Pectoralis major muscle: anatomy and function

The pectoralis major muscle is a multipennate muscle, referring to its feather-like structure, and consists of multiple variably oriented segments of variable lengths [8, 9]. It is this variation in the lengths and orientations of the constituent fibers that results in transmission of differing degrees of stress during contraction, predisposing the more inferior sternal fibers to tear [10,11,12,13,14].

This muscle consists of fibers originating from the anterior chest wall, including the clavicle and sternum, inserting onto the humerus at the lateral lip of the bicipital groove, permitting its functions of adduction, forward flexion, and internal rotation of the shoulder (Fig. 1) [9, 11, 13].

Fig. 1

Illustrations (a, b) showing the U-shaped morphology of the anterior (green) and posterior (blue) tendon layers of the pectoralis major at the humeral insertion. The anterior layer consists of clavicular and sternal segments, while the posterior layer consists of sternal segment contribution. The orientation and contribution of the clavicular and sternal segments (c) to the distal tendon is also shown

The pectoralis major muscle also contributes to stabilization of the glenohumeral joint [15, 16] and is innervated by the medial and lateral pectoral nerves. Variable distal tendon dimensions have been reported, with the width of the insertional footprint ranging between 46 and 77 mm [11].

The pectoralis major muscle consists of a clavicular head arising from the medial clavicle and a sternal (sternocostal) head which can have two to seven segments originating from the anterior manubrium, body of the sternum, first through sixth costal cartilage, and fascia of the external oblique muscle, and some authors note origination also from the fascia of the transversalis abdominal muscles [8]. Some authors subcategorize the sternal head, referring to its most inferior fibers, which arise from the fifth and sixth costal cartilage and the fascia of the abdominal muscles, as the abdominal segment of the sternal head, while others refer to this portion as a separate third, abdominal head of the pectoralis major muscle [8, 9].

Anatomic variability

Anatomic variations of the pectoralis major have also been described and should be recognized to avoid misinterpreting such variants as pathology [7]. For example, there is variability in the overall morphology of the muscle with the clavicular head separated from the sternocostal head in about 1 in 4 cases. Rarely, the cleft is between the portion arising from the manubrium and the remainder of the sternocostal muscle rather than between the clavicular head and the sternocostal head. There is also variation in length of origin of the muscle from the clavicle and from the sternum. Fusion of the clavicular head and the deltoid muscle occurs in about 5% of cases. The number of costal cartilages from which the muscle arises is also variable, and can vary from two to seven. Typically, the muscle arises from 2nd through 6th costal cartilage (in about 60% cases) but can arise from 1st to 6th costal cartilage (about 20% cases), or from 2nd through 7th costal cartilage or from 1st through 5th costal cartilage (about 7.5% each). Variability in the length and width of the anterior and the posterior laminae, and distance of attachment from the top of the humeral head or greater tuberosity have also been described. Accessory muscles in this region including pectoralis quartus, pectoralis intermedius, pectoralis minimus, chondroepitrochlearis, and sternalis have been reported [17].

The layered tendon

While most authors describe a bilaminar medial pectoralis major tendon, others have discussed a trilaminar appearance [1]. The widely accepted morphology of the distal (lateral) pectoralis major tendon has been described as “U-shaped” and is based on the framework of a bilaminar distal tendon attachment (Fig. 1) [2, 15]. In this model, the distal pectoralis major tendon has an anterior layer and a posterior layer, which are continuous inferiorly, creating the “U-shaped” morphology [2, 15]. While the anterior layer consists of fibers from the clavicular head and superior fibers of the sternal head, the posterior layer consists of only fibers from the sternal head [2]. Based on this “U-shaped” bilaminar distal pectoralis major tendon, a sequential order of tearing involving the tendon layers to varying degrees, ranging from partial-thickness partial-width to complete-thickness complete-width tears, has been proposed, as described later. Although this layered anatomy has been described, these layers are often fused and are not separately evident on ultrasound or MRI [15].

A multimodality investigation including histologic analysis of the distal pectoralis major tendon published in 2020 showed a unilaminar fibrocartilaginous humeral attachment on histologic assessment, suggesting that the delaminating injuries described in the literature may have arisen from a different site, and challenging the premise of the bilaminar tendon and the proposed sequence of injury [1].

