Sternal Articular Facets: Articulations, Types, and Clinical Significance

Which structures do the sternal articular facets articulate with?

The sternal articular facets, found along the lateral borders of the sternum (breastbone), primarily articulate with the costal cartilages of the true ribs (ribs 1-7) to form the sternocostal joints, crucial for thoracic cage stability and respiratory mechanics.

Understanding the Sternal Articular Facets

The sternum, a flat, dagger-shaped bone located in the central part of the chest, serves as an anchor point for the ribs via their costal cartilages. Along its lateral borders, the sternum features specific depressions or indentations known as articular facets. These concave surfaces are precisely shaped to receive the ends of the costal cartilages, forming strong yet slightly mobile connections.

The sternum is anatomically divided into three main parts:

• Manubrium: The superior, broadest part, articulating with the clavicles and the first two pairs of ribs.
• Body (Gladiolus): The longest central part, articulating with ribs 2 through 7.
• Xiphoid Process: The smallest, most inferior part, typically cartilaginous in youth and ossifying with age, generally not involved in direct rib articulation.

The sternal articular facets are therefore found predominantly on the manubrium and the body of the sternum.

The Articulating Structures: Costal Cartilages

The structures that directly articulate with the sternal articular facets are the costal cartilages. These are bars of hyaline cartilage that extend from the anterior ends of the ribs, connecting them to the sternum. They play a vital role in providing elasticity to the thoracic cage, allowing for the movements required during respiration.

Specificity of Rib Articulation:

• True Ribs (Ribs 1-7): These ribs articulate directly with the sternum via their own costal cartilages.
• False Ribs (Ribs 8-10): Their costal cartilages articulate indirectly by joining the cartilage of the rib above (e.g., rib 8 joins rib 7's cartilage).
• Floating Ribs (Ribs 11-12): These ribs do not articulate with the sternum at all; their cartilages terminate in the abdominal musculature.

Therefore, the sternal articular facets are specifically designed for articulation with the costal cartilages of the true ribs.

Anatomical Detail of Sternocostal Articulations

The nature of the articulation between the sternum and the costal cartilages varies slightly depending on the rib number:

• First Rib: The costal cartilage of the first rib articulates with the manubrium only. This joint is unique; it is a primary cartilaginous joint (synchondrosis). This means there is no synovial capsule or fluid, making it a very stable, largely immobile joint. Its immobility is crucial for supporting the pectoral girdle and protecting vital structures.

• Second Rib: The costal cartilage of the second rib articulates at the sternal angle (Angle of Louis), the junction between the manubrium and the body of the sternum. This articulation involves facets on both the manubrium and the sternal body. Unlike the first rib, the joint for the second rib is typically a synovial plane joint, allowing for slight gliding movements essential for respiration. The sternal angle is a clinically important landmark for locating the second rib and counting ribs.

• Ribs 3-7: The costal cartilages of these ribs articulate with the lateral borders of the sternal body. These articulations are also synovial plane joints, allowing for controlled gliding and sliding movements. Each costal cartilage fits into a specific sternal facet, forming a distinct sternocostal joint. The joint capsules are reinforced by sternocostal ligaments.

Functional Significance in Respiration

The articulations between the sternal facets and the costal cartilages are fundamental to the mechanics of breathing. While seemingly small, the slight movements permitted by these synovial joints (for ribs 2-7) are crucial for increasing and decreasing the volume of the thoracic cavity during inspiration and expiration:

• Pump-Handle Movement: Primarily involves the sternum and the upper ribs. During inspiration, the sternum moves superiorly and anteriorly, increasing the anterior-posterior diameter of the chest. This movement is facilitated by the sternocostal joints.
• Bucket-Handle Movement: Primarily involves the middle and lower ribs. During inspiration, the ribs elevate and move laterally, increasing the transverse diameter of the chest.

These combined movements allow the lungs to expand and contract, facilitating air exchange. The stability provided by the first sternocostal synchondrosis ensures a fixed point for the more mobile ribs, optimizing the efficiency of respiratory muscle action.

Clinical Relevance and Considerations

Understanding the sternal articular facets and their articulations is vital in clinical practice:

• Costochondritis: Inflammation of the costal cartilages, particularly at the sternocostal joints, is a common cause of chest pain. It can mimic cardiac pain and requires careful differential diagnosis.
• Tietze's Syndrome: A less common condition characterized by painful swelling of one or more costal cartilages, often affecting the second or third sternocostal joint.
• Trauma: Injuries to the sternum or ribs can affect these articulations, leading to pain, instability, or difficulty breathing.
• Physical Assessment: Palpation of the sternal angle helps clinicians accurately count ribs and locate specific intercostal spaces for procedures like listening to heart sounds or performing thoracentesis.

In conclusion, the sternal articular facets articulate with the costal cartilages of the true ribs (1-7), forming a series of sternocostal joints that are indispensable for the structural integrity of the thoracic cage and the dynamic process of respiration.