The radiograph shows knee effusion, which can be seen as water density above the patella and anterior to the fibula. The next best step in treatment is to perform arthrocentesis to evaluate the fluid for infection from a septic joint or another cause.

Knee effusion typically causes pain, swelling, and limits flexion and range of motion. Effusions are usually easily detected on physical examination by feeling for anterior peripatellar swelling that becomes more pronounced when the knee is flexed. Imaging tests can be performed with a bedside ultrasound and with radiograph. Radiography is typically thought to be less sensitive than ultrasound, and may be less sensitive than physical examination of a patient who is not obese. 

The most common cause of knee effusion is likely trauma such as meniscal injury from routine activity, especially in individuals with pre-existing arthritis. The most serious cause of knee effusion, however, is a septic joint; this condition should be suspected when the knee effusion is associated with a history of fever or redness about the knee on examination.

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Arthrocentesis is the test of choice for the diagnosis of a septic joint. Although less likely to cause a fever or an erythrocyte sedimentation rate >30 mm/h, gout can mimic a septic joint. The most reliable way to distinguish the 2 conditions is through arthrocentesis. Both septic joint and gout can cause very elevated white blood cell counts in the synovial fluid and polymorphonuclear neutrophil >75%. However, a septic joint will usually have a synovial fluid glucose level that is <50% of the serum level and crystals will be absent.

In this case, the results of the arthrocentesis diagnosed septic joint. The patient’s white blood cell count was 60,850×109/L, polymorphonuclear neutrophil 95%, and glucose level of 32 mmol/L (serum level 114 mmol/L). Treatment of the septic joint is operative, including a joint washout and broad spectrum antibiotics. The patient was given antibiotics and went into the operating room prior to admission to the hospital. Synovial fluid cultures were positive for Staphylococcus aureus.

Brady Pregerson, MD, is an emergency physician at Cedars-Sinai Medical Center in Los Angeles and at Tri-City Medical Center in Oceanside, California.


Pregerson DB. Orthopedics-infection and overuse. In: Emergency Medicine 1-Minute Consult Pocketbook. 2017;5.