The sternum, commonly known as the breastbone, is a long, flat bone situated at the center of the chest. It serves as the front anchor for the rib cage, offering substantial protection to the organs housed within the thoracic cavity. Breaking this bone is generally an infrequent occurrence that requires a high-magnitude force due to the sternum’s robust structure.
The Sternum’s Design and Protective Role
The sternum is composed of three distinct segments: the manubrium at the top, the body in the middle, and the small, cartilaginous xiphoid process at the bottom. These components are joined together, offering a degree of flexibility. The sternum articulates with the ribs via the costal cartilage, which acts as a shock absorber for the chest cage.
This structure forms a protective shield for vital internal organs. Its primary function is to guard the heart, the great vessels like the aorta, and the lungs from blunt force trauma. The combined rigidity of the bone and the elasticity of the cartilage allows the chest wall to absorb and dissipate considerable energy from an impact before the bone yields.
Biomechanical Forces Required for Fracture
To cause a fracture in a healthy adult sternum, a compressive or blunt anterior force of significant magnitude is necessary. Biomechanical studies suggest that a substantial dynamic force, often exceeding 4,000 Newtons (approximately 900 pounds of force), is required to cause structural failure within the thoracic cage. The mechanism of injury is usually a rapid deceleration or a highly focused impact that overwhelms the bone’s tensile strength.
Because the sternum is a strong, dense cortical bone, a force distributed over a large area is often absorbed without fracture. However, when the force is concentrated over a small surface area, the localized stress rapidly exceeds the bone’s capacity to withstand the load, leading to a fracture.
Common Scenarios Leading to Sternal Trauma
The most frequent scenario generating the high forces necessary to fracture the sternum is a motor vehicle collision (MVC), which accounts for the majority of these injuries. During a frontal crash, the rapid deceleration causes the chest to strike the steering wheel or dashboard, imparting immense energy to the breastbone. Even life-saving seatbelts, particularly the shoulder restraint, can cause a sternal fracture due to the concentrated compression force applied across the chest during a sudden stop.
Sternal fractures are also a recognized complication of Cardiopulmonary Resuscitation (CPR). The focused, repeated manual compression of the chest required to circulate blood can generate enough localized force to break the sternum, especially in individuals with lower bone density. Other high-energy events, such as falls from significant heights or high-impact sports injuries, can also generate the necessary blunt trauma.
Recognizing and Managing a Sternal Fracture
The most immediate symptom of a sternal fracture is severe, localized pain directly over the breastbone, accompanied by tenderness and swelling at the injury site. This pain is worsened by movements of the chest, particularly deep breaths, coughing, or sneezing, as the sternum moves continuously during respiration.
A sternal fracture requires prompt medical attention due to the high risk of associated internal injuries. The force needed to break the sternum is sufficient to cause underlying injuries to the heart or lungs, such as pulmonary or cardiac contusions. Management of an uncomplicated sternal fracture typically involves rest and pain control to ensure the patient can breathe deeply and fully, which helps prevent complications like pneumonia. The fracture itself usually heals spontaneously over several weeks without the need for surgery.