What are the signs and treatment of septic arthritis?

Septic Arthritis: Clinical Signs and Treatment

Clinical Signs and Presentation

Septic arthritis presents as an orthopedic emergency with acute monoarticular joint pain, swelling, and fever, though the classic triad of fever, pain, and diminished mobility occurs in only approximately 50% of cases. 1

Key Clinical Features:

• Acute joint pain and swelling (most commonly knee and hip joints) 1
• Fever (though may be absent in up to 50% of cases) 1, 2
• Inability to bear weight on the affected extremity 1
• Warmth and erythema over the affected joint 2
• Restricted range of motion due to pain 3

Diagnostic Criteria (Kocher Criteria for Hip):

• Fever >101.3°F 1
• Erythrocyte sedimentation rate ≥40 mm/hour 1
• White blood cell count ≥12,000 cells/mm³ 1
• Inability to bear weight 1
• C-reactive protein >2.0 mg/dL (highly predictive) 1

Meeting all criteria approaches 100% likelihood of septic arthritis. 1

Age-Specific Pathogen Considerations:

• Neonates: Group B streptococcus predominates 1, 4
• Children <4 years: Kingella kingae is common 1, 4
• All ages: Staphylococcus aureus is the most common pathogen overall 1, 4, 2
• Sickle cell disease: Salmonella species must be considered 1, 4
• MRSA is increasingly prevalent and requires empiric coverage 5, 4

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Diagnostic Approach

Joint aspiration is the definitive diagnostic procedure, with synovial fluid white blood cell count ≥50,000 cells/mm³ suggestive of septic arthritis. 5

Imaging Algorithm:

• Ultrasound first for hip joints to detect effusions and guide aspiration (sensitivity approaching 95%, false negative rate 5% in patients with symptoms <1 day) 1
• Plain radiographs have low sensitivity but help exclude other conditions 5
• MRI when clinical suspicion remains high despite negative aspiration, or to detect concomitant osteomyelitis (present in >50% of pediatric cases and up to 30% of adult cases) 1, 5

Laboratory Findings:

• Synovial fluid culture positive in approximately 80% of non-gonococcal cases 5
• Elevated CRP and ESR support diagnosis and monitor treatment response 5
• Blood cultures should be obtained before antibiotics 2

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Treatment Algorithm

Immediate surgical drainage combined with appropriate antibiotic therapy is mandatory, as bacterial proliferation can cause irreversible cartilage damage within hours to days. 5, 4, 6

Surgical Management:

• Arthrotomy, irrigation, and debridement for surgical emergencies 5
• Arthroscopic drainage or serial closed-needle aspirations for non-emergency cases 5, 7
• Immediate debridement with intraoperative cultures for surgical emergencies rather than waiting for aspiration results 5

Empiric Antibiotic Therapy (Start Immediately After Cultures):

Adults:

• IV vancomycin 15 mg/kg every 6 hours (30-60 mg/kg/day) as first-line for MRSA coverage 5
• Alternatives: Linezolid 600 mg IV/PO every 12 hours, Daptomycin 6 mg/kg IV daily, or Teicoplanin 5

Pediatrics:

• IV vancomycin 15 mg/kg/dose every 6 hours (40 mg/kg/day) 5
• Clindamycin 10-13 mg/kg/dose IV every 6-8 hours if local clindamycin resistance is low 5

Culture-Directed Therapy:

MSSA (Methicillin-Sensitive S. aureus):

• Switch to Nafcillin or Oxacillin 1-2 g IV every 4 hours, OR
• Cefazolin 1 g IV every 8 hours, OR
• Clindamycin 600 mg IV every 8 hours (if penicillin allergic) 5

MRSA (Methicillin-Resistant S. aureus):

• Continue vancomycin as primary therapy 5
• Consider adding rifampin 600 mg PO daily or 300-450 mg PO twice daily for enhanced bone and biofilm penetration 5

Streptococcal Infections:

• Penicillin G 20-24 million units IV daily, OR
• Ceftriaxone 1-2 g IV every 24 hours 5

Polymicrobial Infections:

• Dual antibiotic coverage is mandatory (e.g., linezolid for MRSA plus ciprofloxacin for Pseudomonas) 5, 4

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Treatment Duration and Route

Oral antibiotics are not inferior to IV therapy and can be initiated after 2-4 days if the patient is clinically improving, afebrile, and tolerating oral intake. 5, 2

Duration Guidelines:

• Uncomplicated bacterial arthritis: 3-4 weeks total 5
• Recent evidence suggests 2 weeks may be adequate after surgical drainage in select cases (predominantly small joints) 5
• Prosthetic joint infections with debridement and retention: 12 weeks (superior to 6 weeks) 5
• Hip prosthesis with exchange: 3 months 5
• Knee prosthesis with exchange: 6 months 5
• Concomitant osteomyelitis requires longer treatment 5

Oral Options for MRSA (After Initial IV Therapy):

• Linezolid 600 mg PO every 12 hours 5
• TMP-SMX (trimethoprim 4 mg/kg/dose) PO every 8-12 hours plus rifampin 600 mg PO daily 5
• Fusidic acid 500 mg PO every 8 hours or 750 mg every 12 hours plus rifampin 5

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Critical Pitfalls and Monitoring

Common Pitfalls:

• Negative synovial fluid culture does not exclude infection (occurs in 20% of cases); consider percutaneous bone biopsy if clinical suspicion remains high 5, 4
• Concomitant osteomyelitis occurs in >50% of pediatric cases and up to 30% of adults, requiring longer treatment and MRI evaluation 1, 5
• Polymicrobial infections require dual coverage 5, 4
• Biofilm formation on joint surfaces creates antibiotic-resistant environments 4, 6

Monitoring:

• Follow CRP and ESR to assess treatment response 5
• Monitor vancomycin trough levels and adjust for toxicity 5
• Be vigilant for drug interactions in elderly patients 5
• Repeat MRI for worsening or persistent symptoms (results in management changes in 21% of cases) 1

Special Considerations:

• For persistent joint swelling after oral antibiotics: Re-treat with another 4-week course of oral antibiotics or 2-4 weeks of IV ceftriaxone 5
• Arthroscopic synovectomy may reduce inflammation duration in persistent synovitis with significant pain 5
• NSAIDs or DMARDs for symptomatic treatment if arthritis persists despite IV therapy and synovial fluid PCR is negative 5