Septic arthritis of the knee
Painful right leg post fall three days ago. Unable to weight bear.
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Right tricompartmental knee replacement. No fracture identified. Alignment within normal limits. Large knee joint effusion containing locules of gas. Prepatellar soft tissue swelling.
The locules of gas seen on the lateral projection are the features most concerning for a septic arthritis in what would otherwise be a standard knee joint effusion. The patient proceeded to washout, with synovial biopsy culturing Streptococcus dysgalactiae (Group C/G). The patient will be on long-term antibiotics.