The extensor indicis is the only finger extensor with an independent muscle belly dedicated to a single finger, providing the ability to extend the index finger alone while the other fingers remain flexed. This independent extension allows pointing without extending all fingers, an important function in human communication and tool use. As the most expendable forearm extensor, it is the standard donor for EPL tendon transfer after rupture.
| Origin | Posterior surface of the ulna distal to the extensor pollicis longus origin and the adjacent interosseous membrane |
|---|---|
| Insertion | Extensor expansion of the index finger, ulnar to the extensor digitorum tendon |
| Nerve Supply | Posterior interosseous nerve (C7, C8) |
| Blood Supply | Posterior interosseous artery |
| Actions | Independent extension of the index finger MCP joint; Assists wrist extension |
|---|
Independent index extension from all other fingers is only possible because the EI has its own muscle belly separate from the extensor digitorum, unlike the juncturae tendinum that limit independent ring and little finger extension.
The extensor indicis is the standard tendon transfer donor for EPL rupture repair because it is expendable, its loss produces no functional deficit as the extensor digitorum continues to extend the index finger, and its tendon reaches the thumb with minimal rerouting. Testing EI function by isolating index extension with all other fingers fully flexed (the index test) confirms posterior interosseous nerve integrity.
The EI tendon is palpable on the dorsum of the index finger just ulnar to the extensor digitorum tendon, becoming distinct during isolated index finger extension with the other fingers held flexed.
Surgical use of the expendable extensor indicis tendon to restore EPL function after rupture, providing excellent cosmetic and functional outcomes with no measurable index extension deficit.
