The sternum, commonly known as the breastbone, is a flat, elongated bone positioned centrally on the anterior side of the chest wall. It serves as a significant component of the human skeletal system, running vertically down the midline of the thorax. This bone provides a central anchor point for the rib cage, forming the front attachment for the majority of the ribs via costal cartilage. Its placement provides both stability and defense for the upper torso.
Anatomical Components
The sternum is not a single, uniform bone but is composed of three distinct segments that fuse together over time. The most superior portion is the manubrium, a roughly trapezoidal structure that connects to the collarbones, forming the sternoclavicular joints. It also articulates with the costal cartilages of the first pair of ribs and a portion of the second pair.
Below the manubrium lies the body, or gladiolus, which is the longest and largest segment of the sternum. The body connects to the costal cartilages of the third through the seventh pairs of ribs along its lateral edges.
The junction between the manubrium and the body forms a slight forward projection called the sternal angle, or the angle of Louis. This angle is an important anatomical landmark because it corresponds precisely to the level of the second rib, which helps healthcare professionals accurately count the ribs during a physical examination. The smallest and most inferior segment is the xiphoid process, a pointed, triangular structure that is largely cartilaginous in youth. This process begins to ossify, or harden into bone, around the age of 40, and its shape can vary significantly among individuals.
Core Functions
The primary function of the sternum is to act as a substantial shield for the organs located directly behind it within the thoracic cavity. Working in conjunction with the ribs, it forms a protective barrier that guards the heart and the major blood vessels of the mediastinum from external physical trauma.
The sternum provides structural support for the entire upper chest and shoulder girdle. It serves as the attachment point for the clavicles and the first seven pairs of ribs, forming the rigid yet flexible structure of the rib cage. This central connection is instrumental in stabilizing the chest wall, which is necessary for proper posture and movement.
The sternum’s connections to the rib cage also play a role in the mechanics of breathing. While the sternum itself has limited movement, the cartilage connecting the ribs allows for the necessary expansion and contraction of the chest wall during respiration. Furthermore, the sternum and the xiphoid process serve as attachment sites for several important muscles, including the pectoralis major and the diaphragm, which contribute to both arm movement and abdominal stability.
Medical Relevance
The sternum holds considerable importance in clinical medicine, particularly in emergency and surgical settings. Its location on the anterior chest makes it the target for life-saving compressions during cardiopulmonary resuscitation. The firm, flat surface of the body of the sternum allows rescuers to apply the force needed to manually circulate blood in a person experiencing cardiac arrest.
Sternal fractures are a common injury, resulting from high-impact trauma such as motor vehicle accidents where the chest strikes the steering wheel or dashboard. Because the sternum protects the vital organs, a fracture often warrants immediate investigation for potential underlying damage to the heart or lungs. In surgical practice, the sternum is the site of a procedure called a sternotomy, where it is vertically divided to allow direct access to the heart, lungs, and great vessels for operations like coronary artery bypass surgery.
The sternum is a site for bone marrow aspiration, a diagnostic procedure, due to the high concentration of red bone marrow it contains. Although the posterior iliac crest is generally the preferred site today, sternal aspiration is still used in certain clinical circumstances when other sites are inaccessible. This procedure requires precision because the sternum is a relatively thin bone, and incorrect technique carries a risk of injury to structures like the ascending aorta or the heart itself.