Management of Septic Knee After Total Knee Replacement
For a septic knee after TKR, proceed directly to surgery when joint aspiration cultures are positive, as no additional imaging is required and surgical intervention (debridement with liner exchange or two-stage revision) combined with pathogen-specific intravenous antibiotics for 4-6 weeks is the definitive treatment. 1, 2
Initial Diagnostic Workup
Laboratory Testing
- Obtain ESR and CRP immediately - these serve as screening tools with CRP showing 73-91% sensitivity and 81-86% specificity when using a cutoff of ≥13.5 mg/L 1, 3
- Perform joint aspiration if ESR or CRP is elevated or if clinical suspicion is high, even with normal inflammatory markers 1
- Withhold antibiotics for at least 2 weeks before aspiration when clinically safe (with careful monitoring for sepsis) to maximize culture yield - sensitivity of microscopy drops from 58% to 12% and culture sensitivity drops from 79% to 28% if antibiotics are given prior to aspiration 1, 4
- Consider synovial fluid alpha-defensin testing which demonstrates 97% sensitivity and 96% specificity, increasing to 100% specificity when combined with synovial CRP 3
Imaging
- Obtain knee radiographs initially to assess for component loosening, osteolysis, or other complications 1
- No additional imaging is needed once cultures are positive - proceed directly to surgical management per AAOS guidelines 1
Surgical Management Algorithm
When Cultures Are Positive
Proceed immediately to surgery - the specific surgical approach depends on timing and component stability 1, 2:
- Early infection (<3 months) with stable components: Debridement, liner exchange, and retention of well-fixed components 5
- Late infection (>3 months) or loose components: Two-stage revision with removal of all prosthetic material, antibiotic-loaded cement spacer placement, 6-week antibiotic course, then reimplantation 5, 6
- Obtain 3-6 periprosthetic tissue samples during surgery for aerobic and anaerobic culture 2
When Initial Cultures Are Negative But Infection Still Suspected
- Perform repeat knee aspiration weekly until infection is confirmed or excluded - false-negative aspirations occur in 16% of cases requiring multiple attempts 1
- Consider intraoperative frozen section analysis of periprosthetic synovial tissue if proceeding to surgery 1
Antibiotic Management
Pathogen-Specific Therapy (Based on Culture Results)
Methicillin-susceptible Staphylococci 2:
- Nafcillin 1.5-2g IV q4-6h, OR
- Cefazolin 1-2g IV q8h, OR
- Ceftriaxone 1-2g IV q24h
Methicillin-resistant Staphylococci 2:
- Vancomycin 15 mg/kg IV q12h
Pseudomonas aeruginosa 2:
- Cefepime 2g IV q12h, OR
- Meropenem 1g IV q8h
Penicillin-susceptible Enterococci 2:
- Penicillin G 20-24 million units IV q24h continuously, OR
- Ampicillin 12g IV q24h continuously
Duration of Therapy
- Standard duration: 4-6 weeks of pathogen-specific IV antibiotics after surgical debridement 2
- For retained prosthesis with staphylococcal infection: 6 months for knee (3 months for hip) 2
- Adjust dosing based on renal and hepatic function 2
Critical Antibiotic Caveats
- Never use rifampin as monotherapy - rapid resistance develops 2
- Monitor for C. difficile colitis with any antimicrobial 2
- Watch for QTc prolongation and tendinopathy with fluoroquinolones 2
Chronic Suppression (If Needed After Initial Treatment)
For methicillin-susceptible Staphylococci 2:
- Cephalexin 500 mg PO tid/qid, OR
- Dicloxacillin 500 mg PO tid/qid, OR
- Clindamycin 300 mg PO qid
For methicillin-resistant Staphylococci 2:
- Co-trimoxazole 1 DS tab PO bid, OR
- Minocycline/Doxycycline 100 mg PO bid
Monitoring During Treatment
- Track CRP and ESR serially - CRP normalizes within 2 months post-surgery under normal circumstances 1, 3
- If inflammatory markers plateau or increase after initial improvement, perform repeat joint aspiration and culture to evaluate for persistent infection 3
- Obtain blood cultures if fever develops or symptoms suggest bloodstream infection 3
Common Pitfalls to Avoid
- Giving empirical antibiotics before aspiration - this dramatically reduces diagnostic yield and may lead to false-negative cultures requiring prolonged empirical therapy 1, 4
- Accepting a single negative aspiration as definitive - "dry taps" do not exclude infection, and multiple aspirations may be necessary 1
- Delaying surgery when cultures are positive - additional imaging provides no benefit and delays definitive treatment 1
- Inadequate tissue sampling at surgery - obtain at least 3-6 samples to maximize organism identification 2