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 |
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| 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 |
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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 (extensor digitorum communis) tendons pass through the fourth extensor compartment under the extensor retinaculum. The juncturae tendinum allow some ring finger extension even after its own tendon is cut, which can hide a complete laceration if only MCP extension is tested. Over each PIP joint the central slip of the extensor mechanism inserts into the middle phalanx; its rupture after a forced flexion injury leads to a boutonniere deformity, with PIP flexion and DIP hyperextension as the lateral bands slip below the joint axis. The Elson test (resisted middle phalanx extension with the PIP bent 90 degrees over a table edge) detects acute central slip rupture, and treatment is splinting the PIP in extension for about 6 weeks while the DIP is left free to move. Lateral epicondylalgia affects the common extensor origin from which the muscle 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.
Avulsion of the central slip at the base of the middle phalanx causes loss of active PIP extension and later a boutonniere deformity, treated with PIP extension splinting for about 6 weeks.
