Flexor Carpi Ulnaris Tendon: Anatomy, Function, Imaging

Topic

Flexor carpi ulnaris tendon

The flexor carpi ulnaris (FCU) tendon is the distal tendinous continuation of the flexor carpi ulnaris muscle, located on the ulnar (medial) side of the forearm. It is the most medial of all wrist flexor tendons and plays a major role in wrist flexion and ulnar deviation.

The FCU tendon inserts primarily into the pisiform bone, with extensions to the hamate (via pisohamate ligament) and the base of the fifth metacarpal (via pisometacarpal ligament). It serves as an important dynamic stabilizer of the ulnar wrist and pisiform complex.

Synonyms

Tendon of flexor carpi ulnaris
Ulnar wrist flexor tendon
Pisiform tendon of FCU

Origin, Course, and Insertion

Origin: The muscle originates by two heads:

Humeral head: From the medial epicondyle of the humerus via the common flexor tendon.
Ulnar head: From the medial margin of the olecranon and the upper two-thirds of the posterior border of the ulna.

Course:

The muscle descends along the anteromedial forearm, becoming tendinous in its distal third.
The tendon passes superficial to the ulnar nerve and artery at the wrist, lying just radial to the pisiform bone.

Insertion:

Primary: Pisiform bone.
Secondary extensions: Through the pisohamate and pisometacarpal ligaments to the hamate and base of the fifth metacarpal, respectively.

Tendon Attachments

The FCU tendon forms a firm fibrous expansion anchoring the pisiform to adjacent carpal bones.
Acts as a dynamic stabilizer of the pisotriquetral and ulnocarpal joints.
The pisiform acts as a sesamoid bone, increasing mechanical leverage for wrist flexion.

Relations

Anteriorly: Skin, fascia, and palmaris longus (more radial).
Posteriorly: Ulnar nerve and artery near the wrist.
Laterally: Flexor digitorum superficialis tendon.
Medially: Ulna and overlying fascia.
Distally: Pisiform bone, pisohamate and pisometacarpal ligaments.

Nerve Supply

Ulnar nerve (C7–T1), branch of the medial cord of the brachial plexus.

Function

Wrist flexion: Primary flexor of the wrist joint.
Ulnar deviation: Acts synergistically with the extensor carpi ulnaris for adduction of the wrist.
Stabilization: Provides dynamic stabilization of the pisiform and ulnar wrist during grip and load-bearing.
Assistive role: Supports the flexor retinaculum and palmar fascia during wrist motion.

Clinical Significance

Tendinitis / Tenosynovitis: Overuse injury, common in racket sports, golf, and manual labor.
Tendon tear or rupture: May occur due to trauma or chronic inflammation near the pisiform.
Ulnar tunnel (Guyon’s canal) syndrome: FCU tendon hypertrophy or inflammation can compress the ulnar nerve.
Pisiform instability: Chronic strain of FCU tendon attachments can destabilize the pisotriquetral joint.
Surgical relevance: Landmark in wrist surgeries and ulnar nerve decompression procedures.

MRI Appearance

T1-weighted images:

Tendon: Low signal (dark linear structure) with smooth margins.
Muscle belly: Intermediate signal proximally.
Surrounding fat and fascia: Bright, enhancing tendon outline.
Chronic tendinopathy: Thickened tendon with intermediate signal intensity.

T2-weighted images:

Normal tendon: Low signal (dark) throughout.
Tendinopathy or partial tear: Focal or diffuse bright signal within tendon substance.
Peritendinous edema: Hyperintense on T2, surrounding the tendon margins.
Complete rupture: Discontinuity with fluid-filled gap and retraction of tendon ends.

STIR:

Normal tendon: Dark signal due to dense collagen.
Pathology: Bright hyperintense areas around or within tendon indicating inflammation, tenosynovitis, or strain.

Proton Density Fat-Saturated (PD FS):

Normal tendon: Dark homogeneous signal.
Pathologic changes: Bright focal signals corresponding to partial tear or edema.
Useful for detecting peritendinous fluid and subtle degeneration.

T1 Fat-Sat Post-Contrast:

Normal tendon: Minimal enhancement.
Tendinitis / Tenosynovitis: Peripheral enhancement of tendon sheath and surrounding tissues.
Chronic tendinopathy: Patchy or rim enhancement indicating fibrosis and neovascularization.

CT Appearance

Non-Contrast CT:

Tendon: Soft-tissue density, distinct linear structure inserting into pisiform.
Bony landmarks (pisiform, hamate, fifth metacarpal): clearly visualized.
Chronic changes: Calcification, ossification, or thickening at insertion.
Acute rupture: Subtle loss of soft-tissue continuity or irregularity at insertion site.

Post-Contrast CT (standard):

Normal tendon: Homogeneous enhancement of surrounding soft tissues only.
Inflammatory tendinitis: Mild peritendinous enhancement or edema.
Calcific tendinitis or chronic scarring: Non-enhancing dense foci with surrounding low-attenuation halo.

MRI image