A Baker’s cyst is a fluid-filled sac that forms in the hollow at the back of your knee, called the popliteal fossa. It shows up as a visible bulge, often about the size of a golf ball, and creates a sensation of tightness or pressure behind the joint. Most Baker’s cysts develop because of an underlying knee problem that causes excess fluid to build up, though some are completely harmless and resolve on their own.
How a Baker’s Cyst Forms
Your knee joint naturally contains a small amount of lubricating fluid that helps the joint move smoothly. Behind the knee, there’s a small fluid-filled sac called a bursa that sits between two tendons. This bursa normally communicates with the knee joint through a small opening in the back of the joint capsule.
When something irritates the knee, whether from injury or arthritis, the joint produces extra fluid. That fluid gets pushed through the opening into the bursa. The key problem is that this connection works like a one-way valve: fluid flows easily from the joint into the bursa but has a hard time flowing back. Over time, the bursa swells and enlarges, forming what you see and feel as a cyst.
Common Causes in Adults and Children
In adults, Baker’s cysts are almost always secondary to another knee problem. The most common triggers include osteoarthritis, meniscus tears, gout, and inflammatory joint conditions like rheumatoid arthritis. Any condition that increases fluid production inside the knee can set the stage for a cyst to develop. This is why treating a Baker’s cyst often means treating the underlying knee issue first.
In children, Baker’s cysts are much rarer and behave quite differently. They’re typically idiopathic, meaning no underlying knee injury or disease is present. Parents usually notice a painless lump behind the child’s knee, or it shows up incidentally during a physical exam. These childhood cysts develop before age 15, tend to stay asymptomatic, and aren’t associated with joint swelling. In most cases, observation and reassurance are all that’s needed, as they frequently resolve on their own.
What a Baker’s Cyst Feels Like
Some Baker’s cysts cause no symptoms at all and are only discovered during imaging for another problem. When symptoms do appear, the most common ones include swelling behind the knee (sometimes extending into the calf), knee pain, and stiffness that makes it difficult to fully bend the knee.
Pain tends to get worse with activity, after prolonged standing, or when you try to fully straighten or bend the joint. The tightness behind the knee can feel like something is “catching” when you move, and larger cysts may limit your range of motion enough to affect walking, squatting, or climbing stairs.
How It’s Diagnosed
A doctor can often suspect a Baker’s cyst during a physical exam by feeling for a soft, fluid-filled mass behind the knee while you flex the joint. But imaging confirms the diagnosis and rules out other conditions.
Ultrasound is the most accessible first step. On ultrasound, Baker’s cysts appear as a crescent-shaped pocket of fluid sitting between two tendons at the back of the knee. When fluid can be seen wrapping around the inner tendon in that characteristic pattern, the finding identifies a Baker’s cyst with 100% accuracy. MRI is considered the gold standard, particularly because it shows not just the cyst itself but also the underlying knee damage (like a torn meniscus or cartilage loss) that’s causing the fluid buildup in the first place.
Treatment Without Surgery
If a Baker’s cyst isn’t causing you problems, no treatment is necessary. Many cysts shrink or disappear when the underlying knee condition is managed.
For symptomatic cysts, the initial approach follows the classic RICE protocol: rest, ice, compression, and elevation. Applying ice for about 15 minutes every four to seven hours helps reduce inflammation and discomfort. Over-the-counter anti-inflammatory medications can also help manage pain during flare-ups.
When conservative measures aren’t enough, a doctor may drain the cyst with a needle (aspiration) and inject a steroid to reduce inflammation. This combination decreases cyst size in roughly two-thirds of patients within two to seven days. Complete disappearance happens in only about 7% of cases, though. Ultrasound-guided aspiration with steroid injection has somewhat better results, but even then, about 19% of cysts recur within six months. The high recurrence rate is a direct consequence of the one-way valve mechanism: if the underlying knee problem keeps producing excess fluid, the cyst tends to refill.
Physical Therapy and Exercises
A physical therapy program can make a real difference in both managing symptoms and preventing recurrence. The focus is on improving knee joint control through range of motion exercises, hamstring stretching, and quadriceps strengthening. These exercises increase joint flexibility and help support the knee so it’s under less mechanical stress.
Your physical therapist will typically design a daily program you repeat several times throughout the day. Gentle stretching of the hamstrings (the muscles running along the back of the thigh) is particularly important because tight hamstrings put extra pressure on the back of the knee, right where the cyst sits. Strengthening the quadriceps (front of the thigh) helps stabilize the knee and can reduce the irritation that drives fluid production. High-impact activities and deep squatting are generally best avoided until symptoms improve, since they increase pressure inside the joint and can push more fluid into the cyst.
When a Baker’s Cyst Ruptures
A Baker’s cyst can rupture, and when it does, the fluid inside spills down into the calf. You’ll typically feel a sharp pain in the knee followed by noticeable swelling in the lower leg. The calf may turn red and feel warm to the touch. These symptoms can be alarming because they closely mimic a deep vein thrombosis (DVT), which is a blood clot in the leg and a medical emergency.
Because the two conditions look so similar, doctors run specific tests to tell them apart. A venous duplex ultrasound can visualize both a ruptured cyst and a blood clot. A D-dimer blood test measures a protein associated with clotting and helps rule out DVT. In some cases, MRI or venography provides additional detail. If you develop sudden calf swelling and redness, it’s important to get evaluated promptly so a blood clot can be ruled out.
Surgical Options and Recovery
Surgery is reserved for cysts that keep coming back, cause significant symptoms despite other treatments, or are associated with a structural knee problem that needs repair. The procedure is usually done arthroscopically, meaning through small incisions with a camera, though some cases require open excision of the cyst.
For straightforward arthroscopic procedures, most people return to normal activities within the first month. More complex cases, especially those involving additional knee repair, may require crutches or a knee brace and limit walking for several weeks. Full recovery from a complicated procedure can take several months to a year, depending on what other work was done inside the joint. Because the cyst is a symptom of a deeper problem, the best long-term outcomes come from addressing the root cause, whether that’s repairing a torn meniscus, managing arthritis, or reducing chronic joint inflammation.