A Baker’s cyst is a fluid-filled sac that forms behind the knee, creating a visible or palpable bump in the hollow at the back of the joint. It’s also called a popliteal cyst, named after the popliteal fossa, the soft space behind your kneecap. The cyst develops when excess fluid from the knee joint pushes backward into a small bursa (a natural cushioning pocket) behind the knee, causing it to swell. Most Baker’s cysts are not dangerous, but they can be uncomfortable and sometimes signal a deeper knee problem worth addressing.
What Causes a Baker’s Cyst
Your knee joint naturally produces a slippery fluid that reduces friction when you bend and straighten your leg. When something irritates or damages the joint, it often responds by making more of this fluid than usual. That extra fluid has to go somewhere, and it frequently migrates to the back of the knee, inflating the bursa there like a small water balloon.
The most common triggers are conditions that cause chronic knee inflammation. Osteoarthritis is a leading cause, and imaging studies show that roughly 30% of people with knee osteoarthritis have a Baker’s cyst. Among those with symptomatic osteoarthritis, about 23% have a moderate or large cyst, compared to just 9% of people without arthritis. Rheumatoid arthritis is another frequent culprit. Meniscus tears and cartilage injuries can also produce the excess fluid that feeds a cyst. In some cases, a Baker’s cyst appears without a clear underlying condition, particularly in children, where the cyst tends to resolve on its own.
How It Feels
Many Baker’s cysts cause no symptoms at all and are discovered incidentally during an exam or imaging for something else. When symptoms do appear, the most noticeable sign is a soft, round lump behind the knee that’s easiest to feel when your leg is fully straight. The bump can range from grape-sized to as large as a golf ball.
Beyond the lump itself, common symptoms include knee pain, stiffness, and a reduced ability to fully bend the knee. Pain typically worsens with activity, prolonged standing, or when you try to fully straighten or bend the joint. Some people also notice swelling that extends into the thigh or calf. Symptoms tend to flare after busy days on your feet and ease with rest.
When a Cyst Ruptures
A Baker’s cyst can rupture, and when it does, the fluid leaks down into the calf. This causes sudden, sharp pain in the back of the knee or calf, noticeable swelling in the lower leg, and sometimes redness. Some people describe a strange sensation of warm water running down the inside of their leg.
The tricky part is that a ruptured Baker’s cyst can look almost identical to a deep vein thrombosis (DVT), a blood clot in the leg. Both cause calf swelling, pain, and redness. Because a DVT is a medical emergency and a ruptured cyst is not, getting the right diagnosis quickly matters. An ultrasound, specifically a venous duplex scan, is the fastest way to tell the two apart. If you develop sudden calf swelling and pain, especially without a known cyst, it’s worth getting evaluated promptly.
In rare cases, a large or persistent cyst can press on nearby nerves or blood vessels, causing numbness, tingling, or circulation problems in the lower leg.
How It’s Diagnosed
A Baker’s cyst can often be identified during a physical exam. Your doctor will feel the back of your knee while you flex and extend the joint, checking for a smooth, fluid-filled mass. Because the symptoms can overlap with more serious conditions like blood clots, aneurysms, or tumors, imaging is frequently ordered to confirm the diagnosis. Ultrasound is quick and effective at showing a fluid-filled sac. MRI provides a more detailed picture and is especially useful when the doctor suspects an underlying meniscus tear or cartilage damage. X-rays don’t show the cyst itself but can reveal arthritis or other bony changes contributing to the problem.
Treatment Without Surgery
Because Baker’s cysts are usually a symptom of something else going on in the knee, the most effective long-term approach is treating the underlying cause. If arthritis is driving the excess fluid production, managing the arthritis often shrinks the cyst. If a meniscus tear is the source, addressing that injury is key.
For immediate relief, the standard approach follows the R.I.C.E. principle: rest, ice, compression, and elevation. Icing the back of the knee for 15 to 20 minutes and keeping the leg elevated can reduce swelling and discomfort. Over-the-counter anti-inflammatory medications help with pain and inflammation.
Physical therapy plays a significant role in managing Baker’s cysts. A typical program focuses on hamstring stretching and quadriceps strengthening, exercises that improve the knee’s range of motion, reduce stiffness, and take pressure off the joint. These exercises are usually performed several times a day and can meaningfully improve how the knee feels and moves over weeks of consistent effort. Improved muscle control around the knee helps reduce the irritation that produces excess fluid in the first place.
When a cyst is large and painful, a doctor may drain the fluid using a needle (aspiration), sometimes combined with a steroid injection to reduce inflammation. This provides faster relief, though the cyst can refill if the underlying cause hasn’t been resolved.
When Surgery Is Considered
Surgery for a Baker’s cyst is rare. It’s generally reserved for two situations: when painful symptoms persist despite nonsurgical treatment, or when the cyst returns repeatedly after multiple aspirations. For large cysts that are compressing nerves or blood vessels, an open surgical procedure to remove the entire cyst may be recommended. Even with surgery, the cyst can recur if the underlying knee problem remains untreated, so the root cause is almost always addressed at the same time.
Living With a Baker’s Cyst
Most Baker’s cysts are manageable and many resolve without intervention, especially in younger people. Staying active with low-impact exercise, keeping the muscles around your knee strong, and managing any underlying arthritis or joint condition are the most practical steps you can take. Gentle activities like swimming, cycling, and walking are generally well tolerated and help maintain joint health without putting excessive stress on the knee.
If you notice a new lump behind your knee, it’s worth having it checked to confirm it’s a cyst and not something else. And if you ever experience sudden sharp pain with calf swelling, getting evaluated quickly ensures a ruptured cyst isn’t mistaken for a blood clot, or vice versa.