The sternum is the bone that lies in the anterior midline of our thorax. It forms part of the rib cage, and the anterior most part of the thorax. Its functions are to protect the thoracic organs from trauma, and also form the bony attachment for various muscles. It is also the centre around which the superior 10 ribs directly or indirectly attach. In this article we will discuss the embryology, anatomy and clinical relevance of the sternum. We will then conclude with some review questions to test the reader’s understanding of the article content.
The sternum develops from a left and right cartilaginous plates that unite in the midline. The ribs also develop from their ossification centres and unite with the sternum in the midline. The manubrium and xiphoid process usually develop from one ossification centre each, but the sternebral centres are commonly paired either symmetric or asymmetric. The xiphoid process does not fully join the body of the sternum until adulthood.
The word sternum originates from the 17th century Greek word ‘Sternon’, meaning chest. The sternum is also known as the breastbone. It is a flat bone that articulates with the clavicle and the costal cartilages of the upper 7 ribs (true ribs), while the 8th, 9th and 10th ribs (false ribs) are indirectly attached with sternum via costal cartilage of the ribs above. The bone is divided into three parts: the manubrium, the body of the sternum (mesosternum) and the xiphoid process (xiphisternum).
The sternum lies very superficially in the anterior thorax, and is easily palpable below the skin of the chest in the midline. The bone covers and protects the heart and great vessels in part, as well as the trachea and oesophagus.
The manubrium is a large quadrangular shaped bone that lies above the body of the sternum. The lower border is narrower, is quite rough, and articulates with the body with a thin layer of cartilage in between. At the superior border of the bone is the jugular notch or suprasternal notch, fibres of interclavicular ligaments are attached here. The clavicular notches for the articulation of clavicles are projected upward and laterally on both sides of jugular notch. The costal cartilages of the first rib and part of the second rib also articulate with the manubrium, and they fit into facets on its lateral border. The sternal fibres of pectoralis major and sternocleidomastoid are attached to the anterior surface. Posterior surface gives attachment to sternohyoid and sternothyroid muscles. The superior sternopericardial ligament connects the pericardium (that lies in the superior part of the middle mediastinum) to the manubrium.
The lower border of the manubrium articulates with the body of the sternum at the sternal angle, it is where the second pair of costal cartilage attaches to the sternum and at the level of inferior border of T4, is also clinically known as the Angle of Louis. The second intercostal space can be palpated on either side of this projection, and is the location for auscultation of the pulmonary and aortic area on left and right respectively.
The body of the bone (also known as the gladiolus) is a long flat structure, with a convex anterior surface, and a concave posterior surface. It has facets on its each lateral border for articulation with the costal cartilage of the 3rd to 7th ribs along with the part of second costal cartilage. The sternocostal head of the pectoralis major muscle attaches the sternum, on the lateral sides of its anterior surface. The posterior surface of the body gives rise to the transversus thoracis muscle (innervated by intercostal nerves). The lower part of the bone is narrower and articulates with the xiphoid process.
The xiphoid process is a small projection of bone which is usually pointed. It possesses demifacets for part of seventh costal cartilage at its superolateral angle. The fibres of rectus abdominis and aponeurosis of internal and external obliques are attached to its anterior surface. Posterior surface gives rise to the inferior sternopericardial ligament. It also is the site of insertion of part of the thoracic diaphragm. Blood supply to the sternum arises from the internal thoracic artery.