A Baker’s cyst is a fluid-filled swelling that forms behind the knee. It develops when excess lubricating fluid from the knee joint pushes through a one-way valve into the space at the back of the knee (called the popliteal fossa), where it pools and creates a visible or palpable bulge. Most Baker’s cysts are not dangerous on their own, but they typically signal an underlying knee problem that’s worth addressing.
How a Baker’s Cyst Forms
Your knee joint contains synovial fluid, a slippery liquid that reduces friction and helps your leg swing smoothly. When something irritates or damages the knee, the joint responds by producing more of this fluid than it needs. That excess fluid migrates to the back of the knee, where it exits through a one-way valve and collects in a small sac called a bursa, located between two muscles behind the knee. As more fluid accumulates, the sac swells outward, forming the cyst.
Because fluid flows through a one-way valve, it enters the bursa easily but has trouble draining back into the joint. This is why Baker’s cysts can persist or grow over time, especially if the underlying cause keeps producing extra fluid.
Common Causes
A Baker’s cyst is almost always a secondary problem, meaning something else in the knee is driving the fluid overproduction. The most common culprits are:
- Osteoarthritis: The most frequent cause in adults. Ultrasound studies have found Baker’s cysts in about 27% of knees with chronic osteoarthritis, compared to just 2% of healthy knees.
- Rheumatoid arthritis: Nearly half of patients with rheumatoid arthritis have been found to have Baker’s cysts on imaging, and about 20% have them in both knees.
- Cartilage tears: A torn meniscus or other knee injury triggers inflammation that increases fluid production.
In children, the picture looks different. Popliteal cysts in kids typically appear without any underlying joint disease or injury history. They tend to resolve on their own and rarely need treatment, unlike adult cases where an identifiable knee condition is almost always present.
What a Baker’s Cyst Feels Like
Many Baker’s cysts cause no symptoms at all. In fact, less than half of cysts found on imaging are noticeable to the person who has them. When symptoms do occur, the most common ones are a sensation of tightness or fullness behind the knee, mild swelling you can see or feel, and stiffness that makes it harder to fully bend or straighten the leg. These symptoms often worsen with activity and improve with rest.
Larger cysts can feel like a water balloon behind the knee. You might notice the swelling more when you’re standing. Some people describe an aching pressure that gets worse after prolonged walking or stair climbing.
When a Cyst Ruptures
A Baker’s cyst can rupture, releasing its fluid into the calf. When this happens, it causes sudden sharp pain behind the knee, swelling and redness in the calf, and sometimes bruising that tracks down toward the ankle. The experience can closely mimic a deep vein thrombosis (blood clot in the leg), which is why imaging is important if you develop these symptoms suddenly. An ultrasound can quickly distinguish between the two.
A ruptured cyst is painful but not usually dangerous. The leaked fluid is eventually absorbed by the body over days to weeks. However, it’s important to rule out a blood clot, because the treatment for each condition is very different.
How It’s Diagnosed
A Baker’s cyst can often be identified during a physical exam, where a healthcare provider feels for a smooth, round lump behind the knee. Because the symptoms can resemble those of a blood clot, an aneurysm, or even a tumor, imaging is frequently ordered to confirm the diagnosis. Ultrasound is the most common first step since it’s quick, noninvasive, and clearly shows fluid-filled structures. An MRI provides more detailed information about the cyst and any underlying knee damage, such as cartilage tears or arthritis, that may be causing it. X-rays can help evaluate the joint for arthritis but won’t show the cyst itself.
Treatment Options
The most effective long-term approach is treating whatever is causing the knee to overproduce fluid. If a meniscus tear is the source, repairing the tear often resolves the cyst. If arthritis is driving the inflammation, managing the arthritis reduces fluid buildup.
For the cyst itself, initial treatment is usually conservative. Rest, ice, compression, and keeping the leg elevated can reduce swelling and discomfort. A physical therapy program focused on hamstring stretching and quadriceps strengthening helps improve knee joint control and flexibility, which can reduce the pressure that feeds the cyst. These exercises are typically done several times a day.
If the cyst is large or painful, a provider may drain it with a needle (aspiration) and inject a steroid medication to reduce inflammation. This provides faster relief, though the cyst can refill if the underlying cause isn’t addressed. Surgery is generally reserved for cysts that cause significant pain, compress nearby nerves or blood vessels, or keep coming back despite other treatments. Arthroscopic approaches that address the internal knee problem at the same time tend to produce better results than simply removing the cyst alone.
How Common Baker’s Cysts Are
Baker’s cysts are surprisingly common, especially among people with knee conditions. Among those with osteoarthritis, studies using ultrasound have found cysts in anywhere from 27% to 42% of patients, depending on disease severity. In rheumatoid arthritis, the prevalence is even higher, with about one in three affected knees showing a cyst on imaging. Many of these cysts are “silent,” discovered incidentally on an MRI or ultrasound ordered for a different reason.
They occur more frequently in adults over 40, which tracks with the age range when arthritis and degenerative knee changes become more common. In children, they’re less common and follow a different pattern, typically appearing between ages 4 and 7 and resolving without intervention over one to two years.