Dorsal Interossei of the hand

The dorsal interossei muscles are four bipennate intrinsic muscles located between the metacarpal bones on the dorsal side of the hand. They function primarily to abduct the fingers away from the midline (the third digit) and also assist in flexion at the metacarpophalangeal joints and extension at the interphalangeal joints. These muscles are essential for coordinated finger movement, particularly during object manipulation and fine motor tasks.

Location

The dorsal interossei lie in the dorsal intermetacarpal spaces, positioned deep within the hand between adjacent metacarpal bones. Despite their location on the dorsal aspect, their insertions and actions are exerted on the palmar side of the fingers through the extensor expansions.

Structure

There are four dorsal interossei muscles. Each arises by two heads from adjacent sides of neighboring metacarpals and inserts into the extensor expansion and base of the proximal phalanx.

• 1st Dorsal Interosseous: Between the first and second metacarpals; inserts on the radial side of the index finger.
• 2nd Dorsal Interosseous: Between the second and third metacarpals; inserts on the radial side of the middle finger.
• 3rd Dorsal Interosseous: Between the third and fourth metacarpals; inserts on the ulnar side of the middle finger.
• 4th Dorsal Interosseous: Between the fourth and fifth metacarpals; inserts on the ulnar side of the ring finger.

Function

The dorsal interossei muscles have the following actions:

• Abduction: Abduct the index, middle, and ring fingers away from the midline of the hand (the third digit).
• Flexion: Assist in flexing the metacarpophalangeal (MCP) joints.
• Extension: Assist in extending the interphalangeal (IP) joints via insertion into the extensor expansion.
• Stabilization: Provide lateral stability and alignment during fine motor activity and power grip.

Innervation

All dorsal interossei are innervated by the deep branch of the ulnar nerve (C8–T1).

Blood Supply

• Dorsal and palmar metacarpal arteries
• Branches from the deep palmar arch

Relations

These muscles lie between the metacarpals and are deep to the extensor tendons on the dorsal surface. Palmar to them are the interosseous spaces, lumbrical muscles, and flexor tendons.

Surface Anatomy

Though not distinctly visible, the 1st dorsal interosseous is palpable in the web space between the thumb and index finger, especially during finger abduction. The others contribute to the fullness and depth of the dorsal hand between the metacarpals.

Development

The dorsal interossei arise from the dorsal mesenchymal mass of the developing limb. Their bipennate structure reflects their role in producing balanced lateral movement and abduction.

Anatomical Variations

• The number of muscles is usually four, but an accessory dorsal interosseous may rarely be present.
• Muscle size and insertion points can vary slightly.
• Some interossei may have slips inserting into adjacent lumbricals or deep fascia.

Clinical Significance

• Ulnar Nerve Injury: Paralysis leads to weakness in finger abduction and loss of MCP flexion/IP extension synergy, contributing to a claw hand deformity.
• Froment’s Sign: Though classically testing the adductor pollicis, weakened dorsal interossei also contribute to compromised pinch grip.
• Hand Function Loss: Difficulty spreading the fingers or performing precision tasks can result from interosseous muscle weakness.

Comparative Anatomy

In non-human primates and quadrupeds, dorsal interossei may be present but are less specialized. In humans, their development supports the refined abduction and coordination needed for advanced hand functions like writing, typing, and musical instrument playing.