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 center around which the superior 10 ribs directly or indirectly attached.
In this article, we will discuss the embryology, anatomy and clinical relevance of the sternum.
- The sternum develops from a left and right cartilaginous plates that unite in the midline.
- The ribs develop from their ossification centers 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).
- 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 esophagus.
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 (of louis), it is where the second pair of costal cartilage attaches to the sternum and at the level of the 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.
Body of Sternum
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.
Open cardiothoracic surgery requires the sternum to be divided and splayed open to access the thoracic organs. This technique can be used for coronary artery surgery, and open abdominal aortic aneurysm repair. However, as minimally invasive radiologically guided techniques such as EVAR (endovascular aneurysm repair) have developed, sternotomy is being used less and less. The newer approaches lead a shorter recovery time and less morbidity for the patient.
Sternoclavicular joint dislocation
This is an uncommon fracture, and due to its location to the great vessels, is potentially rapidly dangerous. Symptoms will include soreness around the area, and if the great vessels are compromised, sudden death.
This is a rare fracture and most commonly results from a motor vehicle accident, or high impact direct trauma of another cause. The manubrium is the most commonly injured part of the bone. Due to their direct connection and proximity, the ribs are also commonly fractured in the process. The vital organs can be compromised.
Bone marrow biopsy
The sternum is used as the site for bone marrow biopsy in obese or overweight patients, where access to the iliac crest is limited.
Pectus excavatum is a condition also known as funnel chest, where the sternum and superior ribs grow abnormally, created a sunken chest appearance. Causes including Marfan syndrome (fibrillin defect) and Ehler’s Danlos syndrome (collagen defect).
This is the opposite of pectus excavatum, and occurs when the ribs and sternum grow abnormally, so the sternum protrudes outwards. The chest is shaped like a bird’s, this condition is also a feature in many syndromes like Down’s syndrome, Marfan syndrome, and osteogenesis imperfecta.