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Difficulty: MediumOrdering Imaging Studies

A primary care physician is evaluating a 62-year-old man who presents to the ambulatory clinic with a nonhealing ulcer on the plantar surface of his left foot. The patient first noticed the lesion four weeks ago. His medical history is significant for poorly controlled type 2 diabetes mellitus, hypertension, and chronic kidney disease with a baseline serum creatinine of 2.2 mg/dL. His temperature is 37.6°C (99.7°F), blood pressure is 145/88 mm Hg, and pulse is 88/min. Physical examination reveals a 3-cm diameter ulceration under the first metatarsal head. There is surrounding erythema, induration, and a moderate amount of purulent exudate. A sterile blunt probe inserted into the ulcer bed easily contacts hard, gritty tissue. Sensation to a 10-g monofilament is absent globally over both feet. Dorsalis pedis pulses are palpable bilaterally. Which of the following is the most appropriate initial imaging study to order for this patient?

  1. A
    Computed tomography angiography of the lower extremities
  2. B
    Magnetic resonance imaging of the foot
  3. Plain radiography of the footAnswer
  4. D
    Point-of-care soft tissue ultrasonography
  5. E
    Three-phase technetium-99m bone scintigraphy

Answer

Plain radiography of the foot
The patient's clinical presentation, particularly the positive probe-to-bone test, strongly suggests osteomyelitis. According to the Infectious Diseases Society of America (IDSA) and the International Working Group on the Diabetic Foot (IWGDF), plain radiography of the affected foot is the most appropriate initial imaging study for all patients presenting with a new diabetic foot infection. It is a rapid, cost-effective method to evaluate for cortical bone destruction, periosteal reaction, soft-tissue gas, and foreign bodies. Although it has low sensitivity in the first two weeks of infection, it establishes a crucial baseline for longitudinal comparison.

Step-by-Step Solution

1
Analyze the patient's clinical presentation.
The patient has a chronic diabetic foot ulcer with purulent drainage and a positive probe-to-bone test.
A positive probe-to-bone test in the setting of a diabetic foot infection is highly predictive of underlying osteomyelitis.
2
Determine the optimal diagnostic imaging sequence for suspected diabetic foot osteomyelitis.
Guidelines recommend starting with plain films before progressing to advanced imaging.
Cost-effective, preliminary imaging is required to identify gross bone destruction, assess for soft-tissue gas, and establish a baseline for future radiographic comparison.
3
Evaluate the patient for specific contraindications to imaging studies.
The patient has chronic kidney disease with an elevated creatinine level (2.2 mg/dL).
This confirms that any modality requiring nephrotoxic intravenous contrast must be avoided.

Key Concept

In patients with suspected diabetic foot osteomyelitis, plain radiography is the recommended initial imaging study, even though advanced modalities are more sensitive.
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Ordering Imaging Studies

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