A Baker’s cyst is a fluid-filled swelling that forms at the back of the knee, in the hollow space behind the joint called the popliteal fossa. It develops when excess lubricating fluid from inside the knee joint gets pushed backward through a one-way valve-like opening, pools in that space, and bulges outward. The result is a soft, sometimes visible lump that can range from barely noticeable to the size of a golf ball or larger.
How a Baker’s Cyst Forms
Your knee joint naturally contains synovial fluid, a slippery liquid that reduces friction when you bend and straighten your leg. When something irritates or damages the joint, the knee responds by producing more of this fluid than usual. That excess fluid doesn’t just sit inside the joint. It can travel to the back of the knee and exit through a passage between two tendons, filling a pouch in the popliteal space. Once there, it tends to stay, because the passage acts like a one-way valve: fluid flows out of the joint easily but has difficulty flowing back in.
This is why Baker’s cysts are almost always a secondary problem. The cyst itself isn’t the disease. It’s a visible consequence of something else going on inside the knee.
Common Underlying Causes
The two most frequent triggers are arthritis and cartilage injuries. Osteoarthritis gradually breaks down the protective tissue inside the joint, and the resulting irritation drives the knee to overproduce fluid. About 30% of people with knee osteoarthritis have a Baker’s cyst on MRI, and moderate-to-large cysts are roughly two and a half times more common in people with symptomatic osteoarthritis than in those without it. Rheumatoid arthritis, which causes a different type of joint inflammation, can do the same thing.
Meniscus tears are the other major cause. A torn piece of cartilage irritates the joint lining, triggering fluid buildup that eventually migrates to the back of the knee. Ligament injuries and other forms of knee trauma can also set the process in motion. In children, Baker’s cysts sometimes appear without an obvious underlying knee problem and often resolve on their own.
What It Feels Like
Many Baker’s cysts cause no symptoms at all and are discovered incidentally during imaging for another issue. When symptoms do appear, the most common ones are stiffness behind the knee, a sensation of tightness or fullness when you bend or fully straighten the leg, and visible swelling in the back of the knee that becomes more prominent when you stand. The lump typically feels soft and slightly squishy.
Pain, when present, tends to be a dull ache that worsens with activity or prolonged standing. Some people notice it most after exercise or at the end of the day. Bending the knee fully, like squatting or kneeling, often increases discomfort because the movement compresses the cyst.
When a Cyst Ruptures
A Baker’s cyst can burst. When it does, the synovial fluid leaks down into the calf, causing sudden sharp pain, swelling, warmth, and sometimes bruising along the back of the lower leg. This matters because the symptoms closely mimic a deep vein thrombosis (DVT), a blood clot in the leg that requires urgent treatment. If you experience a sudden worsening of pain or swelling in your calf, redness or skin discoloration, or the area feels unusually warm, you need medical evaluation quickly. Imaging can distinguish between a ruptured cyst and a clot. A ruptured cyst is painful but not dangerous in the way a DVT can be. The leaked fluid is gradually reabsorbed by the body over days to weeks.
How It’s Diagnosed
A doctor can often identify a Baker’s cyst during a physical exam by feeling the characteristic soft lump behind the knee. Ultrasound is the most common first-line imaging tool and is highly reliable. In one study comparing ultrasound directly against MRI, ultrasound achieved 100% sensitivity for detecting Baker’s cysts, meaning it caught every one that MRI found. When researchers looked at ultrasound’s ability to distinguish Baker’s cysts from other types of soft-tissue masses behind the knee, accuracy remained at 100%.
MRI is typically reserved for cases where the doctor needs a more detailed look at the structures inside the knee, particularly if a meniscus tear or ligament damage is suspected as the underlying cause. It provides a broader picture of what’s driving the fluid production in the first place.
Treatment Options
Because a Baker’s cyst is a symptom of an underlying joint problem, the most effective approach is treating whatever is causing the excess fluid. If osteoarthritis is the culprit, managing the arthritis through physical therapy, anti-inflammatory medications, and activity modification often reduces the cyst over time. If a meniscus tear is responsible, repairing or managing that injury addresses the root cause.
Drainage and Injection
When the cyst is large or painful enough to interfere with daily life, a doctor can drain the fluid using a needle guided by ultrasound. This is frequently combined with a steroid injection into the cyst to reduce inflammation. The procedure provides significant pain relief: in one study, patients’ calf pain scores dropped from an average of 9.5 out of 10 before treatment to 0.5 after injection. Functional scores improved dramatically as well.
The catch is that the cyst itself often persists. In the same study, only about 11% of patients saw complete disappearance of the cyst after drainage and injection, while 88% still had a visible cyst on ultrasound. The symptoms improved markedly even when the cyst remained, but recurrence of fluid buildup is common if the underlying joint problem hasn’t been resolved.
Physical Therapy
Strengthening the muscles around the knee, particularly the quadriceps and hamstrings, helps support the joint, reduce inflammation, and manage symptoms. Gentle range-of-motion exercises can also improve the stiffness that a cyst causes. Physical therapy doesn’t eliminate the cyst directly, but it addresses the mechanical factors that contribute to excess fluid production.
Surgery
Surgery to remove the cyst itself is rare and typically reserved for cases where other treatments haven’t worked and the cyst continues to cause pain or interfere with walking and daily activities. More commonly, if surgery is needed, it targets the underlying joint problem rather than the cyst. Arthroscopic surgery allows a surgeon to repair a torn meniscus or address cartilage damage through small incisions. Once the source of irritation is fixed, the cyst often shrinks or resolves because the knee stops overproducing fluid. Severe knee injuries involving ligament tears or fractures may also require surgical repair.
Long-Term Outlook
Baker’s cysts are not inherently dangerous and many shrink or disappear once the underlying knee condition is managed effectively. In people with chronic conditions like osteoarthritis, the cyst may come and go or persist at a low level indefinitely. This isn’t necessarily a problem if it’s not causing symptoms. The key question is always what’s happening inside the knee joint itself, since that determines both the cyst’s behavior and your overall knee health over time.