The sternum, commonly known as the breastbone, is a flat, dagger-shaped bone positioned centrally in the chest. It forms the front portion of the rib cage, playing a fundamental role in the structure of the upper body.
Structure of the Sternum
The sternum consists of three parts that develop from cartilage in childhood and ossify into bone in adulthood. The uppermost section is the manubrium, a broad, trapezoid-shaped segment that connects to the clavicles (collarbones) and the first pair of ribs. It also forms the superior wall of the anterior mediastinum.
Below the manubrium is the body, the longest and flattest part. This central section articulates directly with the costal cartilages of ribs three through six, and partially with the second and seventh ribs. The manubrium and body join at the sternal angle, also known as the angle of Louis, a palpable ridge and anatomical landmark. This angle corresponds to the level of the second rib and the intervertebral disc between the fourth and fifth thoracic vertebrae.
The lowest and smallest part is the xiphoid process, often pointed, largely cartilaginous, and fully ossifying around age 40. Shape and size vary. This extension serves as an attachment point for certain muscles, including parts of the diaphragm and abdominal muscles.
Roles of the Sternum
The sternum performs several vital functions. It acts as a protective shield for delicate organs like the heart, lungs, and major blood vessels, safeguarding them from external forces. This protective role is achieved by its rigid structure as a central component of the rib cage.
Beyond protection, the sternum provides crucial support for the upper body. It serves as an attachment point for the clavicles and the first seven pairs of ribs, forming the robust rib cage. Various muscles of the chest and upper abdomen also connect to the sternum, further stabilizing the torso. The slight flexibility at the manubriosternal joint, or sternal angle, allows for subtle movements that facilitate respiration.
During breathing, the sternum, in conjunction with the ribs, allows for the expansion and contraction of the chest cavity, particularly increasing the chest’s diameter during inhalation. This dynamic movement is essential for efficient air intake and expulsion. Additionally, like other flat bones, the sternum contains bone marrow, which is involved in hematopoiesis, the production of blood cells.
Common Issues Involving the Sternum
The sternum, despite its protective role, can be susceptible to various issues, including injuries, structural abnormalities, and inflammatory conditions. Sternal fractures, though less common than rib fractures, typically result from significant blunt chest trauma, such as vehicle accidents or direct impacts. These fractures cause localized chest pain, which may worsen with deep breaths or coughing, and can be accompanied by bruising and tenderness. While painful, their significance often lies in the possibility of associated internal injuries to the heart or lungs.
Costochondritis is an inflammatory condition affecting the cartilage connecting the ribs to the sternum. This inflammation causes chest pain, often sharp, aching, or pressure-like, that can mimic cardiac conditions. Pain typically worsens with movement, deep breathing, coughing, or sneezing, and often affects the upper left ribs, though it can occur elsewhere. Cause is often unknown but linked to chest injuries, repetitive movements, or severe coughing.
Pectus deformities are structural abnormalities of the sternum and rib cage present from birth. Pectus excavatum, or sunken/funnel chest, involves an inward depression of the sternum. Conversely, pectus carinatum, or pigeon chest, is an outward protrusion. In severe cases, these conditions can impact thoracic organs.
A sternotomy is a surgical procedure involving a vertical incision through the sternum to access organs like the heart or lungs. Commonly performed for cardiac surgeries, including bypass grafting, heart transplants, or valve repairs. Afterward, the sternum halves are rejoined, often with wires, to facilitate healing.