A Baker’s cyst forms when excess fluid from the knee joint gets pushed through a one-way valve into a small pocket (called a bursa) at the back of the knee. This bursa sits between two tendons behind the knee, specifically between the medial head of the gastrocnemius and the semimembranosus tendons. Fluid can flow into this space, but the valve mechanism makes it difficult for the fluid to drain back out, so it accumulates and forms a fluid-filled swelling known as a popliteal cyst.
The cyst itself isn’t the core problem. It’s a symptom of something else going on inside the knee that’s causing it to produce too much lubricating fluid.
The One-Way Valve Problem
Your knee joint naturally contains a small amount of synovial fluid, a slippery liquid that reduces friction when you bend and straighten your leg. When the knee is healthy, fluid production and reabsorption stay in balance. But when something irritates or damages the joint, the lining of the knee ramps up fluid production as part of the inflammatory response.
That extra fluid has to go somewhere. In many people, there’s a natural connection between the knee joint cavity and the bursa behind the knee. This connection acts as a one-way valve: fluid travels from the joint into the bursa when pressure builds (like when you bend your knee), but it can’t easily flow back. Over time, fluid pools in the bursa, stretching it into the soft, grape-to-golf-ball-sized lump you can feel behind your knee.
Osteoarthritis Is the Most Common Trigger
The single most frequent cause of Baker’s cysts in adults is osteoarthritis. In a study of 399 patients with chronic knee pain, about 73% had ultrasound features of osteoarthritis, and roughly one in four had a Baker’s cyst. The cyst was positively associated with both osteoarthritis and the presence of joint effusion (excess fluid pooling inside the knee). This makes sense: the cartilage breakdown and chronic low-grade inflammation in osteoarthritis keep the joint producing more fluid than it can reabsorb.
Rheumatoid arthritis and other forms of inflammatory arthritis also cause Baker’s cysts through a similar mechanism. The persistent inflammation in these conditions keeps the synovial lining in overdrive, generating a steady stream of fluid that funnels into the bursa. People with rheumatoid arthritis may develop particularly large or recurrent cysts because the inflammation tends to be more aggressive and harder to control.
Meniscus Tears and Other Knee Injuries
You don’t need arthritis to develop a Baker’s cyst. Structural damage inside the knee, especially a torn meniscus, is another well-established trigger. A study of 131 symptomatic knees found Baker’s cysts in 23% of them, with a statistically significant association between cysts and medial meniscus tears. Age was also an independent factor, meaning older adults with meniscus tears were at even higher risk.
Other internal knee injuries can set off the same chain of events. Tears to the anterior cruciate ligament (ACL), damage to the cartilage surface, or loose fragments of bone and cartilage floating inside the joint all provoke inflammation and excess fluid production. Even repetitive overuse injuries that irritate the joint lining without causing a visible tear can generate enough extra fluid to fill the bursa over time.
Baker’s Cysts in Children Are Different
When kids develop a Baker’s cyst, the cause is usually not the same as in adults. In children, the cyst typically does not communicate with the knee joint at all, and it’s rarely associated with any underlying joint damage or disease. The cyst simply forms within the bursa itself, without the excess-fluid-overflow mechanism that drives adult cases. These childhood cysts are generally self-limiting, meaning they tend to shrink and disappear on their own without treatment. Surgical removal is rarely needed.
What a Baker’s Cyst Feels Like
Most people first notice a Baker’s cyst as a firm, smooth swelling in the hollow behind the knee. It’s often most noticeable when you fully straighten your leg. Small cysts may cause no symptoms at all and only show up incidentally on imaging done for another reason. Larger cysts can create a feeling of tightness or pressure behind the knee, especially when bending, kneeling, or walking. Some people describe stiffness that gets worse with activity.
The cyst can fluctuate in size. It may swell during periods of increased joint inflammation and shrink when the underlying condition is better controlled. This waxing and waning is a clue that the cyst is secondary to whatever is happening inside the joint itself.
When a Cyst Ruptures
A Baker’s cyst can rupture, releasing its fluid into the calf tissues. This causes sudden sharp pain behind the knee, followed by swelling, redness, and tenderness that spreads down the calf. The problem is that these symptoms closely mimic deep vein thrombosis (a blood clot in the leg), making the two conditions difficult to tell apart based on physical examination alone. Ultrasound is the quickest way to distinguish between them, which is important because the treatments are completely different and a blood clot left untreated can be dangerous.
How It’s Diagnosed
Ultrasound is the go-to initial test for a suspected Baker’s cyst. It’s fast, inexpensive, and remarkably accurate. One study comparing ultrasound findings to MRI results found that ultrasound detected Baker’s cysts with 100% sensitivity and 94% overall accuracy. When the characteristic finding of fluid between the two tendons behind the knee was present on ultrasound, both sensitivity and specificity reached 100% for distinguishing a Baker’s cyst from other types of masses.
MRI is sometimes used when the clinical picture is more complex, for instance, when a doctor needs to assess the full extent of a meniscus tear or evaluate cartilage damage that might be driving the cyst. But for simply confirming that a lump behind the knee is a Baker’s cyst, ultrasound is typically all that’s needed.
Why Treating the Cause Matters Most
Because a Baker’s cyst is a downstream effect of a knee problem, draining the cyst without addressing its source usually leads to recurrence. The fluid will simply accumulate again as long as the knee keeps overproducing it. This is why treatment focuses on the underlying condition: managing arthritis with anti-inflammatory approaches, repairing a torn meniscus, or reducing joint irritation through physical therapy and activity modification.
When the cyst is large enough to cause significant discomfort, a doctor can drain it with a needle (aspiration), sometimes combined with a steroid injection to reduce inflammation. This provides temporary relief while the root cause is being managed. In persistent cases where the cyst keeps refilling despite treatment of the underlying condition, surgery can address the one-way valve itself, opening the connection between the joint and the bursa so fluid can flow freely in both directions rather than getting trapped.