Adductor Pollicis Muscle: Anatomy, Function, Clinical & Imaging

Topic

Adductor pollicis muscle

The adductor pollicis is a triangular intrinsic muscle of the hand located deep within the palm. It plays a crucial role in adducting the thumb toward the palm, contributing significantly to pinch strength and grip. It lies deep to the flexor tendons and is the most medial of the thenar group, forming the web space between thumb and index finger. The muscle has two distinct heads — oblique and transverse — which converge to insert via a common tendon onto the base of the thumb’s proximal phalanx.

The adductor pollicis is a key muscle for precision grip and opposition and is clinically tested by Froment’s sign in cases of ulnar nerve palsy.

Synonyms

Adductor of the thumb
Adductor pollicis obliquus et transversus

Origin, Course, and Insertion

Oblique Head: Originates from the bases of the second and third metacarpal bones, the capitate, and adjacent carpal ligaments.
Transverse Head: Arises from the anterior surface of the shaft of the third metacarpal bone.
Course: Both heads converge laterally toward the thumb, forming a thick tendon deep to the flexor pollicis longus tendon.
Insertion: The tendon inserts into the medial side of the base of the proximal phalanx of the thumb and into the ulnar sesamoid bone at the first metacarpophalangeal joint capsule.

Tendon Attachments

The tendon of insertion blends with the ulnar sesamoid bone and the fibrous capsule of the thumb’s metacarpophalangeal joint.
Occasionally gives off fibers that blend with the extensor expansion of the thumb.
The adductor aponeurosis covers the deep palmar arch and protects underlying vessels.

Relations

Anteriorly: Flexor tendons of the fingers, superficial palmar arch, and palmar fascia
Posteriorly: Second and third metacarpal bones and interossei muscles
Laterally: Flexor pollicis brevis and first dorsal interosseous muscle
Medially: Deep palmar muscles and ulnar neurovascular bundle
Superiorly: Palmar aponeurosis and skin

Nerve Supply

Deep branch of the ulnar nerve (C8–T1) — this innervation makes the adductor pollicis a key indicator muscle for ulnar nerve integrity.

Arterial Supply

Deep palmar arch (from radial artery) and palmar metacarpal arteries provide the main blood supply.

Function

Adduction of the thumb: Draws the thumb toward the palm in the plane of the hand.
Power grip: Provides stability during pinching and grasping actions.
Opposition assistance: Aids in fine control during thumb–index coordination.
Stabilization: Maintains metacarpophalangeal joint alignment during flexion and opposition.

Clinical Significance

Ulnar nerve palsy: Weakness or paralysis causes loss of thumb adduction (positive Froment’s sign).
Muscle atrophy: Seen in long-standing neuropathy or hand wasting.
Tendon injury: Rare but may impair thumb grip and coordination.
Surgical relevance: Important in reconstructive procedures and tendon transfers for thumb stability.
Imaging importance: MRI evaluation helps assess nerve injury, muscle atrophy, or mass lesions in deep palmar space.

MRI Appearance

T1-weighted images:

Normal muscle: intermediate signal intensity with clear fascicular texture.
Fat planes and neurovascular structures: bright.
Chronic denervation or atrophy: increased fatty signal within muscle belly.
Tendon: low signal (dark linear band) inserting onto base of thumb proximal phalanx.

T2-weighted images:

Normal muscle: intermediate-to-low signal, slightly darker than on T1.
Tendon: low signal (black).
Acute injury, edema, or inflammation: bright hyperintense areas within muscle fibers.
Chronic denervation: heterogeneous signal with fatty replacement.

STIR:

Normal muscle: intermediate-to-dark signal.
Pathology: bright hyperintense signal in acute myositis, contusion, or early denervation changes.
Excellent for identifying subtle edema or soft-tissue inflammation.

Proton Density Fat-Saturated (PD FS):

Normal: intermediate-to-dark homogeneous signal.
Pathologic: focal bright hyperintensity in cases of tendinopathy, strain, or inflammation.
Defines peritendinous fluid and fascial plane thickening clearly.

T1 Fat-Sat Post-Contrast:

Normal muscle: mild uniform enhancement.
Inflammation or infection: diffuse or patchy enhancement.
Neoplasm or vascular lesion: nodular or heterogeneous enhancement.
Denervation atrophy: may show faint enhancement of residual fibers with fatty replacement.

CT Appearance

Non-Contrast CT:

Muscle: soft-tissue density deep in the palm between first dorsal interosseous and flexor pollicis brevis.
Tendon: thin linear soft-tissue band inserting onto ulnar side of thumb proximal phalanx.
Atrophy: visible as reduced muscle bulk and increased intramuscular fat density.
Calcification or ossification: may appear along tendon in chronic injury.

Post-Contrast CT (standard):

Normal muscle: homogeneous enhancement pattern.
Inflamed or fibrotic tissue: focal or diffuse enhancement.
Helpful for assessing mass lesions, abscess, or postoperative scarring within the deep palmar space.

MRI image