Definition
By: Gregory R. Waryasz, MD
The pectoralis major of the musculoskeletal system is characterized by having two heads; sternocostal and clavicular.
It is part of the anterior axioappendicular muscles. It consists of skeletal muscle fibers.
Its unique structural features include having two distinct head, but having a common insertion. The muscle is a large and fan-shaped muscle of the superior thorax. The sternocostal head is the larger of the two heads. The anterior axillary wall is made up of the sternocostal head. The junction of the pectoralis major and the deltoid muscle is known as the deltopectoral interval. The deltopectoral interval is used for surgical access to the glenohumeral joint. The deltopectoral interval also is the location for the cephalic vein.
The origin of the clavicular head is the anterior surface of the medial half of the clavicle. The origin of the sternocostal head is the anterior surface of the sternum, superior six costal cartilages, and the aponeurosis of the external oblique muscle.
The insertion is the lateral lip of the intertubercular groove of the humerus.
The blood supply is from the pectoral branch of the thoracoacromial trunk and venous drainage is from the accompanying veins.
The innervation is from the lateral and medial pectoral nerves.
The pectoralis major muscle as well as all other bones, muscles, and ligaments of the body are derived of mesodermal origin in the embryo.
The function of the pectoralis major is to adduct and medially rotate the humerus and draw the scapula in anteriorly and inferiorly. The clavicular head can flex the humerus. The sternocostal head extends the humerus from the flexed position. The muscle is very strong. The exercise that people do that involves the pectoralis major is the bench press and the push-up.
Common diseases include rupture, tears, and congenital absence.
Pectoralis major rupture can be complete to partial. It is a cause of sudden pain, eccymosis, and swelling along the pectoralis major muscle. The injury usually occurs during the bench press exercise. As the swelling subsides, there may be a sulcus sign and deformity noted. Weakness of the adduction and medial rotation is noted.
Minor tears of the muscle belly can occur from physical activity leading to pain.
Congenital absence can occur of the sternocostal portion of the muscle leading to no significant disability, however the anterior axillary fold may appear abnormal. It is rare to have complete absence of the pectoralis major.
Poland syndrome is a rare birth defect with under development or absence of the pectoralis and webbing/syndactyly of the fingers. It is usually unilateral. In females the breast tissue can be abnormal.
A pectoralis major myocutaneous flap can be used for surgical coverage in reconstructive surgery.
Commonly used diagnostic procedures include clinical history, physical exam, and MRI.
It is usually treated with rest and physical therapy for regaining function for activities of daily living after a tear or rupture. Patients wishing to return to heavy weightlifting after a rupture will require a operative repair. Congenital absence of the pectoralis and Poland syndrome can be treated with a transfer of the latissimus dorsi muscle. Poland syndrome may require its other components to be treated as well such as the syndactyly.
References
Lieberman J (ed), AAOS Comprehensive Orthopaedic Review, American Academy of Orthopaedic Surgeons, 2008.
Moore K, Dalley A (eds), Clinically Oriented Anatomy (5th edition), Lippincott Williams & Wilkins, Philadelphia, PA, 2006.
Wheeless’ Textbook of Orthopaedics: Pectoralis Major (http://www.wheelessonline.com/ortho/pectoralis_major)