The Sternum Is Blank To The Vertebral Column

The Sternum's Connection (or Lack Thereof) to the Vertebral Column: Anatomy, Function, and Clinical Significance

The human sternum, commonly known as the breastbone, is a flat bone located in the anterior thoracic cage. A common misconception is that the sternum is directly connected to the vertebral column (spine). While not directly articulated, the sternum shares a complex and crucial relationship with the vertebral column through a system of interconnected structures and ligaments. Understanding this relationship is vital to comprehending the biomechanics of the thorax, respiratory function, and the clinical implications of injuries or pathologies affecting this region.

Anatomy of the Sternum and its Surroundings

The sternum itself is composed of three parts: the manubrium, body, and xiphoid process. The manubrium, the superior portion, articulates with the clavicles (collarbones) at the sternoclavicular joints and with the first two ribs. The body, the longest section, articulates with ribs 2-7 through costal cartilages. Finally, the xiphoid process, the smallest and most inferior part, is cartilaginous in youth and ossifies with age. It provides attachment points for certain abdominal muscles.

The vertebral column, or spine, is a flexible column of vertebrae extending from the skull to the pelvis. Its thoracic section, specifically, plays a pivotal role in the relationship with the sternum. The thoracic vertebrae articulate with the ribs, forming the posterior aspect of the rib cage. These ribs, in turn, connect anteriorly to the sternum, creating a protective cage around the heart and lungs. This complex network of articulations allows for both stability and flexibility.

Indirect Connections and Supporting Structures

The key to understanding the sternum's relationship with the vertebral column lies in recognizing the indirect connections. There is no direct bony articulation between the two. Instead, their functional unity is mediated by:

• Ribs: Twelve pairs of ribs form the major link. The first seven pairs (true ribs) connect directly to the sternum via costal cartilages. Ribs 8-10 (false ribs) connect indirectly to the sternum through the costal cartilages of the ribs above. Ribs 11 and 12 (floating ribs) lack any sternal connection. However, they are still attached to the vertebral column posteriorly. The ribs' articulation with both the thoracic vertebrae and the sternum creates a semi-rigid structure.

• Costal Cartilages: These hyaline cartilages extend from the anterior ends of the ribs to the sternum, providing a flexible yet strong connection. Their elasticity allows for the rib cage's expansion and contraction during respiration.

• Ligaments: Numerous ligaments contribute to the stability and integrity of the entire thoracic cage. For instance, the costovertebral ligaments connect the ribs to the vertebrae, and the sternocostal ligaments connect the sternum to the ribs. These ligaments reinforce the indirect connections between the sternum and vertebral column, preventing excessive movement and maintaining structural integrity.

• Muscles: Several muscles contribute to the interplay between the sternum and vertebral column. The intercostal muscles, located between the ribs, play a crucial role in respiration. The diaphragm, a dome-shaped muscle separating the thoracic and abdominal cavities, also significantly impacts the movement of the rib cage and consequently, the position of the sternum relative to the vertebral column.

Functional Implications of the Indirect Connection

The indirect connection between the sternum and vertebral column is not a limitation but a design feature optimized for several functions:

• Respiration: The flexible structure allows for expansion and contraction of the thoracic cavity during inhalation and exhalation. The movement of the ribs, facilitated by the intercostal muscles and diaphragm, causes the sternum to move slightly, increasing the lung volume. This mechanism is essential for breathing.

• Protection of Vital Organs: The rib cage, formed by the ribs, sternum, and vertebrae, provides crucial protection to the heart, lungs, and major blood vessels. The indirect connection ensures the stability of this protective cage.

• Postural Support: The entire thoracic cage, including the sternum, contributes to overall postural support. The connection, although indirect, ensures the stability and alignment of the upper body.

• Shock Absorption: The flexible nature of the costal cartilages and the intricate network of ligaments and muscles allows the rib cage to absorb shock and dissipate forces acting on the thorax. This helps to protect the vital organs within.

Clinical Significance of Sternum-Vertebral Column Relationship

Dysfunction or injury to any component of the sternum-vertebral column relationship can have significant clinical implications:

• Rib Fractures: Rib fractures are common, often resulting from trauma. They can compromise the integrity of the thoracic cage and affect respiratory function.

• Sternum Fractures: Sternum fractures are less common but can be serious, potentially causing damage to underlying structures like the heart or lungs.

• Costochondritis: This condition involves inflammation of the costal cartilages, causing pain in the chest. It can be linked to overuse, injury, or underlying medical conditions.

• Pectus Excavatum and Pectus Carinatum: These are congenital deformities of the chest wall that affect the sternum and rib cage. Pectus excavatum is a caved-in chest, while pectus carinatum is a protruding chest. These deformities can impact respiratory function and cardiac output.

• Scoliosis: This is a lateral curvature of the spine. Severe scoliosis can affect the alignment of the rib cage and sternum, leading to respiratory problems and chest pain.

• Kyphosis: This is an excessive curvature of the thoracic spine. It can lead to changes in the position and function of the sternum and rib cage.

• Spinal Injuries: Injuries to the thoracic vertebrae can affect the stability and movement of the rib cage and sternum, impacting respiratory function and potentially leading to significant complications.

Advanced Considerations and Further Research

The intricate biomechanics of the thorax are still an area of ongoing research. Advanced techniques like finite element analysis are used to model the stresses and strains on the sternum and rib cage during various activities. These models provide valuable insights into injury mechanisms and help design better protective equipment. Further research is needed to fully understand the complex interactions between the sternum, vertebral column, and surrounding structures, particularly in the context of aging and disease. A deeper understanding of the biomechanical properties of the costal cartilages and the influence of ligaments and muscles on overall stability is also crucial.

Conclusion

While the sternum is not directly attached to the vertebral column, their indirect connection through ribs, costal cartilages, ligaments, and muscles is essential for respiration, protection of vital organs, postural support, and shock absorption. Understanding this complex relationship is critical for clinicians diagnosing and treating injuries or pathologies affecting the thorax. Future research will continue to refine our understanding of the biomechanics of this crucial region, leading to improved diagnostic tools, treatment strategies, and preventative measures. The seamless interplay between the seemingly separate structures of the sternum and vertebral column highlights the elegance and efficiency of human anatomy.