Swelling behind the knee is most often caused by a Baker’s cyst, a fluid-filled pocket that forms when excess joint fluid gets pushed into the soft space at the back of your knee. In imaging studies, Baker’s cysts show up in 10% to 41% of adult knees, making them by far the most common explanation for this type of swelling. Less often, the cause is a blood clot, a popliteal artery aneurysm, or swelling from an injury to surrounding soft tissue.
Baker’s Cysts: The Most Common Cause
Your knee joint naturally contains a small amount of lubricating fluid that helps the bones glide smoothly. When something irritates or damages the joint, your body produces extra fluid. That fluid can migrate to the back of the knee, settling into the gap between two calf muscles and forming a soft, balloon-like bulge. This is a Baker’s cyst.
The cyst typically feels like a firm, squishy lump that’s most noticeable when you straighten your leg. It can range from the size of a grape to the size of a golf ball. Many people feel tightness or pressure behind the knee, especially when bending or fully extending it. Walking, climbing stairs, and squatting may all feel uncomfortable. In some cases the cyst causes no pain at all and you only notice the visible swelling.
What’s Actually Causing the Extra Fluid
A Baker’s cyst is almost always a secondary problem. Something else inside the knee is producing the excess fluid, and the cyst is just where that fluid ends up. The two biggest culprits are arthritis and meniscus tears.
In a review of 1,760 knee MRIs, researchers found Baker’s cysts in 238 knees. Of those, 47% had complete meniscus tears and another 37% had degenerative tears. Among patients with a tear, 62% had damage specifically to the back portion of the inner meniscus. When a meniscus tears, the torn tissue can act like a one-way valve, allowing joint fluid to squeeze out into the back of the knee but not return easily. Over time, that trapped fluid thickens into a gel-like material that fills the cyst.
Osteoarthritis works differently. Rather than creating a valve effect, arthritis gradually breaks down cartilage, and the body responds by overproducing joint fluid. The extra volume has to go somewhere, and the back of the knee is the path of least resistance.
Children vs. Adults
Baker’s cysts are far less common in children, showing up in roughly 2% to 6% of pediatric knees compared to up to 41% in adults. When kids do develop one, the cause is usually juvenile arthritis or joint hypermobility rather than a torn meniscus. Boys are about three times more likely than girls to have one. The good news for parents: most Baker’s cysts in children shrink on their own or resolve once the underlying issue is treated.
When It Might Be Something More Serious
A ruptured Baker’s cyst and a deep vein thrombosis (DVT, or blood clot in the leg) can look almost identical. Both cause sudden calf pain, swelling, redness, and warmth. In one case series, seven patients referred for suspected blood clots turned out to have Baker’s cysts instead, with no clot present. The overlap in symptoms is real, and the distinction matters because a blood clot can be life-threatening if it travels to the lungs.
Signs that lean more toward a blood clot include swelling that extends well below the knee into the entire calf, skin that looks red or discolored, and symptoms that started after a long period of immobility like a flight or bed rest. If the swelling behind your knee appeared suddenly with significant calf pain and you have risk factors for clots (recent surgery, birth control use, prolonged sitting, or a family history), get evaluated promptly with an ultrasound.
A much rarer possibility is a popliteal artery aneurysm, where the artery behind the knee balloons outward. These account for 85% of all peripheral artery aneurysms but are still uncommon in the general population, occurring in 0.1% to 2.8% of people. They peak in the sixth and seventh decades of life and are often found on both sides. About 60% produce a pulsatile lump you can feel behind the knee. Most are painless, but if the aneurysm clots suddenly, it can cut off blood flow to the lower leg, causing acute pain, numbness, paleness, and weakness.
How It’s Diagnosed
Ultrasound is the first-line imaging tool for posterior knee swelling. It’s quick, inexpensive, and highly accurate. In a study comparing ultrasound to MRI for detecting Baker’s cysts, identifying fluid between the two specific tendons behind the knee in communication with a cyst was 100% accurate for confirming the diagnosis. Ultrasound also readily distinguishes a fluid-filled cyst from a solid mass or a blood clot, which is why it’s often the only test you need.
MRI is reserved for cases where the doctor suspects an underlying meniscus tear, cartilage damage, or another internal knee problem driving the fluid production. It gives a detailed look at all the soft tissue structures inside the joint and helps guide treatment decisions.
Treatment Options
Because a Baker’s cyst is a symptom of a deeper problem, treatment focuses on the underlying cause rather than the cyst itself.
For mild cases, the standard approach is rest, ice, compression, and anti-inflammatory medications to reduce fluid production. Many cysts shrink or disappear once the joint settles down. Physical therapy plays a key role here, particularly exercises that strengthen the muscles around the knee and improve flexibility, which takes pressure off the joint and reduces fluid buildup over time.
If the cyst is large or painful, your doctor can drain it with a needle under ultrasound guidance, often followed by an injection of a steroid to reduce inflammation. This provides relief for most people, though the duration varies. In cases where there’s an ongoing mechanical issue like patellar maltracking or a persistent meniscus tear, symptoms tend to return more quickly because the root cause is still generating excess fluid.
Surgery becomes an option when the cyst keeps coming back despite other treatments. Traditional open surgical removal has relatively high recurrence rates because it doesn’t address what’s happening inside the joint. Arthroscopic approaches, where a surgeon treats the internal damage (like repairing a meniscus tear) and opens the valve-like connection so fluid drains back into the joint normally, tend to have better long-term results. The procedure is typically outpatient, and recovery involves gradually returning to full activity over several weeks.
What a Baker’s Cyst Feels Like Day to Day
Living with a Baker’s cyst is more annoying than dangerous for most people. The tightness behind the knee worsens with activity, particularly anything that requires deep bending. You may notice the swelling increases after a long day on your feet and decreases after rest. Straightening the leg fully can feel stiff or blocked, almost like something is in the way, because the cyst gets compressed between the muscles when the knee extends.
If the cyst ruptures, fluid leaks into the surrounding calf tissue. This causes a sudden, sharp pain behind the knee or in the upper calf, followed by swelling and sometimes bruising that tracks down toward the ankle. A ruptured cyst is painful but not dangerous. The leaked fluid is gradually reabsorbed by the body over days to weeks, though the area can remain sore and swollen during that time.