Muscles of the Thenar Eminence

Thieme Atlas of Anatomy - Thenar and hypothenar muscles, right hand, palmar view

The Three True Thenar Muscles

1. Abductor Pollicis Brevis (APB)

• Position: Most superficial and proximal of the three; overlies the opponens pollicis
• Origin: Tubercles of the scaphoid and trapezium + adjacent flexor retinaculum
• Insertion: Lateral side of the base of the proximal phalanx of the thumb + extensor hood
• Action: Abducts the thumb (principally at the MCP joint); moves the thumb away from the palm in a plane perpendicular to it
• Nerve: Recurrent branch of median nerve (C8, T1)
• Clinical note: The APB is the key muscle tested when assessing median nerve function at the wrist (e.g., in carpal tunnel syndrome)

2. Opponens Pollicis

• Position: Deepest of the three thenar muscles (lies beneath APB and FPB)
• Origin: Tubercle of the trapezium + adjacent flexor retinaculum
• Insertion: Entire length of the lateral margin and adjacent palmar surface of metacarpal I
• Action: Opposition of the thumb - rotates and flexes metacarpal I at the CMC joint, bringing the thumb pad to face the finger pads; the most important muscle for precision grip
• Nerve: Recurrent branch of median nerve (C8, T1)

3. Flexor Pollicis Brevis (FPB)

• Position: Distal to abductor pollicis brevis; lies lateral to opponens pollicis
• Origin:
  • Superficial head: Tubercle of trapezium + adjacent flexor retinaculum
  • Deep head: Capitate, trapezium (variable - may attach to other carpal bones)
• Insertion: Lateral side of the base of the proximal phalanx of the thumb (tendon often contains a sesamoid bone)
• Action: Flexes the MCP joint of the thumb
• Nerve: Superficial head - recurrent branch of median nerve (C8, T1); deep head - often ulnar nerve (C8, T1)

4. Adductor Pollicis

• Position: Deep, not part of thenar eminence but functionally grouped with thumb muscles
• Origin:
  • Transverse head: Anterior aspect of the shaft of metacarpal III
  • Oblique head: Capitate + adjacent bases of metacarpals II and III
• Insertion: Medial side of the base of the proximal phalanx of the thumb + extensor hood (tendon often contains a sesamoid bone)
• Action: Adducts the thumb; powerful muscle for gripping
• Nerve: Deep branch of the ulnar nerve (C8, T1)
• Note: The radial artery passes between its two heads to form the deep palmar arch

Summary Table

Key Clinical Points

• Median nerve palsy / Carpal tunnel syndrome: Loss of the recurrent branch of the median nerve causes wasting of the APB, opponens pollicis, and FPB (superficial head), leading to flattening of the thenar eminence and loss of opposition. The hand adopts an "ape hand" deformity.
• Ulnar nerve palsy: Adductor pollicis is paralyzed - leads to Froment's sign (patient flexes the IP joint of the thumb using flexor pollicis longus to compensate when trying to pinch paper).
• Recurrent branch of median nerve ("the million dollar nerve"): It hooks around the distal edge of the flexor retinaculum to enter the thenar muscles. Its superficial position makes it vulnerable to lacerations at the base of the thumb.
• Opposition is the combination of flexion, abduction, and rotation of the thumb - this uniquely human movement is largely dependent on the opponens pollicis.