SternumEdit

The sternum, commonly known as the breastbone, is a flat, elongated bone that sits along the midline of the anterior chest. It forms a central portion of the rib cage and helps protect vital organs housed in the mediastinum, including the heart and great vessels. Acting as a sturdy anchor point, the sternum also furnishes attachment sites for several muscles involved in respiration and upper limb movement. Its shape and integrity contribute to the overall stability of the chest wall and to the mechanics of breathing.

The sternum is not a single, uniform bone at birth. It develops from cartilaginous precursors that fuse over the first decades of life, ultimately forming three main parts: the upper portion called the manubrium, the longer middle portion known as the body of the sternum (corpus sterni), and the small cartilaginous lower extension called the xiphoid process. The sternum articulates with the clavicles at the sternoclavicular joints and with the first seven pairs of costal cartilages, which connect to the ribs. The joint between the manubrium and the body creates the sternal angle, also known as the Angle of Louis, a clinically important landmark that marks the level of several thoracic structures and the second rib. The sternum, together with the ribs and their cartilages, forms a protective cage around the heart and upper mediastinal contents and helps resist compressive forces.

Anatomy

Structure

The sternum comprises three main components: - the manubrium: the broad, upper section that participates in the sternoclavicular joints and articulates with the first pair of costal cartilages; - the corpus sterni: the central blade-like portion that articulates with costal cartilages of ribs 2 through 7; - the xiphoid process: the small inferior projection that begins as cartilage and ossifies with age.

The upper border of the manubrium provides attachment for several neck and shoulder muscles, while the body serves as the anchorage for the pectoral muscles and the costal cartilages of most ribs. The sternum lies anterior to the heart and great vessels, forming the anterior boundary of the mediastinum.

Articulations

Sternoclavicular joints with the medial ends of the clavicle.
Costosternal joints with the costal cartilages of ribs 1–7.
The joint between the manubrium and body (the sternal angle) is a notable anatomical landmark that aligns with the level of the second rib.

Development and variation

During embryogenesis, the sternum forms from two parallel bands of cartilage that fuse in the midline to become a single, cartilaginous rod that later ossifies. Ossification centers appear in the manubrium and the body, with the xiphoid process developing later and often remaining cartilaginous into adulthood. Complete fusion typically occurs in late adolescence or early adulthood, though there is considerable individual variation. Some anatomical variants exist, such as a persistent sternal foramen (a small congenital hole) in a minority of individuals, which can be relevant for interpretation of radiographs or during invasive procedures.

Function

The sternum contributes to the rigidity and resilience of the chest wall, distributing mechanical loads during movement and respiration. It serves as a strong anterior attachment for muscles such as the pectoralis major and pectoralis minor and provides leverage for movements of the upper limb. During breathing, the rib cage expands and contracts in concert with the sternum and its cartilages, aiding in ventilation.

Clinical relevance

Trauma and fractures

Blunt chest trauma can fracture the sternum, sometimes with injury to underlying mediastinal structures. Management depends on the extent of injury and associated damage to the heart or vessels. Imaging, including radiography and computed tomography, assists in assessment.

Cardiac surgery and sternotomy

One of the most common clinical uses of the sternum is as the access point for open-heart procedures. In cardiac surgery, surgeons perform a median sternotomy, splitting the sternum along its midline to reach the heart and great vessels. After surgery, the sternum is typically closed with wires, though newer fixation methods and plating systems are used in some settings to improve stability and reduce complications. This area is subject to ongoing discussion about technique, safety, and cost-effectiveness, balancing traditional, durable approaches with advances in less invasive methods or alternative access routes when feasible.

Chest wall deformities

Abnormal sternum shape can arise in conditions such as pectus excavatum (a sunken chest) or pectus carinatum (a protruding chest). These deformities may be benign for many individuals but can be associated with cardiopulmonary symptoms in others and may influence decisions about surgical correction or monitoring.

Variants and procedures

The sternum can be a site for medical procedures beyond sternotomy, including bone marrow sampling or biopsy in certain clinical contexts, though such procedures carry risks including injury to mediastinal structures. Radiologic evaluation of the sternum uses X-ray, CT, and MRI to assess structure, alignment, and pathology, with the sternal angle serving as a useful reference point.

Imaging and assessment

Imaging of the sternum is essential in cases of trauma, preoperative planning for cardiac procedures, or evaluation of congenital or acquired chest conditions. The radiographic appearance of the sternum changes with age as ossification progresses, and radiologists balance features such as the sternal angle, the integrity of the manubrium and body, and the position of the xiphoid process when interpreting studies. CT scans offer detailed evaluation of fracture patterns and mediastinal relationships, while MRI can provide tissue characterization in certain scenarios.