A fluid filled sac in the popliteal fossa, causing a lump

Aetiology

Baker's cysts (also known as a popliteal cyst) are not true cysts but rather a distension of the gastrocnemius-semimembranosus bursa. They may be primary or secondary:

• Primary: no underlying pathology, typically seen in children
• Secondary: underlying condition such as osteoarthritis, typically seen in adults

In adults, Baker’s cysts are usually secondary to degenerative changes in the knee joint. Can be assoiciated with:

• Meniscal tears
• OA
• Knee injuries
• Inflammatory arthritis (e.g. RA)

Pathophysiology

Synovial fluid is squeezed out of the knee joint and collects in the popliteal fossa. A connection between the synovial fluid in the joint and the Baker’s cyst can remain, allowing the cyst to continue enlarging as more fluid collects there.

Baker’s cysts are contained within the soft tissues. They do not have their own epithelial lining.

Differential Diagnoses

The key differential diagnoses of a lump in the popliteal fossa are:

• Deep vein thrombosis
• Abscess
• Popliteal artery aneurysm