A Baker’s cyst is a fluid-filled sac that forms behind your knee, creating a noticeable bulge in the hollow at the back of the joint. It develops when excess joint fluid pushes through a small opening into a natural pocket called the popliteal bursa, located between the tendons and muscles behind the knee. The cyst itself isn’t usually the core problem. It’s almost always a signal that something else inside the knee is producing too much fluid.
Why It Forms
Your knee joint naturally contains a small amount of lubricating fluid that helps the joint move smoothly. When the knee is injured or inflamed, it ramps up fluid production. That extra fluid has to go somewhere, and it often migrates to the back of the knee, where a one-way valve-like opening allows fluid into the popliteal bursa but doesn’t let it drain back easily. The bursa swells, and you’ve got a cyst.
The most common triggers are osteoarthritis, rheumatoid arthritis, and cartilage (meniscus) tears. In a study published in RMD Open, moderate-to-large Baker’s cysts appeared in 23% of people with symptomatic knee osteoarthritis, compared to just 9% of those without it. Overall, about 30% of people in the study had some degree of cyst formation, with the larger cysts concentrated in the osteoarthritis group. Essentially, the more irritated your knee joint is, the more likely a cyst will develop.
What It Feels Like
Many Baker’s cysts cause no symptoms at all and are found incidentally during imaging for another issue. When symptoms do appear, the most common is a feeling of tightness or fullness behind the knee, especially when you straighten the leg or stand for long periods. You may notice a visible lump that feels soft and smooth, roughly the size of a golf ball, though they can grow larger.
Bending the knee deeply, squatting, or kneeling tends to increase pressure and discomfort. Some people describe a dull ache that worsens after activity. If the cyst gets large enough, it can limit your range of motion simply by taking up space behind the joint.
When a Cyst Ruptures
Occasionally a Baker’s cyst bursts, releasing its fluid into the calf. This causes sudden, sharp pain behind the knee, along with swelling, redness, and warmth that spreads down the lower leg. The experience closely mimics a deep vein thrombosis (DVT), a blood clot in the leg, and the two are essentially indistinguishable based on symptoms alone. Doctors use ultrasound and sometimes blood tests to tell them apart, because the treatments are very different: a ruptured cyst needs rest and anti-inflammatory care, while a DVT requires blood thinners.
Other possible complications include direct pressure on nearby nerves or blood vessels, which can cause numbness, tingling, or a sensation of heaviness in the lower leg. These situations are uncommon but worth knowing about if your symptoms change suddenly.
How It’s Diagnosed
A physical exam is often enough. Your doctor will feel the back of your knee for a soft, round mass and may shine a light through it to confirm it’s filled with fluid rather than something solid. Ultrasound is the go-to imaging tool because it quickly confirms the cyst and can also check for blood clots. An MRI is sometimes ordered not so much for the cyst itself but to get a detailed look at what’s happening inside the knee, like a torn meniscus or cartilage damage, that’s driving the fluid buildup.
Treatment Without Surgery
Baker’s cysts sometimes disappear on their own, particularly if the underlying knee issue resolves. For mild cases, the first-line approach is straightforward: reduce activity that aggravates the knee, apply ice to the area, and use over-the-counter anti-inflammatory pain relievers like ibuprofen or naproxen to bring down swelling and discomfort.
Gentle exercises play an important role. A typical rehab program focuses on hamstring stretching and quadriceps strengthening, repeated several times a day. These exercises improve the knee’s range of motion, increase flexibility around the joint, and reduce the mechanical stress that contributes to excess fluid production. The goal is better joint control so the knee produces less fluid over time.
If the cyst is large or painful enough to interfere with daily life, a doctor can drain it with a needle (aspiration), sometimes injecting a corticosteroid at the same time to reduce inflammation. This provides quick relief, though the cyst may refill if the underlying knee problem persists.
When Surgery Is Considered
Surgery for a Baker’s cyst is rarely needed. It’s typically reserved for cases where symptoms don’t respond to conservative treatment or the cyst keeps returning after multiple drainings. The procedure is usually arthroscopic, meaning a surgeon works through small incisions with a camera. The operation often targets the root cause inside the knee, such as repairing a meniscus tear, rather than the cyst itself.
In some cases, the surgeon will also decompress the cyst arthroscopically by opening the valve-like connection between the knee joint and the cyst. This allows fluid to flow both directions instead of getting trapped, which helps prevent the cyst from re-forming. Recovery from arthroscopic surgery varies, but most people return to light activity within a few weeks and resume normal function over the following months, depending on what was repaired inside the knee.
The Bigger Picture
A Baker’s cyst is best understood as a symptom rather than a standalone condition. Treating the cyst without addressing the knee problem behind it, whether that’s arthritis, a cartilage tear, or chronic inflammation, usually means the cyst comes back. The most effective long-term strategy combines managing the underlying joint issue with exercises that keep the knee strong, flexible, and producing less excess fluid.