Sternoclavicular Joint Injuries: Mechanisms, Types, and Management

What is the mechanism of injury for the sternoclavicular joint?

The sternoclavicular (SC) joint, a critical link between the axial skeleton and the upper limb, is primarily injured through direct or indirect high-energy trauma that imparts significant force to the shoulder girdle, often resulting in sprains or dislocations.

Anatomy and Function of the Sternoclavicular Joint

The sternoclavicular (SC) joint is a synovial saddle joint located between the medial end of the clavicle and the manubrium of the sternum. It is the only true articulation between the upper limb and the axial skeleton, making it pivotal for all movements of the shoulder girdle.

Key anatomical features contributing to its stability and function include:

• Articular Disc: A fibrocartilaginous disc that separates the joint into two compartments, enhancing congruence and acting as a shock absorber.
• Joint Capsule: A relatively thin but strong fibrous capsule enclosing the joint.
• Ligamentous Support:
  • Anterior and Posterior Sternoclavicular Ligaments: These are the primary stabilizers, preventing anterior and posterior displacement of the clavicle. The posterior ligament is often considered stronger and thicker.
  • Interclavicular Ligament: Connects the medial ends of both clavicles across the suprasternal notch, resisting excessive downward movement and lateral displacement.
  • Costoclavicular Ligament: A strong, short ligament connecting the first rib and its costal cartilage to the inferior surface of the clavicle. It is the strongest stabilizer of the SC joint, preventing superior displacement of the medial clavicle.

Despite its critical role and strong ligamentous support, the SC joint's inherent bony stability is relatively poor, making it susceptible to injury under significant force.

Understanding Sternoclavicular Joint Injuries

Injuries to the SC joint are relatively uncommon, accounting for less than 5% of all shoulder girdle injuries. However, their potential severity, particularly with posterior dislocations, necessitates a thorough understanding of their mechanisms. Injuries typically range from:

• Sprains: Stretching or tearing of the SC joint ligaments (Grade I, II, or III).
• Dislocations: Complete separation of the articular surfaces. These are classified by the direction of the clavicle relative to the sternum:
  • Anterior Dislocation: The medial clavicle displaces anteriorly and superiorly (most common, approximately 90% of SC dislocations).
  • Posterior Dislocation: The medial clavicle displaces posteriorly (less common, but more serious due to proximity to vital mediastinal structures).
• Fractures: Although less common at the joint itself, fractures of the medial clavicle can sometimes involve the joint.

Direct Mechanism of Injury

A direct mechanism involves a force directly applied to the medial end of the clavicle or the anterolateral chest wall.

• Direct Blow to the Medial Clavicle: A direct impact to the anterior aspect of the medial clavicle can drive it posteriorly, causing a posterior dislocation. Conversely, a direct blow to the posterior aspect of the medial clavicle can drive it anteriorly, resulting in an anterior dislocation.
• Direct Blow to the Anterolateral Chest: A force hitting the chest in front of the SC joint can push the sternum posteriorly relative to the clavicle, effectively causing an anterior dislocation of the clavicle.

While less common than indirect mechanisms, direct blows are often associated with high-energy trauma, such as motor vehicle accidents or direct contact in sports.

Indirect Mechanism of Injury

Indirect mechanisms are far more common and involve forces transmitted through the shoulder or arm to the SC joint. These mechanisms leverage the lever arm of the clavicle and the scapula.

