Keywords
anatomic variations - clinical anatomy - supernumerary head of biceps brachii muscle
Introduction
The brachial plexus (C5–T1 vertebrae) is a cervical swelling of the spinal cord that
originates the nerves that supply the upper limbs. The communications between these
5 spinal nerve roots (C5–T1 vertebrae) originates 3 nerve fasciculus, which differ
in terminal branches, including the musculocutaneous nerve (Mc) (C5–C7 vertebrae)
and the median nerve (Me) (C5-T1 vertebrae).
-
The Mc is formed by the terminal portion of the lateral fasciculus and is responsible
for the motor innervation of the muscles of the anterior compartment of the arm and
for the sensory innervation of the lateral skin of the forearm.[1]
-
The Me is formed by a root from the lateral fasciculus and by a root from the medial
fasciculus and is responsible for the motor innervation of a great part of the flexor
muscles of the forearm, of half of the intrinsic muscles of the thenar palm, and of
the skin of the palm.[1]
Therefore, the Mc and the Me nerves do not usually communicate ([Fig. 1]).
Fig. 1 Left drawing: normal arrangement of nerve fibers. Right drawing: variation presented
in the present case report. Fl: lateral fasciculus; Fm: medial fasciculus; Mc: musculocutaneous nerve; Me: median nerve; Rc: communicating branch; Rl: lateral root; Rm: medial root.
-
The biceps brachii muscle has two origins, one in the supraglenoid tubercle, and another
in the coracoid process, both located in the scapula, and their bellies unite to form
a common tendon that inserts into the radial tuberosity and into the forearm fascia,
through the aponeurosis of the biceps brachii.[1]
However, anatomical variations have been reported in the literature, with the occurrence
of communications between the Mc and the Me nerves. Supernumerary heads of the biceps
brachii muscle have also been reported.[2]
However, it is quite unusual for both of these anatomical variations to occur in the
same cadaver and in the same limb. The aim of the present study is to describe the
relationship between multiple anatomical variations found in the right arm of a male
cadaver, which were: a communication between the Mc and the Me nerves associated with
a third head of the biceps brachii. The occurrence of a third head of the biceps brachii
associated with this communication has clinical and surgical importance, and its knowledge
can prevent iatrogenic mistakes.
Case Report
During a routine dissection, the right arm of a male cadaver presented a third head
of the biceps brachii muscle. This variation has an incidence of 14.5% in the Brazilian
population[3] ([Table 1]).
Table 1
Incidence of the Third Head of the Biceps Brachii in Different Populations
|
Population
|
Incidence (%)
|
|
Chinese[a]
|
8
|
|
European[a]
|
10
|
|
African[a]
|
12
|
|
Japanese[a]
|
18
|
|
Brazilian[b]
|
14.5
|
a Data acquired from Jayanthi et al[11].
b Data acquired from Santo Neto et al[3].
The third heads of the biceps were sorted by Rodríguez-Niedenführ et al (1992),[2] according to the location of their origins in the humerus. There are three types
of humeral heads: superior, inferomedial or inferolateral. It was also reported supernumerary
heads arising from the pectoralis major muscle and from the coracoid process. The
variation found in this cadaver, however, has its origin in the anterior third of
the arm, from fibers coming from the brachial muscle, 18.0 cm away of the coracoid
process of the scapula. It crosses the arm, from medial to lateral, joining its body
with the biceps brachii to form a common tendon with this muscle. This third head
has 11.0 cm in length, 1.8 cm wide, and continues inferiorly deep to the biceps brachii
and superficially to the brachialis muscle ([Fig. 2]).
Fig. 2 Bi: biceps brachii; Br: brachial; Me: median nerve; 3c, third head of the biceps brachii.
Besides, this same arm had a communicating branch between the Mc and the Me nerves.
The communication between these nerve was classified by Le Minor (1992)[4] and by Venieratos et al (1998).[5] The classification by Le Minor takes into account the height of this communication
in the arm and how the fibers are arranged. There are five types:
-
Type 1: There is no communication between the Mc and the Me nerves.
-
Type 2: The fibers of the lateral root of the Me nerve go along with the Mc and the
head to encounter and fuse with the Me nerve after a certain distance.
-
Type 3: The fibers of the lateral root of the Me nerve go along with the Mc nerve
and, after a certain distance, leave it to finally form the lateral root of the Me
nerve.
-
Type 4: Some of the fibers of the Mc nerve run through the lateral root of the Me
nerve before merging, finally, in the Mc nerve.
-
Type 5: There is no development of the Mc nerve and every fiber that should be separated
in this nerve is in the Me nerve, and they supply the muscles and the part of the
skin that should be innervated by the Mc nerve.
In addition to the classification by Le Minor, there is also the classification by
Venierators et al, from 1998, defined according to the relationship between the communicating
branch and the coracobrachialis muscle.[4]
[5] It is divided into three types:
-
Type I: The communicating branch is proximal to the drilling of the coracobrachialis
muscle by the Mc nerve.
-
Type II: The communication occurs distal to the perforation of the coracobrachialis
muscle by the Mc nerve.
