Why does a patient have persistent swelling and pain five years after a unicompartmental knee arthroplasty?

Persistent Pain and Swelling 5 Years After Partial Knee Replacement

Aseptic loosening is the most likely cause of persistent pain and swelling five years after unicompartmental knee arthroplasty, followed by polyethylene wear and osteolysis—infection must be ruled out first before pursuing mechanical causes. 1

Immediate Diagnostic Workup

The evaluation must begin with simultaneous imaging and laboratory studies to systematically exclude infection and mechanical failure—the two critical causes of late postoperative pain. 2

First-Line Testing (Obtain Both Immediately)

Laboratory studies:

• Measure ESR and CRP before any treatment—when both are negative, infection is unlikely 2
• CRP >13.5 mg/L has 73-91% sensitivity and 81-86% specificity for prosthetic infection 3, 2
• Infection accounts for 21.9% of revisions performed >2 years after initial surgery 3, 2

Imaging:

• Obtain weight-bearing anteroposterior, lateral, and axial knee radiographs 2
• Look specifically for: periprosthetic lucency >2mm, osteolysis, component loosening, polyethylene wear (joint space narrowing), malalignment, or fractures 2, 4

Clinical Pattern Recognition

Pain characteristics help distinguish the cause:

• Night pain or pain at rest strongly suggests periprosthetic infection 3, 2
• Pain with weight-bearing indicates mechanical loosening or component malposition 2, 5
• Continuous pain can indicate infection or complex regional pain syndrome 5

Critical pitfall: Do not assume normal peripheral WBC count excludes infection—chronic prosthetic infections typically present with normal leukocyte counts 2

Algorithmic Diagnostic Pathway

If ESR/CRP Elevated OR High Clinical Suspicion for Infection:

1. Proceed immediately to joint aspiration (under fluoroscopic or ultrasound guidance) 2

   • Send aspirate for: cell count with differential, aerobic and anaerobic cultures, alpha-defensin, synovial fluid CRP, and crystal analysis 3, 2
   • Alpha-defensin provides 97% sensitivity and 96% specificity for periprosthetic joint infection 3
   • A "dry tap" does not exclude infection—repeat aspiration weekly if clinical suspicion remains high 3

2. If aspiration is equivocal or non-diagnostic:

   • Obtain Indium-111 labeled WBC scan with Tc-99m sulfur colloid bone marrow scan—this combination achieves 96% sensitivity, 87% specificity, and 91% accuracy for diagnosing periprosthetic infection 1, 2

If Infection Excluded (ESR/CRP Normal and Negative Aspiration):

Aseptic loosening is the major cause of late-stage (>2 years) failure and should be the primary consideration. 1

Mechanical causes in order of frequency:

• Aseptic loosening (most common late failure) results from mechanical stresses, osteolysis secondary to particle debris, or poor bone stock 1, 6
• Polyethylene wear and osteolysis occur from macrophage phagocytosis of particle debris—patients may be asymptomatic early but develop pain, swelling, and acute synovitis as osteolysis progresses 1, 6
• Instability from surgical error or poor prosthesis selection, accounting for 7.5% of prosthesis failures—closely interrelated with malalignment and loosening 1, 6
• Patellar maltracking and component malalignment 6, 7

Radiographic signs of loosening:

• Radiolucent lines at cement-bone or metal-cement interface >2mm or increasing in translucency 4
• Component migration 4
• Cement fractures 4

Additional imaging if radiographs are equivocal:

• Artifact-reduced CT can provide additional information regarding periprosthetic osteolysis 4
• Bone scintigraphy may be helpful in diagnosing loosening when obtained many years after surgery (though positive scans occur in 12.5% of asymptomatic patients 2 years post-surgery) 1

Extra-Articular Causes (Often Overlooked)

In 25.6% of cases with persistent pain, the pathology originates from extra-articular sources: 6

• Nerve entrapment in the spine (25.6% of extra-articular cases) 6
• Hip osteoarthritis or femoral head avascular necrosis (15.4% of extra-articular cases) 6
• Complex regional pain syndrome 5

Management Implications

Large areas of osteolysis suggest component loosening and may require revision surgery, while small areas may be monitored. 1 Imaging helps evaluate available bone stock in preparation for potential revision surgery. 1