Swelling behind the knee is most commonly caused by a Baker’s cyst, a fluid-filled sac that forms when excess joint fluid gets pushed to the back of the knee. But it can also signal a blood clot, a muscle strain, or another condition that needs different treatment. Understanding the cause matters because what looks like a simple bump can sometimes point to an underlying knee problem worth addressing.
How a Baker’s Cyst Forms
Your knee joint contains a small amount of lubricating fluid that helps it move smoothly. When something damages the joint or the tissues around it, your body produces extra fluid in response. That fluid can only drain in one direction: out the back of the joint. When enough of it pools there, it forms a sac between two tendons behind the knee. That sac is a Baker’s cyst.
The two most common triggers are arthritis and knee injuries. Arthritis gradually breaks down tissue inside the joint, prompting the body to produce more fluid over time. Injuries, including cartilage tears or ligament damage, create a more sudden wave of swelling that can push fluid backward into the same space. In a study of 276 knee MRIs, 30% of patients who had a tear in the inner cartilage of the knee also had a Baker’s cyst.
A Baker’s cyst typically feels like a soft, fluid-filled lump at the back of the knee. It may be the size of a grape or as large as a golf ball. You might notice it most when you fully straighten or bend your leg, since those positions compress the cyst. Some people have no pain at all, while others feel tightness, aching, or stiffness that gets worse with activity.
Other Causes of Posterior Knee Swelling
Not every bump behind the knee is a Baker’s cyst. A strain of the calf muscle where it attaches behind the knee can produce localized swelling and tenderness, especially after a sudden push-off movement or a change in activity level. This kind of swelling tends to feel more muscular and sore rather than soft and balloon-like.
Less commonly, a popliteal artery aneurysm (a bulge in the artery behind the knee) or a soft-tissue tumor can cause a mass in this area. These are rare but worth knowing about because they feel firmer and may have a noticeable pulse. Your doctor can quickly distinguish these from a cyst with imaging.
When Swelling Could Be a Blood Clot
The concern that keeps doctors attentive to posterior knee swelling is deep vein thrombosis, a blood clot in the leg. A ruptured Baker’s cyst and a DVT can look nearly identical: both cause calf pain, swelling, redness, and tenderness. In fact, only about 25% of people evaluated for DVT symptoms actually have a clot. The rest have other explanations, including ruptured Baker’s cysts.
A few features can help you tell them apart, though none are definitive without testing. A ruptured Baker’s cyst often produces sudden, sharp pain behind the knee that quickly spreads down the calf, sometimes with visible bruising that appears within a day or two. DVT pain tends to develop more gradually, with swelling that extends through the entire lower leg and skin that may turn reddish or bluish. If your calf is warm to the touch, significantly swollen compared to the other leg, or you have risk factors like recent surgery, prolonged immobility, or a history of clots, getting evaluated promptly is important. An ultrasound can confirm or rule out a clot quickly.
How It’s Diagnosed
Ultrasound is the go-to first test for swelling behind the knee. It’s fast, painless, and highly accurate. In a study comparing ultrasound to MRI for detecting Baker’s cysts, ultrasound achieved 100% sensitivity and 94% overall accuracy. When fluid appeared in the characteristic location between the two tendons behind the knee, ultrasound was perfectly accurate at confirming a Baker’s cyst and distinguishing it from other types of masses.
MRI becomes useful when doctors suspect an underlying problem driving the cyst, like a meniscus tear or cartilage damage. It provides a detailed view of the entire knee joint, which helps determine whether the cyst is a symptom of something that needs its own treatment.
Treatment and What to Expect
Many Baker’s cysts resolve on their own once the underlying cause is managed. If arthritis is driving the excess fluid, treating the arthritis with anti-inflammatory measures often shrinks the cyst over time. If a meniscus tear is the culprit, addressing that injury may stop the fluid from accumulating.
For cysts that are painful or limiting your movement, a doctor can drain the fluid with a needle under ultrasound guidance and inject a steroid to reduce inflammation. This procedure relieves symptoms quickly. In one study, 83% of patients had complete resolution of the leaked fluid in the calf within a week of the injection. However, the cyst itself persisted in 88% of cases, meaning it may refill if the underlying joint problem continues. This is why treating the root cause matters more than draining the cyst alone.
In the short term, icing the back of the knee for 15 to 20 minutes at a time, keeping the leg elevated, and using a compression wrap can all reduce swelling and discomfort.
Exercises That Help
Gentle physical therapy can reduce pressure on the back of the knee and help prevent the cyst from recurring. The two priorities are stretching the hamstrings and strengthening the quadriceps. Tight hamstrings pull on the back of the knee and increase compression around the cyst. Stronger quadriceps help stabilize the knee joint and distribute forces more evenly, reducing the irritation that causes extra fluid production.
A simple starting routine: lie on your back and gently straighten the affected leg upward, holding a stretch in the back of the thigh for 20 to 30 seconds. Repeat several times a day. For quad strengthening, seated leg extensions or straight-leg raises are effective and low-impact. Range of motion exercises, like gently bending and straightening the knee through its full comfortable arc, help maintain joint flexibility and control. The key is consistency over intensity. These exercises work best when done multiple times daily rather than in one aggressive session.
Why the Underlying Cause Matters Most
A Baker’s cyst is almost always a secondary problem. It’s your knee’s way of telling you something else is going on inside the joint. Among people who have cysts drained or even surgically removed, recurrence rates are high unless the original issue, whether osteoarthritis, a cartilage tear, or chronic inflammation, gets addressed. Think of the cyst as a symptom rather than the disease itself. Treating only the swelling is like mopping up water without fixing the leak.
If your swelling is small, painless, and not limiting your activity, watchful waiting combined with the stretching and strengthening exercises described above is a reasonable approach. If it’s large, painful, or keeps coming back, imaging to identify the underlying joint problem will guide more targeted treatment.