Flexor Carpi Radialis Muscle: Anatomy, Function, Imaging

Topic

Flexor carpi radialis muscle

The flexor carpi radialis (FCR) is a long, fusiform muscle of the superficial anterior compartment of the forearm. It acts primarily to flex and abduct (radially deviate) the wrist. The FCR is an important dynamic stabilizer of the wrist and is often used as a surgical landmark and tendon graft donor.

The muscle forms a distinct tendon visible and palpable at the wrist, where it passes through a fibro-osseous tunnel in the flexor retinaculum (separate from the carpal tunnel). Its tendon inserts into the base of the second metacarpal, occasionally extending to the third metacarpal.

Synonyms

Radial wrist flexor
FCR muscle
Flexor carpi radialis longus (archaic)

Origin, Course, and Insertion

Origin: Common flexor origin from the medial epicondyle of the humerus via the common flexor tendon, and from the deep fascia of the forearm
Course:
- Runs distally on the anterior surface of the forearm between the pronator teres (laterally) and palmaris longus (medially)
- Forms a distinct tendon in the distal third of the forearm
- Passes through a narrow groove on the trapezium, in its own fascial sheath, outside the carpal tunnel
Insertion: Primarily into the base of the second metacarpal bone, with occasional slips to the third metacarpal

Relations

Superficial: Skin and antebrachial fascia
Deep: Flexor digitorum superficialis, radial artery (laterally), and pronator quadratus (distally)
Medially: Palmaris longus tendon and median nerve
Laterally: Radial artery and brachioradialis tendon
Distally: Flexor retinaculum and trapezium groove

Tendon Attachments

Forms a strong, narrow tendon visible on the anterior radial aspect of the wrist
Enclosed in a synovial sheath separate from the carpal tunnel contents
Inserts firmly onto the base of the second metacarpal for stable wrist flexion and radial deviation
Serves as a guide for locating the radial artery during surgical procedures

Nerve Supply

Median nerve (C6–C7), branch from the lateral cord of the brachial plexus

Arterial Supply

Radial artery and its muscular branches
Additional supply from the anterior ulnar recurrent and anterior interosseous arteries

Venous Drainage

Venae comitantes of the radial artery
Drains into the radial veins and ultimately into the brachial veins

Function

Wrist flexion: Primary flexor of the wrist joint
Radial deviation: Abducts the hand at the wrist (acts with extensor carpi radialis longus and brevis)
Assists in elbow flexion: Secondary role in weak elbow flexion
Stabilization: Maintains tension and control during gripping and wrist stabilization
Clinical test: Prominent when wrist flexion and radial deviation are resisted

Clinical Significance

Tendinitis: Common from repetitive wrist flexion or overuse, known as flexor carpi radialis tendinopathy
Entrapment: FCR sheath inflammation can mimic de Quervain’s syndrome
Laceration or rupture: May occur in distal forearm trauma; surgical repair is crucial for wrist strength restoration
Surgical role: Frequently used as a tendon graft for ligament reconstruction or wrist stabilization procedures
Landmark: Its tendon identifies the plane for radial artery cannulation and volar wrist approaches
Imaging relevance: MRI essential for evaluating tendinopathy, tears, or post-surgical scarring

MRI Appearance

T1-weighted images:
- Muscle belly: intermediate signal intensity
- Tendon: low signal (dark linear band) from mid-forearm to base of metacarpal
- Fat around tendon and sheath: bright
- Chronic atrophy: high intramuscular signal due to fatty infiltration
T2-weighted images:
- Normal muscle: intermediate-to-low signal, slightly darker than T1
- Tendon: dark continuous band
- Pathology: tendinitis or partial tear shows bright hyperintense signal within tendon substance or sheath
- Fluid in sheath: high T2 signal outlining tendon
STIR:
- Normal muscle: intermediate-to-dark signal
- Pathology: bright hyperintense areas represent edema, inflammation, or partial tear
- Useful for detecting peritendinous fluid and reactive muscle edema
Proton Density Fat-Saturated (PD FS):
- Normal muscle: intermediate-to-dark signal
- Normal tendon: very low signal (dark)
- Abnormal: bright hyperintense foci in tendon or sheath from inflammation or small tears
- Ideal for assessing subtle tendinopathy and sheath thickening
T1 Fat-Sat Post-Contrast:
- Normal muscle: homogeneous mild enhancement
- Tendon sheath inflammation: focal or circumferential enhancement
- Chronic tendinopathy: peripheral enhancement with central low-signal fibrosis
- Post-surgical changes: enhancement along scar tissue or grafted tendon

MRA Appearance

Normal: The flexor carpi radialis is visualized indirectly as the radial artery’s companion structure in forearm MRA.
Adjacent arterial anatomy: Radial artery and muscular branches enhance clearly, outlining the muscle’s vascular bed.
Pathology: Reduced perfusion or focal arterial narrowing may appear as signal loss in radial artery territory.

CT Appearance

Non-Contrast CT:

Muscle: homogeneous soft-tissue density in anterior forearm
Tendon: linear soft-tissue band anterior to radius
Fat plane separation: clear around distal tendon
Calcific tendinitis: appears as localized hyperdense foci along the tendon

Post-Contrast CT (standard):

Muscle enhances uniformly
Tendon sheath inflammation: mild to moderate enhancement
Post-traumatic or postoperative fibrosis: irregular enhancing soft-tissue density

CTA Appearance

Normal: Radial artery and branches supplying FCR enhance vividly, with the muscle appearing as a soft-tissue mass lateral to flexor compartment.
Pathology:
- Arterial stenosis: tapering of vessel with reduced muscle perfusion
- Post-traumatic hematoma: non-enhancing peritendinous collection
- Useful in assessing vascular supply integrity during flap planning or trauma