The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 89, Issue 6

Scientific Articles | June 01, 2007

Brachialis Muscle Anatomy: A Study in Cadavers

Domenic T. Leonello, MBBS¹; Ian J. Galley, MBchB²; Gregory I. Bain, MBBS, FRACS(Orth)³; Christopher D. Carter, MBBS⁴

¹ 302/211 Grenfell Street, Adelaide SA 5000, Australia
² Tauranga Hospital, Cameron Road, Private Bag 1204, Tauranga, New Zealand
³ 196 Melbourne Street, North Adelaide SA 5006, Australia. E-mail address: greg@gregbain.com.au
⁴ Level 1, 178 Fullarton Road, Dulwich SA 5065, Australia

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Disclosure: The authors did not receive any outside funding or grants in support of their research for or preparation of this work. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.

Investigation performed at the University of Adelaide, Adelaide, Australia

The Journal of Bone and Joint Surgery, Incorporated

J Bone Joint Surg Am, 2007 Jun 01;89(6):1293-1297. doi: 10.2106/JBJS.F.00343

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Abstract

Background: There have been conflicting descriptions of brachialis muscle anatomy in the literature. The purpose of the present study was to clarify brachialis muscle anatomy in order to refine surgical techniques around the elbow.

Methods: Eleven cadaveric upper limbs were dissected under loupe magnification. The gross morphological characteristics, relationships, and nerve supply of the brachialis muscle were recorded. The nerve supply was examined histologically to confirm the gross findings.

Results: In all specimens, the brachialis muscle had two heads. The larger, superficial head originated from the anterolateral aspect of the humerus, proximal to the smaller, deep head. The superficial head contained longitudinal fibers, which inserted by means of a thick round tendon onto the ulnar tuberosity. The deep head fibers were fan-shaped and converged to insert by means of an aponeurosis onto the coronoid process. In all specimens, a branch of the radial nerve supplied the inferolateral fibers of the deep head.

Conclusions: Our observations of brachialis muscle anatomy differ considerably from the descriptions in the current literature. The larger, superficial head has the mechanical advantage of a more proximal origin and a more distal insertion, which may enable it to provide the bulk of flexion strength. The smaller, oblique, deep head has a more anterior insertion on the coronoid, which may facilitate the initiation of elbow flexion from full extension. The radial nerve-innervated inferolateral fibers of the deep head run in a direction similar to the anconeus muscle, forming a muscular sling around the elbow. This complex may act to dynamically stabilize the ulnohumeral joint.

Clinical Relevance: This information may enhance surgical techniques about the elbow. The identification of an internervous plane may allow for improvement in the current anterior and anterolateral surgical approaches to the humerus. The tendon of the superficial head is well positioned to allow its transfer to the radial tuberosity, potentially improving supination strength in the absence of a distal biceps tendon. It is possible that the tendon of the superficial head might also be used to reconstruct the anular ligament or the medial collateral ligament of the elbow.

Figures in this Article

There have been conflicting descriptions of brachialis muscle anatomy in major anatomical textbooks^1-3. The insertion has been described as being purely muscular, purely tendinous, or mixed. Most texts and published reports^4,5 have described dual innervation from the radial and musculocutaneous nerves, although the functional importance of the radial nerve innervation is not well understood. In addition, we previously observed that the brachialis muscle has multiple heads⁶. The purpose of the present study was to clarify brachialis muscle anatomy in order to potentially improve surgical techniques around the elbow.

Materials and Methods

Materials and Methods | Results | Discussion | References

Eleven cadaveric upper limbs, perfused and fixed in a solution containing 55% ethanol, 15% propylene glycol, 5% phenol, 5% formalin, and 20% water, were dissected under 2.5× loupe magnification. The radial nerve was identified in the interval between the brachialis and the brachioradialis. Specimens that grossly appeared to be radial nerve branches to the brachialis were sent for histological examination in order to confirm that they did indeed represent nerve tissue. The interval between the biceps and the brachialis was explored, and the musculocutaneous nerve contribution to the brachialis was observed and recorded. All muscles except for the brachialis and the anconeus were removed, and the morphological characteristics, relationships, origin, and insertion of the brachialis were recorded. The orientation of the anconeus muscle fibers was compared with that of the brachialis. The tendon of the brachialis was identified, and both the extramuscular and intramuscular lengths of this structure were measured.

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Fig. 1: Lateral view of the arm and elbow. SH = superficial head of the brachialis, DH = deep head of the brachialis, A = anconeus, and C = capitellum.

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Fig. 2: Osteology of the left humerus (anterior view) and ulna (medial view), indicating the origin and insertion of the superficial head (SH) and deep head (DH).

