WHEN THE PROBLEM IS PAINFUL KNEE EFFUSION
Nontraumatic knee effusion can be the primary source of knee pain or the result of underlying pathology. It is mentioned here because clinical suspicion is paramount to diagnosis of a septic joint—a serious cause of painful knee effusion that warrants prompt treatment.
As in other causes of knee pain, a detailed history of the character of the pain is essential. Septic arthritis and crystalline disease (gout, pseudogout) should be suspected in patients without a history of trauma who cannot bear weight. Systemic complaints point to an infection and, with the exception of a possible low-grade fever, are not typically seen in crystalline disease. Notable findings include an erythematous, hot, swollen knee and pain with both active and passive movement.
Plain radiographs of the knee should be ordered to rule out significant trauma or arthritis as the etiology. It is important to perform joint aspiration with synovial fluid analysis. Fluid analysis should include a white blood cell (WBC) count with differential, Gram stain and cultures, and polarized light microscopy (not readily available in an outpatient setting).29
Synovial fluid analysis characteristics suggestive of a septic joint include turbid quality, WBC > 50,000/mL, an elevated protein content, and a low glucose concentration.30 Gram stain and culture will help identify the infectious agent. Orthopedic referral should not be delayed in patients with a suspected infectious joint. Corticosteroids should not be injected during aspiration if infection is being ruled out.
When Jane returns for a follow-up visit eight weeks later, she states that the knee pain has resolved and that she has returned to running. She has lost an additional eight pounds and continues to diet. And, at the advice of her physical therapist, she is continuing her physiotherapy regimen at home to prevent a recurrence of PFPS.
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