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Flexor pollicis brevis muscle (Superficial head)

The superficial head of the flexor pollicis brevis (FPB) is a short intrinsic muscle of the thenar eminence located on the palmar aspect of the hand. It lies superficial and slightly lateral to the deep head of FPB, contributing to the rounded contour of the thenar eminence. Its primary role is flexion of the metacarpophalangeal (MCP) joint of the thumb and stabilization of the proximal phalanx during gripping and pinching actions.

The superficial head originates from the flexor retinaculum and trapezium bone, forming a distinct muscular belly that inserts into the base of the proximal phalanx of the thumb. It functions synergistically with the deep head of FPB, opponens pollicis, and adductor pollicis during thumb flexion and opposition.

Synonyms

  • Superficial head of FPB

  • Thenar superficial flexor of thumb

Origin, Course, and Insertion

  • Origin: Flexor retinaculum and tubercle of the trapezium bone

  • Course: Fibers run obliquely distally and laterally, forming a short thick muscle belly over the deep head

  • Insertion: Lateral base of the proximal phalanx of the thumb via a tendon that blends with the abductor pollicis brevis and contains a sesamoid bone on its lateral side

Tendon Attachments

  • The tendon passes superficial to the first metacarpophalangeal joint capsule

  • A lateral sesamoid bone is embedded within the tendon before insertion

  • Merges partially with fibers of the abductor pollicis brevis and deep head of FPB near the insertion site

Relations

  • Superficial: Skin, palmar fascia, and abductor pollicis brevis

  • Deep: Deep head of FPB and flexor pollicis longus tendon

  • Medially: Adductor pollicis and first palmar interosseous muscle

  • Laterally: Abductor pollicis brevis and the thenar fascia

  • Inferiorly (proximal): Flexor retinaculum covering the carpal tunnel

Nerve Supply

  • Recurrent branch of the median nerve (C8, T1)

Arterial Supply

  • Superficial palmar branch of the radial artery

  • Additional supply from superficial palmar arch branches

Function

  • Flexion: Flexes the thumb at the metacarpophalangeal (MCP) joint

  • Stabilization: Stabilizes proximal phalanx during grasp and opposition

  • Synergistic role: Works with deep head of FPB, adductor pollicis, and flexor pollicis longus

  • Precision control: Essential for fine motor activities such as writing, pinching, and object manipulation

Clinical Significance

  • Thenar atrophy: Occurs in median nerve lesions, leading to weakness of FPB (superficial head) and impaired thumb flexion

  • Tendinopathy: Overuse can cause thenar pain, particularly in repetitive gripping tasks

  • Trigger thumb: Associated with flexor tendon sheath inflammation near FPB insertion

  • Surgical relevance: Important landmark during carpal tunnel release and thenar flap procedures

  • Imaging importance: MRI critical for identifying muscle denervation, tears, or post-traumatic changes in the thenar region

MRI Appearance

  • T1-weighted images:

    • Muscle belly: Intermediate signal intensity

    • Tendon: Low signal (dark band) extending to proximal phalanx

    • Fatty septa and surrounding tissue: Bright

    • Pathology: Denervated or atrophic FPB shows increased fatty infiltration with bright T1 signal

  • T2-weighted images:

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

    • Tendon: Low signal intensity

    • Pathology: Muscle edema, strain, or tear appears as bright hyperintense areas

    • Chronic denervation: Muscle replaced by high T1 and low T2 fatty signal

  • STIR:

    • Normal FPB: Intermediate-to-dark signal intensity

    • Acute injury or inflammation: Bright hyperintense edema pattern

    • Useful for detecting early myositis or focal strain within thenar compartment

  • Proton Density Fat-Saturated (PD FS):

    • Normal muscle: Intermediate-to-dark signal

    • Pathologic muscle: Bright hyperintense foci representing edema or microtear

    • Excellent for subtle tendinous insertional inflammation or early denervation changes

  • T1 Fat-Sat Post-Contrast:

    • Normal FPB: Mild, uniform enhancement

    • Inflammation or tenosynovitis: Intense enhancement around tendon sheath

    • Post-surgical or chronic scar tissue: Peripheral rim enhancement with central low signal

CT Appearance

Non-Contrast CT:

  • Muscle appears as soft-tissue density in the thenar region

  • Tendon seen as a fine low-density linear structure inserting onto base of proximal phalanx

  • Sesamoid bone: small ovoid hyperdense structure adjacent to metacarpal head

  • Useful in detecting calcific tendinitis, sesamoid fracture, or chronic enthesopathy

Post-Contrast CT (standard):

  • Muscle enhances homogeneously

  • Peritendinous enhancement indicates inflammation or postoperative change

  • Helpful for assessing mass lesions, post-traumatic fibrosis, or muscle atrophy in the thenar eminence

MRI images

Flexor Pollicis Brevis (Superficial Head) muscle axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI images

Flexor Pollicis Brevis (Superficial Head) muscle axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00001