Popliteal cyst (baker’s cyst)

Cause, clinical and ultrasound findings

• A Baker’s cyst is a defect in the posterior wall of the knee joint capsule, usually with underlying osteoarthritis. The cyst’s onset can be either spontaneous or gradual.

• It presents with a visible ‘fullness or bulge’ on the posterior aspect of the knee; pain may be present on knee flexion.

• Ultrasound findings are that of a comic magazine “speech bubble” shaped anaechoic structure between the medial head of gastrocnemius muscle and the semimembranosus tendon.