# 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 --- ## Investigations Provided ### Laboratory Results | Test | Result | Normal Range | Interpretation | |------|--------|--------------|----------------| | Hb | 112 g/L | 120-150 g/L | ↓ Mild post-op anaemia | | WCC | 8.5 ×10⁹/L | 4-11 ×10⁹/L | Normal | | CRP | 45 mg/L | <5 mg/L | ↑ Expected post-op elevation | | Creatinine | 72 μmol/L | 45-90 μmol/L | Normal | ### 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 **Diagnosis:** Posterior dislocation of total hip replacement **Mechanism:** **Position:** Flexion + adduction + internal rotation (combined) **Activity:** Bending forward (tying shoelaces) with knees together **Pathomechanics:** Femoral neck impinges on posterior acetabular rim, levers head out **Risk Factors:** **Patient Factors:** - Female sex (more lax soft tissues) - Advanced age - Neuromuscular disease (Parkinson's, stroke) - Previous hip surgery - Hypermobility - Poor compliance with precautions **Surgical Factors:** - Posterior approach (higher risk than anterior) - Inadequate soft tissue repair - Component malposition (cup retroversion, excessive anteversion) - Small head size (22-28mm) - Impingement (bony, soft tissue, component) > **Consultant Tip:** Flexion + adduction + internal rotation = posterior dislocation. Know the positions to avoid. **Resuscitation:** - Analgesia (IV opioids) - Assessment for associated injuries - NBM in case of theatre **Investigations:** **Xray:** - AP pelvis and lateral hip - Confirm dislocation direction - Look for periprosthetic fracture - Assess component position (retroversion?) **Closed Reduction:** **Timing:** Urgent - within 6 hours ideally **Setting:** Theatre or ED with adequate sedation **Anaesthesia:** General anaesthesia preferred, or deep sedation **Technique:** - Patient supine on floor or firm surface - Assistant stabilises pelvis - Hip flexed to 90° - Apply longitudinal traction - Gentle internal then external rotation - Feel for 'clunk' of reduction **Post Reduction:** - Check X-ray to confirm concentric reduction - Assess stability through ROM under fluoro - Document safe ROM > **Consultant Tip:** Reduce urgently. General anaesthesia is preferred for muscle relaxation. Beware the unreducible dislocation (soft tissue interposition). **Acute Management:** **Post Reduction Xray:** Confirm concentric reduction **Stability Assessment:** Intraoperative ROM assessment under fluoro **Immobilisation:** Hip abduction brace for 6-12 weeks **Investigate For Cause:** **Imaging:** - CT: component version assessment - Ideal cup anteversion: 15-20° - Ideal cup inclination: 40-45° (Lewinnek safe zone) **Questions:** - Was this the first dislocation? - What was the position/activity? - Review operative note for approach, component sizes, repair **Conservative Management:** **First Dislocation:** - Hip precautions education - Abduction brace 6-12 weeks - Physiotherapy for hip strengthening - Home modifications (raised toilet, avoid low chairs) **Precautions:** - Avoid combined flexion/adduction/IR - Use pillow between legs when sleeping - Avoid crossing legs - Avoid low chairs > **Consultant Tip:** First dislocation: conservative. Get CT for component position. Review operative notes. **Recurrent Dislocation:** **Definition:** 2 or more dislocations **Significance:** High risk of further recurrence, usually needs surgery **Surgical Options:** **Component Revision:** **Indication:** Malpositioned components **Procedure:** Revise cup/stem to optimal position **Cup Position:** Anteversion 15-20°, inclination 40-45° **Constrained Liner:** **Indication:** Recurrent instability, well-positioned components, abductor deficiency **Mechanism:** Captive polyethylene liner prevents dislocation **Drawback:** Higher forces at bone-implant interface, risk of loosening **Dual Mobility Construct:** **Mechanism:** Large outer head articulating with cup, smaller inner head in outer head **Advantages:** - Increased jump distance - Lower constraint forces than constrained liner - Addresses soft tissue laxity **Indication:** Recurrent instability, first-line surgical option increasingly **Larger Head:** **Mechanism:** Increases jump distance and head-neck ratio **Limitation:** May not be sufficient for recurrent dislocators **Tripolar Construct:** **Indication:** Salvage for multiply recurrent dislocations **Soft Tissue Procedures:** **Indication:** Posterior approach with poor soft tissue repair **Procedure:** Posterior capsular repair/reconstruction **Limitation:** Often insufficient in isolation > **Consultant Tip:** Dual mobility is increasingly first-line for recurrent instability. Constrained liners have higher loosening risk. **CT for Component Assessment:** **Indications:** - Recurrent dislocation - Suspected component malposition - Planning