What causes severe knee pain 2 years post knee replacement?

Evaluation of Severe Knee Pain Two Years After Total Knee Replacement

Begin with knee radiographs (weight-bearing AP, lateral, and axial views) combined with ESR and CRP blood tests to systematically exclude infection and mechanical complications—the two most critical causes of late post-operative pain. 1

Initial Diagnostic Approach

Clinical Assessment Priority

The pattern of pain provides critical diagnostic clues:

• Night pain or pain at rest strongly suggests periprosthetic infection 1, 2
• Pain with weight-bearing is more characteristic of mechanical loosening or component malposition 1, 2
• Infection accounts for 21.9% of revisions performed >2 years after initial surgery 1

Mandatory First-Line Testing

Obtain both imaging and laboratory studies simultaneously to avoid diagnostic delays:

Radiographic Evaluation

• Weight-bearing anteroposterior, lateral, and axial knee views 1
• Look specifically for: periprosthetic lucency, osteolysis, component loosening, polyethylene wear (joint space narrowing), malalignment, or fractures 1
• Full-length hip-to-ankle views if malalignment is suspected 1

Laboratory Testing

• ESR and CRP are essential—when both are negative, infection is unlikely per AAOS guidelines 1
• CRP >13.5 mg/L has 73-91% sensitivity and 81-86% specificity for prosthetic infection 1, 3
• Normal CRP returns to baseline by 2 months post-surgery, so elevation at 2 years is pathologic 1
• Peripheral white blood cell counts are typically NOT elevated in chronic prosthetic infections 1

Critical pitfall: Low-grade chronic infections are difficult to diagnose—53% were not obvious before revision surgery in one series 1. Pain may be the only presenting symptom 1.

Algorithmic Diagnostic Pathway

If ESR/CRP Elevated OR High Clinical Suspicion for Infection

Proceed immediately to joint aspiration (appropriate as initial evaluation per ACR guidelines) 1:

• Joint aspiration combined with CRP testing are the most useful diagnostic tools 1
• Send aspirate for cell count, culture, and crystal analysis
• Consider aspiration under fluoroscopic or ultrasound guidance for accuracy 1

If aspiration is equivocal or non-diagnostic, obtain Indium-111 labeled WBC scan with Tc-99m sulfur colloid bone marrow scan—this is the most robust technique for detecting periprosthetic infection 1:

• Incongruent activity (WBC uptake without marrow uptake) indicates osteomyelitis 1
• Congruent activity (both WBC and marrow uptake) suggests reactive marrow, not infection 1
• Adding SPECT/CT increases specificity and accuracy 1

If Radiographs Show Mechanical Abnormalities

• Component loosening/osteolysis: Surgical consultation for revision 1
• Malalignment: Consider CT for rotational assessment if axial malrotation suspected 1
• Polyethylene wear: Annual weight-bearing radiographs recommended for monitoring 1

If Initial Workup is Negative (Normal Radiographs, Normal ESR/CRP, Negative Aspiration)

Consider non-infectious, non-mechanical causes that account for 10-34% of post-TKR pain 4:

Inflammatory Pain Without Infection

• Patients with chronic post-TKR pain demonstrate elevated high-sensitivity CRP (4.3 mg/L vs 1.7 mg/L in pain-free patients) even without frank infection 5
• Inflammatory cytokines (TNF-α, MMP-13, IL-6) predict ongoing pain 6

Central Sensitization/Neuropathic Pain

• Affects 16-20% of knee surgery patients 7
• Characterized by: diffuse pain, allodynia, pain disproportionate to findings 8, 7
• Temporal summation and inefficient conditioned pain modulation are associated with worse outcomes 8
• Use PainDETECT questionnaire to screen for neuropathic features 5

Soft-Tissue Complications

Obtain MRI without contrast if suspecting 1:

• Quadriceps or patellar tendon tears (0.17-2.5% incidence) 1
• Arthrofibrosis (4.5-6.9% of failures) 1, 4
• Patellar clunk syndrome or soft-tissue impingement 1
• MRI is superior to CT for soft-tissue assessment 1

Other Painful Body Sites

• Number of painful body sites contributes to altered nociceptive signaling and worsens knee pain 8
• Assess for polyarticular pain or fibromyalgia 8

Common Diagnostic Pitfalls

1. Assuming normal WBC count excludes infection—peripheral leukocyte counts are typically normal in chronic prosthetic infections 1

2. Relying on single elevated ESR—ESR can be elevated in uninfected patients with loosening 1. Use combined ESR + CRP per AAOS guidelines 1

3. Ordering bone scan too early—bone scans show increased activity for up to 2 years post-operatively as part of normal healing 1. Serial studies showing increasing activity are more useful 1

4. Missing rotational malalignment—requires CT with specific measurement techniques using anatomic landmarks 1

5. Overlooking psychological factors—pain catastrophizing is significantly higher in patients with persistent pain and predicts worse outcomes 5

Risk Stratification

High probability of infection (requiring more aggressive workup) includes patients with 1:

• Prior knee infection
• Superficial surgical site infection history
• Operative time >2.5 hours
• Immunosuppression
• Early implant loosening/osteolysis on radiographs