Posterior Dislocations: Types, Causes, Diagnosis, and Recovery

What are the different types of posterior dislocations?

Posterior dislocations occur when a bone is displaced backward from its normal articulation within a joint, presenting in various forms depending on the affected joint and the specific mechanism of injury.

Understanding Dislocations: A Brief Overview

A dislocation, medically known as a luxation, is a severe joint injury where the ends of two bones that normally meet within a joint are forced completely out of alignment. This differs from a subluxation, which is a partial dislocation where the bones are only partially displaced. Dislocations often result from significant trauma, leading to damage to the joint capsule, ligaments, and sometimes surrounding soft tissues, nerves, or blood vessels.

A posterior dislocation specifically refers to a displacement where the distal bone in a joint moves posteriorly (backward) relative to the proximal bone. While any joint can theoretically dislocate posteriorly, some are more commonly affected due to their anatomical structure and the typical forces they encounter.

Common Causes and Mechanisms

Posterior dislocations are almost always the result of significant force or trauma. Common mechanisms include:

• Direct Impact: A direct blow to the front of a joint can drive the bone backward.
• Indirect Forces: Often, a combination of rotational and axial loading forces can lead to dislocation.
• Specific Body Positions: Certain positions, especially during high-energy impacts, can predispose a joint to posterior displacement.
• Pathological Conditions: Rarely, underlying conditions like Ehlers-Danlos syndrome (hypermobility) or severe muscle spasms (e.g., from seizures, electrocution) can contribute to dislocations with less force.

Classifying Posterior Dislocations by Joint

The most common way to classify posterior dislocations is by the specific joint involved, as each has unique characteristics, mechanisms, and potential complications.

• Posterior Shoulder Dislocation

  • Description: Occurs when the head of the humerus (upper arm bone) is forced backward out of the glenoid cavity (shoulder socket). This is far less common than anterior shoulder dislocations, accounting for about 2-5% of all shoulder dislocations.
  • Mechanism: Typically results from a direct blow to the front of the shoulder, or more commonly, from an indirect force such as a fall on an outstretched hand with the arm in adduction, internal rotation, and flexion. It's also strongly associated with seizures and electrical shock injuries, where strong muscle contractions pull the humerus posteriorly.
  • Clinical Features: The arm is often held in internal rotation and adduction, and external rotation is severely limited. Diagnosis can be challenging on standard X-rays, often requiring specific views (e.g., axillary view, scapular Y view) or CT scans.
  • Complications: Can include reverse Hill-Sachs lesions (impaction fracture on the anterior humeral head), glenoid rim fractures, and axillary nerve injury.

• Posterior Hip Dislocation

  • Description: Involves the femoral head (top of the thigh bone) being displaced backward out of the acetabulum (hip socket). This is the most common type of hip dislocation, accounting for 80-90% of all hip dislocations.
  • Mechanism: Almost exclusively due to high-energy trauma, most notably motor vehicle accidents where the knee strikes the dashboard ("dashboard injury"). The force drives the flexed knee and femur backward, dislocating the hip. Falls from significant heights can also cause this.
  • Clinical Features: The affected leg typically appears shortened, adducted, and internally rotated.
  • Complications: High risk of sciatic nerve injury (causing foot drop), avascular necrosis of the femoral head (due to disruption of blood supply), and associated fractures of the femoral head or acetabulum. This is considered a medical emergency requiring urgent reduction.

• Posterior Elbow Dislocation

  • Description: Occurs when the ulna and radius (forearm bones) are displaced backward relative to the humerus (upper arm bone). This is the most common type of elbow dislocation.
  • Mechanism: Usually a result of a fall on an outstretched hand (FOOSH) with the elbow in slight flexion, leading to hyperextension and a rotational component.
  • Clinical Features: The elbow appears deformed, often with a prominent olecranon (the bony tip of the elbow) posteriorly.
  • Complications: Can be associated with fractures of the radial head, coronoid process, or epicondyles. Neurovascular compromise is less common than in knee or hip dislocations but can occur.

