Posterior Shoulder Dislocations: Mechanisms, Causes, and Recognition

How do posterior shoulder dislocations happen?

Posterior shoulder dislocations, though less common than their anterior counterparts, typically occur when a significant force drives the humeral head backward out of the glenoid fossa, often involving a combination of axial loading, internal rotation, and adduction, or through uncontrolled, violent muscle contractions.

Understanding Shoulder Anatomy

To grasp how posterior dislocations occur, it's essential to understand the shoulder's unique anatomy. The shoulder is a ball-and-socket joint, specifically the glenohumeral joint, formed by the head of the humerus (arm bone) and the glenoid cavity of the scapula (shoulder blade). This design allows for an extraordinary range of motion, making it the most mobile joint in the body. However, this mobility comes at the expense of stability. The glenoid cavity is relatively shallow, and the joint relies heavily on surrounding soft tissues—the joint capsule, glenoid labrum, ligaments (especially the glenohumeral ligaments), and rotator cuff muscles—for stability.

The Rarity and Mechanism of Posterior Dislocations

Posterior shoulder dislocations account for only 2-5% of all shoulder dislocations. This rarity is primarily due to the anatomical orientation of the glenoid fossa, which faces slightly anteriorly, and the strength of the posterior capsule and rotator cuff muscles (infraspinatus and teres minor) that resist posterior displacement.

The fundamental mechanism involves a forceful posterior translation of the humeral head relative to the glenoid. This typically happens when the arm is in a vulnerable position, allowing the force to bypass or overwhelm the natural stabilizers.

Common Mechanisms of Injury

Posterior shoulder dislocations can result from various traumatic and non-traumatic events, often categorized by the type of force applied:

• Direct Trauma:

  • Fall on an outstretched hand (FOOSH): If the arm is internally rotated and adducted at the time of impact, the force transmitted up the arm can drive the humeral head posteriorly.
  • Direct blow to the anterior aspect of the shoulder: A forceful impact from the front can push the humeral head directly backward.
  • Motor vehicle accidents: Especially when the occupant is braced against the dashboard, leading to an axial load on an internally rotated arm.
  • Sports injuries: Contact sports like rugby or football, particularly during tackles or falls where the arm is pinned or twisted.

• Indirect Trauma / Non-Traumatic Causes:

  • Seizure activity: Uncontrolled, violent muscle contractions, particularly of the internal rotators (subscapularis, pectoralis major, latissimus dorsi, teres major), can overpower the external rotators and pull the humeral head posteriorly. This is a classic cause.
  • Electrocution: Similar to seizures, high-voltage electrical shocks can induce tetanic muscle contractions strong enough to dislocate the joint.
  • Severe muscle spasms: Certain neurological conditions or drug reactions can lead to muscle spasms capable of causing dislocation.
  • Falls from height: Landing awkwardly on the arm.

Specific Biomechanical Pathways

Two primary biomechanical pathways frequently lead to posterior shoulder dislocation:

• Axial Load with Internal Rotation and Adduction: This is the most common traumatic mechanism. Imagine a scenario where someone falls forward, trying to break their fall with an outstretched arm. If the arm is simultaneously twisted inward (internal rotation) and pulled across the body (adduction), the force from the fall (axial load) is directed precisely to push the humeral head out the back of the joint. The humerus acts as a lever, forcing the head posteriorly.

• Violent Muscle Contractions: In cases like seizures or electrocution, the powerful internal rotator muscles of the shoulder (subscapularis, pectoralis major, latissimus dorsi, teres major) contract simultaneously and intensely. If these contractions are strong enough and unopposed by the external rotators, they can overcome the static stabilizers, forcibly rotating the humerus internally and pulling its head posteriorly out of the glenoid.

Risk Factors and Contributing Conditions

While direct mechanisms are key, certain factors can increase the risk of a posterior shoulder dislocation:

• Previous shoulder injury or instability: A history of dislocation or subluxation can weaken the joint capsule and ligaments.
• Ligamentous laxity: Individuals with naturally loose ligaments may be more prone to dislocation.
• Participation in high-impact sports: Sports that involve falls, tackles, or direct blows to the shoulder.
• Underlying medical conditions: Epilepsy, diabetes, alcoholism, and certain psychiatric conditions can increase the risk due to associated falls or seizure activity.
• Osteoporosis: While not a direct cause, weakened bones can contribute to fracture-dislocations.

Recognizing a Posterior Shoulder Dislocation

Recognizing a posterior dislocation can be challenging, as the deformity may be subtle. Key signs include:

• Severe pain: Often excruciating and immediate.
• Limited range of motion: Especially external rotation and abduction. The arm is often locked in internal rotation and adduction.
• Flattening of the anterior shoulder: The deltoid muscle may appear flattened, and the coracoid process may be prominent.
• Posterior prominence: A bulge may be felt posteriorly, representing the displaced humeral head.
• Prominent coracoid process: Due to the anterior displacement of the scapula relative to the humerus.

Prevention and Management

While accidental trauma is difficult to prevent entirely, understanding the mechanisms can inform strategies for high-risk individuals. For those with seizure disorders, adherence to medication regimens is crucial. In sports, proper fall training and protective gear can reduce risk.

Prompt medical evaluation is essential for any suspected shoulder dislocation. Diagnosis is confirmed with X-rays, often requiring specific views (like an axillary view or scapular Y view) to visualize the posterior displacement, as standard anterior-posterior views can sometimes miss it. Treatment typically involves closed reduction (manual manipulation to realign the joint) under sedation, followed by immobilization and rehabilitation.

Conclusion

Posterior shoulder dislocations, though less common, result from significant forces that overwhelm the shoulder's natural stability. The most frequent mechanisms involve an axial load on an internally rotated and adducted arm, or severe, uncontrolled muscle contractions from conditions like seizures or electrocution. Recognizing these specific pathways is crucial for accurate diagnosis and effective management, ultimately leading to better outcomes for individuals experiencing this debilitating injury.