Subclavius posticus muscle: supernumerary muscle as a potential cause for thoracic outlet syndrome.

During routine dissection a subclavius posticus muscle was found on the left side of a male cadaver. This muscle arose from the upper margin of the scapula and transverse scapular ligament, inserted in the superior side of the first rib cartilage, and was innervated by a small branch from the suprascapular nerve. The anatomical relationships of the supernumerary muscle with the brachial plexus and the subclavian artery is suggestive of a possible cause of the thoracic outlet syndrome and therefore of clinical significance.