Should CT be ordered to rule out osteomyelitis of a left BKA stump?
No, CT should not be the initial imaging study to rule out osteomyelitis of a left below-knee amputation stump—start with plain radiographs, and if osteomyelitis remains suspected, proceed to MRI as the definitive advanced imaging modality.
Initial Imaging Approach
Begin with plain radiographs of the left BKA stump as your first-line imaging study 1. The 2022 ACR Appropriateness Criteria explicitly state that radiographs should be used for initial evaluation of musculoskeletal infections, including osteomyelitis 1. While radiographs may be normal in early acute osteomyelitis (<14 days), they can show:
- Periosteal reaction and erosions (acute osteomyelitis)
- Bone sclerosis (chronic osteomyelitis)
- Soft tissue swelling
- Gas in soft tissues
- Foreign bodies
Radiographs also provide an excellent anatomic overview and can exclude other pathology like fractures or heterotrophic ossification—the latter being particularly relevant in amputation stumps 2.
Why CT is Not Appropriate
The ACR guidelines explicitly state there is insufficient evidence to support CT for initial evaluation of osteomyelitis 1. More specifically:
- CT is insensitive for acute osteomyelitis 1
- CT is most useful for characterizing chronic osteomyelitis changes, particularly detecting sequestra (dead bone fragments) 1
- IV contrast administration helps assess soft tissue involvement and abscess formation but does not improve diagnostic sensitivity for acute osteomyelitis 1
When Radiographs Are Normal or Equivocal
If plain films are normal or show findings suggestive of osteomyelitis but you need definitive confirmation, MRI is the advanced imaging modality of choice 3, 4:
- MRI has 95.6% sensitivity and 80.7% specificity for osteomyelitis in adults 3
- MRI provides superior soft tissue detail and accurately defines the extent of infection 4
- MRI is widely available and avoids ionizing radiation 3
Alternative Advanced Imaging Options
If MRI is contraindicated (e.g., pacemaker, severe claustrophobia, metallic hardware causing artifact):
- FDG-PET/CT: 81-100% sensitivity, 87-100% specificity 1
- SPECT imaging: 95.1% sensitivity, 82.0% specificity 3
- Tc-99m HMPAO WBC scintigraphy: 87.3% sensitivity, 94.7% specificity 3
These nuclear medicine studies have diagnostic accuracy comparable to MRI 3.
Critical Pitfall in Amputation Stumps
Beware of heterotrophic ossification in amputation stumps, which can mimic osteomyelitis on bone scans 2. A case report demonstrated that SPECT/CT was able to differentiate heterotrophic ossification from true osteomyelitis in a BKA stump when conventional bone scan showed increased uptake 2. This underscores that if nuclear medicine imaging is used, SPECT/CT hybrid imaging may provide better anatomic localization than planar imaging alone.
Clinical Context Matters
In the amputation stump setting, consider:
- Uncontrolled infection as the primary indication may actually have better healing outcomes than other indications 5
- Vascular patency (particularly profunda femoris artery) significantly impacts stump healing 5
- If deep wound cultures are obtained, be aware that multidrug-resistant organisms like Stenotrophomonas maltophilia can complicate diabetic-related amputations and may require aggressive management 6