Extensor pollicis tertius: an additional extensor muscle to the thumb.

This is the first reported case in the literature of an extra extensor muscle and tendon to the right thumb. It was discovered during routine dissection of a male cadaver. The muscle originated from the dorsal surface of the lower third of the ulna immediately proximal to the origin of the extensor indicis proprius. Its tendon, after passing through the dorsal fourth synovial compartment, turned obliquely and laterally to insert independently into the base of the proximal phalanx of the thumb. The common origin of the muscle with the deep forearm extensors and their common nerve supply suggest a common embryologic origin. For precise descriptive purposes, the name extensor pollicis tertius is proposed. A knowledge of the existence of such anatomic variation may be useful in surgical identification and in planning tendon repairs and/or transfers.