revision surgery - When plain films are inconclusive **CT Protocol:** | Measurement | Technique | Normal Range | |-------------|-----------|--------------| | **Cup anteversion** | Axial cuts through cup | 15-25° | | **Cup inclination** | Reformatted coronal | 30-50° | | **Stem version** | Axial at lesser trochanter | 5-25° anteversion | | **Combined anteversion** | Cup + stem | 25-50° | **Combined Anteversion Concept:** | Combined Anteversion | Stability Risk | |---------------------|----------------| | <25° | ↑ Risk anterior dislocation | | 25-50° | Optimal range | | >50° | ↑ Risk posterior dislocation | **Cup Inclination (Lewinnek):** | Inclination | Implication | |-------------|-------------| | <30° | Risk impingement, ↓ ROM | | 30-50° | Safe zone | | >50° | Risk dislocation, edge loading | **Common Malposition Patterns:** | Position Error | Dislocation Direction | |----------------|----------------------| | Excessive anteversion | Posterior dislocation | | Retroversion | Anterior dislocation | | High inclination | Global instability | | Low offset | Impingement, instability | **CT Findings in This Patient:** - CT would likely show cup retroversion or combined anteversion at lower end of range - Steep inclination (>50°) may also be present > **Consultant Tip:** Combined anteversion matters more than cup version alone. Target 25-50° combined for stability. **Pre-Revision Counselling:** **Understanding the Problem:** - Explain why dislocations are occurring (component position, soft tissue laxity) - Discuss the findings on CT (malposition if present) - Explain that conservative management has failed **Revision Surgery Options:** | Option | When Used | Expected Outcome | |--------|-----------|-----------------| | **Component revision** | Malpositioned components | Good if position corrected | | **Dual mobility cup** | Soft tissue laxity, abductor deficiency | Reduces dislocation to <2% | | **Constrained liner** | Abductor deficiency, revision setting | Stability but ↑ loosening risk | | **Larger head** | Minor instability, cup allows | May be insufficient alone | **Expected Outcomes:** | Outcome | Rate | |---------|------| | Resolution of instability | 85-95% | | Further dislocation | 5-10% | | Re-revision at 5 years | 10-15% | | Infection | 2-3% | | Need for constrained liner in future | 5-10% | **What to Tell Patient:** **Realistic Expectations:** - High chance of resolving instability (~90%) - Revision is bigger surgery than primary - longer recovery - May need constrained implants limiting ROM - Small risk of further problems requiring additional surgery - Abductor weakness may persist (affects gait) **Lifestyle Modifications:** - May need ongoing precautions even after revision - Activity modifications may be permanent - Regular follow-up required **Risk Discussion:** | Risk | Approximate Rate | |------|-----------------| | Recurrent dislocation | 5-10% | | Infection | 2-3% | | Nerve injury | <1% | | Fracture | 1-2% | | Blood transfusion | 20-30% | | Medical complications | 5-10% | **Decision Making:** - Patient must understand this is for quality of life, not survival - Alternative is lifelong bracing and precautions - Shared decision making essential > **Consultant Tip:** Be honest about outcomes. Revision for instability has good results (~90% stable) but is not without risk. Dual mobility has become first-line for most revision scenarios. --- ## 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:** | Feature | Posterior Dislocation | Anterior Dislocation | |---------|----------------------|---------------------| | Position | Flexion, adduction, IR | Extension, abduction, ER | | Mechanism | Bending + crossing legs | Hyperextension + ER | | Approach risk | Posterior approach | Anterior approach | | Frequency | More common | Less common | **Lewinnek Safe Zone:** | Parameter | Target | Safe Range | |-----------|--------|------------| | Cup inclination | 40° | 30-50° | | Cup anteversion | 15° | 5-25° | **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 --- ## 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 --- **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) --- **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 --- **Q: "What are the risk factors for THR dislocation?"** | Category | Risk Factors | |----------|--------------| | **Patient** | Female, advanced age, neuromuscular disease, cognitive impairment, prior hip surgery | | **Surgical** | Posterior approach, inadequate soft tissue repair, component malposition | | **Implant** | Small head size (<32mm), elevated rim liner not used | | **Soft tissue** | Abductor deficiency, capsular damage | --- ## Related Topics - THR Surgical Approaches - Dual Mobility Hip Arthroplasty - Component Positioning in THR - Lewinnek Safe Zone - Revision Hip Arthroplasty