A popliteal cyst, commonly called a Baker’s cyst, is a fluid-filled sac that forms behind the knee. It develops when excess joint fluid pushes into a small pouch (called a bursa) nestled between two muscles at the back of the knee. Most popliteal cysts are painless and discovered by accident, but larger ones can cause tightness, swelling, and discomfort that limits how far you can bend or straighten your leg.
How a Popliteal Cyst Forms
Your knee joint naturally contains a small amount of lubricating fluid that reduces friction when you move. When something irritates the joint, such as arthritis or a cartilage tear, the knee produces more fluid than usual. That extra fluid needs somewhere to go.
Behind the knee, there’s a bursa sitting between the two muscles that make up the upper calf (the gastrocnemius and the semimembranosus). A one-way valve-like opening connects the knee joint to this bursa, allowing fluid to flow in but restricting its return. Over time, the bursa fills and swells into what looks and feels like a small water balloon at the back of the knee. This is why the cyst tends to grow gradually rather than appearing overnight.
Common Causes
In adults, a popliteal cyst is almost always secondary to another knee problem. The most frequent culprits are osteoarthritis, rheumatoid arthritis, and meniscus tears. Any condition that triggers the knee to overproduce fluid can set the stage. Gout, ligament injuries, and cartilage damage are also common triggers.
Children occasionally develop popliteal cysts without any underlying knee problem. These tend to resolve on their own over months to a couple of years without treatment.
What It Feels Like
Many people first notice a soft, round lump behind the knee that’s easier to feel when the leg is straight. This happens because straightening the knee causes the two calf muscles to press together, pushing the cyst against the tissue beneath the skin and making it more prominent. Bending the knee relaxes those muscles and lets the cyst soften, which is why it can seem to “disappear” when you sit down.
When the cyst is large enough to cause symptoms, you might feel stiffness or a sensation of fullness behind the knee, mild to moderate pain that worsens with activity, or difficulty fully bending or extending the leg. Some people describe it as a tight pressure that gets worse after long periods of standing or walking.
When a Cyst Ruptures
Occasionally, a popliteal cyst bursts. When this happens, the fluid inside leaks down into the calf, causing sudden sharp pain, swelling, and redness in the lower leg. The problem is that these symptoms look almost identical to a blood clot (deep vein thrombosis). In one study of over 3,000 patients who underwent vascular imaging, about 3% were found to have Baker’s cysts, and roughly 7% of those patients also had an actual blood clot at the same time. Because the two conditions can coexist and are difficult to tell apart by physical exam alone, sudden calf swelling always warrants imaging to rule out a clot.
How It’s Diagnosed
A doctor can often suspect a popliteal cyst just by feeling the back of your knee with your leg extended. But confirming the diagnosis and ruling out other masses typically involves imaging.
Ultrasound is the go-to first step. It’s fast, widely available, and highly accurate. Research comparing ultrasound directly to MRI found that ultrasound detected Baker’s cysts with 100% sensitivity and 94% overall accuracy. When the radiologist specifically looked for fluid sitting between the two calf tendons, accuracy reached 100%. MRI is reserved for cases where a doctor needs a more detailed look at the structures inside the knee, particularly to identify the underlying cause like a meniscus tear or cartilage damage.
Treatment Without Surgery
Because popliteal cysts are a symptom of a deeper knee issue, treating the underlying cause is the most effective long-term strategy. If arthritis is driving the excess fluid, managing the arthritis often shrinks or eliminates the cyst. If a torn meniscus is responsible, addressing the tear takes away the trigger.
In the meantime, initial management follows the standard rest, ice, compression, and elevation approach. Keeping weight off the knee during flare-ups, icing the area for 15 to 20 minutes at a time, and using a compression wrap can reduce swelling and discomfort. Physical therapy focused on hamstring stretching and quadriceps strengthening helps improve joint control and range of motion. These exercises are typically performed several times a day and aim to reduce the mechanical stress that contributes to fluid buildup.
For cysts that cause persistent pain, a doctor can drain the fluid using a needle guided by ultrasound, often followed by a corticosteroid injection to reduce inflammation inside the bursa. This provides faster relief, but recurrence is possible. In a long-term follow-up study, about 13% of patients who had ultrasound-guided aspiration needed the procedure repeated because the cyst came back.
When Surgery Is Considered
Surgical removal of a popliteal cyst is uncommon. The American Academy of Orthopaedic Surgeons notes that it may be recommended if painful symptoms don’t respond to nonsurgical treatment, or if the cyst keeps returning after multiple aspirations. Surgery is also considered when a large cyst compresses nearby nerves or blood vessels, which can cause numbness, tingling, or circulation problems in the lower leg. The procedure involves removing the entire cyst through an incision behind the knee, and addressing whatever joint problem caused the cyst in the first place to reduce the chance of recurrence.
Living With a Popliteal Cyst
Most popliteal cysts are more of an annoyance than a serious medical problem. Many shrink or disappear entirely once the underlying knee condition is treated. Staying consistent with gentle strengthening exercises, maintaining a healthy weight to reduce joint stress, and addressing knee injuries early all help prevent the cyst from growing or recurring. If you notice the lump getting larger, increasing pain with activity, or any sudden swelling in your calf, those are signs that warrant a closer look.