The extensor indicis provides the independent index finger extension needed for pointing while the other digits are flexed. Its position ulnar to the EDC in the fourth compartment and its independent muscle belly distinguish it from the EDC. The EI proprius sign (maintaining the pointing position with the EDC relaxed) tests its integrity. As the most commonly transferred donor tendon for EPL reconstruction, it is harvested at the index MCP level.
| Origin | Posterior distal ulna and interosseous membrane |
|---|---|
| Insertion | Ulnar side of the index finger extensor expansion (ulnar to the EDC index tendon) |
| Nerve Supply | Posterior interosseous nerve (C7, C8) |
| Blood Supply | Posterior interosseous artery |
| Actions | Independent index finger MCP extension (pointing); Index MCP extension combined with fisting the other digits — tested by the EI sign |
|---|
The EI proprius test confirms tendon integrity — with all digits fisted except the index, the patient maintains index extension; EDC (shared belly) is silenced by the fist position, isolating the EI. At surgery the more ulnar of the two tendons on the index MCP is the EI. Its harvest leaves the EDC intact to maintain index MCP extension.
Both index MCP dorsal tendons are palpable — the ulnar (EI) and radial (EDC). EI is identified by selective index extension with other fingers fisted.
EI tendon harvest at the index MCP and rerouting to the EPL stump restoring thumb IP extension after Lister's tubercle attrition rupture.
