Posterior Superior Hip Dislocations: Damaged Structures, Complications, and Management

Which structure may be damaged in posterior superior hip dislocations?

Posterior superior hip dislocations commonly endanger the sciatic nerve, the femoral head (leading to osteonecrosis or chondral damage), and the acetabulum (fractures), alongside various ligaments and the joint capsule due to the significant forces involved.

Introduction to Hip Dislocations

A hip dislocation is a severe orthopedic injury where the head of the femur (thigh bone) is forcibly displaced from the acetabulum (the socket in the pelvis). This high-energy trauma typically results from significant forces, such as those sustained in motor vehicle accidents, falls from heights, or contact sports. Given the inherent stability of the hip joint—a ball-and-socket joint designed for robust weight-bearing and mobility—dislocation signifies substantial damage to the surrounding bony and soft tissue structures.

Understanding Posterior Superior Hip Dislocation

The vast majority (approximately 90%) of hip dislocations are posterior. A posterior superior hip dislocation occurs when the femoral head is driven posteriorly and superiorly out of the acetabulum. This mechanism most commonly happens when the hip is flexed, adducted, and internally rotated at the moment of impact. A classic example is the "dashboard injury" in a car accident, where the knee strikes the dashboard, transmitting force up the femur and dislocating the hip posteriorly.

Primary Structures at Risk of Damage

The forces involved in a posterior superior hip dislocation can lead to a cascade of injuries to various anatomical structures. Understanding these potential damages is crucial for diagnosis, treatment, and predicting long-term outcomes.

Sciatic Nerve

The sciatic nerve is the most frequently injured nerve in posterior hip dislocations, with an incidence rate ranging from 10% to 20%. Its vulnerability stems from its anatomical proximity as it exits the pelvis through the greater sciatic foramen, passing immediately posterior to the hip joint capsule and deep to the gluteus maximus muscle.

• Mechanism of Injury: The nerve can be stretched, compressed, or directly contused by the dislocated femoral head or by associated bony fragments (e.g., posterior acetabular wall fractures).
• Clinical Presentation: Damage typically manifests as foot drop (weakness in dorsiflexion of the ankle and toes), sensory loss in the posterolateral leg and foot, and often intense pain radiating down the leg.

Femoral Head

The femoral head itself is highly susceptible to damage during a posterior superior dislocation.

• Chondral Damage: The articular cartilage covering the femoral head can be bruised, scuffed, or directly sheared off as it impacts the rim of the acetabulum during dislocation or during the reduction maneuver. This damage predisposes the joint to premature osteoarthritis.
• Osteonecrosis (Avascular Necrosis - AVN): This is a critical long-term complication, occurring in 5% to 40% of cases. The primary blood supply to the femoral head, particularly the medial circumflex femoral artery and its retinacular branches, can be disrupted or thrombosed during dislocation. If blood flow is significantly compromised, the bone tissue of the femoral head dies, leading to collapse and severe osteoarthritis. The risk of AVN increases significantly with delayed reduction of the dislocation.

Acetabulum

The acetabulum, the socket portion of the hip joint, is frequently injured.

• Posterior Acetabular Rim Fractures: As the femoral head is driven posteriorly, it can shear off a portion of the posterior wall or rim of the acetabulum. These fractures are common and can significantly compromise hip stability, leading to recurrent dislocations if not appropriately managed.
• Acetabular Labrum Tears: The acetabular labrum is a fibrocartilaginous ring that deepens the acetabular socket and enhances joint stability. It can be torn or avulsed during the dislocation, contributing to instability and pain.

Ligamentum Teres

The ligamentum teres is an intra-articular ligament connecting the fovea of the femoral head to the acetabular notch. While its primary role in adult hip stability is debated, it contains a small artery (foveal artery) that contributes to the femoral head's blood supply, particularly in children.

• Mechanism of Injury: It is commonly ruptured during hip dislocation.
• Implications: Its rupture can contribute to joint instability and may have implications for femoral head blood supply, especially in younger individuals.

Capsular Ligaments

The strong fibrous joint capsule and its reinforcing ligaments are inevitably stretched or torn during a dislocation.

• Ischiofemoral Ligament: Located on the posterior aspect of the hip joint, this ligament is particularly vulnerable in posterior dislocations.
• Iliofemoral and Pubofemoral Ligaments: While primarily anterior and inferior, respectively, these strong ligaments can also be strained or torn depending on the exact forces and position of the hip during injury, contributing to overall capsular damage.

Other Neurovascular Structures

While less common than sciatic nerve injury, other neurovascular structures may be at risk:

• Superior Gluteal Artery and Nerve: These structures pass superior to the piriformis muscle and can be damaged, particularly if the dislocation involves significant superior displacement or associated pelvic fractures.
• Obturator Nerve and Artery: Located more anteriorly and medially, these are less commonly injured in pure posterior superior dislocations but can be affected in complex hip trauma.

Biomechanical Considerations and Mechanism of Injury

The biomechanics of a posterior superior hip dislocation involve a complex interplay of forces. When the hip is flexed, adducted, and internally rotated, the femoral neck impinges against the posterior acetabular rim. Continued force drives the femoral head posteriorly, causing it to lever out of the socket. This leveraging action, combined with direct impact, creates significant shear and compressive forces that directly impact the posterior acetabulum, sciatic nerve, and the blood supply to the femoral head. The sudden, violent disruption also stretches and tears the strong capsular ligaments and the ligamentum teres.

Clinical Implications and Long-Term Concerns

The comprehensive damage associated with posterior superior hip dislocations carries significant long-term clinical implications:

• Post-traumatic Osteoarthritis: This is a common and debilitating complication, often resulting from initial chondral damage, associated fractures, or subsequent avascular necrosis.
• Avascular Necrosis (AVN): As previously noted, this can lead to collapse of the femoral head and severe joint degeneration.
• Recurrent Dislocation: Especially if there are significant posterior acetabular wall fractures, extensive capsular and labral tears, or inadequate initial reduction.
• Heterotopic Ossification: The abnormal formation of bone in soft tissues around the hip joint, which can limit range of motion and cause pain.

Diagnosis and Management

Prompt diagnosis and management are critical for minimizing long-term complications. X-rays are typically the initial diagnostic tool, followed by CT scans to assess for associated fractures (acetabular, femoral head) and to evaluate the congruity of the reduction. MRI may be used to assess soft tissue damage, including labral tears and early signs of AVN. The primary treatment involves urgent, gentle closed reduction of the hip, ideally within six hours, to restore joint congruity and reduce the risk of AVN. Surgical intervention may be required for irreducible dislocations, incarcerated fragments, or significant acetabular fractures.

Conclusion

Posterior superior hip dislocations are severe injuries that can cause extensive damage to both bony and soft tissue structures surrounding the hip joint. The sciatic nerve, femoral head (with risk of chondral damage and osteonecrosis), and the acetabulum (with risk of fractures and labral tears) are the most critical structures frequently compromised. Understanding the potential for these injuries underscores the importance of prompt diagnosis, careful assessment, and appropriate management to optimize patient outcomes and mitigate long-term disability.