Variations of the extensor indicis muscle and tendon.

Variations of the extensor indicis muscle were examined in 164 hands from 86 Japanese cadavers. Anomalous cases exhibiting supernumerary muscles or tendons were found in 22 hands. These variations were classified into four types: type 1, an additional tendon slip from the extensor indicis tendon; type 2, an extensor indicis radialis or extensor pollicis et indicis accessorius; type 3, an extensor medii proprius with or without extensor medii brevis; and type 4, an extensor indicis radialis and extensor medii proprius. The extensor medii proprius was the most common variation, followed by extensor indicis radialis. There were no clear differences in incidence of variations between men and women or between right and left hands. When variations were bilateral, both sides were identical or similar in type.