Adductor Pollicis

The adductor pollicis is a triangular-shaped intrinsic muscle of the hand responsible for adduction of the thumb — bringing it toward the palm and the second digit. Unlike the thenar muscles, it is not part of the thenar eminence and is unique in having two distinct heads: oblique and transverse. It is essential for strong pinch grip and fine motor tasks involving thumb stability.

Location

The adductor pollicis lies deep in the palm, between the thumb and index finger, forming part of the deep compartment of the hand. It is medial to the first metacarpal and lateral to the third metacarpal, lying deep to the long flexor tendons of the fingers.

Structure

The muscle has two heads:

• Oblique head: Originates from the bases of the second and third metacarpal bones and the adjacent capitate bone.
• Transverse head: Originates from the anterior surface of the shaft of the third metacarpal bone.
• Insertion: Both heads converge to insert into the medial side of the base of the proximal phalanx of the thumb and the extensor expansion.

Function

The adductor pollicis performs several key actions:

• Adduction of the Thumb: Draws the thumb toward the palm and second metacarpal at the carpometacarpal joint.
• Thumb Stabilization: Provides stability during pinching and power grip.
• Assists Flexion: Weakly aids in flexing the thumb at the metacarpophalangeal joint.

Innervation

Supplied by the deep branch of the ulnar nerve (C8–T1), making it an important muscle for evaluating ulnar nerve integrity.

Blood Supply

• Deep palmar arch (primarily from the radial artery)
• Princeps pollicis artery and radialis indicis artery

Relations

The adductor pollicis lies deep to the flexor pollicis brevis and opponens pollicis. The radial artery passes between its two heads before forming the deep palmar arch. It is bordered laterally by the first dorsal interosseous muscle.

Surface Anatomy

The muscle is not visible on the surface but becomes functionally apparent when performing a forceful pinch (e.g., holding a key). Its tendon contributes to the medial contour of the thumb’s base.

Development

The adductor pollicis arises from the deep mesenchymal hand mass and reflects evolutionary advancement for thumb-to-finger opposition and strong grasping. Unlike the thenar muscles (median nerve), its ulnar innervation reflects its deeper, more primitive muscle grouping.

Anatomical Variations

• May be partially fused with the first dorsal interosseous muscle.
• The size and shape of the oblique head may vary considerably between individuals.

Clinical Significance

• Froment’s Sign: In ulnar nerve palsy, loss of adductor pollicis function causes compensatory flexion of the thumb IP joint by flexor pollicis longus (median nerve) during pinch grip.
• Grip Weakness: Loss of this muscle leads to impaired precision grip and difficulty performing tasks like writing or buttoning clothes.
• Ulnar Nerve Entrapment: Common in conditions such as Guyon’s canal syndrome, leading to muscle atrophy and functional deficits.

Comparative Anatomy

In many non-human primates and mammals, the adductor pollicis is less developed or absent due to reduced thumb mobility. In humans, it has evolved to support complex thumb-finger interaction, critical for tool use, writing, and refined manipulation tasks.