What workup is recommended for a patient with a history of total knee replacement and ongoing knee pain not relieved by Non-Steroidal Anti-Inflammatory Drugs (NSAIDs)?

Workup for Persistent Pain After Total Knee Replacement

Begin with standard knee radiographs to evaluate for periprosthetic infection, aseptic loosening, osteolysis, component malalignment, and periprosthetic fracture, followed by joint aspiration if infection is suspected. 1

Initial Imaging

• Obtain weight-bearing knee radiographs (AP, lateral, and merchant views) as the first-line imaging study to assess for component positioning, periprosthetic lucency, osteolysis, hardware failure, and signs of infection 1
• Radiographs should specifically evaluate for:
  • Progressive radiolucent lines at the bone-cement or cement-prosthesis interface (>2mm suggests loosening) 1
  • Focal osteolysis, particularly around femoral condyles and tibial screw holes 1
  • Component subsidence or migration 1
  • Periprosthetic fractures 1
  • Soft tissue gas or abnormal effusions suggesting infection 1

Infection Workup (If Suspected)

If radiographs show concerning features or clinical suspicion exists for infection, proceed immediately to joint aspiration before any additional imaging. 1

Laboratory Studies

• Obtain serum inflammatory markers: ESR and CRP (elevated in infection but non-specific) 1
• Joint aspiration is the critical diagnostic step - send synovial fluid for:
  • Cell count with differential (>3000 WBC/μL or >80% PMNs suggests infection) 1
  • Gram stain and aerobic/anaerobic cultures 1
  • Hold cultures for extended periods if low-virulence organisms suspected 1

Advanced Imaging for Infection (If Aspiration Inconclusive)

• If initial aspiration is negative but infection remains suspected, perform repeat aspiration as recommended by AAOS guidelines 1
• Consider combined In-111 labeled leukocyte/Tc-99m sulfur colloid bone marrow scan if aspiration remains inconclusive - this combination achieves 96% sensitivity and 87% specificity for periprosthetic knee infection 1
• Standard three-phase bone scans have limited utility due to persistent uptake for years after TKR (positive in 20% at 1 year, 12.5% at 2 years in asymptomatic patients) 1

Aseptic Loosening Evaluation (After Infection Excluded)

Once infection is substantially excluded through negative aspiration and laboratory studies, focus on mechanical causes of failure. 1

Radiographic Assessment

• Fluoroscopy can demonstrate lucent lines in profile that may be obscured on standard radiographs and allows real-time assessment of component motion with stress 1
• Serial radiographs comparing to immediate post-operative films help identify progressive lucency or component migration 1

Cross-Sectional Imaging

• CT without IV contrast is usually appropriate for evaluating extent of osteolysis when radiographs suggest particle disease or granuloma formation 1
• CT better demonstrates:
  • Size and extent of osteolytic lesions 1
  • Intraosseous or soft-tissue gas 1
  • Reactive bone formation not evident on radiographs 1
• Metal artifact reduction techniques have improved CT utility in this setting 1

MRI Considerations

• MRI with metal artifact reduction sequences has 86-92% sensitivity and 85-87% specificity for detecting infection when lamellated hyperintense synovitis is present 1
• Can identify extracapsular abscess formation when IV contrast is used 1
• Limited routine use but may be helpful in selected cases 1

Additional Diagnostic Considerations

Instability Assessment

• Fluoroscopy with stress views can demonstrate abnormal component displacement suggesting ligamentous insufficiency 1
• Instability typically manifests within 4 years of primary arthroplasty and often requires revision 1

Soft Tissue Pathology

• Ultrasound can readily assess periprosthetic fluid collections and guide aspiration 1
• Look for signs of extensor mechanism disruption: patella alta/baja, posterior tibial subluxation, soft tissue swelling 1

Common Pitfalls to Avoid

• Do not rely solely on bone scintigraphy in the first 2 years post-TKR - positive uptake is common in asymptomatic patients and has poor specificity 1
• Avoid proceeding to advanced imaging before obtaining joint aspiration when infection is suspected - aspiration is more specific and cost-effective 1
• Do not assume normal radiographs exclude significant pathology - early loosening and infection may not show radiographic changes 1
• Arthrography has poor predictive value for loosening and is not typically recommended 1