A Baker’s cyst (also called a popliteal cyst) is a fluid-filled swelling that forms behind the knee, and it often produces no symptoms at all. When it does cause problems, the hallmark signs are a visible or palpable lump in the back of the knee, stiffness that limits bending, and pain that worsens with activity or prolonged standing.
The Core Symptoms
The most common symptom is swelling behind the knee. You might notice a soft, fluid-filled bulge in the crease at the back of your knee joint, sometimes extending down into the upper calf. In some cases, you can see it clearly when you stand; in others, you only feel it when you press on the area.
Along with the swelling, you may experience:
- Knee pain that gets worse with activity, prolonged standing, or when you fully straighten or bend the knee
- Stiffness that makes it hard to bend the knee through its full range of motion
- A sensation of tightness or pressure behind the knee, especially after sitting for a long time and then standing up
The cyst has an interesting physical property: it feels firm when your leg is straight and softens when your knee is bent to about 45 degrees. This happens because extending the knee compresses the fluid-filled sac between surrounding tissues, while bending it relieves that pressure. If you’re trying to figure out whether a lump behind your knee is a cyst or something else, this firmness change is one of the most reliable clues.
Many Baker’s Cysts Are Silent
It’s worth emphasizing that a large number of Baker’s cysts cause no symptoms whatsoever. Some people discover them incidentally during an MRI or ultrasound ordered for a completely different reason. Even when a cyst is present, the level of discomfort varies widely. It can range from a minor annoyance you barely notice to significant pain that makes it hard to walk or use stairs normally.
What Causes Them in the First Place
Baker’s cysts form when excess fluid from inside the knee joint pushes into a small bursa (a natural fluid-filled cushion) behind the knee. In adults, this almost always happens because something else is going on inside the knee. Osteoarthritis is the most common driver. One imaging study found Baker’s cysts in about 30% of people overall, but moderate-to-large cysts were more than twice as common in people with symptomatic osteoarthritis (23%) compared to those without it (9%). Meniscus tears, cartilage damage, and inflammatory arthritis like rheumatoid arthritis are other frequent triggers.
This means the symptoms you feel may be a mix of the cyst itself and the underlying knee problem. Aching deep inside the joint, grinding sensations, or pain along the joint line are more likely coming from the arthritis or torn cartilage than from the cyst.
Baker’s Cysts in Children
Children develop Baker’s cysts too, most commonly between ages 4 and 8. Unlike in adults, there’s usually no underlying knee injury or arthritis causing the cyst. It appears on its own, and no one knows exactly why.
A painless lump behind the knee is often the only sign. Some kids complain of mild discomfort in the knee or leg, but the cyst rarely limits their activity. Most children with a Baker’s cyst can keep playing sports and running around without any problems, and many of these cysts resolve on their own over time.
When a Cyst Ruptures
The most dramatic symptom shift happens if the cyst bursts. A ruptured Baker’s cyst leaks fluid down into the calf, causing sudden, sharp pain in the back of the lower leg along with swelling, redness, and sometimes bruising that can extend all the way to the ankle or foot.
Here’s the problem: those symptoms look almost identical to a deep vein thrombosis (DVT), which is a blood clot in the leg and a medical emergency. Research confirms that a ruptured Baker’s cyst and DVT are “clinically indistinguishable” based on symptoms alone. Both cause inflammation and acute calf pain, and they require ultrasound imaging to tell apart. This matters because the treatments are completely different, and treating a suspected clot with blood thinners when the real issue is a ruptured cyst can cause serious bleeding complications.
If you develop sudden calf pain and swelling, especially if you know you have a Baker’s cyst, getting an ultrasound quickly is the right move. One subtle clue that may point toward a ruptured cyst rather than a clot is bruising that appears on the top of the foot, though this isn’t reliable enough to skip imaging.
Symptoms That Suggest You Need Treatment
Most Baker’s cysts don’t require any treatment. They often shrink or disappear when the underlying knee condition is managed. But certain symptoms suggest the cyst has become significant enough to address directly: pain severe enough to limit walking or daily activities, stiffness that meaningfully reduces your knee’s range of motion, or swelling that keeps getting larger rather than staying stable.
Surgery is generally reserved for cysts that cause severe pain or make it genuinely difficult to use the knee. For everything in between, treatment typically focuses on the knee problem causing the excess fluid, whether that’s managing arthritis, repairing a torn meniscus, or reducing inflammation. As the underlying issue improves, the cyst often follows.