Thorn injury mimicking a septic arthritis of the knee

On examination in the A&E department he had a swollen, generally tender right knee with decreased range of movement. There was a large effusion with a positive patellar tap. There was also evidence of a small puncture wound in the superomedial aspect of his right knee. He was apyrexic (36.2C) and systemically well. The knee was aspirated under aseptic technique, and 8ml of turbid fluid was sampled. This was sent for direct microscopy and Gram stain as well as for culture for organisms and sensitivities, which ultimately failed to show the presence of organisms. Routine blood testing revealed Haemoglobin of 11.6g/dL, White Cell Count of 14.5x10/mL, and significantly, a normal C-reactive protein (CRP) level of <5mg/L. The possibility of a septic arthritis was raised, but it was felt that on balance, given the clinical condition of the patient, the rapid onset of the effusion, his temperature, routine laboratory results and the results of the aspirate direct microscopy and Gram stain, this diagnosis was less likely. The patient was commenced on intravenous co-amoxiclav and admitted to the ward for observation.