Shoulder Dislocation: Anterior vs. Posterior, Causes, Symptoms, and Treatment

What is the Difference Between Anterior and Posterior Dislocation of the Shoulder Joint?

Shoulder dislocations are common injuries where the head of the humerus (upper arm bone) separates from the glenoid fossa (shoulder socket), with anterior dislocations being significantly more prevalent and typically resulting from different mechanisms and presenting with distinct signs compared to the rarer posterior dislocations.

Introduction to Shoulder Dislocation

The shoulder joint, or glenohumeral joint, is the most mobile joint in the human body, offering an extensive range of motion vital for daily activities and athletic performance. This exceptional mobility, however, comes at the cost of stability, making the shoulder the most frequently dislocated joint. A dislocation occurs when the head of the humerus completely separates from the glenoid fossa. Understanding the specific type of dislocation – anterior or posterior – is critical for accurate diagnosis, appropriate treatment, and effective rehabilitation.

Understanding Shoulder Anatomy

To grasp the mechanics of shoulder dislocation, a brief review of the relevant anatomy is helpful. The glenohumeral joint is a ball-and-socket joint where the large, rounded head of the humerus articulates with the small, shallow glenoid fossa of the scapula (shoulder blade). This inherent bony instability is compensated by a complex network of soft tissues:

• Glenoid Labrum: A fibrocartilaginous rim that deepens the glenoid fossa, enhancing stability.
• Joint Capsule: A fibrous sac enclosing the joint, reinforced by ligaments.
• Glenohumeral Ligaments: Three main ligaments (superior, middle, inferior) that reinforce the anterior aspect of the joint capsule.
• 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.

Dislocation occurs when excessive force overcomes these stabilizing structures, forcing the humeral head out of the glenoid.

Anterior Shoulder Dislocation

Anterior dislocation is by far the most common type, accounting for approximately 95-97% of all shoulder dislocations. In this scenario, the humeral head is displaced forward (anteriorly) and often downward (inferiorly) from the glenoid fossa.

• Mechanism of Injury:
  • Typically results from an indirect force applied to the arm when it is in a position of abduction (arm raised away from the body), external rotation (arm rotated outward), and extension (arm moved backward).
  • Common scenarios include:
    • Falling onto an outstretched arm (FOOSH) with the arm in the vulnerable position.
    • Direct blow to the posterior aspect of the shoulder.
    • Sports injuries involving throwing motions or contact (e.g., football, rugby, basketball).
• Signs and Symptoms:
  • Severe pain in the shoulder, often radiating down the arm.
  • Visible deformity: The shoulder appears "squared off" or flattened anteriorly, with a noticeable bulge of the humeral head below the coracoid process or in the axilla (armpit).
  • Inability to move the arm, especially abduction or external rotation.
  • The arm is often held slightly abducted and externally rotated, with the patient supporting the injured arm with the uninjured hand.
  • Numbness or tingling in the hand or arm due to potential nerve compression (most commonly the axillary nerve, leading to deltoid weakness and numbness over the lateral shoulder).
• Associated Injuries/Complications:
  • Bankart Lesion: A tear of the anterior inferior glenoid labrum, often associated with recurrent dislocations.
  • Hill-Sachs Lesion: A compression fracture on the posterior-superior aspect of the humeral head, caused by impact against the anterior glenoid rim during dislocation.
  • Rotator Cuff Tears: More common in older individuals.
  • Neurovascular Injury: Damage to the axillary nerve or axillary artery, though less common.
  • Recurrent Dislocations: High risk, especially in younger individuals, often requiring surgical stabilization.

Posterior Shoulder Dislocation

Posterior dislocation is significantly rarer, making up only 2-5% of all shoulder dislocations. In this type, the humeral head is displaced backward (posteriorly) from the glenoid fossa. These dislocations are often missed on initial examination or standard X-rays due to their subtlety.

