ANATOMY

The LHB arises from the glenoid labrum and supraglenoid tubercle. It is an intra-articular but extrasynovial structure that is essentially static within the joint, sliding passively on the humeral head during elevation or rotation.¹ The tendon is approximately 9 cm long and 5 to 6 mm in diameter, and the articular portion is flatter and a little larger than the groove portion.

The location of the LHB just anterior to the supraspinatus tendon makes it susceptible to subacromial impingement. In addition, under load, the LHB often bears on the medial aspect of the groove, with the lesser tuberosity and soft tissues acting as a pulley (Fig. 17-1). This renders the tendon highly vulnerable to mechanical attrition and, if the soft tissue restraints of the biceps pulley become incompetent, subluxation.³

FIGURE 17-1 The bicipital groove lies anterior to the axis of the glenohumeral joint, resulting in an oblique line of pull (A), which causes the LHB to bear on the medial side of the groove, predisposing to attrition and subluxation of the tendon (B).

The contributions of the superior glenohumeral ligament and coracohumeral ligament to the biceps pulley mechanism are critical to the stability of the tendon. The transverse humeral ligament is not a significant stabilizer at the entry of the groove, and doubt has been expressed about its existence as a distinct entity, because it may be a continuation of the insertion of the subscapularis.⁴ Posteriorly, the leading edge of the supraspinatus is the primary restraint, although the anterior location of the groove and anterior vector of the pull of the tendon make this less important in most functional positions.

The function of the LHB at the shoulder is still a matter of debate. Some authors have considered it a purely vestigial structure with no true function at the glenohumeral joint, viewing it as the appendix of the shoulder. It functions as a weak abductor of the shoulder, but only when the arm is in external rotation, a position that is rarely used in activities of daily living.⁴^,⁵ It has also been proposed as a depressor of the humeral head and, although some cadaver, biomechanical, and radiologic studies have supported this,⁶^,⁷ electromyographic studies,⁸^,⁹ clinical observation, and the long-term results of tenotomy in the presence of a rotator cuff tear¹⁰ appear to negate this role. The primary function of the biceps is at the elbow, where it acts as a flexor and supinator.

There has been recent interest in the role of the LHB in glenohumeral stability, especially following the description of superior labrum anteroposterior (SLAP) lesions,⁶^,¹¹^–¹³ and mostly in the context of the throwing athlete.¹⁴^,¹⁵ Anterior instability leads to increased activity in the muscle on electromyography, indicating the possible role of the biceps as a secondary glenohumeral stabilizer. However, the magnitude of this function is likely to be small.

PATHOANATOMY

Tenosynovitis, tendinosis, delamination, prerupture, and rupture probably represent the natural history of progressive degeneration of the biceps. The LHB is subject to tenosynovitis because of its anatomy, with a synovial sheath and constrained path in the bicipital groove.¹⁶^–¹⁸ Arthroscopic evaluation shows fluid, adhesions, and increased vascularity around the tendon, but this must not be confused with the normal vascular pattern on the superficial surface of the tendon within the groove. Early tendinosis may be difficult to visualize arthroscopically, and may be best visualized using magnetic resonance imaging (MRI) or ultrasound imaging. Often, the tendon appears macroscopically normal, although there may be some increase in diameter. As the condition progresses, fibrillation, splits, and hypertrophy or attentuation occur, and may be described as delamination or prerupture.

Most degenerative changes in the LHB are associated with pathology of the rotator cuff, as highlighted by Neer.¹⁹ The relative importance of these structures in producing symptoms is still uncertain and will probably continue to be as long as our understanding of the mechanism of pain generation in disease of the rotator cuff remains incomplete.

Spontaneous rupture of the LHB is usually secondary to progression of a chronic partial tear, and may happen during a movement of daily living or a minor injury. Rupture usually occurs in the presence of a cuff tear.¹⁹ The classic Popeye sign of a ruptured LHB is unmistakable, but not all rupture, or tenotomies produce this deformity.¹⁰^,²⁰ Dislocated tendons often become encased in fibrous tissue or adherent to the subscapularis before rupture, and hypertrophic tendons may become fixed within the bicipital groove, resulting in an autotenodesis. Dislocation itself may, by shortening the course of the tendon, produce a low-rising muscle belly, which we call a false Popeye sign (Fig. 17-2).

FIGURE 17-2 CT arthrogram, axial view, demonstrating the LHB (arrow) dislocated in the substance of the subscapularis (A), with resulting loss of contour of the muscle belly in the left arm, or false Popeye sign (B).

Mechanical incarceration of the tendon within the joint has been described as an hourglass biceps,²¹ which is often seen in association with advanced disease of the rotator cuff. The intra-articular portion becomes hypertrophic and is unable to slide into the bicipital groove, resulting in buckling of the tendon on elevation of the shoulder, with painful entrapment of the tendon between the humeral head and glenoid and a mechanical block to terminal elevation. The hypertrophic tendon may also dilate the entry to the groove and lead to subluxation.

Failure of the biceps sling leads to subluxation and eventually dislocation in the presence of a rotator cuff tear. Although the difference may be subtle, it is important to distinguish subluxation from dislocation. A subluxation is defined as a partial and/or transient loss of contact between the tendon and its groove. A dislocation is the complete and permanent loss of contact between the tendon and the groove. Dislocations of the LHB can be classified into intra-articular, intratendinous, and extra-articular subtypes, depending on its position relative to the subscapularis. In cases of subluxation and dislocation, the subscapularis is almost always torn and, most often, the supraspinatus is also torn.³ Instability of the LHB is almost always medial, but lateral instability has been described, mainly in a traumatic context in association with anterior dislocation and/or fracture of the greater tuberosity,²²^,²³ but also in association with supraspinatus tears (see Fig. 17-2B).²⁴

Isolated pathology of the LHB is uncommon and found mostly in younger athletes, especially in throwers, gymnasts, swimmers, and participants in contact sports and martial arts. The LHB is subject to large forces of acceleration and deceleration of the arm as well as torsion and shear stresses, especially when coupled with tension during forced flexion of the elbow or forearm supination. Tenosynovitis, tears, instability of the tendon, and rupture may all occur in the athlete. Prevention of rupture is paramount, as even mild loss of power in elbow flexion or supination will be poorly tolerated, particularly in patients who place high demands on the biceps, such as overhead athletes or manual laborers.