Clavicle Joints: Sternoclavicular (SC) and Acromioclavicular (AC) Anatomy and Function

What is the name of the clavicle joint?

The clavicle, or collarbone, participates in two distinct and crucial joints: the sternoclavicular (SC) joint, where it articulates with the sternum, and the acromioclavicular (AC) joint, where it connects with the scapula.

The Clavicle: A Pivotal Bone in Shoulder Mechanics

The clavicle is a long, S-shaped bone that serves as the only bony connection between the upper limb and the axial skeleton. It acts as a strut, holding the scapula (shoulder blade) and arm away from the trunk, allowing for a wide range of motion. Beyond its structural role, it protects underlying neurovascular structures and transmits forces from the upper limb to the trunk. Its unique position and shape necessitate two primary articulations, each vital for comprehensive shoulder girdle function.

The Sternoclavicular (SC) Joint

The sternoclavicular joint is located at the medial (inner) end of the clavicle, where it meets the manubrium (upper part) of the sternum.

• Anatomy and Classification:

  • Articulating Surfaces: The sternal end of the clavicle articulates with the clavicular notch of the manubrium and the first costal cartilage.
  • Joint Type: Although anatomically classified as a saddle joint, its extensive range of motion and functional capabilities often lead it to be described as functionally a ball-and-socket joint. It features an articular disc that divides the joint into two separate synovial cavities, enhancing congruence and shock absorption.
  • Ligamentous Support:
    • Anterior and Posterior Sternoclavicular Ligaments: Reinforce the joint capsule anteriorly and posteriorly, preventing displacement.
    • Interclavicular Ligament: Connects the medial ends of both clavicles, running across the top of the manubrium, providing superior stability.
    • Costoclavicular Ligament: Extends from the first rib to the inferior surface of the clavicle, acting as a strong primary restraint against superior displacement and limiting clavicular elevation.

• Movements: The SC joint allows for significant movement, including:

  • Elevation and Depression: Upward and downward movement of the clavicle.
  • Protraction and Retraction: Forward and backward movement of the clavicle.
  • Axial Rotation: Rotation along its long axis, which is crucial for full arm elevation. These movements are inextricably linked to and accompany movements of the scapula.

The Acromioclavicular (AC) Joint

The acromioclavicular joint is located at the lateral (outer) end of the clavicle, where it articulates with the acromion process of the scapula.

• Anatomy and Classification:

  • Articulating Surfaces: The lateral end of the clavicle articulates with the medial border of the acromion.
  • Joint Type: Classified as a plane (gliding) synovial joint, allowing for limited gliding movements. It often contains an articular disc, though it may be incomplete or degenerate with age.
  • Ligamentous Support:
    • Acromioclavicular Ligaments (Superior and Inferior): Reinforce the joint capsule directly over the articulation.
    • Coracoclavicular Ligament: This is the primary stabilizer of the AC joint and consists of two distinct parts:
      • Trapezoid Ligament: Lateral and flatter, originating from the coracoid process and inserting onto the trapezoid line of the clavicle.
      • Conoid Ligament: Medial and conical, originating from the coracoid process and inserting onto the conoid tubercle of the clavicle. These ligaments prevent superior displacement of the clavicle relative to the acromion and limit excessive rotation.

• Movements: The AC joint permits subtle movements, primarily:

  • Gliding and Rotation: Small adjustments that allow the scapula to move relative to the clavicle, accommodating the shape of the thoracic cage during arm and shoulder movements. This includes upward/downward rotation, internal/external rotation, and anterior/posterior tilting of the scapula.

Functional Importance of Clavicular Joints

Both the SC and AC joints are indispensable for the full range of motion, stability, and force transmission of the upper limb.

• Shoulder Girdle Mobility: The synergy between the SC and AC joints allows the scapula to move in multiple planes, providing a mobile base for the glenohumeral (shoulder) joint. This combined motion facilitates overhead reaching, throwing, and pushing activities.
• Force Transmission: These joints are critical in transferring forces generated by the upper limb to the axial skeleton and vice versa, allowing for powerful movements and weight-bearing.
• Stability: While providing mobility, the intricate network of ligaments surrounding both joints ensures adequate stability, preventing excessive displacement and injury.

Common Issues and Considerations

Due to their critical roles and exposed positions, both clavicular joints can be susceptible to injury:

• AC Joint Separations: More common than SC joint injuries, these typically result from direct trauma to the shoulder (e.g., falls onto the point of the shoulder). They are graded based on the severity of ligamentous damage (AC ligaments, then coracoclavicular ligaments).
• SC Joint Dislocations: Less common but potentially serious due to proximity to vital structures (trachea, esophagus, major blood vessels). These usually result from high-impact trauma.
• Clavicle Fractures: The clavicle is one of the most frequently fractured bones, often due to a fall onto an outstretched hand or direct impact. Fractures can affect the stability and function of both the SC and AC joints indirectly.

Understanding the distinct anatomy and biomechanics of the sternoclavicular and acromioclavicular joints is fundamental for anyone involved in exercise science, rehabilitation, or sports performance. Their integrated function ensures the remarkable versatility and strength of the human shoulder complex.