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

Topic

Flexor carpi ulnaris muscle (humeral head)

The humeral head of the flexor carpi ulnaris (FCU) is one of the two proximal heads of origin of the muscle, the other being the ulnar head. It arises from the medial epicondyle of the humerus via the common flexor tendon and contributes to the superficial layer of the anterior forearm.

This head plays a crucial role in flexion and adduction of the wrist (ulnar deviation) and stabilizes the wrist joint during grip and forearm rotation. The FCU humeral head is closely related to the ulnar nerve, which passes between the humeral and ulnar heads — a common site of nerve compression or entrapment at the cubital tunnel.

Synonyms

Humeral origin of flexor carpi ulnaris
Humeral head of FCU
Superficial flexor head of the ulnar-side wrist flexor

Origin, Course, and Insertion

Origin: From the medial epicondyle of the humerus via the common flexor tendon, sharing origin with pronator teres, flexor carpi radialis, palmaris longus, and flexor digitorum superficialis.
Course: Fibers descend obliquely along the anteromedial aspect of the forearm, merging with the deeper ulnar head. The combined muscle belly gives rise to a flat tendon in the distal third of the forearm.
Insertion: Tendon attaches to the pisiform bone, and via the pisohamate and pisometacarpal ligaments, continues to the hook of the hamate and base of the fifth metacarpal bone.

Relations

Superficially: Deep fascia of the forearm and skin
Deeply: Flexor digitorum profundus and ulnar artery (distally)
Medially: Ulnar nerve and posterior ulnar recurrent vessels near origin
Laterally: Flexor digitorum superficialis and palmaris longus tendons
Proximally: Common flexor tendon, pronator teres, and humeroulnar joint capsule

Tendinous Attachments

Tendon forms at the junction of humeral and ulnar heads, becoming prominent in the distal forearm.
Inserts into the pisiform, and through ligamentous extensions, connects indirectly to the hamate and fifth metacarpal, contributing to wrist stability during flexion and ulnar deviation.

Nerve Supply

Ulnar nerve (C7–T1), a branch of the medial cord of the brachial plexus.
The FCU is the only superficial flexor muscle of the forearm entirely supplied by the ulnar nerve, not the median nerve.

Arterial Supply

Posterior ulnar recurrent artery and ulnar artery muscular branches supply the proximal portion.
Distal fibers receive minor contributions from the inferior ulnar collateral artery.

Venous Drainage

Drained by venae comitantes of the ulnar artery, which join the brachial veins.
Superficial venous return through the basilic vein system.

Function

Wrist flexion: Acts with flexor carpi radialis to flex the wrist.
Ulnar deviation: Adducts the hand toward the ulnar side.
Joint stabilization: Provides dynamic stability to the ulnar wrist during grasping.
Elbow function: Assists weakly in elbow flexion.
Clinical significance: Overuse can cause medial elbow pain (golfer’s elbow) or cubital tunnel compression.

Clinical Significance

Tendinopathy: Overuse leads to tendinitis at the medial epicondyle (part of medial epicondylitis complex).
Cubital tunnel syndrome: The ulnar nerve runs between the humeral and ulnar heads — frequent site of entrapment.
Avulsion or strain: Seen in throwing athletes or after trauma.
Surgical relevance: Identifying the humeral head is key during ulnar nerve decompression or tendon harvest.
Imaging importance: MRI is used for evaluating tendinopathy, tears, and nerve entrapment adjacent to the humeral head.

MRI Appearance

T1-weighted images:
- Muscle belly: intermediate signal intensity with clear fascicular texture.
- Tendon and common flexor origin: low signal (dark) continuous with the medial epicondyle.
- Perimuscular fat: bright on T1, providing contrast.
- Partial tears: focal intermediate-to-bright signal at the tendon origin.
T2-weighted images:
- Normal muscle: intermediate signal, slightly darker than on T1.
- Tendon: dark (low signal); increased signal indicates tendinopathy or tear.
- Peritendinous fluid or inflammation: bright hyperintensity around origin.
- Distinct visualization of common flexor tendon pathology near medial epicondyle.
STIR:
- Normal muscle: intermediate-to-dark signal.
- Pathology: bright hyperintense signal in tendon or myotendinous junction, typical of acute strain or inflammation.
Proton Density Fat-Saturated (PD FS):
- Normal muscle: intermediate-to-dark homogeneous signal.
- Pathology: focal bright areas along tendon or origin (tendinitis or partial tear).
- Excellent for evaluating subtle myotendinous injury and peritendinous fluid.
T1 Fat-Sat Post-Contrast:
- Normal muscle: mild uniform enhancement.
- Inflamed tendon or enthesis: focal enhancement at the medial epicondyle.
- Chronic tendinopathy: peripheral rim enhancement with fibrotic low-signal center.
- Postoperative or scar tissue: diffuse enhancement around tendon sheath.

CT Appearance

Non-Contrast CT:

Muscle: homogeneous soft-tissue density overlying medial epicondyle.
Tendon origin: dense linear structure along the common flexor tendon.
Chronic tendinopathy: subtle calcifications or cortical irregularity of medial epicondyle.
Useful for evaluating avulsion fractures or ossified tendinous insertions.

Post-Contrast CT (standard):

Enhancing muscle fibers and tendon attachments.
Inflamed or fibrotic tendon origin: shows focal enhancement and adjacent soft-tissue edema.
Helps delineate enthesopathy, ossification, and soft-tissue mass lesions.

MRI image