A Baker’s cyst is a fluid-filled sac that forms at the back of the knee, creating a visible or palpable lump in the hollow behind the joint. Also called a popliteal cyst, it develops when excess joint fluid gets pushed into a small pocket of tissue between two muscles at the rear of the knee. It’s one of the most common knee disorders, and in nearly every case, something else going on inside the knee is driving it.
Where Exactly It Forms
Your knee joint is lined with a slippery membrane that produces synovial fluid, the lubricant that keeps the joint moving smoothly. Small fluid-filled cushions called bursae sit around the joint to reduce friction between muscles, tendons, and bone. One of these bursae sits in the popliteal space, the soft depression behind your knee, nestled between two muscles: the semimembranosus (a hamstring muscle on the inner side) and the medial gastrocnemius (the inner head of your calf muscle).
When the knee is irritated or injured, it ramps up production of synovial fluid. That extra fluid travels toward the back of the joint and exits through a one-way valve into the bursa between those two muscles. Fluid can flow in, but it can’t easily flow back out. Over time, the trapped fluid accumulates and the bursa swells outward, forming the characteristic bulge you can feel or see behind the knee.
What Causes a Baker’s Cyst
A Baker’s cyst is almost always a secondary problem, meaning it’s a symptom of something else happening inside the knee. The cyst itself isn’t the disease. The underlying condition triggers inflammation, and that inflammation causes the knee to overproduce fluid.
The two most common culprits are arthritis and cartilage tears. Osteoarthritis, the wear-and-tear form that develops gradually with age, is the leading cause in adults over 50. Rheumatoid arthritis, which involves the immune system attacking the joint lining, also frequently produces enough excess fluid to form a cyst. In younger or more active people, a torn meniscus (the rubbery cartilage that cushions the knee) is a common trigger. Any knee injury that causes swelling, including ligament sprains, can potentially lead to one.
In children, Baker’s cysts sometimes appear without any identifiable knee problem. These tend to resolve on their own and are generally less concerning than cysts in adults, which almost always point to an underlying joint issue that needs attention.
What It Feels Like
Many Baker’s cysts cause no symptoms at all. You might notice a soft, egg-shaped lump behind your knee and nothing else. When symptoms do appear, they typically include stiffness, a feeling of tightness or pressure behind the knee, and mild to moderate pain that worsens with activity. Straightening your leg fully or bending it all the way can become uncomfortable.
If the cyst grows large enough, it can physically limit your range of motion, making it harder to bend or extend the knee through its full arc. Standing for long periods, walking, or climbing stairs may increase discomfort. The symptoms often fluctuate: the cyst can swell during periods of increased knee inflammation and shrink when things calm down.
When a Cyst Ruptures
Occasionally, a Baker’s cyst bursts. When this happens, the synovial fluid leaks into the calf, causing sudden sharp pain, swelling, and sometimes redness and warmth in the lower leg. The sensation can come on abruptly, often triggered by forceful knee bending or a spike in physical activity.
A ruptured cyst closely mimics deep vein thrombosis (DVT), a blood clot in the leg, because both conditions produce calf pain, swelling, and tenderness. This overlap is well-known in medicine and is sometimes called “pseudothrombophlebitis.” If you develop sudden calf swelling and pain, it’s important to get an ultrasound to determine whether you’re dealing with a ruptured cyst or an actual blood clot, since DVT requires urgent treatment. The fluid from a ruptured cyst is harmless and reabsorbs on its own over days to weeks, though the process can be uncomfortable.
How It’s Diagnosed
A doctor can often suspect a Baker’s cyst during a physical exam by feeling the characteristic lump behind the knee and noting that it’s smooth, somewhat compressible, and most prominent when the leg is straight. But imaging confirms the diagnosis and rules out other possibilities.
Ultrasound is the most common first-line tool. It’s quick, inexpensive, and highly accurate. Research published in the American Journal of Roentgenology found that identifying fluid between the semimembranosus and medial gastrocnemius tendons on ultrasound achieved 100% accuracy in confirming a Baker’s cyst and distinguishing it from other soft-tissue masses. MRI is sometimes ordered as well, particularly when the doctor wants a detailed look at the meniscus, cartilage, or ligaments to identify the underlying cause driving the cyst.
Treatment and What to Expect
Because a Baker’s cyst is driven by an underlying knee problem, treating the root cause is the most effective long-term strategy. If osteoarthritis is producing the excess fluid, managing the arthritis through activity modification, physical therapy, and anti-inflammatory measures will often reduce the cyst over time. If a torn meniscus is the trigger, addressing the tear (sometimes surgically) can resolve the cyst entirely.
For cysts that are painful or limiting mobility, draining the fluid with a needle (aspiration) combined with a steroid injection is a common option. This approach reduces the cyst’s size in roughly two-thirds of patients within two to seven days. Complete disappearance, however, happens in only about 7% of cases with aspiration alone. When the procedure is done under ultrasound guidance, outcomes improve somewhat, though recurrence within six months still occurs about 19% of the time. The high recurrence rate reflects the fundamental issue: if the underlying condition keeps producing excess fluid, the cyst tends to refill.
Surgery to remove the cyst is reserved for cases where it’s very large, persistently painful, or hasn’t responded to other treatments. Even then, it’s most effective when paired with treatment of whatever is going on inside the joint.
Staying Active With a Baker’s Cyst
You don’t need to stop moving, but you may need to adjust how you move. Pain and stiffness typically increase with higher-intensity activity, especially movements that involve deep knee bending or repetitive impact. Low-impact activities like swimming, cycling on a flat surface, or gentle walking tend to be better tolerated. Gentle range-of-motion exercises and quadriceps strengthening can help support the knee and reduce the strain that contributes to fluid buildup.
Avoid pushing through pain during exercise, particularly into full flexion. If a large cyst is mechanically blocking your range of motion, forcing the knee deeper into a bend won’t stretch the cyst away. It’s more likely to increase pressure and discomfort. Ice applied to the back of the knee for 15 to 20 minutes after activity can help manage swelling, and keeping the leg elevated when resting encourages fluid to drain away from the area.