Breastbone

The breastbone, also known as the sternum, is a flat bone located in the anterior part of the thoracic cage. It plays a central role in protecting vital thoracic organs and serves as a point of attachment for several bones and muscles. Understanding its anatomy and development is essential for both clinical and surgical perspectives.

Anatomy of the Breastbone

Location and Orientation

The breastbone lies in the midline of the anterior chest wall, forming the central part of the thoracic skeleton. It is positioned vertically and articulates with the clavicles and the cartilages of the first seven ribs. Its posterior surface faces the mediastinum, which contains the heart, major blood vessels, and other vital structures.

Parts of the Sternum

The sternum consists of three main parts, each with distinct features and clinical significance:

• Manubrium: The superior portion, broad and quadrangular in shape, articulates with the clavicles and the first rib cartilages.
• Body of sternum: The elongated central portion, also called the mesosternum, articulates with the costal cartilages of the second to seventh ribs.
• Xiphoid process: The smallest and most inferior part, often cartilaginous in youth and ossifying with age, provides attachment for the diaphragm and abdominal muscles.

Articulations

The sternum forms several important joints that contribute to thoracic stability:

• Sternoclavicular joint: Articulation between the manubrium and the clavicle, allowing movement of the shoulder girdle.
• Sternocostal joints: Articulations between the sternum and the costal cartilages of the true ribs.
• Xiphisternal joint: The junction between the body of the sternum and the xiphoid process, typically fusing in adulthood.

Surface Landmarks

Several surface features of the sternum are clinically significant:

• The jugular notch, located on the superior border of the manubrium, is palpable at the base of the neck.
• The sternal angle, formed by the junction of the manubrium and body, serves as an anatomical landmark for the second rib and intercostal space.
• The xiphoid process can be palpated at the inferior end and provides guidance in procedures such as cardiopulmonary resuscitation.

Embryology and Development

Formation of the Sternum

The sternum originates from paired mesenchymal structures known as sternal bars, which appear on either side of the anterior thoracic wall during embryogenesis. These bars gradually migrate medially and fuse to form the cartilaginous sternum.

Ossification Centers

Ossification of the sternum occurs through multiple primary centers:

• The manubrium ossifies from one center.
• The body develops from four separate centers arranged in a vertical series.
• The xiphoid process may ossify from one or more variable centers, often later in life.

Fusion and Developmental Anomalies

The ossification centers of the sternum fuse progressively from superior to inferior during childhood and adolescence. Incomplete fusion or abnormal development can result in congenital anomalies such as bifid sternum or sternal clefts. These conditions may affect thoracic integrity and occasionally require surgical correction.

Histology and Structure

Bone Composition

The breastbone is classified as a flat bone. It is composed of two thin layers of compact bone enclosing a central region of cancellous bone. This trabecular structure provides strength while minimizing weight, making the sternum both protective and structurally efficient.

Bone Marrow in Sternum

The sternum contains red bone marrow, which is an important site for hematopoiesis throughout life. Due to its accessibility and rich marrow content, it is commonly used as a site for bone marrow aspiration in diagnostic hematology.

Vascular and Nerve Supply

The blood supply to the sternum is primarily provided by branches of the internal thoracic arteries and veins. Venous drainage parallels the arterial supply and empties into the brachiocephalic veins. Nerve supply is mainly derived from anterior cutaneous branches of intercostal nerves, which also contribute to sensation overlying the chest wall.

Functions of the Breastbone

Protection of Thoracic Organs

The sternum forms the anterior shield of the thoracic cage, protecting the heart, lungs, and great vessels from trauma. Its rigid structure plays a vital role in preventing direct injury to these essential organs.

Attachment for Ribs and Muscles

The sternum provides articulation points for the first seven pairs of ribs, contributing to thoracic stability. In addition, it serves as an attachment site for several important muscles:

• Pectoralis major: Assists in movements of the upper limb.
• Sternocleidomastoid: Contributes to head and neck movement through its manubrial attachment.
• Diaphragm and rectus abdominis: Attach to the xiphoid process, aiding in respiration and trunk stability.

Role in Respiration

Through its connections with the ribs and costal cartilages, the sternum participates in the mechanics of breathing. Movements at the sternocostal joints allow the chest to expand and contract, facilitating effective ventilation.

Bone Marrow Use in Medical Procedures

Because of its accessible red marrow, the sternum is often used for bone marrow biopsy and aspiration. This procedure provides valuable diagnostic information in conditions such as leukemia, anemia, and lymphoma.

