Post-Contrast:
- Normal: uniform mild enhancement of muscle.
- Tendinitis: peripheral or diffuse enhancement of tendon sheath.
- Post-traumatic or inflammatory changes: localized enhancement along myotendinous junction or perimuscular fascia.

CT Appearance

Non-Contrast CT:

Muscle: homogeneous soft-tissue density medial to ulna.
Tendon: dense linear structure attaching to pisiform.
Fat planes: clearly define muscle borders.
Pathology: calcific tendinitis, partial avulsion at pisiform, or chronic scarring visible as focal calcific or fibrotic densities.

Post-Contrast CT (standard):

Muscle: mild homogeneous enhancement.
Pathologic: increased enhancement in inflamed tendon or perimuscular tissue.
Useful in assessing chronic myositis ossificans, calcification, or postsurgical changes near cubital tunnel or pisiform.

MRI image