Are the supernumerary deep extensor tendons to the thumb in man of accessory or rudimentary origin?
Two findings of extensor pollicis et indicis (EPI) were classified referring to the literature of comparative anatomy.
Extensor pollicis longus (EPL) and extensor indicis (EI) developed phylogenetically from EPI. A common origin and proximal muscle belly of EPL and EI is a frequent manifestation of incomplete rudimentary EPI. A complete rudimentary EPI with absent EPL and EI has not yet been found in man. The accessory EPI coexisting with EPL and EI occurs in about 2 % of humans.
During the EI-tendon transposition to EPL, the tendons must be identified very carefully because of the numerous variants of the deep extensors. Transposition of extensor digitorum communis of the index or just one tendon of a double-tendoned EI to EPL by mistake, will make separate extension of the thumb impossible postoperatively.