Posterior Shoulder Dislocation: Understanding Causes, Symptoms, and Treatment

What is a Rear Dislocation of the Shoulder?

A rear, or posterior, dislocation of the shoulder occurs when the head of the humerus (upper arm bone) is forced backward out of the glenoid fossa (shoulder socket), a significantly less common injury compared to its anterior counterpart, often resulting from specific traumatic forces.

Understanding Shoulder Anatomy

To grasp a rear shoulder dislocation, it's essential to understand the intricate anatomy of the shoulder joint. The shoulder, specifically the glenohumeral joint, is a ball-and-socket joint, offering the greatest range of motion of any joint in the body.

• Humeral Head: The "ball" at the top of the humerus.
• Glenoid Fossa: The shallow "socket" of the scapula (shoulder blade) that articulates with the humeral head.
• Glenoid Labrum: A rim of fibrocartilage that deepens the glenoid fossa, enhancing stability.
• Joint Capsule: A fibrous sac enclosing the joint, lined by a synovial membrane that produces lubricating fluid.
• Ligaments: Strong bands of connective tissue (e.g., glenohumeral ligaments) that reinforce the capsule and connect bones.
• Rotator Cuff Muscles: A group of four muscles (supraspinatus, infraspinatus, teres minor, subscapularis) and their tendons that surround the joint, providing dynamic stability and facilitating movement.

While highly mobile, the shoulder's shallow socket makes it inherently less stable, relying heavily on surrounding soft tissues for support.

What Exactly is a Rear Dislocation?

A rear (or posterior) shoulder dislocation is defined by the displacement of the humeral head in a posterior direction relative to the glenoid fossa. This means the ball of the arm bone moves backward, away from the front of the body, and out of its normal resting position in the socket.

• Rarity: Posterior dislocations account for only 2-5% of all shoulder dislocations, making them far less common than anterior dislocations (where the humeral head moves forward).
• Mechanism: Due to the strong posterior capsule and rotator cuff muscles (infraspinatus, teres minor) that resist posterior displacement, a significant force is typically required to cause this injury.

Causes and Mechanisms of Injury

Posterior shoulder dislocations are almost exclusively the result of high-energy trauma or specific muscular contractions.

• Direct Blow: A direct impact to the front of the shoulder.
• Fall onto an Outstretched Arm: Specifically, a fall onto an arm that is adducted (close to the body), internally rotated, and flexed. This position effectively levers the humeral head posteriorly.
• Seizures: Uncontrolled, violent muscle contractions during an epileptic seizure can generate forces strong enough to dislocate the shoulder posteriorly.
• Electric Shock: Similar to seizures, the intense and uncoordinated muscle contractions caused by an electric shock can lead to dislocation.
• Motor Vehicle Accidents: High-impact collisions can transmit forces through the arm, leading to posterior dislocation.
• Sports Injuries: Less common, but can occur in contact sports with specific impact forces.

Signs and Symptoms

Recognizing a posterior shoulder dislocation can be challenging, as the deformity may not be as obvious as with an anterior dislocation, and the injury is often missed in initial assessments.

• Severe Pain: Intense pain in the shoulder, often radiating down the arm.
• Inability to Move the Arm: Significant restriction, particularly with external rotation and abduction.
• Arm Position: The affected arm is typically held in a position of adduction (close to the body) and internal rotation. The patient may support the arm with the opposite hand.
• "Locked" Shoulder: The arm may feel rigidly fixed in its internally rotated position, preventing any attempt at external rotation.
• Flattening of the Anterior Deltoid: While subtle, the front of the shoulder may appear less rounded.
• Prominence of the Coracoid Process: This bony landmark may become more visible.
• Palpable Humeral Head: In some cases, the humeral head may be felt as a prominence posteriorly, beneath the acromion.

Diagnosis

Accurate and timely diagnosis is crucial to prevent complications, but posterior dislocations are notoriously difficult to detect clinically and on standard X-rays.

