Isolated extension of the index finger after extensor indicis proprius (EIP) transfer was studied. In cadavers, the extensor digitorum communis (EDC) muscle of the index finger received distinct innervation from the posterior interosseous nerve and was well separated, with its own innervation, from the other EDC muscle. Examination of von Schroeder's type I juncturae tendinum (JT) between the index and long finger showed two types: von Schroeder type IA (16 of 27 hands; 59%) was fascia only; von Schroeder type IB (11 of 27 hands; 41%) was thin filamentous bands. In clinical cases, 3 hands were type IA and 10 were type IB. Retained independent index finger extension was obtained in all patients with excision of the JT between the index and long finger following EIP transfer.