• Fall onto an Outstretched Hand (FOOSH): This is a classic indirect mechanism. The force of impact travels up the forearm, through the humerus to the scapula, and then along the clavicle to the SC joint.
  • If the force is directed such that the shoulder is driven anteriorly and inferiorly, it can lever the medial clavicle posteriorly, leading to a posterior dislocation.
  • If the force causes the shoulder to be driven posteriorly and superiorly, it can lever the medial clavicle anteriorly, resulting in an anterior dislocation.
• Fall Directly onto the Lateral Aspect of the Shoulder: A fall directly onto the acromion or lateral deltoid region transmits a powerful compressive force medially along the clavicle.
  • If the shoulder is driven medially and anteriorly, the medial clavicle is forced posteriorly, leading to a posterior dislocation. This is often the mechanism in contact sports or falls from height.
  • If the shoulder is driven medially and posteriorly, the medial clavicle is forced anteriorly and superiorly, causing an anterior dislocation.
• Motor Vehicle Accidents (MVAs):
  • Dashboard Injury: In a head-on collision, the occupant's chest may strike the dashboard or steering wheel. If the force is applied to the anterolateral shoulder, it can drive the clavicle posteriorly.
  • Seatbelt Injury: While designed to protect, the diagonal strap of a seatbelt can, in rare instances, apply a compressive force across the shoulder and chest during rapid deceleration, potentially contributing to SC joint injury.
• Sports-Related Trauma: High-impact contact sports like American football, rugby, hockey, and wrestling frequently involve mechanisms that lead to SC joint injuries. These include:
  • Being tackled or falling directly onto the shoulder.
  • Direct blows to the chest or shoulder.
  • Collisions with other players or fixed objects.

Specific Force Vectors and Dislocation Types

The direction of the clavicular displacement is highly dependent on the precise vector of the applied force and the position of the shoulder at the moment of impact.

• Anterior Dislocation: Most commonly results from a force applied to the posterolateral aspect of the shoulder, driving the clavicle anteriorly and superiorly. This can occur with a fall on the side or a blow to the back of the shoulder. The clavicle is levered over the manubrium.
• Posterior Dislocation: Often results from a direct blow to the anteromedial clavicle, or more commonly, an indirect force applied to the anterolateral aspect of the shoulder, driving the shoulder inward and backward. This levers the medial clavicle posteriorly and inferiorly, potentially behind the sternum. This mechanism is particularly concerning due to the close proximity of the clavicle to vital structures in the superior mediastinum, including the trachea, esophagus, great vessels (subclavian artery and vein), and nerves. Compression or laceration of these structures can lead to life-threatening complications.

Contributing Factors and Risk

While high-energy trauma is the primary cause, certain factors can influence the risk or severity of SC joint injury:

• Age: SC joint injuries are more common in younger individuals (under 25) because the physis (growth plate) of the medial clavicle remains open until this age. Injuries in this population are often physeal fractures, which can mimic dislocations.
• Ligamentous Laxity: Individuals with generalized joint hypermobility may have a slightly increased predisposition to sprains or dislocations.
• Previous Injury: A history of SC joint injury can weaken the supporting structures, increasing the risk of recurrence.
• Muscle Imbalances: While not a direct cause, chronic postural issues or muscle imbalances around the shoulder girdle could theoretically alter joint mechanics, though this is less directly linked to acute traumatic injury mechanisms.

Clinical Presentation and Diagnosis

Regardless of the mechanism, SC joint injuries typically present with:

• Severe pain at the base of the neck, radiating to the shoulder.
• Swelling and tenderness over the SC joint.
• Deformity: A palpable prominence (anterior dislocation) or depression (posterior dislocation) at the joint.
• Limited and painful range of motion of the arm, especially elevation and abduction.
• Dysphagia (difficulty swallowing), dyspnea (difficulty breathing), or hoarseness in cases of posterior dislocation due to mediastinal compression.

Diagnosis relies on clinical examination and imaging. Plain radiographs are often insufficient due to bony overlap. Computed Tomography (CT) scans are the gold standard for accurately assessing the degree and direction of displacement, and for identifying any associated mediastinal compression.

Importance of Proper Management

Given the potential for serious complications, particularly with posterior dislocations, accurate diagnosis and prompt management of SC joint injuries are crucial. Treatment ranges from conservative management (rest, ice, immobilization) for sprains and stable anterior dislocations, to urgent closed or open reduction for posterior dislocations and significantly displaced anterior dislocations.

Understanding the specific mechanism of injury is vital for clinicians to anticipate potential complications, guide imaging choices, and plan appropriate management strategies to ensure optimal patient outcomes.