What is the recommended CT scan to order for a patient with a possible infection of a wound to the anterior knee, particularly for adults and adolescents with underlying medical conditions such as diabetes or immunocompromised states?

CT with IV Contrast for Suspected Wound Infection of the Anterior Knee

For a possible infection of a wound to the anterior knee, order CT of the knee WITH intravenous contrast to evaluate the extent of soft tissue infection, identify abscess formation, and guide treatment decisions regarding surgical debridement or percutaneous drainage. 1, 2

Why CT with IV Contrast is the Appropriate Study

CT with IV contrast is specifically designed to assess soft tissue infections and can distinguish between different types of musculoskeletal infection including cellulitis, myositis, tenosynovitis, abscess, and septic arthritis. 1 The contrast enhancement is critical because:

• CT with IV contrast allows evaluation of various soft tissue compartments and helps differentiate cellulitis from deeper infections like abscess or necrotizing fasciitis 1, 2
• Contrast enhancement helps assess the extent of infection and can guide aspiration or surgical debridement 1
• CT provides compartmental anatomy analysis that distinguishes among various types of musculoskeletal infection and guides treatment options 2

Initial Imaging Algorithm

Before ordering CT, you should:

• Obtain plain radiographs first as the initial imaging study to exclude fractures, foreign bodies, gas in soft tissues, and provide anatomic overview 1
• Look specifically for soft tissue swelling, effacement of fat planes, gas, and foreign bodies on radiographs 1
• Recognize that radiographs may be normal in early infection but are essential for excluding other pathology 1

Critical Clinical Features to Assess

When evaluating for wound infection, specifically assess:

• Fever and systemic symptoms, which strongly suggest more serious infection requiring urgent intervention 3
• Inability to bear weight or move the joint, which is a red flag for septic arthritis 3
• Recent bacteremia or infection elsewhere, which increases suspicion of septic arthritis 3
• Presence of joint effusion, which may indicate septic arthritis requiring urgent aspiration 1, 3

Why NOT CT Without Contrast

CT without IV contrast cannot adequately assess soft tissue infections because it lacks the ability to differentiate infected from non-infected tissues and cannot evaluate the vascularity and extent of infection. 1 CT without contrast is appropriate for fracture evaluation but not for infection assessment. 1

Role of Image-Guided Aspiration

If CT identifies a fluid collection or abscess, image-guided aspiration should be performed for culture to identify the infectious organism, which directly affects treatment. 1 Image guidance with CT or ultrasound is ideal because:

• Proper needle placement is confirmed, significantly reducing vascular complications and nerve injury 1
• Needle trajectory can be planned to reduce risk of contaminating normal adjacent tissues 1

Treatment Decision Points Based on CT Findings

CT with IV contrast guides specific treatment pathways:

• If necrotizing fasciitis is identified, emergency surgical debridement is required 2
• If abscess formation is present, percutaneous drainage is indicated 2
• If only cellulitis without deeper involvement, antibiotic therapy alone may be sufficient 2

Common Pitfalls to Avoid

• Do not skip plain radiographs before CT—they provide essential baseline information and may identify foreign bodies or gas that change management 1
• Do not order CT without IV contrast for suspected infection—it lacks the necessary soft tissue detail 1
• Do not delay aspiration if septic arthritis is suspected—imaging should not delay diagnostic aspiration when clinical suspicion is high 1, 3
• Do not assume imaging alone can distinguish infected from non-infected joints—culture is often needed for definitive diagnosis 1