These right knee x-rays are from a 30 year old man who presented with severe pain and swelling in the knee joint and fever.  He had a knee arthrocentesis performed at a rural hospital few days prior. What can you observe in the x-rays?

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The knee x-rays show significant joint effusion and there are areas of gas locules within the effusion. This finding is suspicious for a septic arthritis. There are no lytic areas of bone destruction.

The air bubbles likely entered the joint at the time of arthrocentesis.

The patient grew CoNS (coagulase negative staph), a skin commensal from the synovial fluid taken at the time of joint washout.

In general, plain x-rays are not sensitive in the diagnosis of septic arthritis. Early on in the disease, there may be soft tissue swelling or joint effusion. After about 7-10 days, areas of subchondral lysis or bony erosion may appear. Gas within the joint in a patient with clinical suspicion of septic arthritis suggests an infection with E. Coli, anaerobes.

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