Posterior Hip Dislocation: Causes, Mechanisms, and Management

Why do hips dislocate posteriorly?

Posterior hip dislocations primarily occur due to high-energy trauma, often involving a combination of hip flexion, adduction, and internal rotation, which forces the femoral head out of the acetabulum through the weaker posterior joint capsule.

Understanding Hip Anatomy: A Foundation for Dislocation

The hip joint is a classic ball-and-socket synovial joint, designed for both mobility and stability. It comprises the femoral head (the ball) of the thigh bone (femur) and the acetabulum (the socket) of the pelvis. This deep socket, further deepened by the acetabular labrum (a fibrocartilaginous rim), provides significant inherent stability.

However, stability is also heavily reliant on a robust network of soft tissues:

• Joint Capsule: A strong fibrous sac enclosing the joint, reinforced by distinct ligaments.
• Iliofemoral Ligament (Y-ligament of Bigelow): The strongest ligament in the body, located anteriorly, resisting hip extension.
• Pubofemoral Ligament: Anterior and inferior, resisting hip abduction and extension.
• Ischiofemoral Ligament: Located posteriorly, resisting hip extension and internal rotation. While strong, the posterior aspect of the joint capsule, overall, is generally considered the path of least resistance for dislocation under specific forces.
• Surrounding Musculature: Powerful muscles such as the gluteals (maximus, medius, minimus), hamstrings, and deep external rotators (e.g., piriformis) dynamically stabilize the joint.

The Mechanics of Posterior Hip Dislocation

Posterior dislocation is the most common type of hip dislocation, accounting for over 90% of cases. It is almost always a result of significant, high-energy trauma.

Common Mechanisms and Forces:

The hallmark of a posterior hip dislocation is a force applied along the long axis of the femur while the hip is in a vulnerable position.

• Dashboard Injury (Motor Vehicle Accidents - MVAs): This is the classic mechanism. When an occupant's knee strikes the dashboard during a frontal collision, the impact drives the femur posteriorly. The hip is typically in a position of:
  • Flexion: Usually 60-90 degrees (knees bent).
  • Adduction: Thighs are closer together or crossing the midline.
  • Internal Rotation: The foot may be caught or the leg twisted inward. This combined position creates an opening or "window" for the femoral head to be levered out of the acetabulum posteriorly.
• Falls from Height: Landing directly on the side of the hip or knee in a flexed, adducted position.
• Sports Injuries: High-impact sports like football, rugby, or skiing, where direct blows to the knee or hip occur while the limb is positioned similarly to the dashboard injury scenario.
• Direct Blow to the Greater Trochanter: A forceful impact to the outside of the hip, especially if the limb is internally rotated.

Direction of Force: The force essentially pushes the femoral head directly backward, tearing through the posterior joint capsule and potentially the ischiofemoral ligament, allowing the head to escape the acetabulum and rest posteriorly to it. The strong anterior ligaments (iliofemoral and pubofemoral) help prevent anterior dislocation but do little to resist a posterior force.

Risk Factors and Predisposing Conditions

While high-energy trauma is the primary cause, certain factors can increase susceptibility.

• Traumatic Events: As detailed above, MVAs, significant falls, and high-impact sports are the leading causes. The sheer magnitude of force often overcomes the hip joint's inherent stability.
• Neurological Conditions: Conditions leading to muscle weakness or spasticity around the hip (e.g., cerebral palsy, spinal cord injury) can increase dislocation risk, although these are more commonly associated with chronic or recurrent dislocations rather than acute traumatic ones.
• Congenital Hip Dysplasia: A condition where the acetabulum is abnormally shallow or the femoral head is not well-seated, leading to reduced inherent stability. While often managed in childhood, residual dysplasia can increase vulnerability to dislocation later in life, even with less significant trauma.
• Prior Hip Surgery (e.g., Total Hip Arthroplasty - THA): Patients who have undergone total hip replacement, especially via a posterior surgical approach, have a higher risk of posterior dislocation. This is because the posterior capsule and external rotator muscles are often incised during the surgery, weakening the posterior structures. Post-operative precautions are critical to prevent this.
• Ligamentous Laxity: Rarely, generalized ligamentous laxity (e.g., in conditions like Ehlers-Danlos syndrome) can contribute to an increased risk of dislocation, though traumatic force is usually still required for the hip.

Clinical Presentation and Immediate Concerns

A posterior hip dislocation is a medical emergency. The clinical presentation is typically unmistakable:

• Severe Pain: Intense, debilitating pain in the hip and groin.
• Deformity: The affected limb appears shortened, internally rotated, and adducted. The foot often points inward.
• Inability to Bear Weight: The individual cannot move the leg or put any weight on it.

Immediate concerns include potential damage to surrounding structures:

• Sciatic Nerve Injury: The sciatic nerve runs directly posterior to the hip joint and can be stretched or compressed, leading to numbness, tingling, or weakness in the leg and foot.
• Avascular Necrosis (AVN) of the Femoral Head: Dislocation can disrupt the blood supply to the femoral head, potentially leading to bone death if not reduced promptly. The longer the dislocation persists, the higher the risk of AVN.
• Associated Fractures: Fractures of the acetabulum or femoral head can occur concurrently.

Management and Rehabilitation Principles

Immediate medical attention is crucial for a posterior hip dislocation.

• Reduction: The primary goal is prompt reduction (relocation) of the femoral head back into the acetabulum, usually performed under sedation or anesthesia. The Allis maneuver (traction in line with the deformity, then internal rotation, flexion, and external rotation) or Stimson maneuver (prone, with leg hanging) are common techniques.
• Post-Reduction Assessment: Imaging (X-rays, CT scan) is performed to confirm successful reduction and rule out associated fractures or incarcerated fragments within the joint.
• Rehabilitation: Once reduced, rehabilitation focuses on pain management, restoring range of motion, and gradually strengthening the muscles around the hip to enhance stability. Precautions against re-dislocation (avoiding extreme hip flexion, adduction, and internal rotation) are critical, especially in the initial weeks to months.

Prevention Strategies

While traumatic dislocations are often unavoidable due to the nature of the forces involved, certain measures can reduce risk:

• Vehicle Safety: Consistent use of seatbelts and proper child safety restraints greatly reduces the risk of dashboard injuries.
• Sports Safety: Proper training techniques, appropriate protective gear, and adherence to rules can mitigate injury risk in high-impact sports.
• Post-Surgical Precautions: For individuals with total hip replacements, strict adherence to post-operative movement precautions provided by their surgeon and physical therapist is paramount to prevent re-dislocation.
• Strength and Stability Training: For the general population, maintaining strong core and hip musculature, coupled with good proprioception, contributes to overall joint stability and resilience, potentially reducing injury severity, though not preventing the most extreme traumatic forces.

Conclusion

Posterior hip dislocation is a severe injury resulting from significant external forces that overcome the inherent stability of the hip joint. The typical mechanism involves a combination of hip flexion, adduction, and internal rotation, which leverages the femoral head out through the weaker posterior capsule. Understanding these biomechanical principles is crucial for both immediate clinical management and the development of effective preventative strategies, particularly in high-risk scenarios like motor vehicle accidents and post-surgical recovery.