INTRODUCTION

The biceps brachii muscle (BBM) has two heads, a long head originating from the supraglenoid tubercle and a short head originating from the coracoid process of the scapula. In terms of the number and morphology of its heads, the BBM is one of the most variable muscles in the human body (^{Rodríguez-Niedenführ et al., 2003}). The most common variation is a third head, but four, five, or even seven heads have been reported (^{Asvat et al., 1993}; ^{Nakatani et al., 1998}). The presence of the third head has been reported with varying frequency from 8 % to 37.5 % depending on the population (Asvat et al.; ^{Rincón et al., 2001}; ^{Standring, 2016}). This supernumerary muscle has clinical importance because it may cause the compression of neurovascular structures or confuse a surgeon who performs procedures on the arm (^{Warner et al., 1992}). In this case, we report a unique morphology of the third head of the BBM and discuss the clinical significance of this variation.

DISCUSSION

The BBM exhibits a wide array of variable morphological characteristics (Asvat et al.; Nakatani et al.; Rincón et al.; Rodríguez-Niedenführ et al.; Standring). In detail, the prevalence rates of supernumerary heads of the BBM are as follows: Chinese, 8 %; European whites, 10 %; African blacks, 12 %; Japanese, 18 %; South African blacks, 20.5 %; South African whites, 8.3 %; and Colombian, 37.5 % (Asvat et al.; Nakatani et al.; Rincón et al.; Standring). ^{Lee (2013}) are found this variation in 14 of 214 (6.5 %) individuals in the Korean populations, a lowest prevalence than other populations. They reported that supernumerary head of the BBM occurs unilaterally rather than bilaterally (Asvat et al.; Rincón et al.). Some studies demonstrated that supernumerary heads are more commonly found in males, however, it is controversial until now (^{Vijayabhaskar et al., 2008}).

^{Rodriguez-Niedenfuhr et al. (2003}) classify the type of supernumerary head of the BBM according to its location by dissecting 350 arms. The most common type is the inferomedial humeral head (9 %), originating from the humerus at the level of the coracobrachialis insertion or brachialis. The superior humeral head (1.5 %) is originated from the lesser tubercle, external lip of the bicipital groove, undersurface of the pectoralis major, deltoid muscle, or anterior surface of the shoulder joint capsule. The inferolateral humeral head (0.9 %) is the least common variation, originating from the lateral intermuscular septum, between the insertion of the deltoid and the origin of the brachioradialis, or from the supracondylar ridge.

On the basis of this criterion, the third head in our case was the inferomedial humeral head, the most frequent type of variation in the BBM. The variant origin, insertion, shape, and number of BBM have been frequently reported still (Asvat et al.; Nakatani et al.; Rincón et al.; RodríguezNiedenführ et al.; Standring). However, the musculocutaneous nerve abruptly pierced the third head of the BBM, and this case has not been reported in any articles.

Embryologically, the mesoderm invades the upper limb bud to further condense into ventral and dorsal muscle masses during the fifth week of development (RodríguezNiedenführ et al.). The biceps musculature is derived from the ventral muscle masses of the upper limb bud. The supernumerary head of the BBM may be originated from the incomplete cleavage of the ventral muscle masses. The development of neurovascular structures also may contribute to the cleavage of these muscles. Especially the inferomedial head was suggested to be due to the musculocutaneous nerve piercing the brachialis muscle and producing a separate head. Therefore, these reasons induced a musculocutaneous nerve penetrating the third head of the BBM, as presented in this report.

Here, we report a third head of the BBM penetrated by the musculocutaneous nerve for the first time. The presence of the third head may cause compression of neurovascular structures and can be easily confused with pathological conditions in CT and MR imaging. Therefore, knowledge of the variations of the BBM is important for clinicians because it increases the incidence of iatrogenic injuries during surgery and invasive procedures (Warner et al.; Vijayabhaskar et al.).

Fig. 1 The third head (*) of the biceps brachii muscle in the left arm. The musculocutaneous nerve perforates the third head (arrows). (A) Anterolateral view (B) superior view. B, brachialis; BR, brachioradialis; LH, long head of biceps brachii muscle; SH, short head of biceps brachii muscle.