Baker's Cyst

A Baker's cyst — also called a popliteal cyst — is a fluid-filled pouch that forms in the soft tissue behind your knee (the popliteal fossa). It develops when excess fluid inside the knee joint is pushed backward through a natural one-way valve between the joint capsule and a small bursa that sits between two muscles at the back of the knee (the semimembranosus and the gastrocnemius, the large calf muscle). Fluid can flow out into the bursa, but the valve makes it difficult for the fluid to flow back in — so the pouch fills and stays filled.

The important thing to understand is that a Baker's cyst is almost always a secondary finding — the cyst is not the disease; it is the knee's overflow system telling you something else is wrong inside the joint. The most common drivers are a meniscus tear, osteoarthritis, or an inflammatory condition like rheumatoid arthritis — anything that makes the knee produce more fluid than usual. Treating the cyst alone without addressing the source almost guarantees it will come back.

Many Baker's cysts are discovered by accident — your provider feels a soft lump behind the knee during an exam for something else, or it shows up on an MRI ordered for knee pain. When the cyst is small, you may not notice it at all.

As it enlarges, the most common complaint is a sense of tightness or fullness behind the knee, especially when you bend the knee fully (squatting, kneeling, or pulling your heel toward your buttock). The lump is most visible and easiest to feel when you stand with the knee straight. Some patients describe a vague aching behind the knee after prolonged walking or standing.

If the cyst ruptures — and they do rupture — the fluid spills down into the calf, causing sudden pain, swelling, and sometimes bruising along the inner ankle. A ruptured Baker's cyst can look and feel remarkably like a DVT (a blood clot in a deep vein), which is why your provider may order an ultrasound of the leg veins to rule that out before diagnosing a rupture.

Your provider can often make the diagnosis by feel alone — a soft, smooth mass in the hollow behind the knee that is most prominent when the knee is fully straight and tends to shrink or become harder to find when the knee is bent. That physical finding, combined with knee symptoms that suggest an underlying problem, is usually enough to confirm the diagnosis.

Ultrasound is the most cost-effective imaging study to confirm the cyst, measure its size, and — importantly — distinguish it from a popliteal artery aneurysm (a bulging blood vessel that can feel similar on exam but requires very different treatment). MRI does double duty: it shows the cyst and — more importantly — reveals the intra-articular problem driving it, whether that is a meniscus tear, cartilage loss, or an inflamed joint lining.

Direct surgical removal of a Baker's cyst is rarely needed. In the uncommon case where the underlying joint problem has been treated (for example, an arthroscopic meniscus repair) and the cyst still persists months later — usually because the one-way valve has become a permanent channel — your surgeon can excise the cyst through a small incision behind the knee, often at the same time as the arthroscopic procedure. Most patients never reach this point.

External patient-education references and related OSI pages for additional background:

• Britannica — Knee (anatomy)
• AAOS OrthoInfo — patient education library
• OSI glossary — orthopedic terminology in plain language