Abductor Digiti Minimi

The abductor digiti minimi (ADM) is a superficial intrinsic muscle located on the ulnar side of the hand. It is one of the three hypothenar muscles and is primarily responsible for abduction of the little finger away from the midline of the hand. This muscle contributes to grip strength and hand balance, especially during object manipulation.

Location

The abductor digiti minimi is positioned along the medial border of the palm. It lies superficial and lateral to the flexor digiti minimi brevis and opponens digiti minimi and forms the most prominent part of the hypothenar eminence.

Structure

ADM is a fusiform muscle that originates from the pisiform and inserts onto the proximal phalanx of the fifth digit.

• Origin: Pisiform bone and tendon of the flexor carpi ulnaris.
• Insertion: Medial side of the base of the proximal phalanx of the fifth digit (little finger) and dorsal digital expansion.
• Fiber Orientation: Runs longitudinally from proximal-ulnar to distal-radial along the medial border of the hand.

Function

The abductor digiti minimi has several functions:

• Abduction of the Little Finger: Moves the fifth digit away from the fourth digit in the coronal plane.
• Assists in Flexion: Helps in flexion of the proximal phalanx at the metacarpophalangeal joint.
• Stabilization: Provides medial balance during grip and aids in cupping the palm.

Innervation

Innervated by the deep branch of the ulnar nerve (C8–T1).

Blood Supply

• Ulnar artery: Through branches of the superficial palmar arch and ulnar digital arteries.

Relations

The ADM lies medial to the flexor digiti minimi brevis and superficial to the opponens digiti minimi. It is the most superficial and medial of the hypothenar muscles. The ulnar nerve and artery pass deep to it within Guyon’s canal before entering the palm.

Surface Anatomy

The muscle forms the medial bulge of the hypothenar eminence and is easily palpable on the ulnar side of the palm during resisted abduction of the little finger.

Development

The ADM develops from the ulnar portion of the hand’s mesenchymal muscle mass. It reflects the specialization of the fifth digit in humans for grip and fine adjustment.

Anatomical Variations

• May have accessory slips inserting into the flexor retinaculum or hypothenar fascia.
• Occasionally fused with neighboring hypothenar muscles like the flexor digiti minimi brevis.
• Insertion may vary into the extensor expansion of the fifth digit.

Clinical Significance

• Ulnar Nerve Lesions: Weakness or paralysis of the ADM may result in impaired abduction of the little finger, contributing to weakened grip strength and hypothenar wasting.
• Guyon’s Canal Syndrome: Compression of the ulnar nerve deep to ADM may affect its function, along with other intrinsic hand muscles.
• EMG Testing: The ADM is commonly used in nerve conduction studies to assess ulnar nerve integrity.

Comparative Anatomy

In many quadrupeds, the ADM is less developed due to limited use of the fifth digit. In humans, its evolution supports enhanced hypothenar function, enabling greater dexterity, grip adjustment, and lateral hand stabilization.