Regional anatomy

Lateral lip of the bicipital groove: The distal pectoralis major tendon inserts on this site, distinguishing this as a useful landmark in the assessment for pectoralis major injury (Fig. 2). The insertion of the pectoralis major occurs at the level of the humeral insertion of the latissimus dorsi and teres major, and origin of the lateral head of the triceps muscle [7]. Non-visualization of the pectoralis major insertion between the levels of the humeral attachment of the lateral head of the triceps and the deltoid tuberosity should prompt examination for possible tear and medial retraction on MRI [18].

Long head of the biceps brachii tendon and muscle: The pectoralis major tendon inserts on the lateral lip of the bicipital groove, forming a retinaculum which maintains the long head of the biceps brachii tendon against the anterior humerus at this level (Fig. 2) [19]. The long head of the biceps brachii muscle is deep to the pectoralis major tendon as it courses to its humeral attachment. Anterior displacement of the biceps tendon by more than 4.5 mm relative the humerus is associated with complete full-thickness tears of the pectoralis major tendon [19].

Fig. 2

Normal anatomy in a 21-year-old-female that presented with pain in the clavicular region, with the marker (short arrow in c) overlying the site of pain. (a) and (b) Axial proton-density-weighted fat-saturated MR image shows the pectoralis major muscle (PM) at two levels, with (a) above the level of (b). More cranially in (a), the normal position and appearance of the long head of the biceps tendon (dashed arrow in a) in the bicipital groove is denoted. Also note the tendons of the coracobrachialis and short head of the biceps (black arrow in a). The pectoralis minor muscle (PMin), coracobrachialis muscle (CB), subscapularis muscle (Ssc), infraspinatus muscle (Is) and the deltoid muscle (D) are also shown. More caudally in (b), the PM tendon as it approaches the humeral insertion (arrow in b) at the lateral lip of the bicipital groove is shown. The teres major muscle (TM) is shown, in addition to the other abbreviated muscle from (a). (c) Coronal T1-weighted MR image shows the clavicular (C) and sternal (S) heads of the pectoralis major muscle. Note the deltopectoral groove (long arrow in c), which separates the deltoid muscle (D) from the head of the pectoralis major muscle. The obliquely oriented tubular low signal in the subcutaneous fat in this groove, subjacent to the arrow, is the cephalic vein

Short head biceps brachii muscle: The short head biceps brachii muscle is deep to the pectoralis major tendon and medial to the long head of the biceps tendon.

Coracobrachialis muscle: This muscle is deep to the pectoralis major tendon, originates from the coracoid process of the scapula as a conjoined tendon with the short head of the biceps, and inserts onto the medial humerus.

Deltoid muscle: The deltoid muscle consists of an anterior, middle and posterior components [16]. The anterior deltoid muscle is superficial to the pectoralis major tendon. Fusion of the pectoralis major muscle with the anterior component of the deltoid muscle has been described [1, 7].

Deltopectoral groove: The deltopectoral groove refers to the interval between the pectoralis major muscle belly and the anterior head of the deltoid muscle, serving as a surgical landmark, and contains the cephalic vein [20]. Edema or fluid in this region can be associated with injury of the pectoralis major muscle [7]. On axial images, the pectoralis major myotendinous junction is deep to the deltopectoral groove [18].

Pectoralis minor muscle: This triangular muscle is deep to the pectoralis major muscle. The pectoralis minor muscle originates from the external surface of the third, fourth and fifth ribs, and sometimes also of the second and sixth ribs and inserts at the medial and superior surfaces of the coracoid process [16].

Mechanisms of injury

Indirect trauma, particularly with the bench press maneuver, is the most common mechanism of pectoralis major injury [5, 8, 11]. Males between 20 and 40 years old represent the typical demographic with this injury [11]. In this scenario, with the humerus extended, there is eccentric muscle contraction, and the shorter, maximally stretched inferior sternal head muscle fibers are predisposed to tear [6, 8, 12]. Various types of direct trauma, including occupational injuries, have also been identified as injury mechanisms [11].

Injury classification systems

Multiple imaging-based classification systems of pectoralis major injury have been proposed. There is no current consensus regarding the definition of acute versus chronic injuries of the pectoralis major based on the time interval since injury clinically or by imaging, and various definitions have been used [11]. There is also variability in the degree of detail across radiology and operative reports regarding these injuries. In practice, communication between the ordering clinicians, including surgeons, and radiologists regarding the specific goals of imaging such as reporting details is needed to ensure optimal patient care.

