The long head of the biceps brachii tendon (LHBT) is a common source of pain in the shoulder. Biceps tendon pathology is often associated with rotator cuff (RC) pathology. The spectrum of LHBT injuries includes primary and secondary tendinitis, chronic tendinopathy, superior labrum anterior and posterior (SLAP) lesions, instability, and partial or complete ruptures.[1][2]
Anatomy
The LHBT origin, on average, is 9 cm in length.[3] The tendon is widest at its labral origin, which is primarily posterior about 50% of the time.[4] In 20% of cases, the origin is directly at the supraglenoid tubercle, and the remaining 30% of the time, its origin is seen as a combination originating from the two sites.[5] The tendon itself is intra-articular yet extra-synovial, and it progressively gets narrower as it passes obliquely from its origin and heads toward the bicipital groove. As it exits the distal bicipital groove in the upper arm, the LHBT joins the short head of the biceps tendon (SHBT) as both transition into their respective muscle bellies in the central third of the upper arm. After crossing the volar aspect of the elbow, the biceps brachii inserts on the radial tuberosity and medial forearm fascia. The latter occurs via the bicipital aponeurosis.[4]
Pain Generation
The LHBT is a well-recognized source of anterior shoulder pain. Mechanical causes include repetitive traction, friction, and glenohumeral rotation. The bicipital sheath itself is vulnerable to tenosynovial inflammation by its association as it is contiguous with the synovial lining of the glenohumeral joint.[6] The upper one-third of the LHBT demonstrates a rich sympathetic innervation network, including neuropeptides such as substance P and calcitonin gene-related peptide. These factors are present in the sensory nerves in this region of the tendon. This sympathetic network is known to exhibit vasodilatory changes as part of the neurogenic inflammatory process in the LHBT, which may play a critical role in at least the chronic phase of pathophysiology affecting the LHBT.[7][8][9]
The Bicipital Groove
The bicipital groove is an anatomic landmark that sits between the greater and lesser tuberosities, and its osseous and soft tissue components contribute to the inherent stability of the LHBT. The depth, width, and medial wall angle have been studied relative to overall bicipital groove stability, with significant variability recognized in its components. Many authors attribute these parameters as predisposing factors to pain and instability in both primary and secondary LHBT pathologies.[10][11]
The Soft Tissue Pulley System
The LHBT takes a 30-degree turn as it heads toward the supraglenoid tubercle, relying on the integrity of the enveloping soft tissue sling/pulley system. The most important elements in maintaining stability at this critical turn angle are the most medial structures at the proximal-most aspect of the groove’s exit point. The soft tissue components of the biceps pulley system include the following[12]: Subscapularis. Supraspinatus. The coracohumeral ligament (CHL). The superior glenohumeral ligament (SGHL). The subscapularis has superficial and deep fibers that envelope the groove, creating the “roof” and “floor,” respectively. These fibers also coalesce with those from the supraspinatus and SGHL/CHL complex. These structures attach intimately at the lesser tuberosity to create the proximal and medial aspect of the pulley system, with soft tissue extensions serving to further envelope the LHBT in the bicipital groove. The CHL is a dense fibrous structure connecting the base of the coracoid process to the greater and lesser tuberosities. At its origin, the ligament is thin and broad, measuring about 2 cm in diameter at the base of the coracoid. Laterally, the CHL separates into two distinct bands that envelop the LHBT at the proximal extent of the bicipital groove. Once the LHBT exits the groove, it takes a 30- to 40-degree turn as it heads toward the supraglenoid tubercle and glenoid labrum. Thus, the proximal soft tissue elements of the groove are especially critical for the overall stability of the entire complex. In addition to the CHL, the SGHL reinforces the complex at this proximal exit point. The SGHL travels from the superior labrum to the lesser tuberosity, becoming confluent with the soft tissue pulley as it takes on a U-shaped configuration. Warner and colleagues previously demonstrated that the cross-sectional area of the CHL is, on average, 5 times larger than that of the SGHL.[13][14] Some authors have argued that the CHL is just a thickening of the anterosuperior glenohumeral capsule, while others maintain that the CHL is a unique entity. Advocates supporting the CHL as a distinct anatomic structure cite its role as an important stabilizer to inferior humeral head translation in the adducted shoulder.[15] Studies investigating pathologic changes seen in association with recalcitrant adhesive capsulitis acknowledge the presence of a thickened CHL as the primary source of limitations to the external and internal rotation. Its thickness appears to correlate directly with decreasing degrees of external rotation.[16]
The Transverse Humeral Ligament
Historically, the transverse humeral ligament (THL) was thought to play a primary role in bicipital groove stability. However, more recently, its role in providing stability has been refuted, with many authors questioning its existence as a distinct anatomic structure. The latter remains fairly controversial, with most studies now reporting the THL is, at most, a continuation of fibers from the subscapularis, supraspinatus, and CHL. A histologic study in 2013 identified a distinct fibrous fascial covering in the “roof” of the groove. Neurohistology staining showed the presence of free nerve endings but no mechanoreceptors.[17] Despite the controversial evidence with respect to its definitive existence as an anatomic structure, its location at the distal extent of the bicipital groove inherently refutes the previous dogma of its potential role in LHBT stability. Furthermore, the presence of free nerve endings in the recent histological studies suggests its possible role as a potential pain generator in the anterior shoulder.[17]
Biomechanics
Biomechanically, the LHBT has a controversial role in the dynamic stability of the shoulder joint. It has been demonstrated, mostly in biomechanical cadaveric-based studies and animal models, that the tendon at least plays a passive stabilizing role in the shoulder. Neer proposed in the 1970s that the LHBTs stabilizing role varied depending on the position of the elbow. Several subsequent studies refuted the theory that the LHBT played any active shoulder stabilizing effect.[18] Jobe and Perry evaluated the activation of the biceps during the throwing motion in athletes. The authors reported the peak muscle stimulation occurred in relation to elbow flexion and forearm deceleration, with very little proximal biceps activity during the earlier phases of throwing.[19][20][21][22] In most normal, healthy patient populations, the LHBT plays a negligible role in the dynamic stability of the shoulder. The general consensus concerning its overall function is that it is a strong forearm supinator but a weak elbow flexor.[23]