What type of ankle X-ray should I order to assess for septic arthritis versus osteomyelitis versus gout or pseudogout?

Ankle X-ray for Assessing Septic Arthritis vs Osteomyelitis vs Gout/Pseudogout

Standard radiographs of the ankle (anteroposterior, lateral, and mortise views) should be ordered as the initial imaging study when assessing for septic arthritis, osteomyelitis, or crystalline arthropathies like gout and pseudogout. 1

Initial Imaging Approach

• Standard radiographs of the ankle are the appropriate first-line imaging modality for all suspected musculoskeletal infections and crystalline arthropathies 1
• These radiographs provide an excellent anatomic overview and can help exclude other causes of pain and swelling such as fractures and tumors 1
• The initial radiographic series should include:
  • Anteroposterior (AP) view
  • Lateral view
  • Mortise view 1

Radiographic Findings by Condition

Septic Arthritis

• Joint effusion (key finding) 1
• Soft tissue swelling around the joint 1
• Effacement of fat planes 1
• Early stages may show minimal findings 1
• Later stages may show joint space narrowing and erosions 1

Osteomyelitis

• Early acute osteomyelitis (<14 days): may show only mild soft tissue swelling or appear normal 1
• Acute osteomyelitis: erosions and periosteal reaction 1
• Chronic osteomyelitis: bone sclerosis 1
• Sequestrum (dead bone fragment) in chronic cases 1

Gout/Pseudogout

• Soft tissue swelling 2
• Joint effusion 2
• Tophi (in chronic gout) 2
• Chondrocalcinosis (calcium deposition in cartilage - characteristic of pseudogout) 2
• Erosions with overhanging edges (in chronic gout) 2

Diagnostic Algorithm

1. Start with standard radiographs of the ankle (AP, lateral, mortise views) 1

2. If radiographs are normal or show only joint effusion/soft tissue swelling:

   • For suspected septic arthritis: proceed to joint aspiration for definitive diagnosis 3
   • For suspected osteomyelitis: proceed to MRI without and with contrast 1
   • For suspected crystalline arthropathy: consider joint aspiration for crystal analysis 4, 5

3. If radiographs show specific findings:

   • Bone erosions with periosteal reaction: consider osteomyelitis 1
   • Joint space narrowing with erosions: consider septic arthritis or advanced crystalline arthropathy 2
   • Chondrocalcinosis: suggests pseudogout 2

Important Clinical Considerations

• Septic arthritis and crystalline arthropathies can coexist in up to 5% of cases, making diagnosis challenging 4, 5
• Radiographic findings of gout can sometimes mimic infection, leading to diagnostic confusion 2
• A synovial fluid total nucleated cell count >50,000 cells in a crystal-positive joint should raise suspicion for concurrent septic arthritis 4

Subsequent Imaging After Initial Radiographs

If initial radiographs are inconclusive and clinical suspicion remains high:

• For suspected septic arthritis or soft tissue infection:

  • MRI without and with IV contrast (rated 9/9 for appropriateness) 1
  • Ultrasound-guided aspiration for definitive diagnosis 3

• For suspected osteomyelitis:

  • MRI without and with IV contrast (rated 9/9 for appropriateness) 1
  • CT with IV contrast if MRI is contraindicated (rated 7/9) 1

Common Pitfalls to Avoid

• Relying solely on radiographs to rule out infection, as early septic arthritis and osteomyelitis may have normal radiographs 1
• Failing to recognize that crystalline arthropathy and septic arthritis can coexist 4, 5
• Delaying joint aspiration when septic arthritis is suspected, as it is an orthopedic emergency 3
• Overlooking the need for additional imaging (MRI) when radiographs are normal but clinical suspicion for infection remains high 1