In 2012, ElMaraghy and Devereaux proposed a standardized classification system for pectoralis major injuries, which consisted of three parameters: injury acuity (distinguishing an acute injury as within a timeframe of six weeks), tear location, and tear extent [15]. The authors defined three tear location subcategories: muscle origin or belly, at or between the musculotendinous junction and tendinous insertion, and bony avulsion fracture from the humerus. The parameter of tear extent was based on the framework of a bilaminar distal tendon attachment, described as “U-shaped” [15]. Within this framework, as described above, the distal pectoralis major tendon has an anterior layer and a posterior layer, which are continuous inferiorly, creating the “U-shaped” morphology [2, 15]. Each layer measures approximately 2 millimeters in thickness [2]. According to this classification system, tear extent has two features to characterize: thickness in the anterior-posterior dimension, which can include one or both of the described two layers, and width in the proximal to distal dimension, which can be incomplete or complete [15].

Based on the “U-shaped” pectoralis major common tendon framework, a sequential order of tear progression has been proposed, first involving the posterior layer, which consists of only sternal fibers, at its most inferior components and then progressing to involve its more superior segments, followed by the clavicular head with increasing loads [15]. Although some injuries may not occur in this typical pattern and characterization can be limited by regional edema [7], this classification system proposed by ElMaraghy and Devereaux provides a useful anatomic-based approach to conceptualizing these injuries within this framework.

In this classification system, tears including only a portion of the posterior layer are defined as partial-thickness partial-width, and tears involving the entire posterior layer without involvement of the anterior layer are defined as partial-thickness full-width [7, 15]. Tears involving the entire posterior layer and a portion of the anterior layer, which includes the most superior sternal segments, are classified as full-thickness partial-width, and tears involving the entire posterior layer in addition to the entire anterior layer are classified as full-thickness, full-width injuries [7, 15]. As mentioned previously, the bilaminar appearance is most commonly not evident on ultrasound or MRI. Classification into partial-thickness tears of the tendon is often made at surgery, and an awareness of this imaging limitation is important for radiologists and clinicians.

Prior to this proposal, the Tietjen classification system had been described, without a parameter for acuity, although with distinction between contusion and partial versus complete tear, with subsequent Bak modifications to include bony avulsion and tendon substance rupture [21]. In 2020, Cordasco and colleagues proposed another classification system, which distinguished a type I injury as a contusion, type II as a tear involving one head, and type III as a tear involving multiple heads, with multiple type II and III subtypes based on the location of the tear [21].

According to a critical analysis of the literature published by Magone and colleagues in 2021, however, there is no consensus regarding the definition of acute versus chronic injuries of the pectoralis major or regarding the optimal timeline for surgical intervention with either repair or reconstruction [11]. A combination of case-based factors including the patient’s age and activity level and features of the injury contribute to the decision for non-operative versus operative management [12].

MRI characterization of pectoralis major injuries in practice

As discussed, imaging characterization of pectoralis major injuries is limited in comparison to the detailed histological reference, and partial-thickness tears are often diagnosed intraoperatively. In light of the limitations of imaging, the authors prefer to apply a standard three-part MRI grading system, which will be described in MRI Features of Injury and Tear Characterization, to provide information that can be used to guide treatment. This preferred system of the standard three grade classification incorporates a fundamental element of the Tietjen and ElMaraghy systems: the distinction between a partial versus complete tear. Although the ElMaraghy classification is a more comprehensive system based on the concept of the “U- shaped” tendon, due to the limitation of MRI and ultrasound including difficulty distinguishing the two layers, in practice, the authors prefer this simpler approach of grading.

Locations of Pectoralis Major Tears

The location of pectoralis major injury contributes to clinical decision-making for management [7], and can be described as involving its muscle origin, muscle belly, myotendinous junction, intra-tendinous, or the humeral insertion (Fig. 3) [15].

Fig. 3

Coronal illustrations showing variable sites of pectoralis major injuries, demarcated as an irregular black line. Injuries that are typically managed non-operatively (a) occur at the muscle origin and at the muscle belly. In contrast, injuries that are typically managed operatively (b) occur at the myotendinous junction, are intra-tendinous, or are related to avulsion with or without bone fragment from the humeral insertion

As noted previously, the more inferior sternal head fibers are more predisposed to tear compared to clavicular head fibers [10,11,12,13]. While clinical management is guided by multiple patient-and case-specific factors, cases of strain and tears at the muscle origin or muscle belly are generally categorized as non-operative cases, and surgical management with various operative techniques can be considered for tears at the other locations [7, 15].

Variable relative frequencies of tear locations have been reported in the literature, with tendon avulsion from the humerus or tear at the myotendinous junction as the most common site [2, 7, 11,