Why do we perform a knee X-ray (XR) in the workup of septic arthritis in a pediatric patient with diabetes?

Why Perform Knee X-Ray in Septic Arthritis Workup

Knee X-rays are performed in the workup of septic arthritis primarily to exclude alternative diagnoses (fractures, tumors, other bone pathology) and to identify complications or concurrent conditions, not to diagnose septic arthritis itself, as radiographs are typically normal in early infection. 1

Primary Reasons for Obtaining Knee Radiographs

Exclusion of Alternative Diagnoses

• Plain radiographs serve as the appropriate first-line imaging to exclude other causes of acute knee pain and swelling, including fractures, tumors, and degenerative joint disease that may mimic septic arthritis clinically. 2
• The American College of Radiology recommends radiography as part of the initial imaging algorithm, though it often necessitates additional imaging studies if clinical suspicion for septic arthritis remains high. 1

Detection of Late-Stage Findings and Complications

• Radiographs can identify joint effusion, soft tissue swelling, effacement of fat planes, or gas in soft tissues—findings that increase concern for septic arthritis and warrant immediate joint aspiration. 3, 2
• In later stages of infection (>7-10 days), X-rays may show joint space narrowing, erosions, or periosteal reaction, though these findings require >30% osseous matrix destruction to become visible. 3, 2

Assessment for Concurrent Osteomyelitis

• Radiographs help identify concurrent osteomyelitis, which occurs in approximately 58% of pediatric septic arthritis cases, particularly in the elbow, and is critical to detect as it requires longer antibiotic treatment. 3
• In pediatric patients with diabetes, the risk of concurrent infection is elevated, making this assessment particularly important. 1

Critical Limitations of Knee X-Rays

Poor Sensitivity in Early Infection

• Normal radiographs do NOT exclude septic arthritis, as X-rays are typically completely normal in early acute infection (<14 days), showing at most mild soft tissue swelling. 3, 2
• Radiographic changes do not appear until 7-10 days into the disease course and require substantial bone destruction to become visible. 3

Low Diagnostic Accuracy

• Plain radiographs have low sensitivity and specificity for diagnosing septic arthritis itself. 3
• The definitive diagnostic procedure is joint aspiration with synovial fluid analysis, not imaging. 3

Appropriate Imaging Algorithm for Suspected Septic Arthritis

Initial Imaging Approach

1. Obtain knee radiographs (AP, lateral, and if applicable, sunrise views) as the first imaging study to exclude fractures and assess for gross abnormalities. 2
2. If radiographs are normal or show only joint effusion/soft tissue swelling, proceed immediately to joint aspiration (which can be performed at bedside for the knee, unlike the hip which requires ultrasound guidance). 3

When to Advance to MRI

• Order MRI with and without IV contrast when:

  • Clinical suspicion remains high despite negative aspiration 1, 3
  • Concern exists for concurrent osteomyelitis (particularly if patient has risk factors: CRP >13.8 mg/L, absolute neutrophil count >8.6×10³ cells/mL, platelet level <314 cells/mL, symptoms >3 days, age >3.6 years) 1
  • Patient fails to respond to appropriate antibiotic therapy 3
  • Need to assess for soft tissue abscess or pyomyositis 3

• MRI has 82-100% sensitivity and 75-96% specificity for diagnosing septic arthritis and osteomyelitis. 3

• However, MRI should not delay definitive treatment (surgical drainage and antibiotics) when septic arthritis is strongly suspected clinically. 1

Special Considerations for Pediatric Patients with Diabetes

Increased Risk Profile

• Diabetes mellitus significantly increases the probability of septic arthritis and atypical infections, including mycotic subtypes (such as Candida albicans). 4, 5
• These patients may have more insidious presentations with delayed diagnosis. 5

Higher Risk of Concurrent Infection

• Pediatric patients with confirmed septic arthritis frequently have infection beyond the joint space on MRI, with presence of at least 3 risk factors (including the laboratory values mentioned above) conferring 90% sensitivity for adjacent infection. 1

Common Pitfalls to Avoid

Do Not Rely on Radiographs Alone

• Never use normal radiographs to rule out septic arthritis—this is a critical error that can lead to delayed diagnosis and permanent joint damage. 3, 2
• Bacterial proliferation can cause irreversible cartilage damage within hours to days. 3

Do Not Delay Joint Aspiration

• If clinical suspicion is high (fever >101.3°F, ESR ≥40 mm/hour, WBC ≥12,000 cells/mm³, inability to bear weight, CRP >2.0 mg/dL), proceed directly to joint aspiration regardless of radiographic findings. 3
• Meeting all five criteria approaches 100% likelihood of septic arthritis. 3

Do Not Let Imaging Delay Treatment

• Obtain blood cultures and perform joint aspiration before initiating antibiotics, but do not delay empiric antibiotic therapy waiting for advanced imaging results. 6, 7
• Synovial fluid WBC count ≥50,000 cells/mm³ with >90% polymorphonuclear cells strongly suggests septic arthritis (LR 28.0 for counts >100,000/μL). 3, 4