Baker’s cysts form when excess fluid from inside the knee joint gets pushed into a small pouch at the back of the knee. The underlying cause is almost always something going wrong inside the joint itself, whether that’s arthritis, a cartilage tear, or another condition that triggers inflammation and fluid buildup. About 25% of people who seek care for knee pain turn out to have a Baker’s cyst on ultrasound, and the condition peaks in two age groups: children ages 4 to 7 and adults between 35 and 70.
How a Baker’s Cyst Forms
Your knee joint is lined with a membrane that produces synovial fluid, a slippery liquid that lubricates and cushions the joint during movement. When something irritates or damages structures inside the knee, the joint responds by producing more of this fluid than it normally would. That extra fluid needs somewhere to go.
At the back of the knee, there’s a natural connection between the joint space and a small sac called the gastrocnemio-semimembranosus bursa. This connection acts like a one-way valve: fluid can flow out of the joint into the bursa, but it can’t easily flow back. As fluid pools in this space behind the knee, the bursa swells and forms the bulge known as a Baker’s cyst (also called a popliteal cyst). The cyst is really just a balloon of joint fluid that has been squeezed out through this valve, so the size of the cyst often rises and falls with whatever is happening inside the knee.
The Most Common Underlying Causes
Because a Baker’s cyst is a consequence of excess fluid rather than a disease on its own, identifying the root problem is more important than treating the cyst directly. The most frequent triggers fall into two categories.
Arthritis
Osteoarthritis is the single most common cause in adults. As cartilage wears down over years of use, the resulting friction and debris irritate the joint lining, which responds by overproducing fluid. Rheumatoid arthritis causes the same outcome through a different path: the immune system attacks the joint lining directly, creating chronic inflammation and persistent swelling. Either form of arthritis can keep the joint in a cycle of fluid overproduction that feeds the cyst indefinitely.
Knee Injuries
A torn meniscus (the rubbery cartilage that cushions the knee) is the other major culprit. Meniscal tears are extremely common in active adults and can produce significant joint swelling. Ligament injuries, particularly tears of the anterior cruciate ligament, can also generate enough inflammation and fluid to push into the bursa. Even less dramatic injuries, like a sprain that causes lingering swelling, can trigger cyst formation if the fluid has nowhere else to drain.
Baker’s Cysts in Children
Children between ages 4 and 7 develop Baker’s cysts at a notable rate, but their situation is different from adults. In many pediatric cases, there is no identifiable injury or joint disease. These cysts are considered idiopathic, meaning they form without a clear underlying problem. The prevailing explanation is that the anatomy of the bursal connection is slightly different in young children, allowing fluid to accumulate more easily. Most of these cysts resolve on their own over months to a couple of years without treatment.
What a Baker’s Cyst Feels Like
The hallmark symptom is a feeling of tightness or fullness behind the knee, sometimes accompanied by a visible bulge that’s easiest to see when you’re standing. The swelling often worsens when you fully straighten or bend the leg, since those positions compress the fluid-filled sac. Some people notice that stiffness increases after sitting for a long time or after activity. Smaller cysts may produce no symptoms at all and get discovered incidentally on imaging ordered for something else.
Pain, when present, is usually a dull ache rather than a sharp sensation. It tends to worsen with prolonged standing or walking. If the cyst grows large enough, it can limit your range of motion simply because the swollen sac gets in the way of fully bending the knee.
When a Cyst Ruptures
A Baker’s cyst can burst, releasing its fluid into the calf. When this happens, you may notice sudden sharp pain behind the knee, swelling and redness that spreads down into the calf, and a sensation sometimes described as water running down the back of the leg. The symptoms closely mimic a deep vein thrombosis (DVT), a blood clot in the leg, which is a medical emergency.
Clinically, a ruptured Baker’s cyst and a DVT are nearly indistinguishable based on physical examination alone. Both cause a swollen, painful lower leg, and doctors use the same initial workup (typically a combination of clinical scoring, a blood test called D-dimer, and compression ultrasound of the leg) to tell them apart. This overlap matters because the treatments are very different: a blood clot requires anticoagulation, while a ruptured cyst needs rest, ice, and management of the underlying knee problem. If you develop sudden calf swelling and pain, getting evaluated quickly is important precisely because the two conditions look so similar from the outside.
How Baker’s Cysts Are Diagnosed
Many Baker’s cysts are detectable during a physical exam. Your doctor may feel the characteristic soft, fluid-filled lump behind the knee, which often becomes more prominent when the leg is extended. When the diagnosis is uncertain, or when the cyst’s symptoms overlap with more concerning possibilities like a blood clot or tumor, imaging helps clarify the picture.
Ultrasound is the quickest and most accessible option. It can confirm the cyst is fluid-filled rather than solid and can check for blood clots in nearby veins at the same time. MRI provides a more detailed view and is particularly useful for identifying the underlying cause, like a meniscal tear or cartilage damage, that’s driving the fluid production. X-rays don’t show the cyst itself but can reveal joint narrowing or bone spurs associated with osteoarthritis.
Why Treating the Cause Matters Most
Draining a Baker’s cyst with a needle (aspiration) can provide quick relief, but the cyst frequently refills if the underlying knee problem remains. Think of it like bailing water out of a boat without patching the hole. The one-way valve mechanism means the joint will keep pushing new fluid into the bursa as long as inflammation persists inside the knee.
This is why treatment plans focus on the root cause. For osteoarthritis, that might mean physical therapy to strengthen surrounding muscles, anti-inflammatory medications to reduce swelling, or injections into the joint to calm irritation. For a meniscal tear, repairing or trimming the damaged cartilage through arthroscopic surgery often resolves both the joint swelling and the cyst. In some cases, the surgeon can also open the one-way valve during the procedure, allowing fluid to flow back into the joint rather than getting trapped in the bursa.
When the underlying condition is managed effectively, many Baker’s cysts shrink or disappear entirely on their own. Surgical removal of the cyst itself is rarely needed and is generally reserved for cases where the cyst causes persistent symptoms despite treatment of the joint problem, or where it compresses nearby nerves or blood vessels.