What is the structure and function of the sternum. How does the sternum develop embryologically. Which muscles attach to the sternum. What is the clinical significance of sternal anatomy.

Sternum Anatomy: Structure and Function

The sternum, a crucial component of the anterior chest wall, is a partially T-shaped vertical bone divided into three distinct segments: the manubrium, body, and xiphoid process. This bony structure plays a vital role in protecting the underlying mediastinum and its contents from injury.

Manubrium

The manubrium, the broad superior segment of the sternum, is quadrangular in shape with four borders. Its key features include:

• The suprasternal notch (jugular notch) at the superior segment
• Left and right clavicular notches on either side
• Articulation with the medial end of each clavicle, forming the sternoclavicular joints
• Articulation with the costal cartilages of the first pair of ribs

Body of the Sternum

The body, or mesosternum, is the longest part of the sternum. Its characteristics include:

• A flat surface with depressed ridges along the sides
• Articulation points for the costal cartilages of the 3rd to 7th pairs of ribs
• Junction with the manubrium at the sternal angle, where the 2nd pair of ribs attaches

Xiphoid Process

The xiphoid process, or xiphisternum, forms the distal-most part of the sternum. Its notable features are:

• Triangular shape with highly variable size and form
• Primarily cartilaginous until age 40, becoming fully calcified by age 60
• Significant variability in age-related changes among adults

Embryological Development of the Sternum

The sternum’s development is a fascinating process that begins early in fetal life. How does this complex structure form?

The sternum develops from a pair of sternal bands called “sternal bars,” which are concentrations of mesenchymal cells on either side of the midline. This process initiates during the 6th week of fetal development. By the 10th week, these sternal bands, arising from the parietal layer of the lateral plate mesoderm, transform into pre-cartilaginous structures. These structures then migrate and fuse in a craniocaudal direction, forming the sternal plate.

The primary cartilaginous model of the three sternal segments forms in the 7th week of intrauterine life. This model consists of six horizontal divisions known as sternebrae:

1. The superior-most sternebra becomes the manubrium sterni
2. The four middle sternebrae form the body of the sternum
3. The inferior-most sternebra develops into the xiphoid process

In the sequence of formation, the manubrium sterni is the first to develop, followed by the sternal body and finally the xiphoid process.

Vascular Supply of the Sternum

Understanding the blood supply of the sternum is crucial for surgical procedures and post-operative care. How is the sternum vascularized?

The primary blood supply to the sternum comes from the medial horizontal branches of the right and left internal thoracic arteries. These arteries typically originate directly from the first part of the subclavian arteries bilaterally, though they may occasionally stem from a common trunk.

The internal thoracic artery gives off several branches that contribute to sternal blood supply:

• Sternal branches
• Anterior intercostal branches
• Perforating branches
• Non-collateral branches

The sternal branches, which are the main suppliers of blood to the sternum, are primarily located in the intercostal spaces. However, the perforating branches also play a significant role in sternal blood supply.

For venous drainage, the internal thoracic veins channel blood from the sternum to the brachiocephalic vein on each side.

Muscles Attached to the Sternum

The sternum serves as an attachment point for various muscles of the neck, thorax, and anterior abdominal wall. Which specific muscles connect to this important bony structure?

Neck Muscles

• Sternocleidomastoid
• Sternohyoid
• Sternothyroid

Thoracic Muscles

• Pectoralis major
• Pectoralis minor
• Internal intercostal muscles
• Transversus thoracis

Anterior Abdominal Wall Muscles

• Rectus abdominis

These muscle attachments highlight the sternum’s importance in various bodily functions, including respiration, arm movement, and core stability.

Clinical Significance of Sternal Anatomy

The sternum’s anatomical position and variations make it a structure of significant clinical importance. How does understanding sternal anatomy impact medical procedures and patient care?

Surgical Considerations

Knowledge of sternal anatomy is crucial for various surgical procedures, including:

• Median sternotomy for cardiac surgeries
• Sternal wire placement in chest wall reconstruction
• Bone marrow aspiration from the sternum

The sternal angle, formed by the junction of the manubrium and body of the sternum, serves as an important anatomical landmark. It corresponds anteriorly to the intervertebral disc between the 4th and 5th thoracic vertebrae, guiding clinicians in various diagnostic and therapeutic procedures.

Post-operative Complications

Understanding the blood supply of the sternum is critical in preventing and managing post-operative complications, particularly:

• Sternal infections following coronary artery bypass grafting using the internal thoracic artery
• Delayed healing after sternotomy procedures

Forensic Applications

Sternal anatomy also has forensic implications. The sternum’s unique characteristics and age-related changes can aid in:

• Age estimation in forensic anthropology
• Sex determination in unidentified human remains

Variations and Anomalies of the Sternum

While the basic structure of the sternum is consistent across individuals, there can be significant variations and anomalies. What are some of the common variations in sternal anatomy?

Sternal Foramen

A sternal foramen is a congenital defect resulting in a hole in the body of the sternum. It occurs in approximately 4-8% of the population and can be clinically significant during procedures such as acupuncture or bone marrow aspiration.

Bifid Sternum

In rare cases, the sternum may fail to fuse completely during embryological development, resulting in a bifid (split) sternum. This condition can be associated with other congenital abnormalities and may require surgical correction.

Xiphoid Process Variations

The xiphoid process is particularly variable in its morphology. It may be:

• Pointed
• Bifid
• Perforated
• Deflected anteriorly or posteriorly

These variations are generally of little clinical significance but may be important to recognize during imaging studies or surgical procedures.

Sternal Fractures and Trauma

The sternum’s position in the anterior chest wall makes it vulnerable to trauma. How do sternal fractures occur, and what are their implications?

Mechanisms of Injury

Sternal fractures most commonly result from:

• Motor vehicle accidents (particularly when wearing a seatbelt)
• Direct blows to the chest
• Falls from height

Clinical Presentation

Patients with sternal fractures typically present with:

• Anterior chest pain
• Tenderness over the sternum
• Pain exacerbated by deep breathing or coughing

Associated Injuries

While isolated sternal fractures are often not life-threatening, they can be associated with more severe injuries, including:

• Cardiac contusion
• Pulmonary contusion
• Rib fractures
• Thoracic spine injuries

Therefore, patients with sternal fractures often require thorough evaluation for associated injuries.

The Sternum in Cardiopulmonary Resuscitation (CPR)

The sternum plays a crucial role in cardiopulmonary resuscitation. How does sternal anatomy influence CPR technique and effectiveness?

Chest Compression Location

Current CPR guidelines recommend placing the hands on the lower half of the sternum for chest compressions. This location allows for effective compression of the heart against the spine, maximizing blood flow during resuscitation efforts.

Depth of Compressions

The recommended depth for chest compressions in adults is at least 2 inches (5 cm) but not exceeding 2.4 inches (6 cm). The sternum’s structure and its connections to the ribs allow for this depth of compression without causing excessive damage to internal structures.

Potential Complications

While rare, CPR can sometimes result in sternal fractures or separation of costal cartilages. However, these complications are generally considered acceptable given the life-saving potential of CPR.

Understanding the anatomy and biomechanics of the sternum is crucial for healthcare providers performing CPR, as it helps ensure proper technique and maximizes the chances of successful resuscitation.