A Proposal for a New Classification of the Supernumerary Heads of the Biceps Brachii Muscle

Abstract

Introduction. The anterior compartment of the arm consists of three muscles: the biceps brachii (BB), brachialis, and coracobrachialis muscle. The aim of the present study was to characterize possible variations in the supernumerary heads of the biceps brachii and use these to prepare an accurate classification of the area that could be used for planning surgical procedures in the region. Material and Methods. One hundred (51 left and 49 right, 52 females and 48 males) upper limbs fixed in 10% formalin solution were examined. Results. Four types of supernumerary BB heads were identified, with subtypes. Type I was the most common type, characterized by the two heads (64%); this was subdivided into Type IA, with a single muscle belly, and Type IB with two muscle bellies. The second most common type was Type II, which was characterized by the three BB heads (26%). This type was divided into four subtypes (A-D): Type IIa characterized by attachment to the middle part of the shaft of the humerus; Type IIb characterized by the origin to the coracoid process together with the short head of the BB; Type IIc characterized by origin to the tendon of the pectoralis major muscle; and Type IId characterized by the attachment to the capsule of the humeral joint. The third most common type was Type III, which was characterized by four heads (6%); this was divided into Type IIIa, where two heads originated from the humerus bone, and Type IIIb, where one head originated from the short heads and the second from the long head of the BB. The rarest type was Type IV (4%) which was characterized by five heads: the short head originated from the coracoid process and the long head originated from the supraglenoid tubercle, the third and fourth head originated from the shaft of the humerus, while the fifth head originated from the pectoralis major muscle. Conclusion. The biceps brachii is characterized by very high morphological variability. The new classification proposes four types of supernumerary head arrangement (I-IV), divided into subtypes. This classification has both clinical and anatomical significance.