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Flexor digitorum superficialis muscle (radial head)

The radial head of the flexor digitorum superficialis (FDS) is one of the three heads of origin of the FDS muscle in the forearm. It is the smallest and most lateral portion, arising from the anterior border of the radius, just distal to the radial tuberosity. This head contributes fibers to the deep part of the muscle and assists in flexion of the proximal interphalangeal (PIP) and metacarpophalangeal (MCP) joints of the fingers.

The radial head lies deep to the pronator teres and lateral to the ulnar head of the FDS. It forms part of the intermediate flexor muscle layer and plays a key role in the coordinated flexion of digits II–V.

Synonyms

  • Radial origin of flexor digitorum superficialis

  • Lateral head of FDS

Origin, Course, and Insertion

  • Origin: Anterior border of the radius, below the oblique line and distal to the radial tuberosity.

  • Course: Fibers pass medially and merge with fibers from the humeroulnar head to form the main muscle belly of the FDS in the proximal forearm.

  • Insertion: The combined FDS tendon divides distally into four tendons that pass through the carpal tunnel and insert into the sides of the middle phalanges of the fingers (index to little finger).

Relations

  • Anteriorly: Pronator teres and flexor carpi radialis muscles

  • Posteriorly: Flexor pollicis longus and radial artery (in the proximal forearm)

  • Laterally: Brachioradialis muscle and radial vessels

  • Medially: Humeroulnar head of FDS and median nerve

  • Inferiorly: Continuation with the common FDS tendon entering the carpal tunnel

Tendon Attachments

  • The radial head contributes to the deep tendinous lamina of the FDS belly, reinforcing the fibers destined for the index and middle fingers.

  • The tendons bifurcate near the proximal phalanx, creating a camper’s chiasm, through which the flexor digitorum profundus (FDP) passes to the distal phalanx.

  • The tendons are bound by the fibrous digital sheaths and annular pulleys along the fingers.

Nerve Supply

  • Median nerve (C7, C8, T1) — provides motor innervation via muscular branches in the proximal forearm.

Arterial Supply

  • Ulnar artery: muscular branches supplying the FDS.

  • Radial artery: small proximal muscular branches to the radial head.

  • Anterior ulnar recurrent artery: contributes to proximal vascularization.

Venous Drainage

  • Venae comitantes accompanying the ulnar and radial arteries drain into the brachial veins.

  • Deep venous plexuses within the forearm muscles communicate with the median cubital and basilic veins.

Function

  • Flexion of digits: Flexes the proximal interphalangeal joints of fingers II–V.

  • Wrist flexion: Assists flexor carpi radialis and ulnaris in wrist flexion.

  • Grip strength: Provides controlled flexion during precision grip and fine motor activity.

  • Dynamic stabilization: Maintains tension and joint stability during finger movements.

Clinical Significance

  • Overuse strain: Common in repetitive gripping or typing, producing medial forearm pain.

  • Entrapment of median nerve: The FDS arch (formed partly by the radial and humeroulnar heads) may compress the median nerve — known as pronator syndrome.

  • Tendon injury: Proximal muscle or tendon tears can impair finger flexion.

  • Surgical relevance: Important landmark during decompression for median nerve entrapment.

  • Imaging importance: MRI crucial for detecting partial tears, muscle edema, and nerve impingement beneath the FDS arch.

MRI Appearance

  • T1-weighted images:

    • Muscle belly: intermediate signal intensity with visible fascicular texture.

    • Tendons: low signal (dark bands) continuous with distal FDS tendons.

    • Fat planes: bright, outlining the radial head clearly from adjacent muscles.

    • Chronic atrophy or fatty infiltration appears bright within the muscle.

  • T2-weighted images:

    • Normal muscle: intermediate-to-low signal, darker than on T1.

    • Tendon: very low signal (black line).

    • Pathology: bright hyperintense regions in myotendinous junction indicate strain or partial tear.

    • Adjacent edema or inflammatory changes may appear hyperintense.

  • STIR:

    • Normal muscle: intermediate-to-dark signal.

    • Pathologic regions (strain, edema, myositis): bright hyperintense.

    • Excellent for early detection of overuse injury and subtle inflammation.

  • Proton Density Fat-Saturated (PD FS):

    • Normal: intermediate-to-dark signal, homogeneous.

    • Partial tear or tendinopathy: focal bright hyperintensity at myotendinous junction or tendon origin.

    • Useful for distinguishing muscular injury from surrounding fascial plane changes.

  • T1 Fat-Sat Post-Contrast:

    • Normal: uniform mild enhancement.

    • Inflammation or strain: focal enhancement within muscle fibers.

    • Chronic tendinopathy: peripheral or patchy enhancement with fibrotic center.

CT Appearance

Non-Contrast CT:

  • Muscle: homogeneous soft-tissue density lateral to the ulnar head of FDS.

  • Tendon: thin, linear soft-tissue band continuous with the FDS belly.

  • Chronic strain: may show mild muscle thickening or indistinct borders.

  • Calcific tendinitis: small high-attenuation foci near radial origin.

Post-Contrast CT (standard):

  • Muscle: mild homogeneous enhancement.

  • Inflamed or hypertrophic regions: localized increased enhancement.

  • Useful for identifying myositis, intramuscular hematoma, or post-traumatic scarring.

MRI image

Flexor digitorum superficialis muscle (radial head) axial cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000