A Baker’s cyst is a fluid-filled swelling that forms behind the knee, in the hollow space known as the popliteal fossa. It develops when excess joint fluid gets pushed out of the knee capsule and collects in a small pouch at the back of the leg. The cyst itself isn’t a separate growth or tumor. It’s simply your body’s own lubricating fluid (synovial fluid) that has escaped to a place it doesn’t belong, almost always because something else is going wrong inside the knee joint.
How the Cyst Forms
Your knee joint is lined with cells that produce synovial fluid, a slippery liquid that reduces friction when you bend and straighten your leg. When the knee is healthy, the body produces and reabsorbs this fluid in balance. But when an injury or disease irritates the joint, those lining cells ramp up production, and fluid begins to accumulate faster than the body can clear it.
As pressure builds inside the joint, fluid gets forced through a weak point at the back of the knee. A one-way valve mechanism forms in the joint capsule: fluid can push outward into the space behind the knee, but it can’t easily flow back in. Over time this trapped pocket of fluid becomes the visible, palpable lump known as a Baker’s cyst. Standing with the knee fully straight tends to make it more prominent because that position compresses the joint space and pushes fluid backward.
Common Causes in Adults
In adults, Baker’s cysts are almost always secondary to an existing knee problem. The cyst is the symptom, not the disease. The most frequent culprits are osteoarthritis and rheumatoid arthritis, both of which trigger chronic inflammation and excess fluid production. Rheumatoid arthritis, which affects roughly 3% of women and 1% of men, causes the joint lining to thicken and proliferate, steadily increasing the volume of synovial fluid until it has nowhere to go but out through that valve at the back of the knee.
Cartilage damage is another major driver. Research published in BMC Musculoskeletal Disorders found a moderate, statistically significant correlation between the severity of cartilage lesions and both the amount of knee effusion and the size of the resulting cyst. In practical terms, the worse the cartilage damage, the more fluid the knee produces, and the larger the cyst grows. Meniscus tears can also contribute, particularly when they’re severe enough to reach the joint surface and disrupt normal knee mechanics. Gout, which causes intense inflammatory flare-ups, is another recognized trigger.
Baker’s Cysts in Children
Children can develop Baker’s cysts too, though the mechanism is different. In kids, the cyst often appears without any underlying joint disease. Prevalence in children ranges from about 2.4% in healthy screening populations to 6.3% in those being evaluated for knee pain. These pediatric cysts tend to resolve on their own over time and rarely require treatment.
In adults, the picture is different. Imaging studies report prevalence anywhere from 4% to as high as 41%, depending on the population studied. The rate climbs with age and is highest among people with inflammatory or degenerative joint conditions.
What It Feels Like
Many Baker’s cysts cause no symptoms at all and are discovered incidentally during an MRI for something else. When they do cause trouble, the most common sensation is tightness or fullness behind the knee, as though something is pressing into the back of the joint. You might also notice a visible bulge, especially when standing with the leg straight.
Pain typically worsens with activity, particularly anything involving deep bending or full extension of the knee. Some people find that the cyst limits their range of motion in both directions, making it hard to fully straighten or fully bend the leg. The discomfort is usually described as a dull ache rather than sharp pain, and it tends to be worse after prolonged walking, stair climbing, or squatting.
When a Cyst Ruptures
In rare cases, a Baker’s cyst can rupture, releasing its fluid into the calf muscle compartment. This causes sudden, sharp pain behind the knee and into the calf, along with swelling and redness that can look alarming. Bruising may appear around the calf and sometimes tracks all the way down to the ankle, a finding called the crescent sign, caused by fluid gravitating downward under the skin.
A ruptured Baker’s cyst closely mimics deep vein thrombosis (DVT), a blood clot in the leg. Both conditions cause calf swelling, redness, and pain when flexing the foot upward. Because the symptoms overlap so convincingly, a ruptured cyst is sometimes called pseudothrombophlebitis. Ultrasound is typically the first test used to tell the two apart. It can rule out a blood clot and detect fluid collecting between the calf muscles, which points to a ruptured cyst rather than DVT. If there’s any doubt, MRI provides more detailed imaging.
How It’s Diagnosed
A doctor can often feel a Baker’s cyst during a physical exam, but imaging is needed to confirm the diagnosis and rule out other possibilities. Several conditions can create lumps or swelling behind the knee, including ganglion cysts, meniscal cysts, and rarely, tumors or blood vessel abnormalities. MRI is considered the reference standard because it shows the cyst itself, reveals its internal structure, and provides a detailed look at the knee joint to identify whatever underlying problem is driving the fluid production. Ultrasound is a quicker, less expensive alternative that’s effective for confirming the cyst and checking for DVT, though it provides less information about the joint as a whole.
Treatment and Recurrence
The single most important thing to understand about treating a Baker’s cyst is that it will often come back if the underlying cause hasn’t been addressed. A cyst caused by osteoarthritis, for example, will likely refill as long as the arthritic knee keeps producing excess fluid. That’s why treatment usually focuses on the root problem: managing arthritis, repairing a torn meniscus, or reducing joint inflammation.
For cysts that are painful or interfering with daily life, aspiration (draining the fluid with a needle) is a common first step. This is typically done under ultrasound guidance. After the fluid is removed, a steroid medication is often injected into the cyst space to reduce inflammation and slow the fluid from returning. The procedure is done under local anesthesia and is generally well tolerated.
Long-term follow-up data shows a recurrence rate of about 12.7% after aspiration and injection. Notably, every recurrence in one study was associated with advanced osteoarthritis, a complex cyst (one with thickened walls, debris, or internal dividers), or both. In other words, the simpler the cyst and the healthier the knee, the better the odds that one drainage procedure will be enough. For people with severe, ongoing joint disease, repeated aspirations or even surgical intervention may eventually be needed.
Small, painless cysts that don’t limit your movement generally don’t need any treatment. Many remain stable or even shrink on their own, particularly if the underlying knee condition is well managed through physical therapy, weight management, or appropriate medication for arthritis.