CIM Case: Hip Dislocation After THR

Clinical Scenario

Patient: 72-year-old woman Presentation: Acute right hip pain and inability to move leg after bending to tie shoelaces, 6 weeks post-THR Relevant history: Posterior approach THR 6 weeks ago for primary OA, 28mm femoral head, uncomplicated surgery, first dislocation, had been compliant with precautions, lives with husband Examination findings:

• Hip held in flexion, adduction, and internal rotation (classic posterior dislocation position)
• Leg appears shortened compared to contralateral
• Unable to actively or passively range the hip
• Severe pain with any attempted movement
• Neurovascularly intact distally (palpable pulses, sensation intact, able to move toes)
• No wound complications visible

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Investigations Provided

Laboratory Results

Imaging

Image 1: AP Pelvis Radiograph

Radiological features:

• Right THR with femoral head dislocated posteriorly and superiorly
• Femoral head sitting above and posterior to acetabular component
• No periprosthetic fracture visible
• Acetabular cup appears retroverted (opening facing posteriorly)
• Estimated cup inclination appears steep (>50°)
• Left hip normal

Image 2: Post-reduction AP Pelvis and Lateral Hip

Post-reduction findings:

• Femoral head concentrically reduced within acetabular liner
• No new fracture
• Cup version assessment: appears retroverted on AP and lateral views

Questions & Model Answers

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Key Teaching Points

Pattern Recognition

This pattern suggests Posterior THR Dislocation:

• Hip held in flexion, adduction, internal rotation
• Leg shortened and internally rotated
• Mechanism: flexion + adduction + internal rotation combined (bending forward with knees together)
• Posterior approach has higher dislocation risk than anterior approach

Distinguish Anterior vs Posterior Dislocation:

Lewinnek Safe Zone:

Note: Lewinnek safe zone has been questioned - many dislocations occur within these parameters. Combined anteversion (stem + cup) may be more important.

Critical Management Points

1. Reduce urgently - within 6 hours ideally
2. General anaesthesia preferred - adequate muscle relaxation
3. Check stability under fluoro - document safe ROM
4. First dislocation = conservative - bracing, precautions, physio
5. Recurrent dislocation = surgery - dual mobility increasingly first-line
6. CT for component position - assess version, especially if recurrent

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Common Examiner Follow-ups

Q: "What positions should patients avoid after posterior approach THR?"

Avoid combined flexion + adduction + internal rotation:

• Flexion >90° - avoid low chairs, deep squatting
• Adduction across midline - pillow between legs when sleeping
• Crossing legs - keep legs apart
• Bending forward with knees together - use long-handled reaching aids
• Duration: typically 6-12 weeks, some surgeons for 3 months

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Q: "What is dual mobility and how does it work?"

Dual mobility construct:

• Design: Small inner head articulating with a large outer head, which articulates with a metal shell
• Mechanism: Two articulations increase total arc of motion and "jump distance"
• Jump distance: Distance head must travel before dislocating (larger head = greater distance)
• Advantages: Reduces dislocation risk, lower constraint than constrained liner
• Drawbacks: Potential for intraprosthetic dislocation, polyethylene wear concerns (improved with modern PE)

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Q: "What is a constrained liner and when would you use it?"

Constrained liner:

• Design: Polyethylene liner with a mechanical locking ring that captures the head
• Mechanism: Physical barrier prevents dislocation
• Indication: Recurrent instability, abductor deficiency, neuromuscular disorders
• Drawbacks:
  • Higher forces transmitted to bone-implant interface
  • Increased risk of loosening
  • If constraint fails, dislocation can still occur
  • Essentially "uses up" a surgical option

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Q: "What are the risk factors for THR dislocation?"

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Related Topics

• THR Surgical Approaches
• Dual Mobility Hip Arthroplasty
• Component Positioning in THR
• Lewinnek Safe Zone
• Revision Hip Arthroplasty