A popliteal cyst, commonly called a Baker’s cyst, is a fluid-filled sac that forms behind the knee. It develops when excess lubricating fluid from inside the knee joint gets pushed through a one-way valve into the hollow space at the back of the knee, called the popliteal space. The cyst sits specifically between two muscles: the semimembranosus (a hamstring muscle) and the inner head of the calf muscle. On MRI scans of the knee, these cysts show up in roughly 5 to 18 percent of patients, making them one of the most common soft-tissue findings behind the knee.
How a Popliteal Cyst Forms
Your knee joint contains a clear, straw-colored fluid called synovial fluid that reduces friction when you bend and straighten your leg. When something irritates the joint, the lining responds by producing more of this fluid than the joint can hold. The excess fluid migrates to the back of the knee and pools there, gradually forming a visible or palpable bulge.
The cyst itself is essentially a herniation of the joint capsule’s inner lining. Once the fluid escapes through the one-way valve, it doesn’t flow back easily, so the pocket tends to persist or grow until the underlying cause of inflammation is addressed.
Common Causes in Adults and Children
In adults, a popliteal cyst is almost always secondary to another knee problem. The two most frequent triggers are osteoarthritis and cartilage (meniscus) tears. Rheumatoid arthritis and other inflammatory joint conditions can also drive fluid overproduction. Essentially, anything that creates chronic irritation inside the knee can set the stage for a cyst to develop behind it.
Children are a different story. Popliteal cysts in kids are usually not associated with meniscal tears or degenerative joint disease. Instead, they tend to arise on their own from the joint capsule lining without a clear internal knee injury. Most resolve without treatment over time, which is the opposite of the adult pattern, where the cyst typically sticks around until its root cause is treated.
What It Feels Like
Many popliteal cysts cause no symptoms at all and are discovered incidentally on imaging ordered for something else. When symptoms do appear, the most common sensation is tightness or fullness behind the knee, especially when you fully straighten or deeply bend the leg. This makes sense mechanically: when the knee extends, the two muscles surrounding the cyst squeeze together and compress it against the deeper tissue, raising the pressure inside the sac. Bending the knee does the opposite, allowing the cyst to relax. Doctors sometimes use this characteristic finding on physical exam, checking whether the cyst feels hard with the leg straight and softer with it bent.
Larger cysts can cause a visible lump behind the knee, mild pain with activity, and stiffness that limits how far you can bend or straighten the joint. If a cyst ruptures, fluid leaks into the calf, causing sudden sharp pain, swelling, and redness that closely mimics the symptoms of a blood clot in the leg. That similarity is important to know about because it often sends people to the emergency room, where imaging is needed to tell the two apart.
How It’s Diagnosed
A doctor can often identify a popliteal cyst during a hands-on exam by feeling for a soft mass behind the knee. Because the symptoms can overlap with more serious conditions like a blood clot, an aneurysm, or even a tumor, imaging is frequently ordered to confirm the diagnosis. Ultrasound is quick, inexpensive, and very effective at distinguishing a fluid-filled cyst from a solid mass. MRI provides a more detailed picture and is particularly useful when the doctor suspects an underlying meniscus tear or cartilage damage that might be driving the fluid buildup. X-rays won’t show the cyst itself but can reveal arthritis or bone changes contributing to the problem.
Treatment Without Surgery
Because popliteal cysts are a downstream effect of a knee problem, treating the underlying cause is the most effective long-term strategy. If the cyst is small and painless, observation alone is reasonable.
For symptomatic cysts, initial management follows the familiar rest, ice, compression, and elevation approach. Physical therapy plays a central role: a typical program includes hamstring stretching, quadriceps strengthening, and range-of-motion exercises repeated several times a day. The goal is to improve joint mechanics and reduce the intra-articular pressure that pushes fluid into the cyst.
When conservative measures aren’t enough, a doctor can drain the cyst using a needle guided by ultrasound. A steroid injection is often given at the same time to reduce inflammation. In clinical studies, the recurrence rate after aspiration with a steroid injection was about 19 percent within six months, meaning the majority of patients get lasting relief from this approach. It’s a meaningful improvement, though roughly one in five people will see the cyst refill.
When Surgery Is Considered
Surgery enters the picture when the cyst keeps coming back despite repeated drainage, when it’s large enough to compress nearby nerves or blood vessels, or when the underlying knee condition itself requires surgical repair. There are two main approaches, and the differences between them are stark.
Traditional open removal of the cyst carries a recurrence rate around 50 percent, with about a quarter of patients experiencing ongoing motion limitations and over a third developing wound healing problems or significant calf swelling. Joint pain lasting more than two days affects roughly 75 percent of patients after open surgery.
Arthroscopic treatment, which addresses the cyst from inside the joint using small incisions, has shown dramatically better results. Studies found no recurrence on ultrasound at 6 and 12 months. About 28 percent of patients had pain lasting more than three days, and minor bruising occurred in roughly 7 percent. A small percentage of arthroscopic cases (around 7 percent) needed to be converted to an open procedure. When surgical treatment is warranted, the arthroscopic approach has largely become the preferred option because of these outcomes.
Why Treating the Root Cause Matters
The single most important thing to understand about a popliteal cyst is that it’s a symptom, not a standalone disease. Draining or removing the cyst without addressing the arthritis, torn cartilage, or inflammation that created it is like mopping a floor while the faucet is still running. If you’ve been told you have a Baker’s cyst, the more relevant question is usually what’s going on inside the knee joint itself. Solving that problem is what keeps the cyst from coming back.