When is an X-ray of the buttocks indicated to rule out osteomyelitis (OM)?

X-Ray of the Buttocks for Diagnosing Osteomyelitis

Plain radiographs (X-rays) should be the initial imaging modality for suspected osteomyelitis in the buttocks region, as they provide an excellent anatomical overview and can help exclude other conditions, though they have limited sensitivity in early infection. 1

Initial Imaging Approach

Plain Radiography (X-ray)

• The American College of Radiology (ACR) recommends radiographs as the first-line imaging for suspected osteomyelitis in any body region, including the buttocks 1
• X-rays provide valuable baseline information and can detect:
  • Erosions and periosteal reaction (common in acute osteomyelitis)
  • Bone sclerosis (common in chronic osteomyelitis)
  • Soft tissue swelling
  • Presence of gas or foreign bodies
  • Fractures or tumors that might mimic infection 1

Limitations of X-rays

• Low sensitivity in early osteomyelitis (first 1-2 weeks)
• May show only mild soft tissue swelling in early stages
• Bone destruction requires 30-50% mineral loss to be visible on X-ray 1, 2
• Radiographic changes often lag behind clinical progression 3

Follow-up Imaging When X-rays Are Normal or Inconclusive

If initial X-rays are normal but clinical suspicion for osteomyelitis remains high:

MRI (Preferred Next Step)

• MRI with and without contrast is the most accurate follow-up imaging modality with sensitivity and specificity >90% 1, 4
• Provides excellent evaluation of:
  • Bone marrow changes (earliest sign of osteomyelitis)
  • Soft tissue involvement
  • Abscess formation or fistulas
  • Extent of infection for surgical planning 1
• Can detect osteomyelitis before radiographic changes appear 5

Alternative Advanced Imaging Options

1. CT scan:

   • Useful when MRI is contraindicated
   • Better than X-ray for detecting sequestrum, foreign bodies, or gas
   • Less sensitive than MRI for early osteomyelitis 1, 5

2. Nuclear Medicine Studies:

   • 3-phase bone scan: High sensitivity (83%) but low specificity (45%) 1
   • FDG-PET/CT: High sensitivity (81-100%) and specificity (87-100%) 1, 4
   • WBC scan with SPECT/CT: Useful for more accurate localization of infection 1

Special Considerations for Buttock Region

• Deeper soft tissue structures in the buttocks may limit ultrasound evaluation 1
• Metal hardware (if present) can create artifacts on MRI and CT 1
• Pressure ulcers in the buttock region overlying bony prominences increase suspicion for underlying osteomyelitis 4
• Probe-to-bone test can be performed if an ulcer is present (positive test strongly suggests osteomyelitis) 1, 4

Diagnostic Algorithm

1. Start with plain X-rays of the buttock region (multiple views)
2. If X-rays are positive with characteristic findings of osteomyelitis:
   • Proceed with treatment or consider MRI to determine extent of infection
3. If X-rays are negative or inconclusive but clinical suspicion remains high:
   • Proceed to MRI with and without contrast (preferred)
   • If MRI is contraindicated, consider CT or nuclear medicine studies
4. If diagnosis remains uncertain after advanced imaging:
   • Consider image-guided bone biopsy for definitive diagnosis 1

Pitfalls to Avoid

• Do not rely solely on X-rays to exclude osteomyelitis, especially in early disease
• Do not delay advanced imaging if clinical suspicion is high despite normal X-rays
• Avoid misinterpreting normal variants or non-infectious changes (like Charcot osteoarthropathy) as infection 1
• Do not forget to mark the site of any ulcer with a radio-opaque marker during X-ray to allow accurate assessment of structures adjacent to the ulcer 1
• Recognize that recent surgery or trauma in the buttock region can mimic infection on imaging studies 1

By following this evidence-based approach, clinicians can appropriately utilize X-ray imaging as the initial step in diagnosing osteomyelitis in the buttock region, while understanding its limitations and knowing when to proceed to more advanced imaging modalities.