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Difficulty: MediumOrthopedic Surgery

A 9-year-old boy is brought to the emergency department by his parents due to a 3-day history of worsening left leg pain and fever. The parents report that he initially started limping but now refuses to bear any weight on the left leg. There is no history of recent trauma, tick bites, or travel. His temperature is 38.8°C (101.8°F), blood pressure is 105/65 mmHg, heart rate is 115/min, and respiratory rate is 18/min. On physical examination, the child is ill-appearing. There is exquisite point tenderness, localized warmth, and mild swelling over the proximal anterior aspect of the left tibia. Range of motion of the left knee and ankle is fully preserved and does not elicit pain. Laboratory studies reveal a leukocyte count of 15,500/mm³ (85% neutrophils), an erythrocyte sedimentation rate (ESR) of 65 mm/hr, and a C-reactive protein (CRP) level of 82 mg/L. Plain radiographs of the left tibia and knee show no obvious abnormalities. Which of the following is the most appropriate next step in management?

  1. Obtain magnetic resonance imaging of the left lower extremityAnswer
  2. B
    Administer empiric intravenous vancomycin and ceftriaxone
  3. C
    Discharge home with a prescription for oral NSAIDs and close follow-up
  4. D
    Transport the patient for a computed tomography scan of the left tibia
  5. E
    Apply a supportive splint and admit for non-weight-bearing observation

Answer

Obtain magnetic resonance imaging of the left lower extremity
The most appropriate next step in evaluation is to obtain a magnetic resonance imaging (MRI) scan of the lower extremity. This pediatric patient presents with localized bone pain, high fever, and elevated inflammatory markers (WBC, ESR, CRP), which is highly suspicious for acute hematogenous osteomyelitis. In children, this infection typically seeds in the highly vascular metaphysis of long bones. Plain radiographs are typically normal during the first 10 to 14 days of infection because approximately 30% to 50% of bone mineral must be lost before lytic changes become radiographically apparent. MRI, with and without contrast, is the most sensitive and specific imaging modality for early osteomyelitis, capable of detecting bone marrow edema and surrounding soft tissue involvement (e.g., subperiosteal abscess) within 3 to 5 days of onset.

Step-by-Step Solution

1
Analyze the clinical presentation.
The patient is a 9-year-old boy with acute onset of localized proximal tibial pain, refusal to bear weight, high fever, and no history of trauma.
Identifying these classic symptoms strongly points to acute hematogenous osteomyelitis.
2
Evaluate the laboratory and imaging findings.
Elevated WBC, ESR, and CRP confirm an acute inflammatory/infectious process. Normal plain radiographs are noted.
Plain radiographs are typically normal in the first 10-14 days of osteomyelitis because significant bone demineralization has not yet occurred, making further imaging necessary.
3
Determine the next best step in management.
MRI of the affected limb is indicated.
In a hemodynamically stable patient, MRI is the gold standard to confirm early bone marrow edema and evaluate the extent of the infection before initiating antibiotic therapy.

Key Concept

Early acute hematogenous osteomyelitis typically presents with normal plain radiographs; MRI is the most sensitive imaging modality and is the next best step for definitive diagnosis in a stable patient.
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Orthopedic Surgery

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