Clinical Significance

Congenital Abnormalities

• Pectus excavatum: A deformity where the sternum is depressed inward, often giving the chest a sunken appearance. It can cause cosmetic concerns and, in severe cases, cardiopulmonary compromise.
• Pectus carinatum: A condition in which the sternum protrudes outward, sometimes referred to as pigeon chest. It may affect breathing mechanics and body image.

Fractures of the Sternum

Sternal fractures typically occur due to direct trauma, such as in motor vehicle accidents where the chest strikes the steering wheel. These fractures may be associated with injuries to underlying organs, including cardiac contusions or pulmonary damage, and require careful evaluation.

Infections and Inflammatory Conditions

Osteomyelitis of the sternum is a rare but serious condition, often developing after sternotomy or chest trauma. Inflammation of the costosternal junctions, known as costochondritis, can mimic cardiac chest pain, making accurate diagnosis essential.

Tumors and Metastasis Involvement

Primary tumors of the sternum are uncommon, but the bone may be a site of secondary involvement by metastases, particularly from breast, lung, and prostate cancers. Sternal involvement often signals advanced disease and carries prognostic significance.

Palpation and Diagnostic Relevance

The sternum is easily palpable and serves as a landmark for clinicians during physical examination. Its surface features help locate adjacent ribs and intercostal spaces, which are crucial in procedures such as thoracentesis and auscultation of heart sounds.

Surgical and Procedural Importance

Sternal Biopsy

The sternum is a preferred site for bone marrow aspiration and biopsy due to its superficial location and high marrow content. This procedure is valuable in diagnosing hematological disorders, including leukemias and anemias.

Sternotomy in Cardiac Surgery

A median sternotomy is a standard surgical approach for open-heart procedures. By dividing the sternum longitudinally, surgeons gain direct access to the heart and great vessels. Following surgery, the sternum is typically reapproximated with wires or plates.

Sternal Fixation Techniques

To promote healing after sternotomy or in cases of sternal fracture, fixation methods are employed. These may include stainless steel wires, plates, or bioabsorbable devices, which help restore stability and reduce complications such as nonunion or infection.

Imaging and Diagnostic Evaluation

Radiography

Plain chest radiographs can demonstrate the sternum, although visualization may be limited by overlapping thoracic structures. Lateral views provide better assessment of sternal fractures, deformities, and post-surgical healing.

CT and MRI

Computed tomography (CT) offers detailed cross-sectional images of the sternum, making it the gold standard for evaluating fractures, tumors, and postoperative complications. Magnetic resonance imaging (MRI) is particularly valuable in assessing marrow involvement, soft tissue extension, and detecting infections.

Ultrasound Applications

Ultrasound can be used to evaluate superficial sternal lesions, guide biopsies, and assess costosternal joint inflammation. It provides a safe and non-invasive method for real-time imaging without radiation exposure.

Comparative Anatomy

Variations in Other Vertebrates

In birds, the sternum is highly developed with a keel-like structure called the carina, which provides extensive surface area for the attachment of flight muscles. In amphibians and reptiles, the sternum is often cartilaginous and less ossified compared to mammals.

Evolutionary Considerations

The evolution of the sternum is closely linked to respiratory and locomotive adaptations across species. The development of a robust, ossified sternum in mammals and birds reflects the need for efficient respiration and, in birds, powerful flight mechanisms. Comparative anatomy highlights the significance of the sternum in maintaining thoracic stability across different classes of vertebrates.

References

1. Standring S. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. London: Elsevier; 2020.
2. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. 9th ed. Philadelphia: Wolters Kluwer; 2021.
3. Rohen JW, Yokochi C, Lütjen-Drecoll E. Color Atlas of Anatomy: A Photographic Study of the Human Body. 8th ed. Philadelphia: Wolters Kluwer; 2016.
4. Drake RL, Vogl W, Mitchell AWM. Gray’s Atlas of Anatomy. 2nd ed. Philadelphia: Elsevier; 2019.
5. Snell RS. Clinical Anatomy by Regions. 10th ed. Philadelphia: Wolters Kluwer; 2019.
6. Marieb EN, Hoehn K. Human Anatomy & Physiology. 11th ed. Boston: Pearson; 2020.
7. Moore KL. Essential Clinical Anatomy. 6th ed. Philadelphia: Wolters Kluwer; 2020.
8. Standring S, ed. Pelvic and Thoracic Anatomy. In: Gray’s Anatomy. 42nd ed. London: Elsevier; 2020:1102-1115.
9. Hollinshead WH. Textbook of Anatomy. 4th ed. New York: Harper & Row; 1982.
10. Rogers LF. Radiology of the Thorax and Thoracic Skeleton. 2nd ed. Philadelphia: Saunders; 2017.