• Clinical Examination: Assessment of pain, range of motion, and observation of arm position. Suspect a posterior dislocation if the arm is fixed in internal rotation.
• Imaging Studies:
  • X-rays:
    • Anteroposterior (AP) View: Often appears normal or shows subtle signs like a "lightbulb sign" (humeral head appears perfectly round due to internal rotation) or loss of overlap between the humeral head and glenoid.
    • Axillary View: This view is critical for diagnosing posterior dislocations as it directly shows the anterior-posterior relationship of the humeral head to the glenoid.
    • Scapular Y-View: Another specialized view that can help identify posterior displacement.
  • Computed Tomography (CT) Scan: Often necessary to confirm the diagnosis, especially when X-rays are inconclusive, and to assess for associated bone fractures (e.g., reverse Hill-Sachs lesion, glenoid rim fracture).
  • Magnetic Resonance Imaging (MRI): Useful for evaluating soft tissue injuries, such as rotator cuff tears or labral damage, once the dislocation is reduced.

Treatment and Management

The primary goal of treatment is prompt reduction of the dislocation, followed by rehabilitation to restore function and prevent recurrence.

• Acute Management (Reduction):
  • Closed Reduction: The most common approach, performed under sedation or general anesthesia. The shoulder is manipulated back into place using specific maneuvers to overcome muscle spasm and guide the humeral head into the glenoid.
  • Open Reduction: If closed reduction is unsuccessful due to significant soft tissue interposition or bone fragments, surgical intervention is required to realign the joint.
• Post-Reduction Management:
  • Immobilization: The arm is typically immobilized in a sling, often in slight external rotation, for a period (e.g., 3-6 weeks) to allow the joint capsule and ligaments to heal. The duration depends on factors like age, associated injuries, and risk of recurrence.
  • Pain Management: Analgesics and anti-inflammatory medications are used to control pain and swelling.
• Rehabilitation: A structured physical therapy program is essential for a successful recovery.
  • Phase 1 (Early Mobilization): Gentle range-of-motion exercises, avoiding positions that stress the posterior capsule.
  • Phase 2 (Strengthening): Progressive strengthening of the rotator cuff muscles (especially external rotators), deltoid, and scapular stabilizers.
  • Phase 3 (Return to Activity): Sport-specific or activity-specific drills, proprioceptive training, and gradual return to full activity.
• Surgical Intervention: May be considered in cases of:
  • Recurrent posterior dislocations.
  • Significant bone loss from the humeral head (reverse Hill-Sachs lesion) or glenoid.
  • Irreducible dislocations.
  • Associated significant soft tissue injuries that require repair.

Potential Complications

While rare, posterior shoulder dislocations can lead to several complications if not managed appropriately.

• Recurrence: Although less common than with anterior dislocations, re-dislocation can occur, especially in younger, active individuals.
• Nerve Damage: The axillary nerve is most commonly at risk, potentially leading to weakness in the deltoid muscle and numbness over the lateral shoulder.
• Vascular Injury: Extremely rare, but damage to blood vessels around the shoulder is possible.
• Rotator Cuff Tears: Can occur concurrently with the dislocation, particularly in older individuals.
• Bone Fractures:
  • Reverse Hill-Sachs Lesion: An impaction fracture on the anterior aspect of the humeral head, caused by its forceful impact against the posterior glenoid rim during dislocation.
  • Glenoid Fracture: A fracture of the posterior rim of the shoulder socket.
• Chronic Instability: Persistent feeling of the shoulder "giving way."
• Stiffness (Adhesive Capsulitis/Frozen Shoulder): Can develop if immobilization is too prolonged or rehabilitation is inadequate.
• Undiagnosed Dislocation: If missed, it can lead to chronic pain, stiffness, and degenerative changes in the joint.

Prevention and Prognosis

Preventing a posterior shoulder dislocation often involves avoiding the specific high-force trauma mechanisms. For individuals prone to seizures or those working in environments with electric shock risk, appropriate safety measures are paramount.

The prognosis following a posterior shoulder dislocation varies based on several factors:

• Severity of the initial injury: Presence of associated fractures or soft tissue damage.
• Age of the patient: Younger patients tend to have a higher risk of recurrence.
• Adherence to rehabilitation: A dedicated and progressive physical therapy program is critical for restoring strength, stability, and full range of motion.
• Timeliness of diagnosis and reduction: Prompt treatment minimizes complications.

While recovery can be challenging and requires patience, most individuals can achieve a good functional outcome with appropriate management and rehabilitation.