From the Guidelines
For diagnosing knee infection, I recommend starting with plain radiographs (X-rays) as the initial study, followed by MRI which is the most sensitive and specific imaging modality. X-rays should be obtained first to evaluate for bony changes, joint space narrowing, soft tissue swelling, and joint effusion, though they may appear normal in early infection. MRI with contrast is the gold standard for diagnosing knee infections, as it can detect bone marrow edema, soft tissue abscesses, synovitis, and osteomyelitis with high sensitivity 1.
Key Imaging Modalities
- Ultrasound is useful for detecting joint effusions and guiding aspiration procedures.
- CT scans are helpful when evaluating bony destruction or planning surgical intervention, especially in cases with hardware.
- Nuclear medicine studies like three-phase bone scans or labeled leukocyte scans may be considered when MRI is contraindicated or for patients with prosthetic joints 1.
Importance of Joint Aspiration
These imaging studies complement clinical evaluation and laboratory tests, but joint aspiration remains essential for definitive diagnosis through synovial fluid analysis, Gram stain, culture, and cell count 1. The choice of imaging should be guided by clinical presentation, with more advanced imaging reserved for cases where diagnosis remains uncertain after initial evaluation.
Considerations for Specific Patient Populations
In certain cases, such as young children, ultrasound may be more valuable due to its lack of radiation, no need for sedation, and bedside imaging capability 1. However, for most patients, MRI remains the preferred imaging modality for diagnosing knee infections due to its high sensitivity and specificity.
From the Research
Imaging Modalities for Diagnosing Knee Infection
The following imaging modalities are used to diagnose knee infection (septic arthritis):
- Magnetic Resonance (MR) imaging: useful in identifying cellulitis, abscess, septic arthritis, and osteomyelitis 2
- Computed Tomography (CT): used to evaluate the extent of disease and to assess arthritis within knee compartments, patellofemoral joint, and implant sizing prediction 3, 4
- Radiographs: used as an initial imaging modality, but may not be sensitive or specific for infection 3, 5
- Ultrasound (US): used to evaluate the extent of disease and to guide joint aspiration or tissue sampling 3
- Nuclear studies: used to diagnose prosthetic joint infection, including bone scintigraphy, combined bone gallium imaging, and combined leukocyte/marrow imaging 5
Specific Imaging Modalities for Postoperative Infection
For postoperative infections of the knee, the following imaging modalities are used:
- Radiographs: as an initial imaging modality 3
- MR imaging: to evaluate the extent of disease and to identify complications such as abscess or osteomyelitis 3
- CT: to assess implant positioning and to evaluate the extent of disease 3, 4
- US: to guide joint aspiration or tissue sampling 3
- Nuclear studies: to diagnose prosthetic joint infection 5