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Abductor pollicis brevis muscle

The abductor pollicis brevis (APB) is the most superficial and lateral muscle of the thenar eminence in the hand. It plays a key role in abducting the thumb at the carpometacarpal and metacarpophalangeal joints, allowing opposition and fine grasping movements. It also assists in stabilizing the thumb during pinch and grip.

The APB is essential in hand function and is often involved in conditions such as carpal tunnel syndrome, where its innervating branch — the recurrent branch of the median nerve — is affected. Its prominence defines the contour of the thenar eminence on the radial aspect of the palm.

Synonyms

  • Short abductor of the thumb

  • Lateral thenar muscle

Origin, Course, and Insertion

  • Origin: Flexor retinaculum and the tubercles of the scaphoid and trapezium bones

  • Course: Fibers pass distally and laterally over the radial aspect of the first metacarpal

  • Insertion: Lateral side of the base of the proximal phalanx of the thumb and the extensor expansion of the thumb

Tendon Attachments

  • Forms a flat tendon inserting into the lateral base of the proximal phalanx of the thumb

  • Partially blends with the aponeurosis of the extensor pollicis longus and flexor pollicis brevis for coordinated thumb motion

  • Reinforced by the fibrous capsule of the metacarpophalangeal joint

Relations

  • Superficially: Palmar fascia and skin of the thenar eminence

  • Deeply: Flexor pollicis brevis, opponens pollicis, and the radial artery

  • Medially: Flexor pollicis brevis

  • Laterally: Thenar fascia and palmar skin

  • Posteriorly: First metacarpal bone

Nerve Supply

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

Arterial Supply

  • Superficial palmar branch of the radial artery

  • Additional contributions from the superficial palmar arch

Function

  • Abduction of the thumb: Moves the thumb away from the palm at the carpometacarpal and metacarpophalangeal joints

  • Opposition assistance: Aids in positioning the thumb for gripping and pinching

  • Stabilization: Maintains thumb alignment during fine motor activity

  • Clinical importance: Atrophy or weakness of APB is a key clinical sign in carpal tunnel syndrome

Clinical Significance

  • Carpal tunnel syndrome: Compression of the median nerve causes APB weakness and thenar atrophy

  • Tendon or muscle tear: May occur from overuse or trauma, presenting as weakness in thumb abduction

  • Atrophy evaluation: MRI helps quantify muscle bulk in chronic neuropathies

  • Surgical relevance: Landmark during median nerve decompression; often used as a donor in reconstructive procedures

MRI Appearance

  • T1-weighted images:

    • Normal muscle: Intermediate signal intensity with well-defined margins

    • Tendon: Low signal (dark linear band) inserting into base of proximal phalanx

    • Fatty planes: Bright, highlighting muscle borders

    • Atrophy: Muscle replaced by bright fatty signal

  • T2-weighted images:

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

    • Tendon: Very low signal

    • Pathology: Bright hyperintense signal in tendinopathy, strain, or denervation edema

  • STIR:

    • Normal muscle: Intermediate-to-dark signal

    • Pathology: Bright hyperintense signal in acute denervation, strain, or inflammation

  • Proton Density Fat-Saturated (PD FS):

    • Normal: Intermediate-to-dark homogeneous signal

    • Pathologic: Bright focal or diffuse hyperintensity in muscle (myositis, denervation, or edema)

    • Ideal for evaluating subtle muscle fiber disruption or inflammation

  • T1 Fat-Sat Post-Contrast:

    • Normal: Homogeneous mild enhancement

    • Inflamed muscle or tendon: Focal or diffuse enhancement

    • Chronic denervation or fibrosis: Minimal or no enhancement with fatty infiltration

CT Appearance

Non-Contrast CT:

  • Muscle: Homogeneous soft-tissue density lateral to the thenar eminence

  • Tendon: Linear soft-tissue band inserting into thumb base

  • Pathology: Fatty atrophy appears as low-density replacement; calcific tendinitis appears as small hyperdense foci

Post-Contrast CT (standard):

  • Muscle: Homogeneous enhancement

  • Inflammation or myositis: Focal hyperenhancement

  • Atrophy: Minimal enhancement with fatty replacement evident as low-density areas

MRI image

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

MRI image

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

MRI image

Abductor pollicis brevis axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00002

MRI image

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

CT image

Abductor pollicis brevis ct axial image