Hypothenar Muscles

Introduction

Introduction to the Hypothenar Muscles

The hypothenar muscles are a group of three short muscles located on the ulnar (medial) aspect of the palm, forming the fleshy hypothenar eminence at the base of the little finger. They are responsible for the fine movements of the fifth digit, including abduction, flexion, and opposition. Together with the thenar muscles, they contribute to the concavity of the palm, enhancing grip and dexterity. A fourth muscle, the palmaris brevis, lies superficially over the hypothenar eminence and is functionally related to it.

All hypothenar muscles are innervated by the deep branch of the ulnar nerve (C8, T1). Their vascular supply derives from the deep palmar branch of the ulnar artery. Lesions of the ulnar nerve lead to wasting of the hypothenar eminence and impaired fine motor control of the little finger.

Study

1. The Hypothenar Eminence and Its Muscles

The hypothenar eminence is the medial muscular elevation of the palm, overlying the fifth metacarpal. It consists of three muscles arranged in two layers: superficially, the abductor digiti minimi and flexor digiti minimi brevis; deep to them lies the opponens digiti minimi. Palmaris brevis is a small cutaneous muscle located superficial to the hypothenar muscles, not strictly part of the deep group but clinically associated with the eminence.

2. Abductor Digiti Minimi (ADM)

Origin: Pisiform bone, the tendon of flexor carpi ulnaris, and the pisohamate ligament. Insertion: Ulnar side of the base of the proximal phalanx of the little finger, and the dorsal digital expansion. Action: Abduction of the little finger at the metacarpophalangeal (MCP) joint (movement away from the ring finger). It also assists in flexion of the MCP joint. Innervation: Deep branch of the ulnar nerve (C8, T1).

3. Flexor Digiti Minimi Brevis (FDMB)

Origin: Hook of the hamate and the flexor retinaculum. Insertion: Ulnar side of the base of the proximal phalanx of the little finger. Action: Flexion of the little finger at the MCP joint. Innervation: Deep branch of the ulnar nerve (C8, T1).

4. Opponens Digiti Minimi (ODM)

Origin: Hook of the hamate and flexor retinaculum. Insertion: Ulnar border of the fifth metacarpal shaft. Action: Opposition of the little finger (medial rotation and slight flexion of the fifth metacarpal, deepening the palmar concavity and bringing the little finger towards the thumb). Innervation: Deep branch of the ulnar nerve (C8, T1).

5. Palmaris Brevis

Origin: Flexor retinaculum and palmar aponeurosis. Insertion: Skin of the hypothenar eminence. Action: Wrinkles the skin of the palm, improving grip security. Innervation: Superficial branch of the ulnar nerve (C8, T1).

6. Blood Supply

The hypothenar muscles receive their arterial supply from the deep palmar branch of the ulnar artery and perforating branches from the deep palmar arch. Venous drainage accompanies these arteries into the deep palmar venous network.

7. Clinical Considerations

Ulnar nerve palsy: Causes wasting of the hypothenar eminence and loss of abduction, flexion, and opposition of the little finger. Combined with interosseous and lumbrical weakness, it produces a claw hand deformity (hyperextension at MCP joints and flexion at IP joints of ring and little fingers).

Wartenberg’s sign: In ulnar nerve palsy, the little finger rests in an abducted position due to unopposed action of extensor digiti minimi (radial nerve) when the palmar interosseous muscle (adductor of little finger) is paralysed.

Hypothenar hammer syndrome: Thrombosis or aneurysm of the ulnar artery in the hypothenar region due to repetitive trauma, leading to digital ischemia.

Fasciectomy for Dupuytren’s contracture: Diseased tissue often involves the hypothenar aspect; knowledge of hypothenar muscle anatomy is essential to preserve motor branches of the ulnar nerve.

Summary

Summary of Hypothenar Muscles

The hypothenar eminence comprises abductor digiti minimi, flexor digiti minimi brevis, and opponens digiti minimi, all supplied by the deep branch of the ulnar nerve. They abduct, flex, and oppose the little finger, and contribute to the transverse metacarpal arch. Palmaris brevis is a superficial muscle over the eminence, innervated by the superficial branch of the ulnar nerve. Ulnar nerve lesions cause hypothenar wasting and characteristic finger deformities.