Clavicle: Joints, Ligaments, Muscles, and Functional Significance

What joins the clavicle?

The clavicle, or collarbone, is a crucial S-shaped bone that serves as the sole bony connection between the upper limb and the axial skeleton. It articulates with the sternum medially and the scapula laterally, supported by a complex network of ligaments and serving as an attachment point for several vital muscles.

Introduction to the Clavicle

The clavicle is a long, slender bone positioned horizontally at the base of the neck, forming the anterior portion of the shoulder girdle. Its unique S-shape allows it to function as a strut, holding the scapula and upper limb away from the trunk, thereby maximizing the range of motion of the arm. It also plays a significant role in transmitting forces from the upper limb to the axial skeleton and protecting underlying neurovascular structures.

Joints of the Clavicle

The clavicle forms two primary synovial joints, each critical for shoulder function and stability:

• Sternoclavicular (SC) Joint: This highly mobile joint is formed by the articulation of the medial (sternal) end of the clavicle with the manubrium of the sternum and the first costal cartilage. It is classified as a synovial saddle joint, allowing for elevation, depression, protraction, retraction, and circumduction of the clavicle. The SC joint is the only direct bony connection between the entire upper limb and the axial skeleton, making it fundamental for transferring forces from the arm to the trunk.
• Acromioclavicular (AC) Joint: Located at the lateral aspect of the shoulder, this joint is formed by the articulation of the lateral (acromial) end of the clavicle with the acromion process of the scapula. It is classified as a synovial plane joint, allowing for subtle gliding movements that facilitate scapular rotation and adjust the position of the scapula relative to the clavicle during arm movements.

Ligamentous Support of the Clavicle

The stability of the clavicle's articulations is heavily reliant on a robust network of ligaments. These strong fibrous bands connect bones, reinforcing the joints and limiting excessive movement.

• Ligaments of the Sternoclavicular (SC) Joint:

  • Anterior and Posterior Sternoclavicular Ligaments: These reinforce the joint capsule anteriorly and posteriorly, limiting anterior and posterior displacement of the clavicle.
  • Interclavicular Ligament: Spans the jugular notch, connecting the medial ends of both clavicles and attaching to the manubrium. It helps prevent superior displacement of the clavicle.
  • Costoclavicular Ligament: A strong, short ligament connecting the inferior surface of the medial clavicle to the first rib and its costal cartilage. It is a primary stabilizer of the SC joint, limiting elevation and excessive medial displacement.

• Ligaments of the Acromioclavicular (AC) Joint:

  • Acromioclavicular Ligament: Reinforces the AC joint capsule superiorly and inferiorly, helping to keep the acromion and clavicle in apposition.

• Coracoclavicular Ligaments: While technically not part of the AC joint capsule, these powerful ligaments are crucial for its stability and the overall integrity of the shoulder girdle. They connect the coracoid process of the scapula to the inferior surface of the lateral clavicle. They are composed of two distinct parts:

  • Conoid Ligament: Medial and conical, it primarily resists superior displacement of the clavicle relative to the scapula.
  • Trapezoid Ligament: Lateral and flat, it resists posterior and medial displacement of the clavicle relative to the scapula.

Muscular Attachments to the Clavicle

Beyond its bony and ligamentous connections, the clavicle serves as an important attachment site for several muscles that contribute to shoulder movement, neck stability, and respiration.

• Muscles Originating from the Clavicle:

  • Pectoralis Major (Clavicular Head): Originates from the medial half of the clavicle, contributing to shoulder flexion, adduction, and internal rotation.
  • Deltoid (Anterior Fibers): Originates from the lateral third of the clavicle, primarily responsible for shoulder flexion and internal rotation.
  • Sternocleidomastoid: The sternal head originates from the manubrium, and the clavicular head originates from the medial third of the clavicle. Both insert on the mastoid process and superior nuchal line, contributing to neck flexion, rotation, and lateral flexion.

• Muscles Inserting onto the Clavicle:

  • Trapezius (Upper Fibers): Inserts onto the lateral third of the clavicle and acromion. It elevates, retracts, and rotates the scapula, and extends the neck.
  • Subclavius: A small muscle located inferior to the clavicle, originating from the first rib and inserting onto the inferior surface of the clavicle. It depresses the clavicle and stabilizes the SC joint.
  • Sternohyoid: A strap muscle of the neck that originates from the sternum and medial clavicle, inserting onto the hyoid bone. It depresses the hyoid.

Functional Significance of the Clavicle and its Connections

The intricate connections of the clavicle are vital for the biomechanics of the upper limb. It acts as a pivot, allowing the scapula to move freely across the posterior thoracic wall, thereby increasing the arm's range of motion. It also functions as a protective barrier for the neurovascular bundle (brachial plexus and subclavian vessels) passing into the upper limb. Its connections allow it to dissipate forces transmitted from the arm, protecting the delicate structures of the shoulder and neck.

Clinical Relevance and Common Injuries

Given its exposed position and critical role, the clavicle is susceptible to injury.

• Clavicle Fractures: These are among the most common fractures, often resulting from a fall onto the outstretched hand, direct trauma to the shoulder, or a fall directly onto the shoulder. The middle third of the clavicle is the most common site of fracture due to its curvature and lack of muscular support.
• Acromioclavicular (AC) Joint Separations: Often referred to as "shoulder separations," these injuries involve damage to the AC and/or coracoclavicular ligaments. They are common in contact sports and falls, leading to pain, swelling, and sometimes a visible deformity ("step-off" deformity).
• Sternoclavicular (SC) Joint Dislocations: Less common than AC joint injuries but potentially more serious due to proximity to vital structures (trachea, esophagus, major vessels). These usually result from high-energy trauma and can be anterior or posterior.

Conclusion

The clavicle, though seemingly simple, is a highly integrated component of the shoulder girdle. Its connections with the sternum and scapula via the sternoclavicular and acromioclavicular joints, reinforced by a complex web of ligaments, are indispensable for the remarkable mobility and stability of the human upper limb. Understanding these anatomical relationships is fundamental for comprehending shoulder function, diagnosing injuries, and designing effective rehabilitation strategies.