Infected Baker’s cyst, diagnosed in the emergency department using POCUS

Abstract

Baker’s cyst is a closed collection of fluid that forms in the posterior aspect of the knee. Usually, it appears as a non-painful inflammation in the popliteal fossa. In adults, its aetiology is secondary to problems that cause distension of the knee joint. It is often associated with rheumatoid arthritis and osteoarthritis. Occasionally, the cyst may become oversized and rupture with the consequent leakage of synovial fluid into adjacent tissues, presenting a clinical course similar to acute thrombophlebitis. Infection of a popliteal cyst is an uncommon complication and is associated with septic arthritis. In this paper, we present the case of a patient, an intravenous drug user (IVDU), who developed a spontaneous infection of a Baker’s cyst secondary to Staphylococcus aureus, which was diagnosed in the emergency department (ED) using point-of-care-ultrasound (POCUS). Figure 1. Within the posterior aspect of the left calf on the medial aspect there is an extremely large cystic lesion measuring 18.7 cm in length and 4 cm in width with no adverse features. The cystic lesion is communicating with the semimembranosus/medial head gastrocnemius bursa more proximally in the knee where it demonstrates internal echoes and synovial thickening and a single septation.