Manubriosternal Joint: Anatomy, Function, and Clinical Significance

What is the joint between the manubrium and the sternum?

The joint between the manubrium and the body of the sternum is known as the manubriosternal joint, also commonly referred to as the sternal angle or the Angle of Louis. It is classified as a secondary cartilaginous joint, or symphysis, characterized by a fibrocartilaginous disc uniting the two bony segments.

Anatomy of the Sternum and Its Articulations

The sternum, or breastbone, is a flat bone located in the central part of the chest. It comprises three main parts:

• Manubrium: The broad, superior portion that articulates with the clavicles (collarbones) and the first two pairs of ribs.
• Body (Gladiolus): The elongated, central part of the sternum, articulating with ribs 2 through 7.
• Xiphoid Process: The small, inferior projection that serves as an attachment point for various muscles.

The joint in question specifically connects the inferior border of the manubrium with the superior border of the sternal body.

The Manubriosternal Joint Defined

The manubriosternal joint (MSJ) is a critical anatomical landmark and a fascinating example of a joint with limited, yet vital, movement.

• Formal Name: Manubriosternal joint.
• Common Synonyms: Sternal angle, Angle of Louis (named after Antoine Louis, a French surgeon).
• Joint Type: It is a secondary cartilaginous joint, also known as a symphysis. This means that the articulating surfaces of the manubrium and sternal body are covered by hyaline cartilage, but they are united by a strong disc of fibrocartilage. In some older individuals, this joint may even fuse completely, forming a synostosis.
• Movement: While not a synovial joint designed for extensive movement, the manubriosternal joint allows for slight angular movement. This subtle hinge-like motion is crucial during respiration.

Anatomical and Clinical Significance

The manubriosternal joint is an indispensable anatomical landmark for both healthcare professionals and those studying human anatomy and physiology. Its prominence makes it easily palpable and useful for various clinical assessments.

• Palpation: You can typically feel the sternal angle as a distinct ridge or projection approximately 5 cm below the jugular notch (the depression at the top of the sternum).
• Second Rib Attachment: A key clinical significance is that the second costal cartilage (from the second rib) consistently articulates with the sternum precisely at the level of the manubriosternal joint. This allows clinicians to accurately count ribs, as the first rib is often obscured by the clavicle.
• Mediastinal Division: Internally, the sternal angle marks the transverse plane that divides the superior mediastinum from the inferior mediastinum.
• Great Vessels: It lies at the level where the arch of the aorta begins and ends, and where the trachea bifurcates (splits) into the right and left main bronchi.
• Clinical Assessment: Its palpability aids in diagnosing conditions like costochondritis (inflammation of the costal cartilages) and assessing chest wall symmetry.

Biomechanical Role in Respiration

Despite its limited range of motion, the manubriosternal joint plays a vital biomechanical role in the mechanics of breathing.

• "Pump-Handle" Motion: During inspiration (inhalation), the sternum elevates and moves anteriorly (forward), increasing the anteroposterior diameter of the thoracic cavity. This motion is often described as a "pump-handle" action. The slight hinge-like movement at the manubriosternal joint facilitates this elevation and allows the sternum to move as a unit, contributing to efficient lung expansion.
• Force Distribution: It helps distribute the forces exerted by the ribs and respiratory muscles, ensuring smooth and coordinated movement of the rib cage during the respiratory cycle.

Clinical Considerations and Conditions

While robust, the manubriosternal joint can be subject to various conditions, particularly in the context of trauma, inflammation, or degenerative changes.

• Trauma: Direct impact to the sternum can cause dislocation or fracture at the manubriosternal joint, though these are relatively uncommon due to its strong ligamentous support.
• Inflammation (Costochondritis/Tietze's Syndrome): Inflammation of the cartilage at or near the sternal angle can cause localized chest pain, often exacerbated by movement or pressure. While costochondritis typically affects multiple costochondral junctions, Tietze's syndrome specifically involves swelling and inflammation, often at a single joint, including the manubriosternal joint.
• Degenerative Changes: Like other cartilaginous joints, the fibrocartilage disc can undergo degenerative changes (osteoarthritis) with age, potentially leading to pain and reduced flexibility.
• Ankylosing Spondylitis: In systemic inflammatory conditions like ankylosing spondylitis, the manubriosternal joint can be affected, leading to pain, stiffness, and in severe cases, fusion (ankylosis) of the joint, which can further restrict chest wall expansion and breathing.

Conclusion

The joint between the manubrium and the sternum, known as the manubriosternal joint or sternal angle, is far more than just a connection point. As a secondary cartilaginous joint, it provides a subtle yet essential degree of flexibility that is critical for the biomechanics of respiration. Furthermore, its consistent anatomical location makes it an indispensable landmark for identifying other structures and for clinical assessment, underscoring its importance in both exercise science and healthcare. Understanding this joint enhances our appreciation of the intricate design and function of the human musculoskeletal system.