European Journal of Cardiovascular Medicine

Upper limb muscles can be classified by origin, topography, and innervation. Grouping by innervation reveals embryological and phylogenetic origins. The Triceps brachii gets its name from tri, referring to “three" muscle heads or points of origin (with Brachii referring to the arm). These include the Medial head, Lateral head, and Long head.

The brachial plexus provides motor and sensory supply to the upper limb, and the ventral rami of C5, C6, C7, C8, and T1 form it. These rami arise in the posterior triangle of the neck and pass through the axilla to innervate the muscles of the anterior and posterior compartments of the limb. In the axilla, cords are formed, which split into branches.

The upper limb muscles are innervated segmentally from proximal to distal so that the proximal muscles are innervated by higher segments (C5-C6), and lower segments innervate the distal muscles. The triceps or triceps brachii is a large muscle on the back of the upper limb of many vertebrates. It consists of 3 heads:  medial, lateral and longhead¹. All three heads of the triceps brachii muscle are classically said to be innervated by radial nerve ^2,3.

The axillary and Ulnar collateral branches of the radial nerve are previously unrecognized sources of triceps brachii innervations. However, some recent studies claimed the presence of innervation from the axillary nerve to the Long head and from the Ulnar nerve to the medial Head of the triceps. When examining patients with a traumatic injury involving the axillary nerve, it is important to search for paralysis of the long head of the triceps brachii. If present, it is a sign of a severe axillary nerve lesion requiring early repair at 3 months.

A total of 52 formalin-embalmed superior extremities were dissected and studied. The age, gender and cause of death is unknown. The period of study is for one year.

Inclusion criteria: Formalin embalmed upper limb specimens irrespective of age, gender and cause of death.

Exclusion criteria: Upper limbs showing gross asymmetry. Any injury to the nerve of the triceps will be excluded.

During routine dissection, the triceps brachii was exposed. Its innervation, the radial nerve, was traced for its branches in the spiral groove. The branches of the radial nerve are followed till their innervations distally. The axillary nerve, seen in the axilla, is also followed posteriorly along the surgical neck of the humerus in the Quadrangular space, and its branches are traced distally. Any variation in the triceps innervations was noted, and the nerve was followed proximally to identify its origin.

Out of 52 dissected cadaveric limbs, 46 long-head triceps have shown a normal motor innervation of the radial nerve, 6 long-head triceps have shown axillary nerve innervations, and none of the specimens have shown motor branches arising from posterior cords of brachial plexus.

The triceps are innervated by the radial nerve [C6,7 and 8] with separate branches for each head. Before the spiral groove, it gives branches to the long head and medial head. The branches of the radial nerve ^1,2,3,4 classically innervate all three heads. However, various studies have been conducted, and cases have been reported suggesting the axillary nerve supplying the long head of the triceps and the Ulnar nerve supplying the medial head of the triceps.^5,6,7,8,9 The present study was done to identify the innervations of all three heads’ triceps and to notice if any variation in their innervations exists. Any variation in the innervations pattern of the 3 heads of triceps brachii was noted and compared with previous anatomical studies. Nerves supplied to the long head of the triceps were traced, and the observations noted are compared with previous studies in the following table.

A study by Alexander J Erhdt et al. states that 71% of the 62 cadaveric limbs have classical radial nerve innervation patterns. However, the remaining 29% were found to have non-classical patterns, which include purely axillary or directly from posterior cords of the brachial plexus.

A study by Gyata Mehta et al. states that the axillary nerve innervates 8 Long Heads of Triceps (LHT) out of 30 cadaveric limbs. A study done by Maichel D Wade et al. states that other nerves supplied none of the LHT. The radial nerve itself supplied all 27. However, in the present study, it was found that 5 out of 52 LHT.

Radial nerve ¹⁰ supplied all three heads of the triceps. Each of the triceps' motor branches might be used as a donor for transfer. The long triceps head motor branch should be used preferentially when the intention is to establish triceps reinnervation. Nerve fascicles to the Flexor carpi ulnaris and the Posterior Branch of the Axillary Nerve are viable options for obtaining meaningful triceps muscle recovery in a select group of patients¹¹. Thorough anatomical knowledge of innervation of triceps brachii is essential for assessing the functional loss, planning reconstructive surgeries, and preventing iatrogenic injury to the nerve while fixing humerus fractures¹². The long head of the triceps is occasionally found to be supplied by the axillary nerve. The present study has revealed the motor innervations of triceps brachii long head by the radial nerve in 47 cases (90.38%) and by the axillary nerve in 5(9.62%) cases. The commonest variation of nerve supply to the long head of the triceps is the axillary nerve.

The inconsistency in the variations of innervations to triceps compared with similar studies could be due to differences in the population and race under study. These findings could be relevant for the surgical treatment of Shoulder dislocations, Humerus fractures, nerve transplant procedures, etc. The present study showed that the radial nerve supplied the motor branch of the triceps brachii's long head in 47(90.38%) cases and the axillary nerve in 5(9.62%) cases. The axillary nerve or brachial plexus can sustain injuries from trauma, traction injuries, shoulder dislocation or due to complications that occur during shoulder surgery¹³. The knowledge of the above study can be of great help to anatomists, orthopaedics, neuro physicians and surgeons for academic, diagnostic, therapeutic and surgical purposes. The knowledge of the innervation patterns of triceps brachii and their variations is important in investigating the lesions, planning appropriate treatment, and avoiding iatrogenic injuries to radial nerves during surgeries.

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