Palmar Interossei Muscles

The palmar interossei muscles are small, deep intrinsic muscles of the hand that lie between the metacarpal bones. Their primary function is to adduct the fingers toward the axial line (middle finger) and assist in flexion at the metacarpophalangeal joints and extension at the interphalangeal joints. These muscles are critical for precise finger movements and coordinated grip.

Location

The palmar interossei are located in the palmar side of the hand, lying between the metacarpal bones of the fingers (excluding the middle finger). They are positioned deep to the flexor tendons and lumbricals.

Structure

There are typically three palmar interossei muscles, though some individuals may have a fourth (usually associated with the thumb or little finger). Each muscle arises from a single metacarpal bone and inserts onto the base and extensor expansion of the proximal phalanx of the corresponding digit.

• 1st Palmar Interosseous: Arises from the ulnar side of the second metacarpal and inserts into the medial side of the proximal phalanx of the index finger.
• 2nd Palmar Interosseous: Arises from the radial side of the fourth metacarpal and inserts into the lateral side of the ring finger.
• 3rd Palmar Interosseous: Arises from the radial side of the fifth metacarpal and inserts into the lateral side of the little finger (may be rudimentary or absent).

Function

The palmar interossei muscles perform the following actions:

• Adduction: Bring the fingers (index, ring, and little) toward the middle finger (the hand’s midline).
• Flexion: Assist in flexing the metacarpophalangeal (MCP) joints of the corresponding fingers.
• Extension: Assist in extending the interphalangeal (IP) joints via their insertion into the extensor expansion.

Innervation

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

Blood Supply

• Palmar metacarpal arteries (branches of the deep palmar arch).
• Common palmar digital arteries (from the superficial palmar arch).

Relations

The palmar interossei lie deep to the lumbrical muscles and flexor tendons. They run alongside the sides of the metacarpals and blend into the lateral bands of the extensor expansions.

Surface Anatomy

Due to their deep location, the palmar interossei are not visible or palpable on the surface. However, their function can be demonstrated by adduction of the fingers toward the midline of the hand.

Development

These muscles arise from mesenchymal condensations in the hand's dorsal and palmar muscle masses. Their ulnar nerve innervation corresponds with their deep location and intrinsic function in the hand.

Anatomical Variations

• Occasionally a fourth palmar interosseous muscle is present, typically related to the thumb or fifth digit.
• In some individuals, one or more interossei may be absent or merged with adjacent structures.

Clinical Significance

• Ulnar Nerve Palsy: Weakness or paralysis of the palmar interossei leads to impaired finger adduction, loss of precision grip, and intrinsic minus hand deformity.
• Froment’s Sign: In ulnar nerve injury, the adductor pollicis and palmar interossei weakness causes the thumb to flex excessively at the IP joint during pinch testing (due to FPL compensation).
• Claw Hand: Interossei weakness contributes to hyperextension of the MCP joints and flexion at the IP joints in ulnar nerve damage.

Comparative Anatomy

In non-human primates and other mammals, the interossei are generally less differentiated. In humans, their development supports the evolution of fine motor skills, precision pinch, and complex hand gestures necessary for tool use and communication.