• Posterior Knee Dislocation (Tibiofemoral)

  • Description: The tibia (shin bone) is displaced backward relative to the femur (thigh bone). This is a rare but extremely severe injury.
  • Mechanism: Typically requires extremely high-energy trauma, such as a direct blow to the anterior tibia while the knee is flexed, or a "dashboard injury" where the tibia is forced backward.
  • Clinical Features: Gross deformity of the knee.
  • Complications: Due to the close proximity of the neurovascular structures, there is an extremely high risk of popliteal artery injury (leading to limb ischemia) and peroneal nerve damage (causing foot drop). Ligamentous damage (ACL, PCL, MCL, LCL) is extensive, often involving multiple ligaments. This is a limb-threatening emergency.

• Posterior Sternoclavicular (SC) Joint Dislocation

  • Description: The medial end of the clavicle (collarbone) is displaced backward relative to the sternum (breastbone). This is a rare injury but potentially life-threatening.
  • Mechanism: Often results from a direct blow to the anterior aspect of the shoulder, pushing the clavicle posteriorly, or from indirect forces from a fall or motor vehicle accident.
  • Clinical Features: Pain, swelling, and sometimes a visible depression at the SC joint.
  • Complications: The posterior displacement can cause compression or injury to vital structures in the mediastinum, including the trachea (leading to breathing difficulties), esophagus, subclavian artery and vein, and major nerves.

Other Classification Factors

Beyond the specific joint, posterior dislocations can also be categorized by other important characteristics:

• Completeness:
  • Complete Dislocation (Luxation): The joint surfaces are entirely separated.
  • Subluxation: The joint surfaces are only partially separated but remain in some contact.
• Open vs. Closed:
  • Closed Dislocation: The skin remains intact over the dislocated joint.
  • Open (Compound) Dislocation: The skin is broken, and the joint communicates with the external environment, significantly increasing the risk of infection.
• Acute vs. Chronic/Recurrent:
  • Acute Dislocation: A new, single episode of dislocation.
  • Chronic/Recurrent Dislocation: Repeated dislocations of the same joint, often due to underlying ligamentous laxity or unaddressed structural damage from a previous injury.

Diagnosis and Importance of Prompt Medical Attention

Diagnosis of a posterior dislocation typically involves:

• Clinical Examination: Assessment of pain, swelling, deformity, range of motion, and neurovascular status (pulses, sensation, motor function).
• Imaging Studies:
  • X-rays: Initial imaging to confirm dislocation and identify associated fractures. Special views may be necessary, especially for shoulder dislocations.
  • CT Scans: Often used to provide detailed bony anatomy, assess for subtle fractures, and better visualize complex dislocations like those of the hip or SC joint.
  • MRI Scans: Useful for evaluating soft tissue injuries, such as ligament tears, joint capsule damage, and nerve involvement.

Prompt medical attention is crucial for all dislocations, particularly posterior ones. Delayed reduction (putting the bone back into place) can lead to:

• Increased swelling and pain.
• Greater difficulty in reduction.
• Higher risk of avascular necrosis (bone death due to lack of blood supply), especially in the hip.
• Increased potential for permanent nerve or blood vessel damage.

Recovery and Rehabilitation Principles

Once a posterior dislocation has been reduced, the immediate focus is on protecting the joint and allowing initial healing. This typically involves:

• Immobilization: Using a sling, brace, or cast for a prescribed period to prevent re-dislocation and allow soft tissues to heal.
• Pain and Swelling Management: Rest, ice, compression, and elevation (RICE) alongside pain medication.
• Gradual Rehabilitation: Under the guidance of a physical therapist, rehabilitation progresses through stages:
  • Restoring Range of Motion (ROM): Gentle exercises to regain joint flexibility.
  • Strengthening: Progressive resistance exercises to rebuild muscle strength around the joint for stability.
  • Proprioception and Balance Training: Exercises to improve the body's awareness of joint position and enhance dynamic stability.
  • Return to Activity: Gradual progression back to daily activities, work, and sport, often with specific guidance on preventing re-injury.

Understanding the specific type of posterior dislocation, its mechanism, and potential complications is vital for effective diagnosis, treatment, and rehabilitation, ultimately aiming to restore joint function and prevent recurrence.