What is the appropriate evaluation and management for an adult with knee pain, swelling, and an open wound over the knee?

Knee Pain with Swelling and Broken Skin Integrity: Evaluation and Management

Immediately obtain plain radiographs (AP and lateral views) of the knee, followed by urgent arthrocentesis under sterile technique to rule out septic arthritis, which is a medical emergency requiring prompt diagnosis and treatment. 1

Initial Clinical Assessment

The presence of an open wound over a swollen, painful knee creates high suspicion for infection, including septic arthritis, osteomyelitis, or soft-tissue infection. 1, 2

Key risk factors to identify:

• Age >60 years, diabetes, immunosuppression, recent bacteremia, rheumatoid arthritis, prior joint surgery, corticosteroid use, or recent trauma 3, 4
• Fever, inability to bear weight, warmth, erythema, and limited range of motion suggest acute infection 3, 4
• Night pain or pain at rest is more characteristic of infection than mechanical loosening 1

Diagnostic Algorithm

Step 1: Plain Radiographs (First Study)

Obtain AP and lateral radiographs of the knee immediately as the first imaging study for any patient with suspected osteomyelitis, septic arthritis, or soft-tissue infection. 1, 5

• Radiographs may show joint effusion, periosteal reaction, bone destruction, or gas in soft tissues 1
• Negative radiographs do NOT exclude infection, as early septic arthritis and osteomyelitis may have normal plain films 1, 6

Step 2: Joint Aspiration (Concurrent with Radiographs)

Perform arthrocentesis immediately if joint effusion is present to analyze synovial fluid for cell count with differential, Gram stain, and culture. 1, 2

Synovial fluid interpretation:

• WBC count >50,000/μL has likelihood ratio (LR) of 7.7 for septic arthritis 4
• WBC count >100,000/μL has LR of 28.0 for septic arthritis 4
• Polymorphonuclear cells ≥90% suggests septic arthritis (LR 3.4) 4
• Aspiration can be performed under ultrasound or fluoroscopic guidance if needed 1

Step 3: Laboratory Studies

Obtain blood cultures, ESR, and C-reactive protein (CRP) before initiating antibiotics. 1, 3

• CRP >13.5 mg/L has sensitivity of 73-91% and specificity of 81-86% for prosthetic knee infection 1
• ESR is abnormal in infected patients but has limited specificity 1
• Peripheral leukocyte counts are often normal and should not be relied upon 1

Step 4: Advanced Imaging (After Radiographs)

If radiographs are negative or equivocal but clinical suspicion for infection remains high, obtain MRI of the knee without and with IV contrast. 1

MRI is rated 9/9 (usually appropriate) for:

• Soft-tissue or juxta-articular swelling with cellulitis and skin lesion/wound with suspected osteomyelitis 1
• Evaluation of extent of infection, fluid collections, areas of necrosis, and bone marrow involvement 1
• Contrast enhancement helps delineate abscess formation and necrotic tissue 1

Alternative imaging if MRI is contraindicated:

• CT with IV contrast (rated 7/9) can assess soft-tissue infection and bone involvement 1
• MRI without contrast (rated 7/9) is acceptable if contrast cannot be given 1

Step 5: Foreign Body Evaluation

If there is history of puncture wound through the skin, obtain ultrasound (rated 8/9) or CT without contrast (rated 7/9) to assess for retained foreign body. 1

• Ultrasound is favored for radiolucent foreign bodies (wood, plastic) 1
• CT without contrast is preferred for radiopaque foreign bodies 1
• Contrast may obscure identification of foreign bodies 1

Immediate Management

Empiric Antibiotic Therapy

Initiate broad-spectrum IV antibiotics immediately after obtaining cultures, covering MRSA and gram-negative/gram-positive organisms. 2, 3

• MRSA has become a major cause of septic arthritis in the United States and is associated with worse outcomes 2
• Tailor antibiotics based on culture results once available 2, 3
• Duration: 3-4 weeks for uncomplicated bacterial arthritis; extend to 6 weeks if osteomyelitis is present 2

Surgical Intervention

Prompt evacuation of infected joint is mandatory through arthrocentesis, arthroscopic drainage, or open arthrotomy. 2, 3

• Arthroscopic or open irrigation and debridement with removal of all infected tissue is required for septic arthritis 3
• Imaging-guided drainage may be appropriate for soft-tissue abscesses 2

Critical Pitfalls to Avoid

Never assume normal radiographs exclude infection in a patient with an open wound, swelling, and pain—early septic arthritis and osteomyelitis may have normal plain films. 1, 6

Do not delay arthrocentesis if joint effusion is present, as synovial fluid analysis is essential for diagnosis and guides treatment. 1, 4

Do not order MRI before obtaining plain radiographs, as radiographs are the required first imaging study. 1, 5

Do not rely on peripheral WBC count or ESR alone to exclude infection, as these are often normal or nonspecific in septic arthritis. 1, 4

Avoid attributing symptoms to osteoarthritis or mechanical causes without first excluding infection in the presence of broken skin integrity. 7, 3