-
Type III: There is no drilling of the coracobrachialis muscle by the Mc nerve or by
the communicating branch.
The measurement of the third head of the biceps and of the communicating branch between
the Mc and the Me nerves took as reference the distance from the coracoid process
of the scapula. Based on these classification standards in the literature, the variation
found in this body is type 2, according to Le Minor, and type II, according to Veniertators
et al.
The communicating branch (Rc) was found 16.0 cm away from the coracoid process of
the scapula drilling in coracobrachialis muscle (the branch’s length was 8.0 cm).
The length of the Mc was 17.5 cm (from its origin as a terminal branch of the lateral
fasciculus, until the fibers that was provied for the formation of Rc). The Me in
this cadaver was formed by a lateral root (derived from the lateral fasciculus), with
4.5 cm, and by a medial root (derived from the medial fasciculus), with 3.0 cm. From
the location where the Me is formed (from the medial and lateral roots) to where it
receives the Rc, the length of the Me was 20.0 cm ([Fig. 3]).
Fig. 3 Ax: axilar Artery; Mc: musculocutaneous nerve; Me: median nerve; Rc: communicating branch; 3c: third head of the biceps brachii.
Discussion
The presence of a third head of the biceps associated with a variation of the Mc nerve
may result from embryonic alterations during the development of the upper limb.[2]
[4]
[6]
[7]
[9] The mesenchymal forming muscles of the upper limb are pierced by primary ventral
spinal nerve branches, whose contact is required for the mesenchymal condensation
to the muscles.
As the development goes on, the somatic mesoderm invades the anlage, producing two
condensations, a dorsal one and a ventral one, which will origin, respectively, the
supinator-extensor muscle group and the flexor-pronator group. The nerves that invade
the anlage of the developing limb avoid or do not penetrate the dense mesenchymal
region or tissues with high rates of glycosaminoglycans. The places where a nerve
can penetrate will collaborate with the development of muscles by signaling produced
by the muscle itself. This may help to explain the mechanism of neuromuscular anatomical
variations.[6]
The development of a third head of the biceps brachii can influence these nerve branching
patterns, due to the close relationship of the mesenchyme with the primary ventral
branches of the spinal nerves.[7]
[8]
[9]
[10]
[11]
[12] It is speculated that changes in muscle growth regulatory genes, such as Pax 3 and Myf 5, and transcription factors, such as Myo D, may be involved in this kind of variation.[11]
In most cases, the communication between the Mc and the Me nerves, as well as the
third head of the biceps brachii, are asymptomatic variations and incidental findings
during surgeries or imaging studies.[7] The third head of the biceps brachii can, however, simulate a soft tissue tumor
when it is unilateral.[6] It can also complicate surgical procedures and confuse surgeons; as well as promote
vascular and/or nerve compression in the arm, causing ischemia of the irrigation of
the territory of the brachial artery and of its branches, or even pinching the Mc,
causing paresis and paresthesia in its territory of innervations.[7]
[8]
[10]
[11]
[13]
The third head may still be responsible for the compression of the Me nerve, which
runs through the arm and penetrates the region of the elbow in close relation with
the brachial muscle and with the tendon of the biceps, with common symptoms when associated
with doubling of the lacertus fibrosus. On physical examination, the compression of
the Me can be evidenced by elbow flexion against resistance, with the supinated forearm,
which will cause tension in the lacertus fibrosus associated with accessory aponeurosis
and a third head of the biceps brachii, triggering neurological symptoms in the nerve
territory, similar to pronator syndrome.[14]
The simultaneous presence of the communication between the Mc and the Me nerves and
of the third head of the biceps brachii could also increase the incidence of compressive
phenomena due to the proximity of these two variations. An hypertrophy of the third
head of the biceps brachii that compresses the communicating branch between the Mc
and the Me nerves could even simulate carpal tunnel syndrome. It could be difficult
to diagnose and it could not be corrected by conventional surgical methods if the
possibility of these variations is not borne in mind.[7] Humeral fractures in patients with a third head of the biceps brachii can be moved
depending on the type and location of the fracture and on the presence of fragments.[9]
[11]
[15] Nerve damage from communicating branches between the Mc and the Me nerves may cause
weakness in the compartment of the anterior arm, compromising the flexors of the forearm,
as well as loss of skin sensitivity on the lateral side of the forearm and on the
palm, depending on the fibers that pass through the injured communicating branch.
The third head of the biceps brachii can also generate extra flexion and supination
force of the forearm, as well as elbow flexion independent of the position of the
shoulder joint.[4]
[5]
[6] Some authors suggest that, depending on the position of the third head, it can contribute
to the pronation of the forearm.[16] There have been also suggested that acessory heads with an accompanying artery or
nerve may be useful in reconstructive surgery with flap removal.[11]
Conclusion
Due to the frequency of diagnoses and the number of surgical procedures performed
in the upper limbs, it is extremely important for orthopedists, surgeons, neurologists,
and general physicians to know the anatomical variations described in the present
study. The information in the present report, therefore, helps in understanding these
variations so that iatrogenic complications can be avoided.[7]
[15]
