Anatomy, Shoulder and Upper Limb, Pectoral Muscles

Structure and Function

The pectoralis major constitutes the bulk of the chest muscles, lying underneath the breast. It is thick, fan-shaped, and originates from multiple places along the medial and superior chest: anterior sternum, clavicular head, sternal end of rib 6, superior six costal cartilages, and from the aponeurosis of the external oblique. All the fibers converge and end in a flat tendon, inserting into the lateral lip of the bicipital groove of the humerus. The actions of the pectoralis major are dependent on which parts of the muscle are activated. The fibers attaching to the clavicular head allow for the flexion of the humerus, seen as in lifting a glass for a toast. However, the muscle fibers attaching to the sternocostal head permit the horizontal and vertical adduction, extension, and internal rotation of the upper extremity. Among these motions specific to the shoulder is the muscle's contraction as it depresses and abducts the scapula concomitantly.[2] 

There is a fascial continuation between the fascia of the pectoralis major muscle, the brachial fascia up to involving the bands of the radial and flexor muscles of the elbow and wrist. The tension of the pectoralis major muscle can affect the tone and function of the muscle parts of the upper limb and vice versa.

Compared to the pectoralis major, the pectoralis minor is much thinner and triangular in shape and resides below the major. It originates from the margins of the third to fifth ribs adjacent to the costochondral junction. The fibers consequently pass upward and laterally to insert into the medial border and superior surface of the coracoid process. It is crucial in stabilizing the scapula by pulling it downward and anteriorly against the thoracic wall.[1]

The serratus anterior is a thin muscular sheet originating from the outer surface of the first to eighth ribs and then inserting into the costal aspect of the medial margin of the scapula. All parts of the muscle help pull the scapula forward around the thorax, permitting anteversion of the upper extremity. An example is when someone throws a punch, occasionally calling the muscle the "big swing muscle" or "boxer's muscle." [3]

The subclavius, anatomically, is a tiny triangular muscle originating from the first rib and its respective cartilage anterior to the costoclavicular ligament. It then inserts into the subclavian groove of the clavicle. In other words, the inferior surface of the middle one-third of the bone. The muscle helps in depressing the shoulder as well as in elevating the first rib. These actions instinctively help protect the brachial plexus running underneath, along with the subclavian vessels, during the injury from a broken clavicle.[4]

Embryology

Skeletal muscle tissue derives from the mesoderm of the original three germ layers. From here, the mesoderm develops into the paraxial and the lateral plate mesoderm. The paraxial mesoderm, precisely those organized in the trunk, is scattered into individual tissue blocks known as somites. These somites that are concentrated more so dorsolaterally aggregate into the dermatomyotome and undergo induction into myoblasts.[2] 

Differentiation of the mesoderm to transition to eventual myoblasts is theorized to be through the modifications employed by a family of basic Helix-Loop-Helix transcription factors, most notably through MyoD1. Continuous expression of MyoD transcription factors is essential for the genetic expression of genes involved in muscle development.[5]

The cells of the dermatomyotome subdivide with one of the resulting structures to be the hypomeres. The hypomeres differentiate into three additional layers, representing the external intercostals, internal intercostals, as well as the innermost intercostals or the transverse thoracic muscle. A portion of the myoblasts originating from the hypomeres also give rise to the pectoral muscles in the anterior chest, including the pectoralis major, pectoralis minor, serratus anterior, and the subclavius.[6]

Blood Supply and Lymphatics

Pectoralis Major

• Arterial supply to the pectoralis major comes from the pectoral branch of the thoracoacromial trunk.[2]

Pectoralis Minor

• Arterial supply to the pectoralis minor also derives from the pectoral branch of the thoracoacromial trunk.[2]

Serratus Anterior

• Arterial supply to the serratus anterior includes the lateral thoracic artery, superior thoracic artery (upper half), and thoracodorsal artery (lower half).[3]

Subclavius

• The arterial supply of the subclavius is from the clavicular branch of the thoracoacromial trunk.[7]

Nerves

Pectoralis Major

• The nerve supply of the pectoralis major is via the lateral pectoral nerve and medial pectoral nerve.[2]

Pectoralis Minor

• The nerve supply of the pectoralis minor is a function of the lateral pectoral nerve and the medial pectoral nerve.[1]

Serratus Anterior

• The nerve supply of the serratus anterior is provided by the long thoracic nerve, rising from the C5, C6, and C7 roots of the brachial plexus.[3]

Subclavius

• The subclavian nerve provides the nerve supply of the subclavius.[7]

Muscles

The muscles discussed in this section include the fibers that make up the pectoral muscles:

• Pectoralis major
• Pectoralis minor
• Serratus anterior
• Subclavius

Physiologic Variants

Variations of the pectoralis major seem to primarily encompass the extent of attachment to the sternum and ribs, differences in the size of the sections to the abdomen, the degree of separation among the sternocostal and the clavicular segments, the fusion of the clavicular fibers with the deltoid, and in the decussation anterior to the sternum. Also seen is the absence of the sternocostal fibers, more so than the clavicular fibers.[8]

