Sternoclavicular Joint: Movement, Anatomy, and Clinical Significance

How Does the Sternoclavicular Joint Move?

The sternoclavicular (SC) joint, a unique saddle-shaped synovial joint, facilitates movement of the clavicle in three planes, enabling the wide range of motion essential for the entire shoulder girdle and upper limb.

Understanding the Sternoclavicular (SC) Joint

The sternoclavicular (SC) joint is a critical articulation, serving as the sole bony connection between the upper limb and the axial skeleton. It is formed by the medial end of the clavicle and the manubrium of the sternum (the superior part of the breastbone), along with a small portion of the first costal cartilage. Despite its small size, its mobility is paramount for the overall function and positioning of the shoulder.

Key Anatomical Features:

• Articular Disc: A fibrocartilaginous disc is interposed between the articulating surfaces. This disc divides the joint into two separate synovial cavities, enhancing congruence, distributing forces, and allowing for complex movements.
• Joint Capsule: A fibrous capsule encloses the joint, providing stability.
• Ligaments: Several strong ligaments reinforce the joint capsule and limit excessive motion:
  • Anterior and Posterior Sternoclavicular Ligaments: Strengthen the capsule anteriorly and posteriorly, limiting protraction/retraction and anterior/posterior displacement.
  • Interclavicular Ligament: Connects the medial ends of both clavicles across the top of the manubrium, limiting excessive depression of the clavicles.
  • Costoclavicular Ligament: An extremely strong ligament connecting the inferior aspect of the medial clavicle to the first rib and its cartilage. This ligament is a primary stabilizer, limiting elevation, protraction, and medial displacement of the clavicle.

Degrees of Freedom and Movement Types

The SC joint is classified as a saddle joint, which typically allows two degrees of freedom. However, due to the presence of the articular disc and the unique configuration of its articulating surfaces, it effectively functions as a multi-axial joint with three degrees of freedom, enabling movement in all three cardinal planes.

Primary Movements of the Clavicle at the SC Joint:

1. Elevation and Depression: Movement in the frontal (coronal) plane.
2. Protraction and Retraction: Movement in the horizontal (transverse) plane.
3. Anterior and Posterior Rotation: Rotation along the longitudinal axis of the clavicle.

Detailed Description of SC Joint Movements

Each movement at the SC joint contributes to the overall mobility of the shoulder girdle and is often coupled with movements at the acromioclavicular (AC) joint and the glenohumeral joint.

Elevation and Depression

• Description:
  • Elevation: The superior movement of the clavicle, raising the shoulder girdle.
  • Depression: The inferior movement of the clavicle, lowering the shoulder girdle.
• Axis of Motion: Occurs predominantly around an oblique anteroposterior axis that passes through the costoclavicular ligament.
• Range of Motion (ROM):
  • Elevation: Approximately 30-45 degrees.
  • Depression: Approximately 10-15 degrees.
• Kinematics: During elevation, the medial end of the clavicle glides inferiorly on the sternum. During depression, it glides superiorly. The costoclavicular ligament becomes taut during elevation, limiting further upward movement.

Protraction and Retraction

• Description:
  • Protraction: The anterior movement of the clavicle, moving the shoulder forward.
  • Retraction: The posterior movement of the clavicle, moving the shoulder backward.
• Axis of Motion: Occurs predominantly around a vertical axis passing through the medial end of the clavicle.
• Range of Motion (ROM): Approximately 15-30 degrees in each direction.
• Kinematics: During protraction, the medial end of the clavicle glides anteriorly on the sternum. During retraction, it glides posteriorly. The anterior and posterior sternoclavicular ligaments limit these movements.

Anterior and Posterior Rotation

• Description: This movement refers to the rotation of the clavicle along its own longitudinal axis, a critical motion for full upper limb elevation.
  • Posterior Rotation: The superior surface of the clavicle rotates posteriorly (upward rotation of the distal end).
  • Anterior Rotation: The superior surface of the clavicle rotates anteriorly (downward rotation of the distal end).
• Axis of Motion: Occurs around the longitudinal axis of the clavicle itself.
• Range of Motion (ROM): Approximately 20-30 degrees.
• Kinematics: Posterior rotation of the clavicle occurs automatically and passively during shoulder flexion and abduction (e.g., raising your arm overhead). This rotation is essential for allowing the scapula to fully upwardly rotate, preventing impingement. Anterior rotation is the reverse movement as the arm returns to the anatomical position. The costoclavicular ligament plays a crucial role in limiting excessive posterior rotation.

Clinical Significance and Functional Role

The SC joint's comprehensive range of motion is fundamental to the overall mobility and stability of the shoulder girdle. It acts as a pivot point for the entire upper limb, enabling the scapula (shoulder blade) to move freely across the posterior thorax, which is crucial for:

• Scapulohumeral Rhythm: The coordinated movement of the scapula and humerus (upper arm bone) during shoulder elevation. SC joint movements are synchronized with AC joint and glenohumeral joint movements to achieve full range of motion without impingement. For instance, during full arm elevation, the clavicle elevates and posteriorly rotates at the SC joint, simultaneously contributing to scapular upward rotation.
• Force Transmission: The SC joint transmits forces from the upper limb to the axial skeleton, absorbing shock and distributing loads.
• Optimal Muscle Function: Proper SC joint movement ensures that the muscles acting on the scapula and humerus can function efficiently across their full length-tension relationships.

Given its vital role and unique biomechanics, injuries to the SC joint, though less common than other shoulder injuries, can significantly impair upper limb function and require careful clinical attention.

Conclusion

The sternoclavicular joint, while often overlooked, is a marvel of anatomical engineering. Its unique saddle-joint structure, reinforced by a critical articular disc and strong ligaments, grants it three degrees of freedom, allowing for elevation/depression, protraction/retraction, and anterior/posterior rotation of the clavicle. These intricate movements are indispensable for the coordinated function of the entire shoulder girdle, facilitating the wide range of motion required for daily activities, athletic performance, and overall upper limb health. Understanding its mechanics is foundational for anyone involved in exercise science, rehabilitation, or human movement.