What is the treatment for a septic knee?

Treatment of Septic Knee

Septic arthritis of the knee requires immediate surgical drainage combined with empiric IV vancomycin, followed by culture-directed antibiotics for 3-4 weeks, as this approach prevents rapid cartilage destruction and optimizes functional outcomes. 1, 2

Immediate Management (Within 1 Hour)

Diagnostic Confirmation

• Perform joint aspiration immediately for synovial fluid analysis including Gram stain, culture, cell count (≥50,000 cells/mm³ suggests septic arthritis), and crystal analysis to exclude gout 3, 1, 2
• Obtain blood cultures before initiating antibiotics 4
• Plain radiographs should be obtained initially to exclude fractures and assess for joint effusion, though they have low sensitivity for early infection 3
• Ultrasound can detect effusions and guide aspiration, particularly useful for hip joints 1

Critical pitfall: Negative synovial fluid culture does not exclude infection—cultures are positive in only ~80% of non-gonococcal septic arthritis cases 1, 2. If clinical suspicion remains high despite negative aspirate, consider percutaneous bone biopsy 1

Warning: Coexistent septic arthritis and gout can occur in the same joint; always send synovial fluid for culture even if MSU crystals are identified 3

Surgical Intervention

Immediate surgical drainage is mandatory and should not be delayed 1, 2, 5. The choice of surgical approach depends on disease stage:

Surgical Options

• Arthroscopic debridement and lavage is the preferred initial approach for most cases 1, 6
• Open arthrotomy with debridement is indicated for advanced disease (Gächter stage III-IV), when arthroscopic drainage fails, or when there is significant synovial hypertrophy 6
• Synovectomy should be considered as a primary procedure when significant synovial hypertrophy is present or when conservative treatment fails to show rapid improvement 6

Key evidence: A 2021 study comparing medical (arthrocentesis alone) versus surgical treatment showed similar failure rates (39.2% vs 30.4%), but this should be interpreted cautiously as the medical group had 12 days shorter hospital stay 7. However, guideline recommendations strongly favor surgical drainage for definitive source control 1, 2, 5.

Timing is critical: Delay between symptom onset and surgery is the major prognostic factor for success (P=0.023), with early intervention correlating with better functional outcomes 6

Empiric Antibiotic Therapy

First-Line Empiric Treatment

Start IV vancomycin immediately after obtaining cultures to cover MRSA, which has become a major cause of septic arthritis in the United States 1, 2, 4, 5:

• Adults: Vancomycin 15 mg/kg IV every 6 hours (or 30-60 mg/kg/day in 2-4 divided doses) 1
• Children: Vancomycin 15 mg/kg/dose IV every 6 hours (40 mg/kg/day in 4 divided doses) 1

Alternative empiric options if MRSA is less likely based on local epidemiology 1:

• Linezolid 600 mg IV/PO every 12 hours
• Daptomycin 6 mg/kg IV daily
• Teicoplanin 6-12 mg/kg IV every 12 hours for 3 doses, then daily

Culture-Directed Therapy

Once culture results are available, narrow antibiotics based on susceptibility 3:

For Methicillin-Susceptible Staphylococcus aureus (MSSA):

• Switch to: Nafcillin or oxacillin 1-2 g IV every 4 hours 1
• Alternative: Cefazolin 1 g IV every 8 hours 1, 8
• If penicillin allergic: Clindamycin 600 mg IV every 8 hours 1

For Methicillin-Resistant Staphylococcus aureus (MRSA):

• Continue vancomycin as primary therapy 1, 2
• Consider adding rifampin 600 mg PO daily or 300-450 mg PO twice daily for enhanced bone and biofilm penetration 1, 2, 4

For Streptococcal Infections:

• Penicillin G 20-24 million units IV daily (continuous infusion or divided doses) 1
• Alternative: Ceftriaxone 1-2 g IV every 24 hours 1, 9

