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Difficulty: MediumOrdering Imaging Studies

An endocrinology clinic evaluates a 68-year-old individual who complains of a non-healing sore on the bottom of the right foot that has been present for the past month. Medical history is significant for long-standing type 2 diabetes mellitus, severe peripheral neuropathy, and chronic kidney disease stage 4 (estimated glomerular filtration rate of 22 mL/min/1.73 m²). The patient reports noticing increased swelling and a foul-smelling discharge on the dressing over the past three days. Vitals show a temperature of 37.6°C (99.7°F), blood pressure of 138/82 mmHg, and heart rate of 88/min. Inspection of the plantar aspect of the right forefoot reveals a 2.5-cm, deep ulceration with a necrotic base directly overlying the first metatarsophalangeal joint. A sterile blunt probe easily contacts hard, gritty tissue at the depth of the wound. Which of the following is the most appropriate initial radiologic investigation for this patient?

  1. Plain radiography of the footAnswer
  2. B
    Magnetic resonance imaging of the foot with and without intravenous contrast
  3. C
    Magnetic resonance imaging of the foot without intravenous contrast
  4. D
    Three-phase technetium-99m bone scintigraphy
  5. E
    Computed tomography of the foot without intravenous contrast

Answer

Plain radiography of the foot is the most appropriate initial imaging modality.
The correct answer is plain radiography of the foot. When a patient presents with clinical signs highly suggestive of diabetic foot osteomyelitis (such as a positive probe-to-bone test), plain radiography is the universally recommended first imaging step. While it lacks sensitivity for early infection—as visible cortical destruction or periosteal reaction typically takes 10 to 14 days to develop—it is widely available, cost-effective, and essential for establishing a baseline architecture. Importantly, it can rapidly identify other critical pathologies, such as underlying Charcot neuroarthropathy, retained foreign bodies, or soft tissue gas produced by anaerobic organisms, which would prompt immediate surgical debridement.

Step-by-Step Solution

1
Identify the primary clinical suspicion.
The patient has a chronic diabetic foot ulcer with a positive probe-to-bone test, which is highly sensitive and specific for underlying osteomyelitis.
Recognizing the high pre-test probability of osteomyelitis guides the diagnostic workup.
2
Determine the initial imaging modality of choice according to standard guidelines.
Plain radiography is always the first-line imaging study.
Although plain films have low sensitivity for acute osteomyelitis (changes take 10-14 days to appear), they are essential to establish a baseline, evaluate for Charcot arthropathy, identify foreign bodies, and check for soft-tissue gas indicating a necrotizing infection.
3
Evaluate the role of advanced imaging.
Advanced imaging (like MRI) is deferred until after plain films are obtained and reviewed.
MRI is the most sensitive imaging modality and is indicated subsequently if plain radiographs are negative or inconclusive in the setting of high clinical suspicion.

Key Concept

In patients with suspected diabetic foot osteomyelitis, plain radiography of the affected area is always the most appropriate initial imaging step, regardless of the high sensitivity of MRI.
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Ordering Imaging Studies

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