The extensor digitorum is the primary finger extensor, running from the common extensor origin on the lateral epicondyle to the extensor expansions of all four fingers. Its tendons are connected by juncturae tendinum, fibrous bands that link adjacent tendons on the dorsal hand, which limit independent extension of the middle and ring fingers and are responsible for the fact that most people cannot fully extend the ring finger independently with the middle finger held flexed.
| Origin | Lateral epicondyle via the common extensor tendon |
|---|---|
| Insertion | Extensor expansions (dorsal aponeurosis) of fingers 2 through 5, extending to the bases of the middle and distal phalanges |
| Nerve Supply | Posterior interosseous nerve (C7, C8) |
| Blood Supply | Posterior interosseous artery |
| Actions | Extension of the metacarpophalangeal joints of fingers 2 to 5; Assists extension of the interphalangeal joints through the extensor expansion; Assists wrist extension |
|---|
It extends primarily at the MCP joint; IP joint extension is completed by the intrinsic muscles of the hand acting through the extensor expansion. This is why radial nerve palsy produces MCP extension loss but preserved IP extension, as the intrinsics remain intact.
The extensor digitorum tendons pass through the fourth extensor compartment at the wrist under the extensor retinaculum. The juncturae tendinum allow partial ring finger extension even after extensor digitorum ring finger tendon laceration, which can delay diagnosis of complete laceration if only MCP extension is tested. Lateral epicondylalgia affects the common extensor origin from which extensor digitorum arises.
The four extensor digitorum tendons are clearly visible on the dorsum of the hand during active finger extension and can be individually palpated as they cross the metacarpal heads toward each finger.
Avulsion of the terminal extensor tendon at the distal phalanx, usually from a forced flexion injury, producing inability to actively extend the DIP joint and a flexed resting posture of the fingertip.
Tendon division on the dorsal hand or wrist producing MCP extension loss in the affected finger, requiring surgical repair or splinting depending on zone and completeness.
