Popliteal (Baker) Cyst

Popliteal (Baker) cyst is a fluid-filled swelling that develops behind the knee, involving the musculoskeletal system. It occurs when excess joint fluid accumulates and forms a sac in the popliteal fossa, the area behind the knee joint. This cyst can cause pain, swelling, and a feeling of tightness or stiffness in the knee. It often results from underlying knee problems such as arthritis or meniscal tears that increase fluid production. The cyst may limit knee movement and sometimes rupture, causing sharp pain and swelling in the calf. Diagnosis is usually made by physical exam and imaging, and the condition primarily affects knee function and comfort.

Popliteal (Baker) cyst is a synovial fluid-filled cyst located in the popliteal fossa, formed by distension of the gastrocnemio-semimembranosus bursa. It typically arises secondary to intra-articular knee pathology such as osteoarthritis, rheumatoid arthritis, or meniscal injury, which leads to increased production of synovial fluid and subsequent bursal distension. The cyst communicates with the knee joint through a valvular opening, allowing fluid to flow into but not easily out of the bursa, causing enlargement. Clinically, it presents with a palpable, often painless swelling behind the knee, sometimes accompanied by posterior knee pain, stiffness, or restricted range of motion. Complications include cyst rupture, mimicking deep vein thrombosis, and compression of neurovascular structures. Diagnosis is confirmed by ultrasound or MRI, which delineate the cyst and underlying joint pathology.