Results

Materials and Methods | Results | Discussion | References

In all specimens, the brachialis muscle had two heads: a superficial head and a deep head. The larger, superficial head originated from the anterolateral aspect of the middle third of the humerus and the lateral intermuscular septum. Proximally, it originated posterior, distal, and anterior to the deltoid tuberosity, partially encircling the deltoid muscle insertion. The insertion continued across the anterolateral aspect of the middle third of the humerus. The origin from the lateral intermuscular septum extended from the level of the deltoid tuberosity superiorly to the origin of the brachioradialis inferiorly. Its parallel fibers ran in a line from the deltoid tuberosity to the ulnar tuberosity (Figs. 1 and 2). The ulnar tuberosity was situated on the anterior aspect of the ulna, immediately distal to the coronoid process.

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Fig. 3: Medial view of brachialis insertion. TSH = tendon of the superficial head, DHA = deep head aponeurosis, U = ulna, and R = radius.

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Fig. 4: Lateral view of the relationship between the brachialis muscle and the radial nerve. SH = superficial head, DH = deep head, LIMS = lateral intermuscular septum, RN = radial nerve, and BI = biceps.

Distally, the superficial head terminated in a thick circular tendon (Fig. 3). The median length of tendon external to the muscle belly was 44 mm (range, 28 to 53 mm). The median total length of tendon, including the intramuscular portion, was 87 mm (range, 69 to 101 mm). The tendon inserted onto the ulnar tuberosity. The tendon of the superficial head lay anterior to the deep head at the level of the elbow and was invaginated by the muscle fibers of the deep head.

The smaller, deep head originated from the distal third of the anterior aspect of the humerus and the medial intermuscular septum. The oblique, fan-shaped muscle occupied the entire anterior aspect of the metaphysis of the distal part of the humerus (Figs. 1 and 2). There was no origin from the lateral intermuscular septum. Most fibers converged onto an aponeurosis, which, at its insertion on the ulna, was oriented in the sagittal plane and attached to all but the tip of the coronoid process. The aponeurotic insertion lay medial to the insertion of the tendon of the superficial head (Fig. 3). The inferolateral fibers ran more obliquely than the superomedial fibers and were similarly oriented to the fibers of the anconeus. A small number of fibers from the deep aspect of the deep head directly attached to the anterior elbow joint capsule. We have defined these fibers as articularis cubitus.

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Fig. 5: Lateral view of the brachialis, demonstrating the two heads after separation. SH = superficial head, and DH = deep head.

In all specimens, the superficial head was pierced by the musculocutaneous nerve in its proximal midline. In ten specimens the musculocutaneous nerve supplied one main branch to the superficial head, and in one specimen this main branch was noted to divide into several smaller branches prior to piercing the muscle belly. Intramuscularly, the musculocutaneous nerve divided into branches. A variable number of these branches crossed the interval between the two heads to supply the deep head. These bridging nerve connections entered the deep head at its proximal aspect and in the approximate midline of the humerus.

A groove was formed on the lateral aspect of the brachialis muscle between its two heads. The brachioradialis and extensor carpi radialis longus overlay this groove. The radial nerve, after piercing the lateral intermuscular septum, ran in the apex of this groove, sending a small branch to the obliquely oriented inferolateral fibers of the deep head (Fig. 4). This finding was observed in all specimens. The small branch was histologically examined to confirm that it was indeed nerve tissue. A radial nerve supply to the superficial head was not observed.

The plane between the two heads (Fig. 5) was easily developed from the lateral side at the level of the distal aspect of the humeral metaphysis as the muscle bellies were loosely adherent and easily identified by the above-mentioned groove. Distally, the two heads were more difficult to separate, with more adherent fibrous tissue attaching the tendon of the superficial head to the muscle fibers and aponeurosis of the deep head. In five of the eleven specimens, there were communicating slips of muscle between the two heads.

Discussion

Materials and Methods | Results | Discussion | References

Our observations regarding brachialis muscle anatomy differ considerably from the descriptions in the current literature. Gray's Anatomy described a variation of the muscle being in "two or more" parts¹; however, the consistent presence of two separate heads has not been described previously, to our knowledge. The noticeable difference in the morphology of these two heads suggests that they may have different functions.

With the elbow fully extended, the brachialis has its poorest mechanical advantage because the lever arm to create flexion is small. This finding is due to the fact that, with the elbow extended, the line of the ulna is nearly parallel with the line of the humerus and the brachialis fibers, so the flexion moment is smallest. The most biomechanically favorable position is reached when the ulna is perpendicular to the brachialis fibers at 90° of elbow flexion.

