What is the treatment for a suspected right knee infection?

Treatment of Suspected Right Knee Infection

The treatment of a suspected right knee infection requires prompt joint aspiration for synovial fluid analysis and culture, followed by immediate empiric antibiotic therapy targeting Staphylococcus aureus while awaiting culture results, and surgical intervention (arthroscopic or open debridement) in most cases.

Diagnostic Approach

Initial Assessment

• Pain, swelling, warmth, erythema, and limited range of motion are classic signs of knee joint infection
• Night pain or pain at rest is characteristic of infection, whereas pain on weightbearing is more characteristic of mechanical issues 1
• Fever may be present in acute infections but can be absent in chronic infections 1

Laboratory Testing

• Obtain serum inflammatory markers:
  • C-reactive protein (CRP) - sensitivity 73-91%, specificity 81-86% for prosthetic knee infection 1
  • Erythrocyte sedimentation rate (ESR)
  • Note: Peripheral leukocyte counts are often not elevated in joint infections 1

Joint Aspiration

• Mandatory when infection is suspected
• Should be performed using sterile technique with ultrasound or fluoroscopic guidance 2
• Synovial fluid analysis should include:
  • Cell count and differential (WBC ≥50,000 cells/mm³ with >60% neutrophils indicates high suspicion for septic arthritis) 2
  • Gram stain (high specificity but limited sensitivity)
  • Culture (definitive test, positive in ~80% of non-gonococcal septic arthritis) 2
  • Crystal analysis to rule out crystalline arthropathy

Imaging

• Plain radiographs are the initial imaging modality for suspected knee infection 1
• MRI may be helpful in detecting extracapsular spread of infection and abscess formation 1
• Advanced imaging (nuclear medicine studies) may be considered for equivocal cases 1

Treatment Algorithm

1. Native Knee Infection

Medical Management

• Start empiric intravenous antibiotics immediately after obtaining cultures
• Initial therapy should target Staphylococcus aureus (most common pathogen in 85-95% of joint infections) 3
• Recommended empiric regimen:
  • For methicillin-sensitive organisms: Nafcillin 1.5-2g IV q4-6h or Cefazolin 1-2g IV q8h 1
  • For suspected methicillin-resistant organisms: Vancomycin 15 mg/kg IV q12h 1
  • Ceftriaxone 1-2g IV q24h is an alternative option 4
• Adjust antibiotics based on culture results and susceptibilities

Surgical Management

• Surgical drainage is essential in most cases of septic arthritis
• Options include:
  • Arthroscopic debridement and lavage for early or less severe infections 3
  • Open arthrotomy for severe infections or when arthroscopic treatment fails 5
• The Gächter classification can guide the extent of debridement needed 5

Duration of Therapy

• Minimum 2-3 weeks of antibiotics after signs and symptoms of infection have resolved 4
• Total duration typically 4-6 weeks depending on clinical response 1

2. Prosthetic Knee Infection

Surgical Options

• Debridement, antibiotics, and implant retention (DAIR)
  • For acute infections (<3 weeks) with stable implants 1
  • Higher failure rates (up to 60%) with coagulase-negative staphylococci 6
• Two-stage exchange arthroplasty
  • Indicated for chronic infections or when DAIR fails 1
  • First stage: Remove prosthesis, debride, place antibiotic-impregnated spacer
  • Second stage: Reimplantation after infection control (typically 6-12 weeks)
• One-stage exchange
  • May be considered when pathogens are known preoperatively and susceptible to oral antibiotics 1
• Permanent resection arthroplasty or amputation
  • For patients with limited bone stock, poor soft tissue coverage, or highly resistant organisms 1

Antibiotic Therapy

• For staphylococcal prosthetic joint infections:
  • 2-6 weeks of pathogen-specific IV antibiotics plus rifampin 300-450mg orally twice daily
  • Followed by rifampin plus an oral companion drug (preferably ciprofloxacin or levofloxacin)
  • Total duration: 3 months for hip infections, 6 months for knee infections 1

Special Considerations

• If initial cultures are negative but clinical suspicion remains high, consider:

  • Repeat aspiration (especially if patient was on antibiotics prior to initial aspiration) 2
  • Extended incubation for slow-growing organisms
  • Specific cultures for fungi or mycobacteria if epidemiologically indicated 7, 8

• Biofilm-producing pathogens like coagulase-negative staphylococci have higher treatment failure rates (only 47% success at one year) and often require more aggressive surgical intervention 6

Follow-up

• Monitor clinical response (pain, swelling, range of motion)
• Serial CRP and ESR to assess treatment response
• Consider repeat aspiration if clinical improvement is not observed

Remember that knee joint infection is an orthopedic emergency requiring prompt diagnosis and treatment to prevent joint destruction and systemic complications.