Other Muscle Injuries

Anatomy

The pectoralis major muscle covers the anterior chest wall and is composed of two separate heads (clavicular and sternal) that serve to adduct, internally rotate, and flex the shoulder. The muscle originates as a broad sheet from the clavicle, sternum, ribs, and external oblique fascia, with the two heads converging and inserting over a 5-cm area on the lateral lip of the bicipital groove of the humerus. The tendons are hypothesized to twist 90 degrees, 180 degrees, or simply to overlap at their insertion, resulting in the inferior (sternal) segment inserting proximal to the more distal insertion of the superior (clavicular) segment on the humerus. Because of this configuration, the fibers of the sternal segment experience increased excursion with the arm in 0 to 30 degrees of extension, as experienced during bench press, resulting in maximal stretching of the tendon fibers and increasing the risk for rupture. The pectoralis major muscle is innervated by the medial (lower sternocostal segments) and lateral (clavicular segments) pectoral nerves arising from the C5 to T1 nerve roots.

Mechanisms

The majority of injuries to the pectoralis major are secondary to substantial contraction applied to the distal tendon while it is maximally stretched, with direct trauma to the shoulder and arm being less common. Multiple studies have found that the most common mechanism of injury was the bench press, in which the abducted, extended, and externally rotated arm is eccentrically contracted, placing the pectoralis tendon under maximal tension. Owing to the anatomic configuration of the tendons distally, application of maximal load results in a significant mechanical disadvantage to the fibers of the inferior sternal segment of the pectoralis major, thus often causing them to rupture and accounting for the high incidence of sternal head ruptures. Although partial ruptures are more common, up to 65% of complete ruptures have been reported to occur at the humeral insertion, with up to 29% occurring at the musculotendinous junction. Moreover, distal tendon injuries are more likely to be complete ruptures. Injuries to the muscle belly or the clavicular and sternal origins of the pectoralis major are less common and usually result from direct trauma.

Other mechanisms—including direct trauma sustained during American football and rugby along with other sporting activities associated with a risk of rapid, forceful shoulder abduction, such as rodeo, windsurfing, sailing, handball, wrestling, hockey, and artistic gymnastics —have been described. In addition, injuries have also been described in individuals attempting to break a fall in which the force of impaction on the ground on the contracted muscles results in injury.

Abuse of anabolic steroids has been tied to an increased risk for tendon ruptures. Pochini et al. have reported that in 60 patients with complete ruptures, more than 90% endorsed anabolic steroid use, whereas Aarimaa et al. point to a 36% rate of abuse in their case series of 33 patients. Other authors have proposed that steroids weaken and stiffen the tendon fibers secondary to tendinopathy. Concomitant with disproportionate gains in muscle mass and force, the weakened tendon is overwhelmed, resulting in rupture with continued stress placed across the tendon with weight lifting.

Classification

Injuries to the pectoralis major are generally classified using the classification system proposed by Tietjen et al., which describes the site of the lesion without accounting for the severity (partial versus complete), timing (acute versus chronic), or segments of involvement (sternal versus clavicular head). A recent meta-analysis by El Maraghy and Devereaux proposed a new classification scale focusing on lesion acuity (acute or chronic), the qualitative degree of tendon involvement (incomplete versus complete), and the location of the rupture (muscle, musculotendinous junction, or humerus).

Physical Examination

In addition to a thorough history, a comprehensive physical examination is essential for diagnosis and to ensure proper clinical decision-making while ruling out concomitant injuries in and around the shoulder. Acute ruptures to the distal tendons present with edema, ecchymosis, and hematoma to the lateral chest and proximal arm, while injuries to the sternal or clavicular origins are isolated over the anterior chest wall. Patient typically report hearing or feeling a popping or tearing sensation at the time of injury, accompanied by pain in the chest wall, axilla, or down the arm. Examination will demonstrate weakness with concomitant discomfort with resisted adduction, forward flexion, and internal rotation, along with tenderness at the axilla and humeral insertion. Further inspection may reveal an asymmetric prominence from hematoma or from retraction of the muscle belly near the axilla. As swelling subsides, examination of the axillary fold may demonstrate a palpable defect in the anterior axilla, accompanied by thinning, hollowing, or loss of the fold accentuated with isometric contraction with adduction as the patient presses the hands against one another in the front of the chest. Patients may also demonstrate a visibly retracted stump within the anterior axillary fold on lateral inspection of the arm during forward flexion, known as the “S” sign . Asymmetric webbing of the axilla is commonly reported to describe the loss of musculature from the anterior wall of the axilla. Whereas some authors describe the ability to differentiate full-thickness tears from partial tears by the presence of a distinct gap or defect, lack of a palpable defect in the axilla has not been shown to be a reliable indicator of pectoralis continuity. In cases of chronic tendon injuries, patients may develop a prominent skin fold, known as cicatricial fibrosis , along with webbing and prominence of the distal deltoid insertion. Retraction of the tendon and muscle with appreciable asymmetry is more pronounced as compared with the intact contralateral side in chronic tears as well.