Posterior Hip Dislocation: Common Complications, Sciatic Nerve Injury, and Prognosis

What is the most common complication of a posterior hip dislocation?

The most common complication of a posterior hip dislocation is sciatic nerve injury, affecting a significant percentage of patients and potentially leading to long-term functional deficits.

Understanding Hip Dislocation

A hip dislocation occurs when the head of the femur (thigh bone) is forced out of its socket (the acetabulum) in the pelvis. This is a severe injury, typically resulting from high-impact trauma such as motor vehicle accidents, falls from height, or contact sports. While dislocations can be anterior (femoral head moves forward) or posterior (femoral head moves backward), posterior dislocations are by far the most common type, accounting for approximately 90% of all hip dislocations. This is due to the typical mechanism of injury: a force applied to the knee with the hip and knee flexed, driving the femoral head posteriorly.

The Most Common Complication: Sciatic Nerve Injury

The sciatic nerve is the largest nerve in the human body, originating from the lower back and extending down the back of the leg. Its anatomical course makes it highly vulnerable in cases of posterior hip dislocation.

• Anatomical Vulnerability: The sciatic nerve passes directly posterior to the hip joint. When the femoral head dislocates posteriorly, it can directly compress, stretch, or even sever the sciatic nerve.
• Mechanism of Injury: The forceful displacement of the femoral head out of the acetabulum, particularly in a posterior direction, can cause direct impingement or traction injury to the nerve.
• Symptoms: Sciatic nerve injury manifests primarily as neurological deficits in the affected leg. These can include:
  • Foot drop: Weakness or inability to dorsiflex the ankle (lift the front part of the foot), leading to a characteristic "steppage gait." This is often due to damage to the common fibular (peroneal) division of the sciatic nerve.
  • Sensory deficits: Numbness, tingling, or altered sensation along the distribution of the sciatic nerve (typically the posterior thigh, lower leg, and foot).
  • Muscle weakness: Difficulty with knee flexion, ankle dorsiflexion, eversion, inversion, and toe movements.
  • Pain: While pain from the dislocation itself is severe, nerve pain can be distinct, described as burning, shooting, or radiating.
• Prognosis: While many sciatic nerve injuries associated with hip dislocation are neurapraxias (temporary nerve conduction block) or axonotmesis (axon damage with intact sheath) that can resolve partially or fully over weeks to months, a significant percentage can result in permanent neurological deficits, especially if the injury is severe or nerve reduction is delayed.

Other Potential Complications

While sciatic nerve injury is the most frequent, posterior hip dislocations can lead to several other serious complications:

• Avascular Necrosis (AVN) of the Femoral Head: This is a severe complication where the blood supply to the femoral head is disrupted, leading to the death of bone tissue. The hip joint receives its primary blood supply through vessels that run along the femoral neck and capsule. Dislocation, especially if reduction is delayed, can stretch or tear these vessels. AVN can result in collapse of the femoral head and severe osteoarthritis, often requiring total hip replacement. The risk increases with the duration of dislocation.
• Post-Traumatic Osteoarthritis: Even with successful reduction, the forceful impact and potential cartilage damage sustained during the dislocation can accelerate the development of osteoarthritis in the affected hip joint over time. This is due to direct cartilage injury, altered joint mechanics, and inflammatory responses.
• Recurrent Dislocation: Once dislocated, the hip joint may be more prone to future dislocations, especially if there is associated soft tissue damage (e.g., labral tear, capsular disruption) or bony abnormalities from the initial injury (e.g., acetabular fracture).
• Associated Fractures: It is common for hip dislocations to occur concurrently with fractures of the acetabulum (hip socket) or the femoral head or neck. These fractures can complicate treatment, increase the risk of other complications (like AVN), and worsen the long-term prognosis.
• Heterotopic Ossification: This is the abnormal formation of bone in soft tissues around the hip joint after trauma. While less common than nerve injury or AVN, it can limit range of motion and cause pain.

Immediate Management and Long-Term Implications

Prompt reduction of a dislocated hip is critical to minimize the risk of complications, particularly AVN and sciatic nerve injury. The longer the femoral head remains dislocated, the higher the risk of adverse outcomes. Following reduction, comprehensive rehabilitation is essential to restore strength, range of motion, and stability to the hip joint. Long-term follow-up is necessary to monitor for the development of AVN, osteoarthritis, or persistent neurological deficits.

Conclusion

A posterior hip dislocation is a medical emergency with significant potential for complications. While various issues can arise, sciatic nerve injury stands out as the most common immediate complication, directly impacting patient mobility and sensation. Understanding these risks underscores the importance of rapid diagnosis, emergent reduction, and diligent follow-up care to optimize outcomes and mitigate long-term disability.