The fourth dorsal interosseous is the most ulnar of the dorsal interossei, abducting the ring finger toward the little finger. It lies between the fourth and fifth metacarpals and is the smallest of the four DI muscles. Its action is balanced by the second palmar interosseous, which adducts the ring finger back toward the middle finger.
| Origin | Adjacent sides of the fourth and fifth metacarpals (bipennate) |
|---|---|
| Insertion | Ulnar base of the proximal phalanx of the ring finger and extensor hood |
| Nerve Supply | Deep branch of the ulnar nerve (C8, T1) |
| Blood Supply | Fourth dorsal metacarpal artery |
| Actions | Abducts the ring finger ulnarly (toward the little finger); Flexes the MCP and extends the IP joints of the ring finger |
|---|
The fourth dorsal interosseous is assessed in ulnar nerve palsy by resisted ring-little finger spreading. Its atrophy produces guttering between the fourth and fifth metacarpals. The fourth interosseous space is the target for fourth web space Morton's neuroma excision in the foot but the hand equivalent is relevant to ring finger corrective surgery. In severe ulnar clawing, the fourth DI loss compounds the ring finger intrinsic minus posture.
Palpated between the fourth and fifth metacarpals on the dorsal hand during resisted ring finger ulnar abduction.
Fourth dorsal interosseous denervation from ulnar nerve palsy producing inability to spread the ring finger from the little finger, visible as loss of fourth interspace muscle bulk on the dorsal hand.
