The palmar interossei adduct the fingers. There are three (some authors count four including the thumb palmar interosseous). They are the intrinsic muscles most affected by ulnar nerve palsy, contributing to the claw deformity alongside the lumbricals.
| Origin | Palmar surface of metacarpals 1, 2, 4, and 5 — the sides facing the midline (middle finger) |
|---|---|
| Insertion | Bases of proximal phalanges 1, 2, 4, and 5 — and the extensor expansions (adducting toward midline) |
| Nerve Supply | Deep branch of ulnar nerve (C8, T1) — all three (note: palmar interosseous I to the thumb is sometimes median) |
| Blood Supply | Palmar metacarpal arteries |
| Actions | Adduct the fingers toward the middle finger — PAD mnemonic (Palmar ADduct); Flex the MCP joints and extend the IP joints via the extensor expansion |
|---|
Palmar interosseous testing by resisted finger adduction — the card test (squeezing a card between adjacent fingers) is the classic screening test for palmar interosseous weakness. In ulnar claw hand, the palmar interossei are denervated, reducing the MCP flexion force and allowing the long extensor to hyperextend the MCP joint.
Tested by the card test — inability to grip a piece of paper between adjacent fingers without the thumb.
Failure to hold a piece of card between adjacent fingers in ulnar nerve palsy from palmar interosseous denervation, a screening test for intrinsic muscle function.
