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Flexor carpi ulnaris muscle

The flexor carpi ulnaris (FCU) is a long, fusiform muscle of the superficial flexor compartment of the forearm. It lies on the ulnar (medial) side, forming the most medial margin of the forearm, and is a strong flexor and adductor (ulnar deviator) of the wrist. The FCU is unique among the flexor muscles in that it is exclusively innervated by the ulnar nerve.

Its tendon forms a key palpable landmark along the medial wrist, serving as a guide for the ulnar artery and nerve, which pass lateral and deep to it. The FCU is clinically significant in cases of medial epicondylitis, ulnar nerve entrapment, and wrist flexor tendinopathy.

Synonyms

  • Ulnar wrist flexor

  • Ulnar carpal flexor

Origin, Course, and Insertion

  • Origin:

    • Humeral head: Medial epicondyle of the humerus via the common flexor tendon

    • Ulnar head: Medial margin of the olecranon and the proximal two-thirds of the posterior border of the ulna

  • Course:

    • Fibers descend along the medial forearm, converging into a thick tendon in the distal third.

    • The tendon passes superficial to the ulnar nerve and artery at the wrist.

  • Insertion:

    • Pisiform bone

    • Hook of hamate (via pisohamate ligament)

    • Base of the fifth metacarpal (via pisometacarpal ligament)

Tendon Attachments

  • Main tendon inserts on the pisiform, acting as a sesamoid-like bone that transmits tension to the hamate and fifth metacarpal.

  • Forms the pisohamate and pisometacarpal ligaments, which function as fibrous extensions of the tendon.

  • Distally, the tendon contributes to the flexor retinaculum and blends with hypothenar fascia.

Relations

  • Superficial: Skin, fascia, and subcutaneous fat of the forearm and wrist

  • Deep: Flexor digitorum profundus and ulna

  • Medially: Posterior border of ulna

  • Laterally: Flexor digitorum superficialis and ulnar artery

  • At wrist: Ulnar nerve and artery pass deep to its tendon before entering Guyon’s canal

Nerve Supply

  • Ulnar nerve (C7, C8, T1) — the only muscle in the superficial anterior compartment not supplied by the median nerve

Arterial Supply

  • Ulnar artery: Primary supply via muscular branches

  • Posterior ulnar recurrent artery: Contributes proximally

  • Inferior ulnar collateral artery: Minor proximal contribution

Venous Drainage

  • Venae comitantes of the ulnar artery draining into the brachial veins

  • Superficial veins: Median antebrachial and basilic veins overlie the muscle and drain into the axillary system

Function

  • Wrist flexion: Primary flexor of the wrist joint

  • Ulnar deviation: Adducts the hand toward the ulnar side

  • Stabilization: Acts synergistically with extensor carpi ulnaris during ulnar deviation to stabilize the wrist

  • Assists in grip: Provides steady wrist base for finger flexors during grasping

Clinical Significance

  • Tendinopathy: Common in repetitive wrist flexion and ulnar deviation (golfers, racket sports)

  • Medial epicondylitis: FCU origin often involved in “golfer’s elbow”

  • Ulnar nerve entrapment: Compression beneath FCU aponeurosis at the cubital tunnel causes sensory and motor deficits

  • Trauma: Lacerations at the wrist can injure the FCU tendon

  • Imaging relevance: Used as an anatomical landmark in ultrasound and MRI for locating the ulnar nerve and artery

MRI Appearance

  • T1-weighted images:

    • Muscle belly: Intermediate signal intensity

    • Tendon: Low signal (dark linear band)

    • Perimuscular fat: Bright, providing clear delineation

    • Pathology: Tendon thickening or discontinuity with intermediate-to-bright signal in partial tear or strain

  • T2-weighted images:

    • Muscle: Intermediate signal, slightly darker than T1

    • Tendon: Very low signal intensity

    • Pathology: Tendinitis or partial tear appears as bright hyperintense regions in tendon or at myotendinous junction

    • Surrounding edema: Bright, hyperintense signal within peritendinous fat

  • STIR:

    • Normal muscle: Intermediate-to-dark signal

    • Pathology: Bright hyperintense regions in muscle or tendon indicating edema, strain, or tenosynovitis

    • Distinguishes acute inflammation from chronic fibrosis

  • Proton Density Fat-Saturated (PD FS):

    • Normal: Muscle and tendon show intermediate-to-dark homogeneous signal

    • Tendinopathy: Focal bright hyperintensity at tendon or insertion

    • Tear: Linear fluid-filled hyperintense clefts within tendon substance

  • T1 Fat-Sat Post-Contrast:

    • Normal muscle: Mild uniform enhancement

    • Inflamed tendon sheath or enthesopathy: Focal enhancement along tendon

    • Chronic tendinopathy: Peripheral enhancement with central low-signal fibrotic core

CT Appearance

Non-Contrast CT:

  • Muscle: Soft-tissue density, homogeneous

  • Tendon: Linear dense band inserting onto pisiform

  • Bone attachment: Well-defined cortical surface at pisiform and hamate

  • Calcific tendinitis: May appear as focal high-attenuation deposits along tendon path

Post-Contrast CT (standard):

  • Muscle: Homogeneous enhancement

  • Inflamed or injured tendon: Focal enhancement or surrounding soft-tissue swelling

  • Useful for: Identifying calcifications, myositis ossificans, or post-traumatic scarring

MRI image

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MRI image

Flexor carpi ulnaris muscle axial cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000_00001

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Flexor carpi ulnaris muscle axial cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000_00003

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Flexor carpi ulnaris muscle axial cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000_00004

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Flexor carpi ulnaris muscle axial cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000_00005

CT image

Flexor carpi ulnaris ct axial image