The deep head inserted onto the anterior prominence of the coronoid, anterior to the axis of rotation of the ulnohumeral joint, whereas the superficial head attached more distally, past the anterior prominence of the coronoid. With the elbow in extension, the anterior insertion of the deep head should enable it to provide a greater flexion moment on the ulna than the superficial head. In contrast, with the elbow in flexion, the distal insertion of the superficial head should increase the lever arm and the mechanical advantage of the superficial head. Therefore, we postulate that the deep head is more important for the initiation of flexion from full extension and that the superficial head provides greater power once the elbow is flexed.

We have termed the deep head fibers that directly attach to the anterior elbow joint capsule as articularis cubitus. These fibers may be important for preventing capsular impingement during elbow flexion by bringing the anterior elbow joint capsule more anterior when the brachialis contracts.

The inferolateral muscle fibers of the deep head of the brachialis muscle run in a direction similar to the anconeus muscle. Both muscles are innervated by the radial nerve, originate from the lateral aspect of the distal part of the humerus, run parallel to the lateral ulnar collateral ligament, and insert onto the proximal part of the ulna. The anconeus is posterior to the radial head, and the brachialis is anterior. Both muscles are well positioned to provide a dynamic restraint to rotatory instability. These two muscles together provide a muscular sling, which conceivably can provide a rotatory torque on the proximal part of the ulna and are well positioned to dynamically stabilize the forearm against rotation. We postulate that this muscular sling may be important to actively "lock" the elbow when it is threatened with rotatory instability⁷. Contraction of both aspects of the dynamic sling should provide a valgus and approximating force on the elbow to load and should stabilize the ulnohumeral joint. The radiocapitellar joint will also be loaded, with the radial head capturing the capitellum, thereby indirectly stabilizing the radial head to the capitellum.

The tendon of the superficial head of the brachialis is well positioned to allow for its transfer to the radial tuberosity. This technique may improve supination strength when the distal biceps tendon is not reconstructable. The tendon of the superficial head also could be harvested proximally, with preservation of its insertion onto the ulna, and used to reconstruct the anular ligament or the medial collateral ligament of the elbow.

The descriptions of the musculocutaneous nerve were similar across the textbooks and reports in the published literature^1-3,8. Our observations of the relationship of the musculocutaneous nerve to the brachialis muscle were also consistent with the descriptions in the published literature.

A better understanding of the true internervous plane could be used to improve the current anterior and anterolateral surgical approaches to the humerus⁹. Instead of splitting the superficial head in the midline, as in the anterior approach, the interval between the two heads of the brachialis could be used. This interval could be identified easily as the radial nerve lies in the apex created between the two heads. The inferolateral fibers of the deep head (innervated by the radial nerve) could then be split from the remaining fibers of the deep head (innervated by the musculocutaneous nerve) in order to minimize denervation of the muscle. ?

References

Materials and Methods | Results | Discussion | References

Strandring S, editor. Gray's anatomy: the anatomical basis of clinical practice. 39th ed. New York: Elsevier Churchill Livingstone; 2005. 2005

Sinnatamby CS. Last's anatomy: regional and applied. 10th ed. Edinburgh: Churchill Livingstone; 1999. 1999

Rosse C, Gaddum-Rosse P, editors. Hollinshead's textbook of anatomy. 5th ed. Philadelphia: Lippincott-Raven; 1997. 1997

Mahakkanukrauh P, Somsarp V. Dual innervation of the brachialis muscle. Clin Anat. 2002;15: 206-9.15206 2002 [PubMed][CrossRef]

Yan J, Aizawa Y, Honma S, Horiguchi M. [Re-evaluation of the human brachialis muscle by fiber analysis of supply nerves]. Kaibogaku Zasshi. 1998;73: 247-58. Japanese.73247 1998 [PubMed]

Bain GI, Eames MH, Fogg Q. Distal biceps tendon: anatomy, endoscopy and endobutton repair (VHS). Rosemont, IL: American Academy of Orthopaedic Surgeons.

O'Driscoll SW, Bell DF, Morrey BF. Posterolateral rotatory instability of the elbow. J Bone Joint Surg Am. 1991;73: 440-6.73440 1991

Yang ZX, Pho RW, Kour AK, Pereira BP. The musculocutaneous nerve and its branches to the biceps and brachialis muscles. J Hand Surg [Am]. 1995;20: 671-5.20671 1995 [PubMed][CrossRef]

Hoppenfeld S, DeBoer P. Surgical exposures in orthopaedics: the anatomic approach. 3rd ed. Philadelphia: Lippincott Williams and Wilkins; 2003. 2003

Topics

cadaver ; elbow region ; humerus ; radial nerve ; tendon ; brachialis muscle

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Domenic T. Leonello, Ian J. Galley, Gregory I. Bain, Christopher D. Carter; Brachialis Muscle AnatomyA Study in Cadavers. The Journal of Bone & Joint Surgery. 2007 Jun;89(6):1293-1297.

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