The extensor medii proprius and anomalous extensor tendons to the long finger.

The extensor medii proprius is a muscle analogous to the extensor indicis proprius in that it has a similar origin, but inserts into the long finger. The extensor indicis et medii communis muscle is an extensor indicis proprius muscle that splits to insert into both the index and long fingers. The extensor tendons to the fingers were dissected in 58 adult hands to determine the incidence and anatomy of the extensor medii proprius and extensor indicis et medii communis. The incidence of the extensor medii proprius was 10.3% and the incidence of the extensor indicis et medii communis was 3.4% in this series. The extensor medii proprius and extensor indicis proprius muscles had a common origin in all cases. The insertion of the extensor medii proprius into the dorsal aponeurosis was palmar and ulnar to the extensor digitorum communis of the long finger, but in one case, the extensor medii proprius inserted into the deep fibrous tissue proximal to the metacarpophalangeal joint. The tendon slip of the extensor indicis et medii communis inserting into the long finger did not insert into the dorsal aponeurosis, but into the deep fibrous tissue near the metacarpophalangeal joint. Both the extensor medii proprius and the extensor indicis et medii communis may represent evolutionary remnants. Awareness of their potential presence and anatomy should be helpful in extensor identification, repair, and transfer.