Popliteal Cyst (Baker's Cyst)

A popliteal (Baker's) cyst is a fluid-filled distension of the gastrocnemius-semimembranosus bursa in the posteromedial popliteal fossa.

In adults it is almost always secondary to intra-articular pathology that produces an effusion, most often knee osteoarthritis (OA), a meniscal tear or inflammatory arthritis. In children it is usually primary and resolves without treatment. In adults the priority is to identify and treat the underlying knee disorder, because aspiration alone is followed by recurrence. A ruptured cyst is a classic mimic of deep vein thrombosis (DVT).

The gastrocnemius-semimembranosus bursa lies in the posteromedial popliteal fossa, between the medial head of gastrocnemius and the semimembranosus tendon. It commonly communicates with the knee joint through a slit-like opening in the posteromedial capsule.

The opening acts as a one-way valve: effusion fluid enters the bursa during flexion, but its return is impeded in extension, so the bursa distends. This explains why the cyst enlarges with large or recurrent effusions, refills after aspiration unless the joint is treated, and often fluctuates in size with activity and rest.

Adult causes:

• •OA, the most common
• •Meniscal tears and anterior cruciate ligament injury
• •Inflammatory arthritis, including rheumatoid arthritis (RA) and gout; the cyst can be the presenting feature of RA

Rupture is the most common complication, and may be asymptomatic in up to 80%. It is more likely with inflammatory arthritis than with OA. Fluid tracks into the calf, causing acute swelling that mimics DVT, and bruising may track to the medial malleolus (the crescent sign). A large cyst can also compress the popliteal vein (causing DVT), the tibial nerve or, rarely, the popliteal artery. Compartment syndrome after rupture and infection are rare.