Flexor Pollicis Brevis

The flexor pollicis brevis (FPB) is one of the intrinsic muscles of the hand and a key component of the thenar eminence. It functions primarily to flex the thumb at the metacarpophalangeal joint and assists in thumb opposition. It often consists of two distinct heads — superficial and deep — with separate innervation, making it a unique muscle in the hand.

Location

The FPB lies medial to the abductor pollicis brevis and superficial to the opponens pollicis. It spans from the flexor retinaculum and carpal bones to the base of the thumb’s proximal phalanx.

Structure

The flexor pollicis brevis typically has two heads:

• Superficial head: Arises from the flexor retinaculum and trapezium.
• Deep head: Often arises from the trapezoid and capitate bones (and sometimes from the palmar ligaments).

Both heads insert together into the lateral base of the proximal phalanx of the thumb and sometimes into the extensor expansion.

Function

The flexor pollicis brevis performs:

• Flexion of the Thumb: Flexes the thumb at the metacarpophalangeal joint.
• Assists Opposition: Helps in bringing the thumb across the palm toward the fingers.
• Thumb Stabilization: Contributes to grip strength and fine motor control by stabilizing the thumb during object manipulation.

Innervation

• Superficial head: Recurrent branch of the median nerve (C8–T1)
• Deep head: Deep branch of the ulnar nerve (C8–T1)

This dual innervation is an important anatomical detail and has clinical relevance in nerve injury assessment.

Blood Supply

• Superficial palmar arch (mainly from the ulnar artery)
• Branches of the radial artery

Relations

The FPB lies between the abductor pollicis brevis (laterally) and the adductor pollicis (medially). The tendon of the flexor pollicis longus passes deep to it. The muscle contributes to the bulk of the thenar eminence along with the abductor pollicis brevis and opponens pollicis.

Surface Anatomy

The FPB contributes to the rounded contour of the thenar eminence. It lies deeper than the abductor pollicis brevis but becomes palpable during active thumb flexion.

Development

The muscle develops from the mesenchyme of the thenar compartment. The dual innervation arises from the early merging of tissues from both median and ulnar nerve domains in embryonic development.

Anatomical Variations

• The deep head may be absent, incomplete, or fused with adjacent muscles like the adductor pollicis.
• The superficial and deep heads may have variable origins or insertions.
• Sometimes has additional slips inserting into the extensor expansion or first metacarpal bone.

Clinical Significance

• Median or Ulnar Nerve Lesions: Injury to either nerve can partially impair thumb flexion depending on which head is affected.
• Thenar Atrophy: Long-standing median nerve injury can lead to wasting of the thenar muscles, including the FPB, leading to weakened thumb flexion and opposition (commonly seen in carpal tunnel syndrome).
• Grip Weakness: Loss of FPB function can significantly affect grip involving pinching or buttoning.

Comparative Anatomy

In most non-human primates, the FPB is either not as well developed or is merged with the opponens muscles. In humans, its independent function and dual innervation reflect its evolution to support precision grip and complex thumb movements.