The first palmar interosseous is unique in that it adducts the thumb toward the palm, working with adductor pollicis in key pinch and opposition. Unlike the other palmar interossei which are unipennate from single metacarpals, the first is often described as having dual origins. It lies deep to the adductor pollicis in the first web space.
| Origin | Ulnar side of the first metacarpal and base of the proximal phalanx of the thumb |
|---|---|
| Insertion | Ulnar base of the proximal phalanx of the thumb and extensor hood |
| Nerve Supply | Deep branch of the ulnar nerve (C8, T1) |
| Blood Supply | First palmar metacarpal artery |
| Actions | Adducts the thumb toward the second finger axis; Assists MCP flexion of the thumb |
|---|
The first palmar interosseous is assessed with adductor pollicis in ulnar nerve palsy, where both muscles produce the characteristic Froment sign (compensatory IP joint flexion during key pinch due to adductor and palmar interosseous weakness). It is distinguished from adductor pollicis by its smaller size and location. Its preservation during first web space release in contracture ensures thumb adduction is maintained.
Palpated deep in the first web space on the ulnar aspect of the first metacarpal during resisted thumb adduction.
Combined weakness of the first palmar interosseous and adductor pollicis from ulnar nerve injury producing positive Froment's sign and impaired key pinch, requiring tendon transfer for reconstruction.