The pectoralis minor has been seen to occasionally originate from the second rib in addition to the third to fifth ribs. The insertion of the tendon can also range towards the coracoid process to reach the greater tubercle in some cases as well.[9]

Variations of the serratus anterior include attachments to the tenth rib and a deficiency of the attachments to the first rib. There have also been cases reporting the serratus anterior to be in union with the levator scapulae, external intercostals, or the external oblique.[3]

The subclavius has been reported to occasionally insert into the coracoid process rather than the clavicle or even into the clavicles and the coracoid process at the same time.[10]

Rarely it is possible to find a pectoral muscle (fifth and sixth ribs at the cost-chondral junction) between the pectoralis major muscle and the pectoralis minor muscle; this strip of muscle merges with the small pectoral muscle, with a vertical and lateral direction. It can be 14 centimeters long and only on one side of the rib cage. This rare variation does not have a name.

The sternalis muscle is a strip of anterior and lateral muscle to the sternum and lateral to the pectoralis major muscle, with a population response of 5 to 8%. It is inserted at the level of the manubriosternal junction and arises from the sternal handlebar and with myofascial continuity with the sternocleidomastoid muscle. Its insertion is quite variable and not always located on both sides of the sternal bone. It can be innervated by the lateral pectoral branches or by the intercostal nerves of the anterior branches. The sternalis muscle can be about 9 centimeters long.

The chondrocoracoideus muscle or Wood's muscle is a rare variant of the pectoral muscles.[11] It is born on the side of the pectoralis major muscle (sixth to eighth ribs), exchanging fibers with the external oblique muscle aponeurosis, and continues in the abdominal muscles, starting from the coracoid process.

A strip of muscle tissue known as an axillary arch in the axillary portion can merge with the pectoralis major muscle.

The subclavian muscle may be absent (agenesis) from one side of the body.[12] The subclavius posticus muscle is an alteration of the subclavian muscle and arises from the first rib and moves towards the scapula (upper edge), passing under the clavicle. The subclavian muscle can also occur supernumerary.

Surgical Considerations

Based on its size and location, the pectoralis significant merits the most frequent consideration of the pectoral muscles in surgical situations, most commonly used as a flap for reconstruction of other systems due to the extensive amount of vascularity leading to reduced incidences of necrosis following the formation of flaps. It has demonstrated effectiveness in maxillofacial defects, transected arteries, skull base defects, as well as pharyngoesophageal abnormalities.[2]

The pectoralis minor is considered a target of interest recently in patients suffering from neurogenic thoracic outlet syndrome due to nerve root compression of the brachial plexus running either through the neck or through the interscalene triangle. The symptoms of nerve compression in these patients can occur within the subcoracoid space underneath the pectoralis minor muscle, requiring a pectoralis minor tenotomy. This procedure entails the detachment of the pectoralis minor tendon to relieve the symptoms.[13]

Some surgeons use strips of the serratus anterior muscle for the aesthetic and functional repair of the face.[14]

Clinical Significance

Some disease processes consisting of abnormalities of the pectoral muscles should merit consideration when assessing patients with musculoskeletal limitations. Poland syndrome is a congenital unilateral deficiency in the pectoralis major muscle. Though most commonly arises for cosmetic or aesthetic complaints, there are some cases of Poland syndrome presenting with cardiopulmonary issues as well as dextroposition and lung herniation.

On physical exam, patients will also demonstrate osseous and cartilaginous irregularities of the rib cage, nipple absence or hypoplasia, breast asymmetry in females, and concomitant hypoplasia of the pectoralis minor, serratus anterior, latissimus dorsi, and trapezius muscles as well. If necessary, surgical intervention can be used to treat severe cases; however, virtually all cases are asymptomatic, and invasive measures are only for cosmetic indications.[15]

We must remember that the pectoral muscles influence the mobility and functionality of the shoulder. Dysfunction of these muscles could cause defects and shoulder pain.

The pectoralis minor muscle creates a passage between the ribs for the transit of the vascular-nerve brachial system. Its abnormal tension, as well as negatively affecting the position of the scapula (upwards) and the movement of the shoulder, can cause thoracic outlet syndrome.

Other Issues

For palpation of the pectoralis major muscle, the musculature is accessible with the hand, between the axilla and the nipple.

For a palpatory clinical visit, the pectoralis minor muscle can be palpated by placing a finger in the axilla and pushing obliquely towards the coracoid process of the scapula. If it is sore, the muscle is in spasm.

To palpate the subclavian muscle, look for the angle between the clavicle and the sternum; if it is sore, then the muscle may be in spasm. This situation means that there could be a problem with the passage of the vascular system and the brachial nervous system, causing thoracic outlet syndrome.