What is the initial treatment for a patient with a periprosthetic knee infection?

Initial Treatment for Periprosthetic Knee Infection

The initial treatment for periprosthetic knee infection requires immediate clinical assessment with ESR and CRP testing, followed by joint aspiration (holding antibiotics for at least 2 weeks if clinically safe), and then urgent surgical debridement with implant retention if the infection is within 30 days of implantation or within 3 weeks of symptom onset, combined with collection of multiple intraoperative tissue specimens for culture. 1, 2, 3

Diagnostic Workup Before Treatment

Clinical Assessment and Laboratory Testing

• Begin with clinical examination focusing on pain, swelling, warmth, erythema, and fever - these findings are common in acute infection, while chronic infections may present with pain alone 1
• Order ESR and CRP immediately - when both are negative, infection is unlikely; positive results for either warrant joint aspiration 1
• CRP has 73-91% sensitivity and 81-86% specificity for prosthetic knee infection when cutoff is ≥13.5 mg/L 1
• The combination of ESR and CRP as screening tools misses only one infected knee when both tests are negative 2

Joint Aspiration Protocol

• Perform joint aspiration when ESR or CRP is elevated or clinical suspicion is high 1, 2
• Critically important: Hold antibiotics for at least 2 weeks before aspiration if clinically feasible - false-negative cultures occur with recent antibiotic use 1, 2, 3
• Obtain blood cultures if fever is present or acute symptom onset 2, 3
• Send aspirate for: cell count with differential (>65% neutrophils or >1700 cells/μL suggests infection), aerobic and anaerobic cultures, and alpha-defensin testing 1, 2
• If initial aspiration is negative but suspicion remains high, perform repeat aspiration - in 16% of cases, more than three aspirations were necessary to obtain positive culture 1

Imaging

• Obtain plain radiographs initially - both radiographs and joint aspiration are appropriate for initial evaluation 1
• Radiographs alone cannot distinguish infection from loosening, making aspiration superior 2
• Advanced imaging (MRI, CT) is not indicated for initial diagnostic workup once infection is confirmed 1

Surgical Treatment Strategy

Timing-Based Algorithm

For infections within 30 days of implantation OR within 3 weeks of symptom onset:

• Perform debridement, antibiotics, irrigation, and implant retention (DAIR) if the prosthesis is well-fixed and there is no sinus tract 1, 3
• This approach offers the advantage of component retention and maximum knee function 4

For infections beyond these timeframes:

• Two-stage exchange is the gold standard - involves resection arthroplasty, removal of all prosthetic components and cement, placement of antibiotic-impregnated spacer, followed by pathogen-specific IV antibiotics 3, 4
• Two-stage revision has higher success rates than single-stage exchange 4

Intraoperative Tissue Collection

• Collect at least 3-5 (optimally 5-6) periprosthetic tissue specimens from different suspicious sites during debridement for aerobic and anaerobic culture 1, 3
• Submitting fewer than 5-6 specimens decreases sensitivity of culture as a diagnostic test 1
• Send specimens with explicit instructions to culture for mycobacteria - nontuberculous mycobacterium infections can present with identical findings and are often culture-negative initially 3
• Consider prolonged incubation up to 14 days for Propionibacterium species 1

Antibiotic Management

Empiric Therapy Considerations

• Withhold antimicrobial prophylaxis at time of revision surgery if preoperative risk of infection is high - this maximizes tissue culture yield 1
• If prophylaxis is given, vancomycin combined with ciprofloxacin or third-generation cephalosporin provides highest antimicrobial coverage in early PJI 5
• Vancomycin as perioperative prophylactic antibiotic decreases overall PJI rate (1.0% to 0.5%) and MRSA PJI (0.23% to 0.07%) 6

Pathogen-Directed Therapy

• Once cultures return, tailor antibiotics to specific organisms - Staphylococcus aureus and coagulase-negative Staphylococcus species are most common 1, 3
• For S. aureus knee PJI: continue IV therapy for 2-6 weeks, then transition to rifampin plus companion drug for 6 months total 3
• Monitor CRP weekly during IV therapy - should decrease by 50% within 1 week if treatment is effective 3

Critical Pitfalls to Avoid

• Never interpret "dry tap" as excluding infection - absence of fluid at aspiration does not indicate absence of infection 2
• Do not aspirate patients currently on antibiotics - wait at least 2 weeks off antibiotics when clinically safe 1, 2
• Do not rely on peripheral WBC count - it may be normal in joint infections despite significant intra-articular inflammation 2
• Do not delay surgical intervention once infection is confirmed - early debridement leads to significant reduction in treatment duration and better outcomes 2
• Gram stain has relatively poor sensitivity and specificity, so do not rely on it alone 2