During routine anatomical dissection of the left supraclavicular and infraclavicular regions of a male cadaver, a supernumerary muscle was observed, which, by its position and insertions, consistent with the subclavius posticus muscle (posterior subclavius muscle). It had its anterior insertion by a thin tendon in the cranial surface of the first costal cartilage next to the subclavius muscle’s insertion, and ran dorso-laterally crossing over the brachial plexus and subclavian vessels to end on the posterior surface of the serratus anterior muscle’s fascia near to the superior margin of the scapula, without taking insertion on it, which differentiates it from the muscles described in the bibliography. This aberrant muscle has clinical implication since it has been described as a cause of thoracic outlet syndrome and it may appear in diagnostic imaging techniques simulating different pathological processes.
KEY WORDS: Subclavius posticus muscle; Thoracic outlet syndrome; Anatomic variation; Aberrant muscle; Subclavius muscle; Suprascapular region.
