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Flexor pollicis brevis muscle

The flexor pollicis brevis (FPB) is one of the intrinsic muscles of the thenar eminence in the hand. It lies superficial and medial to the abductor pollicis brevis, acting as the principal flexor of the metacarpophalangeal (MCP) joint of the thumb. It consists of two heads — superficial and deep — which embrace the tendon of the flexor pollicis longus (FPL) as it passes to the distal phalanx of the thumb.

The FPB plays a critical role in thumb flexion, grip, and fine motor control, enabling actions like pinching and writing. Its anatomy is important in carpal tunnel and thenar muscle pathology, as well as during reconstructive and tendon transfer surgeries.

Synonyms

  • Short flexor of the thumb

  • Brevis flexor of pollex

Origin, Course, and Insertion

Origin

  • Superficial head: flexor retinaculum and trapezium bone

  • Deep head: trapezoid and capitate bones

Course

  • Fibers run distally and medially from the carpal bones, forming a muscle belly that covers the medial aspect of the thenar eminence.

  • The two heads converge and pass alongside the flexor pollicis longus tendon.

Insertion

  • Both heads insert into the lateral base of the proximal phalanx of the thumb and into the radial sesamoid bone at the MCP joint.

Tendon Attachments

  • A sesamoid bone lies within the FPB tendon at its insertion on the lateral aspect of the MCP joint.

  • FPB blends with the abductor pollicis brevis and adductor pollicis tendons for coordinated thumb motion.

  • The flexor pollicis longus tendon passes deep between the two heads.

Relations

  • Superficially: Skin and palmar fascia of the thenar eminence

  • Deeply: Flexor pollicis longus tendon

  • Laterally: Abductor pollicis brevis

  • Medially: Adductor pollicis (oblique head)

  • Proximally: Flexor retinaculum and carpal bones

  • Distally: Proximal phalanx and sesamoid bone of the thumb

Nerve Supply

  • Superficial head: Median nerve (recurrent branch, roots C8–T1)

  • Deep head: Deep branch of ulnar nerve (C8–T1)

Arterial Supply

  • Superficial palmar branch of radial artery

  • Contributions from the superficial palmar arch

Function

  • Flexion: Primary flexor of the thumb at the MCP joint

  • Opposition: Assists opponens pollicis during opposition

  • Grip strength: Stabilizes thumb during pinching and grasping

  • Fine motor control: Coordinates with other thenar muscles for precise thumb positioning

Clinical Significance

  • Thenar atrophy: Seen in median nerve compression (carpal tunnel syndrome) causing thumb weakness

  • Ulnar nerve injury: May affect deep head, leading to weak thumb flexion

  • Tendinitis or strain: Overuse in gripping or repetitive motion causes thenar pain

  • Surgical relevance: FPB tendon used in tendon transfers and thumb reconstruction

  • Imaging role: MRI essential in evaluating muscle integrity, nerve injury, and soft tissue masses in the thenar region

MRI Appearance

T1-weighted images

  • Muscle belly: intermediate signal intensity with clear fascicular pattern

  • Tendons: low signal (dark) inserting on the proximal phalanx

  • Sesamoid bone: low-signal cortex with intermediate marrow signal

  • Surrounding subcutaneous fat: bright signal outlining muscle borders

T2-weighted images

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

  • Tendons: uniformly dark

  • Pathology: bright hyperintense areas in tendon or muscle from edema, strain, or partial tear

  • Joint capsule or sesamoid inflammation appears bright and ill-defined

STIR

  • Normal muscle: intermediate-to-dark signal intensity

  • Pathologic muscle or tendon: bright hyperintense signal (edema, strain, inflammation)

  • Useful for detecting early myotendinous injury and peritendinous edema

Proton Density Fat-Saturated (PD FS)

  • Normal: intermediate-to-dark homogeneous signal

  • Tendon tears or tendinitis: focal bright signal near insertion

  • Identifies subtle myofascial inflammation or partial-thickness injury

T1 Fat-Sat Post-Contrast

  • Normal: mild uniform enhancement of muscle

  • Tendinitis or synovitis: focal enhancement along tendon sheath

  • Chronic tendinopathy or fibrosis: peripheral rim enhancement with central low signal

  • Mass lesions: show nodular or heterogeneous enhancement depending on vascularity

CT Appearance

Non-Contrast CT

  • Muscle: soft-tissue density forming medial mass of thenar eminence

  • Tendon: fine soft-tissue band inserting on proximal phalanx

  • Sesamoid bone: dense ovoid opacity adjacent to MCP joint

  • Chronic tendinopathy: tendon thickening or small calcifications

Post-Contrast CT (standard)

  • Normal muscle: homogeneous enhancement

  • Inflamed tendon or peritendinous tissues: show increased enhancement

  • Mass or fibrotic change: irregular or nodular enhancement pattern

  • Provides excellent evaluation of osseous detail and adjacent joint pathology

MRI image

Flexor pollicis brevis muscle axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI image

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

MRI image

Flexor pollicis brevis muscle coronal cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

CT image

Flexor pollicis brevis musclect axial iamge