Femoral Dislocations: Types, Mechanisms, Complications, and Treatment

What are the different types of femoral dislocations?

Femoral dislocations, more commonly known as hip dislocations, occur when the head of the femur separates from the acetabulum of the pelvis, and are primarily categorized by the direction of displacement: posterior, anterior, or, less commonly, central.

Understanding the Hip Joint Anatomy

To fully grasp femoral dislocations, a brief review of the hip joint's anatomy is essential. The hip is a classic ball-and-socket joint, renowned for its stability and wide range of motion. The "ball" is the femoral head, the uppermost part of the femur (thigh bone), which fits snugly into the "socket," known as the acetabulum. The acetabulum is a concave depression in the pelvis.

This stability is reinforced by a strong joint capsule, powerful surrounding ligaments (iliofemoral, pubofemoral, ischiofemoral), and robust musculature. Despite these stabilizers, extreme forces can overcome the joint's integrity, leading to a dislocation.

What is a Femoral Dislocation?

A femoral dislocation is a severe orthopedic injury where the head of the femur is forcibly displaced from the acetabulum. Unlike many other joint dislocations, hip dislocations almost invariably result from high-energy trauma due to the inherent stability of the joint. Common causes include motor vehicle accidents, falls from significant heights, and high-impact sports injuries.

Prompt diagnosis and reduction are critical to minimize complications, as the blood supply to the femoral head can be compromised, leading to long-term issues.

Primary Classification: Posterior vs. Anterior

The most fundamental classification of femoral dislocations is based on whether the femoral head displaces behind (posterior) or in front of (anterior) the acetabulum.

Posterior Dislocation

• Prevalence: This is by far the most common type, accounting for approximately 85-90% of all hip dislocations.
• Mechanism of Injury: Typically results from a force applied to the knee while the hip is in a flexed, adducted, and internally rotated position. A classic example is the "dashboard injury" in a car accident, where the knee strikes the dashboard, driving the femur posteriorly.
• Clinical Presentation: The affected leg usually appears shortened, internally rotated, and adducted. The patient experiences severe pain and an inability to move the hip.
• Associated Injuries:
  • Sciatic Nerve Injury: The sciatic nerve runs close to the posterior aspect of the hip joint and is susceptible to stretching or compression, leading to foot drop or sensory deficits.
  • Posterior Wall Acetabular Fractures: Fragments of the acetabulum's posterior rim can be avulsed or fractured.
  • Femoral Head Fractures: Less common, but can occur due to direct impact.

Anterior Dislocation

• Prevalence: Less common than posterior dislocations, comprising about 10-15% of cases.
• Mechanism of Injury: Occurs when the hip is in abduction, external rotation, and often extension, with a force applied along the long axis of the femur or a direct blow to the back of the hip. Examples include falls with the leg splayed or specific sports injuries.
• Clinical Presentation: The affected leg typically appears shortened, externally rotated, and abducted. The femoral head may be palpable in the groin region.
• Associated Injuries:
  • Femoral Head Fractures: Can occur due to direct impact.
  • Neurovascular Compromise: Though less common than sciatic nerve injury in posterior dislocations, the femoral artery and nerve are located anteriorly and can be at risk.

Subtypes of Anterior Dislocation

Anterior dislocations are further sub-classified based on the final resting position of the femoral head relative to the pelvis:

• Obturator Dislocation: The femoral head dislocates inferiorly and medially, resting near the obturator foramen. The leg is typically markedly abducted and externally rotated.
• Pubic Dislocation: The femoral head dislocates superiorly and medially, resting on the pubic ramus or superior pubic bone. The leg is usually extended, externally rotated, and moderately abducted.
• Iliac Dislocation: The femoral head dislocates superiorly and laterally, coming to rest near the anterior superior iliac spine. This is a rare subtype.

Central Dislocation (Acetabular Fracture-Dislocation)

While sometimes listed as a type of dislocation, central dislocations are more accurately described as acetabular fracture-dislocations. In this scenario, the femoral head is driven directly through the acetabulum, fracturing the pelvic bone in the process.

• Mechanism of Injury: Typically results from a direct lateral impact to the greater trochanter (e.g., a side impact in a car accident or a fall directly onto the hip).
• Severity: Always involves a significant acetabular fracture, making it a more complex injury often requiring surgical intervention.
• Clinical Presentation: Variable, but often presents with significant pain, swelling, and limited hip movement.

Rarity: Inferior Dislocation

Extremely rare, an inferior dislocation (also known as a luxatio erecta of the hip) occurs when the femoral head dislocates directly downwards.

• Mechanism of Injury: Usually involves extreme flexion and abduction of the hip, often with a rotational component.
• Clinical Presentation: The leg is typically held in a "frog-leg" position, severely flexed, abducted, and externally rotated, often with the foot resting near the head.

Risk Factors and Complications

Understanding the potential ramifications of femoral dislocations underscores the importance of prompt treatment.

Risk Factors

• High-Energy Trauma: Motor vehicle accidents are the leading cause.
• Falls from Height: Significant impact can overcome joint stability.
• Sports Injuries: Especially in contact sports or activities with high-impact falls.
• Pre-existing Hip Conditions: Conditions like hip dysplasia or previous hip surgery can increase vulnerability.
• Joint Laxity: Rarely, individuals with hypermobility may be more prone.

Complications

• Avascular Necrosis (AVN) of the Femoral Head: The most severe complication. Dislocation can disrupt the blood supply to the femoral head, leading to bone death. The risk increases with the duration of dislocation.
• Post-traumatic Osteoarthritis: Damage to the articular cartilage during dislocation or reduction can lead to premature joint degeneration.
• Sciatic Nerve Injury: Particularly common in posterior dislocations, leading to weakness or numbness in the leg and foot.
• Femoral Nerve/Artery Injury: Possible in anterior dislocations.
• Recurrent Dislocation: Although rare after a primary traumatic dislocation, it can occur, especially if there are associated fractures or ligamentous damage.
• Heterotopic Ossification: Abnormal bone formation in the soft tissues around the hip joint.

Diagnosis and Treatment

Diagnosis typically involves a thorough clinical examination, assessing the limb's position and neurovascular status, followed by imaging studies. X-rays (anterior-posterior and lateral views) are usually sufficient to confirm the diagnosis and classify the type of dislocation. A CT scan may be performed after reduction to assess for associated fractures (e.g., acetabular wall, femoral head) or incarcerated fragments within the joint.

Treatment is an orthopedic emergency. The primary goal is prompt reduction of the dislocation, ideally within 6 hours, to minimize the risk of avascular necrosis.

• Closed Reduction: Most dislocations are managed with closed reduction under sedation and muscle relaxation. Specific maneuvers are used to guide the femoral head back into the acetabulum.
• Open Reduction: If closed reduction fails, or if there are associated fractures or incarcerated fragments, open surgical reduction may be necessary.
• Post-Reduction Care: Immobilization, pain management, and a structured rehabilitation program are crucial for recovery and to restore strength and mobility.

Conclusion

Femoral dislocations are severe injuries primarily categorized as posterior or anterior, with less common central and inferior types. Each type presents with distinct mechanisms of injury and clinical signs, often accompanied by significant associated trauma. Due to the high risk of complications such as avascular necrosis and nerve damage, immediate medical attention and prompt reduction are paramount for optimizing outcomes and ensuring the best possible recovery for the affected individual.