Background Some leading anatomy texts state that all three heads of the triceps brachii are innervated by the radial nerve. The posterior cord of the brachial plexus bifurcates to terminate as the radial and axillary nerves. Studies have noted the presence of axillary innervation to the long head of the triceps brachii muscle, patterns different from the classic exclusive radial nerve supply. An understanding of these variations may assist the clinician in the assessment of shoulder weakness and in preoperative and operative planning of radial and axillary neuropathies. Questions/Purpose We aimed to further investigate, in cadaver dissections, the prevalence of axillary nerve contribution to the innervation of the long head of the triceps brachii. Methods We performed bilateral brachial plexus dissections on 10 embalmed cadavers combining anterior axillary and posterior subscapular approaches. Two additional unilateral cadaveric brachial plexuses were dissected. The posterior cords were fully dissected from the roots distally. The radial and axillary nerves were followed to their muscle insertion points, the dissections were photographed, and the length of branching segments were measured.

All three heads of the Triceps brachii are classically described as being innervated by the radial nerve. A previous study of 20 cadaveric shoulders revealed that in 13 specimens the long head of the triceps (LHT) was supplied by a branch of the axillary nerve, not the radial nerve. In our current study 20 cadaveric shoulders were dissected via a combined anterior and extensile dorsal approach. The dissections were photographed and nerve maps were created. Distances from proximal LHT attachment to the point of neuromuscular insertion were measured with calipers. The results indicated that in 60% of the specimens the axillary nerve contributed to the innervation of the LHT; in 15% axillary nerve alone and in 45% combined axillary and radial nerve. These findings have clinical significance in understanding why triceps brachii dysfunction is sometimes associated with axillary nerve injuries.