How should I evaluate and manage a child with acute onset knee pain and fever?

Acute Onset Knee Pain with Fever in a Pediatric Patient

Septic arthritis of the knee must be assumed until proven otherwise in any child presenting with acute knee pain and fever, as bacterial proliferation can cause irreversible cartilage damage within hours to days. 1, 2

Immediate Clinical Assessment

Key Clinical Predictors to Evaluate

The presence of specific clinical and laboratory findings dramatically increases the likelihood of septic arthritis:

• Fever >38.3°C (101.3°F) at presentation is a critical predictor 2, 3, 4
• Inability to bear weight on the affected limb 2, 3
• Joint swelling and warmth (present in 72% of cases) 5
• Severe limitation of joint movement (present in 77% of cases) 5
• Pain with passive range of motion (present in 92% of cases) 5

Critical pitfall: The classic triad of fever, pain, and diminished mobility occurs in only approximately 50% of cases, so absence of all three does not exclude septic arthritis 2, 6

Essential Laboratory Studies

Obtain immediately:

• Erythrocyte sedimentation rate (ESR): ESR ≥40 mm/hr is a validated predictor of septic arthritis 2, 3, 4
• C-reactive protein (CRP): CRP >2.0 mg/dL (or >20 mg/L) is highly predictive 2, 3, 4
• White blood cell count: WBC ≥12,000 cells/mm³ supports the diagnosis 2, 3
• Blood cultures before initiating antibiotics 2, 7

When all four Kocher criteria are present (fever, non-weight-bearing, ESR ≥40, WBC ≥12,000), the likelihood of septic arthritis approaches 100%. 2, 3

Diagnostic Imaging Algorithm

Step 1: Plain Radiographs (Initial Study)

• Obtain knee radiographs first to exclude fractures, tumors, and other bony pathology 1, 2
• Radiographs have low sensitivity for septic arthritis (only 32% show diagnostic findings) and are often normal in early infection 1, 5
• Do not delay further workup if radiographs are normal but clinical suspicion remains high 1, 2

Step 2: Ultrasound of the Knee

• Ultrasound can detect joint effusions and guide aspiration if needed 1, 2
• False-negative ultrasound can occur if performed within 24 hours of symptom onset 1, 3
• Ultrasound has limited utility for the knee compared to the hip, as knee aspiration can be performed at bedside without imaging guidance 2

Step 3: Joint Aspiration (Definitive Diagnostic Procedure)

Perform arthrocentesis immediately - this is the gold standard for diagnosis:

• Synovial fluid WBC count ≥50,000 cells/mm³ is highly suggestive of septic arthritis 2
• Send fluid for Gram stain, culture and sensitivity, and cell count with differential 2, 7
• Staphylococcus aureus is the most common pathogen across all pediatric age groups (39% of isolates) 6, 7, 5
• In children under 4 years, Kingella kingae should be strongly considered 2, 6, 3
• Culture is positive in approximately 80% of non-gonococcal cases, but negative culture does not exclude infection 2, 6

Step 4: MRI with Contrast (When Indicated)

Order MRI when:

• Clinical suspicion remains high despite negative or equivocal aspiration 1, 2
• Concern for concurrent osteomyelitis exists (occurs in >50% of pediatric cases) 2, 3
• Symptoms persist or worsen despite initial treatment 2

MRI findings in septic arthritis include:

• Joint effusion with synovial thickening and enhancement 2
• Bone marrow edema in adjacent bones 2
• Decreased femoral head enhancement on early post-contrast imaging (specific for septic arthritis, not seen in transient synovitis) 1, 2, 3
• MRI has 82-100% sensitivity and 75-96% specificity for septic arthritis 2

Critical consideration: Over 50% of pediatric patients with septic arthritis have infection beyond the joint space on MRI, including concurrent osteomyelitis 2, 3

Immediate Management

Surgical Intervention

Immediate surgical drainage (arthrotomy or arthroscopy) is mandatory in all cases of confirmed or highly suspected septic arthritis:

• Surgical drainage combined with antibiotics is essential to prevent irreversible cartilage damage 2, 7, 8
• Most patients do not require additional surgical intervention after initial debridement 7
• Arthroscopic lavage is as effective as open arthrotomy for the knee joint 7, 8

Empiric Antibiotic Therapy

Start empiric antibiotics immediately after obtaining cultures:

• Vancomycin 15 mg/kg/dose IV every 6 hours for MRSA coverage 2
• Consider adding coverage for gram-negative organisms based on age and risk factors 8
• Clindamycin 10-13 mg/kg/dose IV every 6-8 hours is an alternative if local clindamycin resistance is low 2
• Adjust antibiotics based on culture results and sensitivities 2, 7

Antibiotic Duration and Route

• Transition to oral antibiotics after 2-4 days if patient is clinically improving, afebrile, and tolerating oral intake 2
• Total treatment duration: 3-4 weeks for uncomplicated bacterial arthritis 2
• Longer treatment (6+ weeks) required if concurrent osteomyelitis is present 2
• Monitor CRP and ESR to assess treatment response 2

Critical Pitfalls to Avoid

• Do not delay treatment waiting for imaging results - septic arthritis is an orthopedic emergency 2, 6, 3
• Do not assume transient synovitis in a febrile child without ruling out infection first 1, 3
• Do not rely on negative cultures alone - 20% of septic arthritis cases have negative cultures 2, 6
• Always consider concurrent osteomyelitis - present in >50% of pediatric septic arthritis cases and requires longer antibiotic treatment 2, 3
• In patients with sickle cell disease, consider Salmonella species as a causative pathogen 6, 9
• Negative ASO titer helps distinguish septic arthritis from reactive arthritis 4