Posterior Dislocation: Mechanisms, Common Causes, and Immediate Steps

How Does a Posterior Dislocation Happen?

A posterior dislocation occurs when a bone forming a joint is forcibly displaced backward from its normal anatomical position, most commonly due to a high-energy traumatic event that applies a significant posterior force, often in conjunction with specific joint positions like internal rotation and adduction.

Understanding Joint Dislocation

A joint dislocation, or luxation, is a complete separation of the two bones that form a joint. This means the articular surfaces of the bones are no longer in contact. It differs from a subluxation, which is a partial or incomplete dislocation where the joint surfaces remain partially in contact. Posterior dislocations are less common than anterior dislocations for most major joints but can be particularly serious due to the forces involved and potential for associated injuries.

The Mechanics of Posterior Dislocation

Posterior dislocations typically result from a powerful, external force that drives the joint backward. The specific mechanism often involves a combination of:

• Direct Posterior Force: A blow to the front of the joint, pushing the bone directly backward.
• Leverage and Rotational Forces: The joint is often in a specific position (e.g., flexed, adducted, internally rotated) when the force is applied, creating a lever arm that facilitates the posterior displacement.

Let's examine common examples:

• Shoulder (Glenohumeral Joint): This is the most frequently dislocated joint posteriorly, though still less common than anterior shoulder dislocations.
  • Mechanism: Typically occurs when a direct blow impacts the front of the shoulder, or when the arm is in a position of flexion, adduction, and internal rotation (e.g., reaching across the body) and an axial load is applied through the humerus. This can happen during a fall onto an outstretched hand, a direct blow to the front of the shoulder (e.g., during a tackle), or an uncoordinated seizure or electrocution where strong internal rotators contract forcefully.
  • Anatomy: The humeral head is forced backward out of the glenoid fossa.
• Hip (Femoral-Acetabular Joint): Posterior hip dislocations are the most common type of hip dislocation and are considered a medical emergency.
  • Mechanism: Almost exclusively due to high-energy trauma, most notably Motor Vehicle Accidents (MVAs). This is often referred to as a "dashboard injury" where the knee strikes the dashboard, driving the femur posteriorly. The hip is typically in a position of flexion, adduction, and internal rotation at the moment of impact.
  • Anatomy: The head of the femur is forced backward out of the acetabulum.
• Elbow (Ulnohumeral Joint): While elbow dislocations are common, posterior dislocations are relatively rare compared to posterolateral.
  • Mechanism: Usually results from a fall onto an outstretched hand, where the elbow is hyperextended and a significant axial load or rotational force is applied, driving the ulna and radius posteriorly relative to the humerus.
• Knee (Tibiofemoral Joint): Posterior knee dislocations are extremely rare but devastating injuries.
  • Mechanism: Involves massive force, often from high-velocity trauma (e.g., MVAs, direct impact in contact sports). The tibia is driven posteriorly relative to the femur, often tearing multiple ligaments (ACL, PCL, MCL, LCL) and potentially causing neurovascular damage.

Common Causes and Scenarios

Posterior dislocations are almost always the result of significant trauma. Common scenarios include:

• Motor Vehicle Accidents (MVAs): The leading cause of posterior hip dislocations (dashboard injury).
• Sports Injuries:
  • Contact Sports: Football, rugby, hockey, where direct impacts or falls can occur.
  • High-Impact Sports: Skiing, snowboarding, gymnastics, where falls or awkward landings are common.
• Falls from Height: Landing awkwardly after a fall.
• Industrial Accidents: Crushing injuries or falls from machinery.
• Seizures or Electrocution: Involuntary, powerful muscle contractions can sometimes generate enough force to dislocate a joint, particularly the shoulder.

Risk Factors for Posterior Dislocation

While severe trauma is the primary cause, certain factors can increase susceptibility:

• Previous Dislocation: Once a joint has been dislocated, the surrounding ligaments and capsule may be stretched or damaged, making it more prone to future dislocations.
• Joint Anatomy: Some individuals may have naturally shallower joint sockets or ligamentous laxity, predisposing them to instability.
• Muscle Weakness/Imbalance: Inadequate strength in stabilizing muscles (e.g., rotator cuff for the shoulder) can reduce joint stability.
• Underlying Medical Conditions: Certain connective tissue disorders (e.g., Ehlers-Danlos syndrome) can lead to generalized joint hypermobility and increased risk of dislocation.
• High-Risk Activities: Participation in contact sports or occupations with a high risk of falls.

Immediate Signs and Symptoms

If a posterior dislocation has occurred, the signs are usually unmistakable:

• Severe Pain: Intense and immediate pain at the joint.
• Obvious Deformity: The joint will look visibly out of place. For a posterior shoulder dislocation, the shoulder may appear flattened anteriorly, with the coracoid process more prominent. For a posterior hip dislocation, the leg will typically be shortened, internally rotated, and adducted.
• Loss of Function: Inability to move the affected limb.
• Swelling and Bruising: May develop rapidly around the joint.
• Neurological or Vascular Compromise: Crucially, nerve or blood vessel damage can occur. This might manifest as numbness, tingling, weakness, or a loss of pulse in the limb distal to the dislocation. This is a medical emergency.

Importance of Prompt Medical Attention

Any suspected joint dislocation, especially a posterior one, requires immediate medical attention. Do not attempt to reduce the dislocation yourself, as this can cause further damage to nerves, blood vessels, or surrounding tissues. Prompt diagnosis and reduction are vital to:

• Minimize Pain: Reduce discomfort and muscle spasm.
• Prevent Complications: Avoid long-term damage such as nerve injury, avascular necrosis (death of bone tissue due to lack of blood supply, particularly a risk in hip dislocations), or chronic joint instability.
• Restore Function: Allow for proper healing and rehabilitation.

Prevention Strategies

While not all traumatic dislocations can be prevented, certain measures can reduce the risk:

• Proper Sports Technique: Learning and practicing correct form in sports to minimize awkward landings or impacts.
• Strength and Conditioning: Developing strong muscles around joints, particularly stabilizing muscles, to enhance joint integrity.
• Protective Gear: Wearing appropriate safety equipment in contact sports or high-risk activities.
• Awareness and Caution: Being mindful of surroundings and avoiding situations that could lead to severe trauma.
• Rehabilitation After Previous Injury: Thoroughly completing rehabilitation programs after any joint injury to restore full strength and stability.

Understanding the mechanisms behind posterior dislocations is crucial for both prevention and recognizing the urgency of immediate medical care should such an injury occur.