A Baker’s cyst forms when excess fluid from inside the knee joint pushes through a natural opening in the back of the knee and pools there, creating a fluid-filled sac. The underlying cause is almost always a problem inside the knee itself, most commonly osteoarthritis, rheumatoid arthritis, or a torn meniscus cartilage.
How the Cyst Actually Forms
Your knee joint is lubricated by a slippery liquid called synovial fluid. When something irritates or damages structures inside the knee, the joint responds by producing more of this fluid than usual. That extra fluid needs somewhere to go.
At the back of the knee, there’s a small bursa (a fluid-filled cushion) sitting between two tendons. This bursa connects to the knee joint through what functions as a one-way valve. Fluid can flow out of the joint and into the bursa, but it has a hard time flowing back. As fluid accumulates, the bursa swells and bulges outward, forming the soft lump you can feel behind your knee. That lump is the Baker’s cyst.
The cyst itself isn’t really the problem. It’s a downstream effect. Whatever is producing the excess fluid inside the knee is the actual cause, and treating the cyst without addressing that underlying issue usually means it comes back.
The Most Common Underlying Causes
Three knee conditions account for the vast majority of Baker’s cysts in adults:
- Osteoarthritis. The gradual wearing down of cartilage in the knee triggers chronic, low-grade inflammation. The joint compensates by overproducing synovial fluid, and over time that fluid finds its way into the popliteal space behind the knee.
- Rheumatoid arthritis. This autoimmune condition causes the immune system to attack the joint lining itself, producing significant inflammation and fluid buildup. Baker’s cysts are particularly common in people with active rheumatoid arthritis affecting the knee.
- Meniscus tears. A torn meniscus, the rubbery cartilage that cushions your knee, irritates the joint and increases fluid production. In one study of patients with symptomatic knee pain, Baker’s cysts were found in about 23% of knees evaluated by ultrasound, and the cysts were significantly associated with tears of the inner meniscus even after accounting for other factors like age, joint effusion, and osteoarthritic changes.
Other less common triggers include gout, ligament injuries, and infections inside the knee joint. Essentially, anything that causes the knee to swell from within can lead to a Baker’s cyst.
Why Baker’s Cysts Are Different in Children
In adults, a Baker’s cyst is almost always secondary to another knee problem. In children, it’s a different story. Kids frequently develop popliteal cysts that have no connection to a meniscal tear, arthritis, or other intra-articular damage. These pediatric cysts tend to arise from the joint capsule lining itself, essentially a herniation of the tissue at the back of the knee, rather than from fluid being pushed out of a damaged joint.
This distinction matters because pediatric Baker’s cysts often resolve on their own over a year or two without treatment, while adult cysts tend to persist as long as the underlying knee condition remains active.
What a Baker’s Cyst Feels Like
Many Baker’s cysts cause no symptoms at all and are discovered incidentally on imaging done for another reason. When they do cause symptoms, you’ll typically notice a soft, fluid-filled bulge behind the knee that’s easier to feel when you fully straighten your leg. Larger cysts can create a sense of tightness or stiffness, especially when bending the knee or during activity. Some people describe an aching pressure behind the knee that worsens after standing for long periods.
The cyst can change size over time. It may swell when the knee is more inflamed and partially shrink during quieter periods.
When a Cyst Ruptures
Occasionally, a Baker’s cyst bursts. When this happens, the synovial fluid leaks down into the calf, causing a sharp pain in the knee followed by swelling, redness, and tenderness in the calf muscle. This can look and feel remarkably similar to a deep vein thrombosis (DVT), a blood clot in the leg, which is a medical emergency.
Because the two conditions share symptoms like calf swelling and pain, doctors typically use ultrasound to tell them apart. Venous duplex scanning can check for blood clots, while the same ultrasound session can identify whether the cyst has ruptured. Imaging of a Baker’s cyst shows fluid sitting in a characteristic crescent shape between two specific tendons behind the knee, a pattern that’s essentially unmistakable on ultrasound. MRI may be ordered if there’s uncertainty or if the doctor wants to evaluate the internal knee damage driving the cyst.
How Baker’s Cysts Are Treated
Because the cyst is a symptom rather than a standalone condition, treatment focuses on the underlying cause. If osteoarthritis is driving the fluid production, managing the arthritis with physical therapy, anti-inflammatory medications, or injections into the joint can reduce fluid buildup and shrink the cyst over time. If a meniscus tear is responsible, repairing or addressing that tear often resolves the cyst.
For cysts that are large or painful, a doctor can drain the fluid with a needle, sometimes combined with a corticosteroid injection to reduce inflammation. This provides relief, but the cyst frequently refills if the underlying knee problem isn’t also treated.
Surgery to remove the cyst is rarely necessary and is generally reserved for cases where the cyst is very large, keeps coming back despite treatment of the underlying condition, or is compressing nerves or blood vessels behind the knee. Even with surgical removal, recurrence is possible if the knee continues to overproduce fluid.