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Flexor pollicis brevis muscle (Deep head)

The deep head of the flexor pollicis brevis (FPB) is a small but functionally significant component of the thenar muscle group in the hand. It lies deep to the superficial head of the FPB and medial to the tendon of the flexor pollicis longus. This head contributes to flexion and stabilization of the metacarpophalangeal (MCP) joint of the thumb and assists in opposition and precision grip.

Anatomically, the deep head is often smaller, variably developed, and more medially positioned than the superficial head. It plays a critical synergistic role with adductor pollicis and opponens pollicis, providing controlled thumb movement during fine motor activities.

Synonyms

  • Profundus head of flexor pollicis brevis

  • Deep portion of FPB

  • Deep thenar flexor

Origin, Course, and Insertion

Origin: From the trapezoid and capitate bones and the palmar ligaments of the distal carpal row.
Course: Fibers pass obliquely distally and laterally, deep to the flexor pollicis longus tendon, forming a short muscular belly.
Insertion: Joins the medial side of the base of the proximal phalanx of the thumb, often blending with the tendon of the adductor pollicis.

Tendon Attachments

  • The tendon of the deep head merges partially with the adductor pollicis insertion on the ulnar sesamoid and base of the proximal phalanx.

  • The superficial head of FPB inserts on the radial sesamoid; together, they stabilize the MCP joint during thumb flexion.

  • The deep head forms part of the ulnar sesamoid complex on the volar surface of the MCP joint.

Relations

Superficially: Flexor pollicis longus tendon and superficial head of FPB
Deeply: Adductor pollicis oblique head and first palmar interosseous muscle
Medially: Ulnar side of thumb base and adductor pollicis tendon
Laterally: Tendon of flexor pollicis longus
Anteriorly: Skin, palmar fascia, and superficial palmar arch
Posteriorly: First metacarpal and MCP joint capsule

Nerve Supply

  • Deep branch of the ulnar nerve (C8–T1) — unlike the superficial head, which is supplied by the median nerve.

Arterial Supply

  • Deep palmar arch (branch of radial artery)

  • Minor contributions from the princeps pollicis and radialis indicis arteries

Function

  • Flexion: Flexes the proximal phalanx at the metacarpophalangeal joint of the thumb.

  • Stabilization: Provides medial support to the MCP joint and balances forces of the superficial head.

  • Opposition: Assists in thumb opposition and grip precision.

  • Synergistic action: Works with adductor pollicis for strong thumb pinch and grasp.

Clinical Significance

  • Ulnar nerve injury: Paralysis or weakness leads to reduced thumb flexion strength and impaired pinch function.

  • Atrophy: Seen in chronic ulnar neuropathy or Guyon’s canal syndrome.

  • Tendinopathy or strain: Occurs with repetitive gripping or fine manipulation activities.

  • Surgical relevance: Important landmark in carpal tunnel and deep palmar dissections; must be preserved during thenar repair surgeries.

  • Imaging importance: Differentiation between superficial and deep heads on MRI aids in evaluating focal muscle denervation or mass lesions in the thenar region.

MRI Appearance

T1-weighted images:

  • Muscle belly: intermediate signal intensity (isointense to other thenar muscles).

  • Distinct separation from superficial head and adductor pollicis by thin bright fat planes.

  • Fatty infiltration or atrophy in denervation: bright signal replacing normal intermediate texture.

T2-weighted images:

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

  • Tendon: low signal (dark linear band).

  • Pathology (strain or inflammation): focal or diffuse hyperintense areas in muscle belly or tendon insertion.

STIR:

  • Normal muscle: intermediate-to-dark signal intensity.

  • Acute pathology: bright hyperintense signal from edema, myositis, or muscle strain.

  • Denervated muscle: early high signal, chronic stages show volume loss with dark fibrotic tissue.

Proton Density Fat-Saturated (PD FS):

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

  • Pathologic changes: bright focal or patchy hyperintensity representing edema, partial tear, or peritendinous fluid.

  • Excellent for evaluating subtle soft-tissue inflammation and fascial plane edema.

T1 Fat-Sat Post-Contrast:

  • Normal muscle: uniform mild enhancement.

  • Tendinous or fascial inflammation: focal linear or rim enhancement.

  • Denervation or chronic fibrosis: minimal enhancement with decreased muscle bulk.

CT Appearance

Non-Contrast CT:

  • Muscle: soft-tissue density, blending with other thenar muscles.

  • Tendon: linear, slightly higher density structure extending toward base of proximal phalanx.

  • Normal architecture: smooth contours, uniform density.

  • Pathology: shows calcification, fatty atrophy, or post-surgical scarring.

Post-Contrast CT (standard):

  • Normal muscle enhances homogeneously.

  • Inflammation or post-surgical granulation: focal or diffuse increased enhancement.

  • CT delineates bony attachments at trapezoid, capitate, and base of proximal phalanx clearly for trauma or enthesopathy assessment.

MRI image

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

MRI image

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