Variation in the triceps brachii muscle: a fourth muscular head.

Routine cadaver dissection has resulted in the identification of a fourth head of the triceps brachii muscle on the left side in one specimen. This novel arrangement demonstrated a single tendon arising from the proximal posteromedial aspect of the humeral shaft, distal to the shoulder capsule. The tendon of this fourth head passed along the medial aspect of the humerus and gave way to a muscle belly on the medial surface of the distal one-third of the humerus. The tendon of the fourth head passed directly over the neurovascular bundle containing the radial nerve and deep brachial artery at approximately the point where the neurovascular bundle entered the radial sulcus. This close positional relationship between the tendon of the fourth head, the radial nerve, and the deep brachial artery has prompted us to speculate on the possible clinical significance of this finding in relation to radial nerve palsy and arterial compression. Additionally, the position of the muscle belly, lying in close proximity to the ulnar groove, invites speculation on the role of the fourth head in cases of snapping elbow. To the authors' knowledge, a description of the muscular fourth head of the triceps as seen in the present work has not been noted in previous literature.