For Polymicrobial Infections:

• Dual antibiotic coverage is mandatory when multiple organisms are identified (e.g., linezolid for MRSA plus ciprofloxacin for Pseudomonas aeruginosa) 1, 2

Duration and Route of Therapy

Treatment Duration

• Uncomplicated bacterial arthritis: 3-4 weeks total duration 1, 2, 5
• Recent evidence suggests 2 weeks may be adequate after surgical drainage in select cases, predominantly small joints 1
• With concomitant osteomyelitis: Extend to 6 weeks 1, 5
• Prosthetic joint infections with debridement and implant retention: 12 weeks is superior to 6 weeks 1, 2

Important consideration: Concomitant osteomyelitis occurs in up to 30% of children with septic arthritis and requires longer treatment 1, 2, 4. MRI has high sensitivity for detecting concomitant osteomyelitis when clinical suspicion remains high 3, 1

Transition to Oral Antibiotics

Oral antibiotics are not inferior to IV therapy for most cases 1. Switch to oral can be made after 2-4 days if:

• Patient is clinically improving
• Afebrile
• Can tolerate oral intake 1

Oral options for MRSA (after initial IV therapy) 1:

• Linezolid 600 mg PO every 12 hours
• TMP-SMX (trimethoprim 4 mg/kg/dose) PO every 8-12 hours plus rifampin 600 mg PO daily
• Fusidic acid 500 mg PO every 8 hours or 750 mg every 12 hours plus rifampin

Monitoring and Follow-Up

Clinical Monitoring

• Never leave the septic patient alone—ensure continuous observation 3
• Monitor for vancomycin toxicity and adjust dosing based on trough levels 1
• Follow CRP and ESR to monitor treatment response 1
• Be vigilant for drug interactions and adverse effects, especially in elderly patients 1, 4

Treatment Failure

Worsening or ongoing organ dysfunction and persistence of fever for more than 48-72 hours should question adequacy of therapy 3. Common causes of treatment failure include 3:

• Inadequate empirical anti-infective therapy (wrong pathogen targeted, too narrow spectrum, primary resistance, counterfeit/expired drugs)
• Missed or insufficient control of infectious focus
• Insufficient supportive therapy
• Development of new antimicrobial resistance
• New hospital-acquired infection
• Non-infectious disease mimicking sepsis

Persistent Symptoms

For persistent or recurrent joint swelling after a course of oral antibiotics 1, 2:

• Re-treatment with another 4-week course of oral antibiotics OR
• 2-4 weeks of IV ceftriaxone
• Arthroscopic synovectomy may reduce duration of joint inflammation in cases of persistent synovitis with significant pain or functional limitation 1, 2

Special Populations and Considerations

Pathogen-Specific Considerations

• Children <4 years: Consider Kingella kingae as potential causative organism 1, 2
• Patients with sickle cell disease: Consider Salmonella species 1, 2
• Post-operative infections: Staphylococcus aureus and epidermidis are most common (23/40 cases in one series) 6

Systemic Inflammatory Response Syndrome (SIRS)

Patients meeting SIRS criteria require hospital admission for immediate surgical debridement and IV antibiotic therapy due to high risk of rapid cartilage damage and systemic complications 4. SIRS criteria indicate potential progression to sepsis with significant morbidity and mortality risks 3, 4.

Pregnancy Considerations

When injecting a joint during pregnancy, most compounds in routine practice can be used except radiopharmaceuticals, which are contraindicated 3

Prognosis and Outcomes

Early diagnosis and prompt treatment are essential for protecting articular cartilage and achieving full recovery 10. Through early diagnosis and prompt treatment, infection can be successfully eradicated with stability of the knee and full range of motion achieved 10.

Age is an independent risk factor for treatment failure (P=0.04), with male sex also associated with worse outcomes in univariate analysis 7. MRSA joint infection appears to be associated with worse outcomes compared to methicillin-sensitive infections 5.