Interossei

Introduction

Introduction to the Interossei of the Hand

The interossei are a group of seven small intrinsic muscles situated between the metacarpal bones of the hand. They are divided into four dorsal interossei and three palmar interossei. These muscles are the prime abductors and adductors of the fingers relative to the axial line of the middle finger and, like the lumbricals, flex the metacarpophalangeal (MCP) joints while extending the interphalangeal (IP) joints. All interossei receive their motor supply from the deep branch of the ulnar nerve (C8, T1). Their dysfunction is a hallmark of ulnar nerve lesions, producing weakness in finger abduction and adduction, clawing deformity, and characteristic clinical signs.

A thorough understanding of the interossei is essential for diagnosing nerve injuries, understanding hand deformities, and performing reconstructive procedures.

Study

1. General Features and Classification

The interossei are intrinsic muscles lying in the intermetacarpal spaces. They are bipennate (dorsal) or unipennate (palmar), depending on their function. A convenient mnemonic is DAB (Dorsal ABduct) and PAD (Palmar ADduct). The axis of abduction and adduction runs through the middle finger, so abduction moves a finger away from the middle finger, adduction moves it toward the middle finger.

2. Dorsal Interossei (4 muscles)

The dorsal interossei are bipennate and occupy the dorsal aspect of the intermetacarpal spaces. They arise by two heads from the adjacent metacarpal shafts and insert into the proximal phalanx and dorsal digital expansion.

First dorsal interosseous: The largest, located between the first and second metacarpals. It inserts onto the radial side of the base of the proximal phalanx of the index finger. Action: abducts the index finger (radial deviation) and flexes the MCP joint. Innervation: deep branch of ulnar nerve (C8, T1).

Second dorsal interosseous: Between the second and third metacarpals, inserting onto the radial side of the middle finger proximal phalanx. It abducts the middle finger radially.

Third dorsal interosseous: Between the third and fourth metacarpals, inserting onto the ulnar side of the middle finger proximal phalanx. It abducts the middle finger ulnarly.

Fourth dorsal interosseous: Between the fourth and fifth metacarpals, inserting onto the ulnar side of the ring finger proximal phalanx. It abducts the ring finger.

3. Palmar Interossei (3 muscles)

The palmar interossei are unipennate, lying on the palmar surfaces of the metacarpals. They adduct the digits toward the middle finger.

First palmar interosseous: Arises from the ulnar side of the second metacarpal and inserts onto the ulnar side of the proximal phalanx of the index finger. Adducts the index finger.

Second palmar interosseous: Arises from the radial side of the fourth metacarpal and inserts onto the radial side of the proximal phalanx of the ring finger. Adducts the ring finger.

Third palmar interosseous: Arises from the radial side of the fifth metacarpal and inserts onto the radial side of the proximal phalanx of the little finger. Adducts the little finger.

The middle finger has no palmar interosseous because it is the axis of movement, but it has two dorsal interossei that allow abduction in both directions.

4. Actions of the Interossei

All interossei flex the MCP joints and extend the IP joints via their insertion into the extensor hood, a function shared with the lumbricals. Their specific abduction/adduction action is as follows: Dorsal interossei (DAB) – first (index), second (middle to radial side), third (middle to ulnar side), fourth (ring). Palmar interossei (PAD) – first (index), second (ring), third (little).

5. Nerve Supply

All seven interossei are innervated by the deep branch of the ulnar nerve (C8, T1). The deep branch reaches them after passing between the two heads of adductor pollicis. The first dorsal interosseous is often the earliest muscle to show wasting in ulnar nerve compression.

6. Blood Supply

The interossei receive blood from the deep palmar arch, the palmar metacarpal arteries, and perforating branches from the dorsal metacarpal arteries. Venous drainage is into the deep palmar venous arch.

7. Clinical Significance

Ulnar nerve palsy: Paralysis of all interossei leads to inability to abduct or adduct the fingers, with a characteristic claw hand deformity (MCP hyperextension, IP flexion) most pronounced in ring and little fingers. Wartenberg's sign (abducted little finger due to unopposed extensor digiti minimi) and Froment's sign (thumb IP flexion during pinch due to adductor pollicis weakness) are key signs. The card test evaluates palmar interosseus strength (patient holds a card between fingers while examiner pulls).

First dorsal interosseous wasting: Visible atrophy in the web space between thumb and index finger is an early sign of ulnar neuropathy.

Claw hand correction: Tendon transfers often aim to restore intrinsic function (MCP flexion, IP extension) lost with interosseous paralysis.

Summary

Summary of the Interossei

The dorsal interossei (4) abduct, the palmar interossei (3) adduct the fingers relative to the middle finger. All interossei flex the MCP joints and extend the IP joints via the extensor hood. They are all innervated by the deep branch of the ulnar nerve (C8, T1). Interosseous paralysis produces loss of finger abduction/adduction, clawing, and visible wasting of the first dorsal interosseous, serving as a reliable clinical marker of ulnar nerve dysfunction.