The three palmar interossei are unipennate muscles adducting the index, ring, and little fingers toward the middle finger (there is no palmar interosseous to the middle finger itself, which is adducted by the dorsal interossei). Their PAD mnemonic (Palmar interossei ADduct) complements the DAB of the dorsal interossei. Like the dorsal interossei, they contribute to the lumbrical action of MCP flexion with IP extension through the extensor hood.
| Origin | Palmar surfaces of the second, fourth, and fifth metacarpals (single head from the side facing the middle finger) |
|---|---|
| Insertion | Bases of the proximal phalanges and extensor expansions of fingers 2, 4, and 5 on the side facing the middle finger |
| Nerve Supply | Deep branch of the ulnar nerve (C8, T1) |
| Blood Supply | Palmar metacarpal arteries |
| Actions | Adduction of the fingers toward the middle finger axis (PAD mnemonic); Flexion of the MCP joints; Extension of the IP joints through the extensor expansion |
|---|
Adducting the fingers allows them to squeeze together for firm grip around a tool handle and closes the finger spaces to prevent small objects from slipping between fingers.
The palmar interossei cannot be individually tested or palpated, but their collective function is assessed by testing finger adduction strength. Loss of palmar interosseous function in ulnar nerve palsy allows the fingers to spread apart in the clawed position, contributing to the intrinsic minus posture. The Wartenberg sign of persistent little finger abduction is caused by loss of little finger palmar interosseous function combined with intact extensor digiti minimi pull.
The palmar interossei are not directly palpable as they lie between the metacarpals in the palm, but their function is assessed by resisting finger adduction from an abducted position.
Loss of palmar and dorsal interosseous function producing finger spreading and inability to squeeze fingers together, contributing to the clawed hand posture of advanced ulnar nerve palsy.
