What are the potential causes and management options for a patient experiencing a hot knee and pain after knee surgery?

Hot Knee and Pain After Knee Surgery

Obtain plain radiographs of the knee immediately, followed by joint aspiration if infection is suspected, as periprosthetic infection is the most serious complication requiring urgent diagnosis and carries significant morbidity and mortality risk. 1, 2

Immediate Diagnostic Priority: Rule Out Infection First

The combination of heat and pain after knee surgery demands urgent evaluation for infection, which occurs in 0.8-1.9% of total knee arthroplasties and represents the most catastrophic complication if missed. 1

Clinical Differentiation

• Infection characteristics: Night pain or pain at rest, combined with warmth, erythema, swelling, and fever suggest acute infection, though chronic infections may present with pain alone. 1
• Mechanical loosening characteristics: Pain primarily on weight-bearing rather than at rest points toward aseptic loosening instead of infection. 1
• Critical pitfall: Never attribute persistent pain to other causes before completing the full infection workup, as delayed diagnosis can result in catastrophic outcomes. 2

Laboratory Workup

• Obtain ESR and CRP immediately if any concerning symptoms exist; CRP >13.5 mg/L has 73-91% sensitivity and 81-86% specificity for prosthetic knee infection. 1
• Interleukin-6 combined with CRP shows excellent sensitivity for detecting infection after total knee arthroplasty. 1
• Abnormal results for at least 2 of 3 tests (CRP cutoff 0.93 mg/L, ESR cutoff 27 mm/h, and fibrinogen cutoff 432 mg/dL) yields 93% sensitivity, 100% specificity, and 97% accuracy for infection diagnosis. 1
• Peripheral leukocyte counts are NOT elevated in most patients with infected prostheses, so normal white blood cell count does not exclude infection. 1

Imaging Algorithm

Step 1: Plain Radiographs (Always First)

• Weight-bearing knee radiographs assess component positioning, alignment, periprosthetic lucencies, soft-tissue swelling, and potential complications. 1, 2, 3
• Radiographic signs of infection include periprosthetic lucencies, soft-tissue swelling, and intraosseous gas. 1

Step 2: Joint Aspiration (If Infection Suspected)

• Perform joint aspiration after reviewing radiographs for cell count, culture, and sensitivity testing. 2, 3
• If aspiration cultures are positive, proceed directly to surgical management per AAOS guidelines without additional imaging. 1, 3
• If aspiration cultures are negative or inconclusive but infection still suspected, reaspiration of the knee is appropriate per AAOS guidelines. 1

Step 3: Advanced Imaging (Based on Initial Results)

• If infection excluded and aseptic loosening suspected: CT of the knee without IV contrast is the preferred modality to evaluate for loosening, osteolysis, and component positioning. 1, 3
• Fluoroscopy can demonstrate lucent lines in profile and loosening under real-time manipulation. 1
• Tc-99m three-phase bone scan may be helpful for diagnosing loosening, especially when obtained many years after surgery, though positive scans occur in 20% of asymptomatic knees at one year and 12.5% at two years postoperatively. 1

Management Based on Diagnosis

If Infection Confirmed

• Proceed to surgical management immediately as per AAOS guidelines when aspiration cultures are positive. 1, 3
• Staphylococcus aureus and coagulase-negative Staphylococcus species (including S. epidermidis) are the most common organisms. 1

If Infection Excluded

• For aseptic loosening with imaging confirmation: Orthopedic surgery referral for revision consideration. 2, 4
• For pain control: Use NSAIDs while avoiding long-term opioids. 2, 4
• For persistent pain despite initial management: Referral back to the orthopedic surgeon for further evaluation. 3

Common Pitfalls to Avoid

• Never obtain MRI before recent radiographs, as this deviates from standard protocols and may miss critical bony pathology. 2
• Failing to obtain weight-bearing radiographs can miss component subsidence or malalignment. 2
• Do not attribute pain to psychological factors without first completing the full diagnostic workup for infection, loosening, and mechanical failure. 2
• CRP can be elevated after surgery but generally returns to baseline within 2 months under normal circumstances. 1