Sternum Anatomy and Clinical Significance

Understanding the sternum is fundamental for any aspiring clinician. This central bone of the anterior chest wall is not just a passive shield; it serves as a critical anatomical landmark for internal structures and is a key site for vital diagnostic and therapeutic procedures. Mastering its anatomy will enhance your physical examination skills and deepen your comprehension of thoracic trauma, surgical approaches, and hematologic diagnostics.

Anatomical Structure: The Three-Part Breastbone

The sternum, or breastbone, is a flat, elongated bone that forms the middle anterior portion of the thoracic cage, acting as its keystone. It develops in three distinct segments that fuse over time: the manubrium, the body, and the xiphoid process.

The manubrium is the broad, superior segment. Its concave superior border, the jugular (suprasternal) notch, is easily palpable. On either side, you find the clavicular notches for articulation with the clavicles, forming the sternoclavicular joints. The first pair of costal cartilages also articulates directly with the lateral edges of the manubrium.

Inferior to the manubrium lies the body (or gladiolus), the longest part of the sternum. Its lateral borders have articular facets for the costal cartilages of ribs 2 through 7. The body connects to the manubrium at the manubriosternal joint, a secondary cartylaginous joint (symphysis).

The most inferior and variable part is the xiphoid process. This small, cartilaginous projection ossifies later in life and provides attachment for important abdominal muscles, including the linea alba and the diaphragm. It is a crucial landmark for procedures like CPR, as improper hand placement can lead to its fracture and life-threatening visceral injury.

Key Articulations and the Sternal Angle

The sternum’s primary function is to provide a stable anterior attachment point for the thoracic skeleton. The costal cartilages of the true ribs (1-7) connect directly to the sternum. Ribs 8-10 (false ribs) attach to the costal cartilage of the rib above, and ribs 11-12 (floating ribs) have no anterior attachment. This arrangement creates the protective thoracic cage that houses the heart, lungs, and great vessels.

The most significant external landmark on the sternum is the sternal angle (Angle of Louis). This palpable transverse ridge is formed by the anterior projection of the manubriosternal joint. It is an indispensable clinical compass because it reliably marks the anatomical level of:

• The articulation of the second costal cartilage, providing the starting point for counting ribs and intercostal spaces during examination.
• The bifurcation of the trachea into the right and left main bronchi.
• The superior border of the aortic arch, where it transitions into the descending thoracic aorta.
• The approximate plane between the superior and inferior mediastinum.

Consider this clinical vignette: A patient presents with a persistent cough. On auscultation, you hear wheezing that is louder on the right side. Palpating the sternal angle gives you a confident starting point to localize your examination to the right main bronchus, guiding your subsequent diagnostic reasoning.

Clinical Procedures: The Sternum as a Portal

The sternum’s superficial and accessible location, coupled with its composition of cancellous (spongy) bone rich in red marrow, makes it a prime site for essential clinical interventions.

A bone marrow biopsy and aspiration is frequently performed at the sternum, specifically on the manubrium or the body. This procedure is critical for diagnosing hematologic conditions like leukemia, lymphoma, or myelodysplastic syndromes. The clinician inserts a specialized needle (e.g., a Jamshidi needle) through the outer cortical bone to extract a sample of marrow. The sternum is chosen because it is easily accessible with the patient supine, and its thin anterior cortex allows for relatively straightforward penetration while the posterior cortex protects underlying structures. However, extreme caution is required to avoid plunging through the bone and injuring the heart or great vessels.

In cardiothoracic surgery, access to the heart and mediastinum is achieved via a median sternotomy. This involves a vertical incision along the midline and splitting the sternum with a specialized saw. This "open-heart surgery" approach provides excellent exposure for coronary artery bypass grafting (CABG), valve replacements, and heart transplants. Post-operative stability relies on the sternum being securely rewired with stainless steel sutures to allow for proper healing.

Trauma, Pathology, and Assessment

Given its prominent position, the sternum is vulnerable to injury. Sternal fractures most commonly result from blunt chest trauma, such as a steering wheel impact in a car accident. While isolated sternal fractures can be managed conservatively with pain control, their real significance lies in the potential for associated injuries. The transmitted force can cause myocardial contusion, pulmonary contusion, or aortic injury. Your clinical priority is never just the fractured bone itself but a thorough evaluation for these life-threatening internal complications.

A flail chest is a severe, life-threatening condition that often involves the sternum. It occurs when a segment of the rib cage—typically involving multiple adjacent ribs fractured in two places—becomes detached. If this segment includes the sternal attachments, the detached "flail segment" exhibits paradoxical movement: it moves inward during inspiration and outward during expiration. This severely impairs ventilation and requires urgent respiratory support.

Developmental variations also occur. A pectus excavatum ("funnel chest") is a congenital deformity where the sternum, particularly the body and xiphoid process, is depressed inward. Conversely, pectus carinatum ("pigeon chest") involves a protrusion of the sternum. These can have cosmetic and, in severe cases, cardiopulmonary implications.

Common Pitfalls

1. Misidentifying Ribs: Starting a rib count from the clavicle or the first palpable rib (which is often the second) is a common error. Correction: Always begin your count at the sternal angle, the unambiguous landmark for the second rib. Move laterally and inferiorly from this point to accurately number ribs and intercostal spaces.

1. Neglecting Associated Injuries: Treating a sternal fracture as an isolated orthopedic injury is dangerous. Correction: In any case of sternal trauma, actively rule out associated myocardial contusion (with ECG and cardiac enzymes), pulmonary injury (with imaging and oxygenation assessment), and great vessel trauma.

1. Improper CPR Hand Placement: Placing the heel of the hand too low on the sternum during CPR compressions increases the risk of xiphoid process fracture and laceration of the liver. Correction: Place the heel of the dominant hand on the lower half of the sternal body, roughly in line with the nipples, ensuring the xiphoid process is clear.

1. Overlooking Sternal Angle Landmarks: Forgetting the multiple structures transected by the sternal angle plane limits diagnostic utility. Correction: Use the mnemonic "RAT B" to remember: Rib 2, Aortic arch, Tracheal bifurcation, and the boundary of mediastinal divisions.

Summary

• The sternum is the keystone of the anterior thoracic cage, composed of three parts: the manubrium, body, and xiphoid process.
• The sternal angle at the manubriosternal joint is a critical landmark, indicating the level of the second rib, tracheal bifurcation, and aortic arch.
• Its cancellous bone makes it a primary site for bone marrow biopsy, a key diagnostic procedure for hematologic diseases.
• The sternum is a surgical gateway via median sternotomy and is vulnerable to traumatic fractures that mandate evaluation for associated cardiac and pulmonary injuries.
• Accurate knowledge of sternal anatomy is essential for safe and effective physical examination, procedural skill, and clinical management of thoracic conditions.