What is a dislocation of the femoral head component after hip replacement surgery?

Dislocation of Femoral Head Component After Hip Replacement

A dislocation of the femoral head component after hip arthroplasty occurs when the prosthetic femoral head separates from the acetabular component, resulting in loss of normal articulation and requiring urgent reduction to prevent complications including pain, functional limitation, and potential damage to the prosthesis.

Definition and Mechanism

Dislocation of the femoral head component is one of the most common complications following total hip arthroplasty (THA). It occurs when the femoral head component separates from the acetabular socket, resulting in:

• Loss of normal articulation between components
• Abnormal positioning of the femoral head relative to the acetabulum
• Significant pain and functional limitation for the patient

Dislocations are classified according to the direction of displacement:

• Posterior dislocations (most common): Femoral head displaces posteriorly, typically occurring with hip flexion, adduction, and internal rotation
• Anterior dislocations (less common): Femoral head displaces anteriorly, typically occurring with hip extension and external rotation 1

Risk Factors

Several factors increase the risk of femoral head component dislocation:

• Surgical approach: Posterolateral approaches have higher dislocation rates compared to anterior, anterolateral, and straight lateral approaches 2
• Femoral head size: Smaller head sizes (22-28mm) have higher dislocation rates than larger sizes (32mm or 36mm) 2, 3
• Component positioning: Excessive anteversion or retroversion of either the acetabular or femoral components 1
• Patient factors: Advanced age, neuromuscular disorders, cognitive impairment, previous hip surgery
• Post-operative compliance: Failure to follow hip precautions during recovery

Clinical Presentation

Patients with a dislocated femoral head component typically present with:

• Sudden onset of severe hip pain
• Visible deformity of the affected limb
• Inability to bear weight
• Limb shortening (in posterior dislocations)
• Limb held in external rotation (in posterior dislocations) or internal rotation (in anterior dislocations)
• Limited and painful range of motion

Diagnosis

Diagnosis is confirmed through:

• Physical examination: Assessing limb position, length discrepancy, and range of motion
• Radiographic evaluation: AP pelvis and lateral hip radiographs to confirm dislocation and assess component positioning
• CT scan: May be used to evaluate component position, version, and to plan management 1

Management

Acute Management

1. Closed reduction: Initial treatment involves closed reduction under sedation or anesthesia

   • Posterior dislocations: Longitudinal traction with internal rotation 4
   • Anterior dislocations: Inline traction with external rotation 4, 1

2. Post-reduction care:

   • Immobilization in a safe position (for anterior dislocations, the "deck chair position" of 45° flexion, 10-20° abduction, and neutral rotation for approximately 2 weeks) 1
   • Hip precautions to prevent recurrence
   • Post-reduction radiographs to confirm proper reduction and component position

Management of Recurrent Dislocations

For patients with recurrent dislocations, options include:

• Conservative management: Bracing, activity modification, and physical therapy
• Surgical intervention:
  • Revision of malpositioned components
  • Exchange to larger femoral head size (32mm or 36mm) 2, 3
  • Conversion to dual mobility components, which have shown lower dislocation rates 3
  • Use of constrained liners in cases of severe instability

Complications of Untreated Dislocations

Chronic dislocations can lead to serious complications:

• Damage to prosthetic components
• Soft tissue damage and scarring
• Neurovascular injury
• Pressure sores in bedridden patients
• In extreme cases, protrusion of the femoral head through the skin 5

Prevention

Preventive measures include:

• Optimal component positioning: Proper anteversion and inclination of acetabular and femoral components
• Appropriate femoral head size selection: Using 32mm or larger heads when possible 2, 3
• Consideration of surgical approach: Anterior or anterolateral approaches may reduce dislocation risk compared to posterolateral approach 2
• Patient education: Clear instructions on hip precautions and movement restrictions
• Early identification of at-risk patients: Closer monitoring for those with risk factors

Key Pitfalls to Avoid

• Delayed diagnosis: Prompt recognition and reduction are essential to prevent complications
• Inadequate imaging: Ensure proper radiographic evaluation before and after reduction
• Failure to identify the cause: Recurrent dislocations require thorough evaluation of component positioning and patient factors
• Overlooking soft tissue tension: Proper soft tissue balance is crucial for hip stability
• Neglecting patient education: Clear instructions on hip precautions are essential for preventing dislocations