Infection of Baker's Cyst without Bacteremia, Sepsis, or Extraneous Source: A Case Report

Baker's cysts are benign fluid-filled cysts that commonly form around knee joints which typically resolve spontaneously. Infection of baker's cysts is uncommon, but most often associated with septic arthritis or bacteremia. We describe a unique case of an infected Baker's cyst that presented without bacteremia, septic knee, or external source of infection. This is a rare manifestation that has not been described in the current literature.

This is a case of a 46 year-old woman who developed an infected Baker's cyst without bacteremia or septic arthritis. She initially presented with the right knee pain, swelling, and limited range of motion. Blood work and synovial fluid aspiration of her right knee showed no source of infection. The patient subsequently developed erythema and tenderness over her right knee. This prompted MRI imaging that demonstrated a complex Baker's cyst. The patient later developed a fever, tachycardia, and worsening anion-gap metabolic-acidosis. Aspiration of the fluid collection was performed and revealed purulent fluid which grew pansensitive Methicillin-Sensitive Staphylococcus aureus, blood and knee aspiration cultures remained negative. The patient was treated with antibiotics and debridement procedures and her symptoms/infection resolved.

Given that isolated infections of Baker's cysts are rare, the localized aspect of this infection makes this case quite unique. Development of an infected Baker's cyst after negative aspiration cultures, combined with the presence of systemic symptoms including fever, without evidence of systemic spread, has not been seen before in the literature to our knowledge. The unique presentation of this case is important for future analysis of Baker's cysts as it introduces the possibility of localized cyst infections as a possible diagnosis for physicians to consider.