• Mechanism of Injury:
  • Typically results from a direct blow to the anterior aspect of the shoulder or an indirect force with the arm in a position of adduction (arm close to the body), internal rotation (arm rotated inward), and flexion (arm moved forward).
  • Common causes include:
    • Seizures (due to strong muscle contractions, particularly internal rotators).
    • Electrocution injury.
    • Fall onto a flexed and internally rotated arm.
    • Motor vehicle accidents (dashboard injury).
• Signs and Symptoms:
  • Often less obvious deformity than anterior dislocations, making diagnosis challenging.
  • Severe pain in the shoulder.
  • The arm is typically held in adduction and internal rotation, with marked inability to externally rotate the arm.
  • Prominence of the coracoid process anteriorly and a flattening of the posterior deltoid.
  • A "locked" internal rotation, where the patient cannot externally rotate the arm beyond neutral.
  • On X-ray, the "lightbulb sign" (due to fixed internal rotation of the humeral head) or "rim sign" (increased distance between the anterior glenoid rim and humeral head) may be present.
• Associated Injuries/Complications:
  • Reverse Hill-Sachs Lesion: A compression fracture on the anterior-medial aspect of the humeral head.
  • Reverse Bankart Lesion: A tear of the posterior inferior glenoid labrum.
  • Fracture of the posterior glenoid rim.
  • Fractures of the humeral neck or tuberosities.
  • Increased risk of avascular necrosis of the humeral head if reduction is delayed.

Key Differences Summarized

Diagnosis and Treatment

Regardless of the type, a suspected shoulder dislocation requires immediate medical attention.

• Diagnosis:
  • Clinical examination: Assessment of pain, deformity, range of motion, and neurovascular status.
  • Imaging: X-rays (AP, Y-view, and an axillary view are crucial to confirm the direction of dislocation and rule out fractures), CT scans (for complex fractures or subtle posterior dislocations), and MRI (to assess soft tissue damage like labral tears or rotator cuff injuries).
• Treatment:
  • Reduction: The primary treatment is to manually maneuver the humeral head back into the glenoid fossa. This is typically done under sedation and muscle relaxation.
  • Immobilization: After reduction, the arm is usually immobilized in a sling for a period (e.g., 1-3 weeks) to allow soft tissues to heal. The position of immobilization might vary slightly (e.g., some evidence suggests external rotation for anterior dislocations).
  • Pain Management: Analgesics are prescribed to manage pain.
  • Surgery: May be necessary for recurrent dislocations, significant associated fractures, or extensive soft tissue damage that compromises joint stability.

Rehabilitation and Prevention

Following immobilization, a structured rehabilitation program is essential to restore full function and prevent recurrence.

• Rehabilitation:
  • Phase 1 (Early Mobility): Gentle range of motion exercises to prevent stiffness.
  • Phase 2 (Strengthening): Progressive strengthening of the rotator cuff muscles and scapular stabilizers to improve dynamic stability.
  • Phase 3 (Proprioception and Functional Training): Exercises to improve joint position sense and prepare for return to activity or sport.
  • Gradual Return to Activity: A phased approach to resuming normal activities and sports, guided by a physical therapist.
• Prevention:
  • Strengthening the muscles surrounding the shoulder (rotator cuff, deltoids, scapular stabilizers).
  • Maintaining good posture and joint mobility.
  • Using proper technique in sports and activities.
  • Avoiding high-risk positions, especially if there's a history of dislocation.

When to Seek Medical Attention

Any suspected shoulder dislocation should be treated as a medical emergency. Prompt diagnosis and reduction are crucial to minimize pain, prevent complications (such as nerve or blood vessel damage), and improve long-term outcomes. Do not attempt to reduce a dislocation yourself, as this can cause further injury.

Conclusion

While both anterior and posterior shoulder dislocations involve the displacement of the humeral head from the glenoid, they differ significantly in their prevalence, typical mechanisms of injury, clinical presentation, and associated complications. Anterior dislocations are far more common, usually resulting from forces that externally rotate and abduct the arm. Posterior dislocations are rarer, often due to seizures or direct anterior impacts, and present with the arm internally rotated and adducted, making them more challenging to diagnose. Accurate identification of the dislocation type is paramount for effective treatment and a targeted rehabilitation strategy, ultimately aiming to restore shoulder stability and function.