Interossei muscles of the hand

The interossei muscles of the hand are intrinsic muscles situated between the metacarpal bones. They are divided into two groups: the dorsal interossei and the palmar interossei. These muscles are primarily responsible for abduction and adduction of the fingers and play a crucial role in fine motor coordination, MCP joint flexion, and IP joint extension through their action on the extensor expansion.

Location

The interossei occupy the spaces between the metacarpals and lie deep to the lumbricals and flexor tendons. The dorsal interossei are located on the dorsal aspect of the hand, while the palmar interossei lie on the palmar side. All interossei are innervated by the deep branch of the ulnar nerve (C8–T1).

Dorsal Interossei

• There are four dorsal interossei, each arising from two adjacent metacarpal bones.
• They are bipennate muscles, which gives them greater strength for finger abduction.
• They insert into the base of the proximal phalanx and the extensor expansion of the associated finger.

Insertions

• 1st dorsal interosseous – lateral base of proximal phalanx of the index finger (digit 2)
• 2nd – lateral base of digit 3
• 3rd – medial base of digit 3
• 4th – medial base of digit 4

Actions

• Abduct fingers away from the midline of the hand (middle finger axis)
• Assist lumbricals in flexing the MCP joints and extending the IP joints

Palmar Interossei

• There are three palmar interossei, each arising from a single metacarpal.
• They are unipennate muscles and smaller than the dorsal interossei.
• Each inserts into the base of the proximal phalanx and extensor expansion of the same finger.

Insertions

• 1st palmar interosseous – medial side of digit 2
• 2nd – lateral side of digit 4
• 3rd – lateral side of digit 5

Actions

• Adduct digits 2, 4, and 5 toward the middle finger axis
• Contribute to MCP flexion and IP extension via the extensor mechanism

Function

• Digit positioning: Control precise finger movements such as spreading or squeezing fingers together.
• Assist lumbricals: Work with lumbricals to produce the “tabletop” position—MCP flexion with IP extension.
• Stabilize metacarpophalangeal joints: Prevent excessive lateral deviation during fine motor tasks.

Physiological Role(s)

• Enable typing, playing musical instruments, and coordinated gripping actions.
• Maintain proper hand posture during precision tasks (e.g., drawing, suturing).
• Support digital balance in power grip and pinch grip positions.

Development

Interossei muscles develop from the intrinsic mesenchymal core of the hand during fetal limb development. These muscles are derived from the ventral muscle mass and are innervated by the ulnar nerve, which is established during early embryogenesis. The extensor hood and lumbrical-interosseous connections develop in tandem, allowing coordinated digit extension and precision movement by birth.

Innervation

• All interossei muscles (both palmar and dorsal) are innervated by the deep branch of the ulnar nerve (C8–T1).
• This makes interossei function a key indicator in diagnosing ulnar nerve integrity in clinical exams.

Clinical Significance

• Ulnar nerve injury: Loss of interosseous function results in weakened finger abduction/adduction and the classic “claw hand” deformity (especially in digits 4 and 5).
• Intrinsic minus hand: Absence of interossei and lumbrical activity causes MCP hyperextension and IP flexion.
• Froment’s sign: In adductor pollicis dysfunction (also ulnar-innervated), compensation occurs with flexor pollicis longus due to weakened interossei stability.
• Paralysis of interossei: Results in difficulty spreading fingers or performing coordinated finger movements.

Surface Anatomy

• Dorsal interossei can be palpated on the dorsum of the hand between metacarpals during finger abduction.
• Palmar interossei are not palpable due to their deep location on the palmar side beneath neurovascular structures.
• Wasting of dorsal interossei is an early sign of chronic ulnar nerve compression and appears as hollowing between the metacarpals.

Imaging

• Ultrasound: Useful for evaluating muscular integrity and guiding ulnar nerve injections near Guyon’s canal.
• MRI: Essential for detailed visualization of muscle structure, nerve entrapment, and pathology involving intrinsic hand muscles.

Anatomical Variations

• The number and arrangement of palmar interossei may vary; a fourth muscle is rarely present for the thumb.
• Occasionally, dorsal interossei may insert differently along the extensor expansion or metacarpal bases.
• Accessory slips may connect interossei to adjacent lumbricals or flexor tendons.