P18) Knee pain and fever in an infant

Review the Learning Outcomes, Hx, PE and Labs, and begin the module with your Provisional Diagnosis. Keep hitting "Next" to move through the module.

Learning Outcomes

  1. Articulate your relationship with the consulting diagnostic radiologists in the evaluation of suspected musculoskeletal infection.
  2. Review some of the forms of musculoskeletal infection.
  3. Identify the role of different imaging modalities for evaluating patients with suspected musculoskeletal infection.


Physical Exam


Provisional Diagnosis

Select the Dx you believe is most appropriate
In the setting of pain, fever, and elevated inflammatory markers, an infectious process must be considered first.
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Potential Acuity

What is your assessment of the likely acuity for this patient?

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Septic arthritis leads to rapid cartilage destruction and must be treated rapidly if present. Due to high clinical concern for septic arthritis in this case, bedside arthrocentesis was performed. No fluid was aspirated. Concern for infection remains high.

First Imaging Study

What is the first imaging study you will order?

Radiographs are the standard initial imaging modality in the assessment of most musculoskeletal complaints, particularly suspected infection.
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Second Imaging Study

What is the next imaging study you will order?

Clinical concern for infection remains high even after a dry joint aspiration and a radiograph showing only soft tissue edema. Manifestations of musculoskeletal infection include cellulitis, abscess, septic arthritis, septic tenosynovitis, and osteomyelitis. MRI is the only modality that provides reliable assessment for each of these categories. If MRI is not available, CT would be a reasonable option though is much less sensitive, particularly for osteomyelitis. In addition, CT would expose this young patient to ionizing radiation. Ultrasound may show soft tissue edema, fluid collections, and joint effusions but is otherwise nonspecific. PET/CT and indium scans can show manifestations of infection, but can be nonspecific, would delay care in this emergent situation, and would expose the patient to radiation.
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What is your Diagnosis now that you have seen the imaging results?

This most likely reflects a hematogenous infection. Typically, patients suffer from a mild trauma (e.g. ground level fall) causing mild osseous injury and hyperemia. This occurs days to weeks before presentation and is often forgotten. Subsequently, the patient has a mild transient bacteremia such as from a superficial soft tissue wound. The pathogens then deposit in the region of relative osseous hyperemia from prior trauma. Often the pathogens are managed effectively by the immune system. If not, osteomyelitis can be established. If the disease process progresses, the pathogen can decompress through the overlying bone cortex either into the joint space if intra-articular or be contained by the periosteum if extra-articular. In the later case, a subperiosteal abscess forms.

Current Acuity

Initially, you selected and we suggested acuity.

Has your concern for this patient changed?

Subperiosteal abscess likely requires surgical decompression.

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