A Baker cyst (also called a popliteal cyst) is a fluid-filled swelling that forms behind the knee. It develops when excess joint fluid gets pushed into the space at the back of the knee, called the popliteal fossa, creating a visible or palpable bulge. Most Baker cysts are not dangerous on their own, but they signal that something else is going on inside the knee joint.
How a Baker Cyst Forms
Your knee joint naturally contains synovial fluid, a slippery liquid that lubricates the joint and reduces friction during movement. When the knee becomes inflamed or injured, it responds by producing extra fluid. That excess fluid can travel to the back of the knee through a one-way valve mechanism, pooling in the popliteal space rather than draining back into the joint. Over time, this trapped fluid forms the cyst.
Because the valve only lets fluid flow one direction, the cyst tends to grow as more fluid accumulates but has no easy path back out. This is why Baker cysts can gradually enlarge, especially if the underlying knee problem persists.
Common Causes
In adults, Baker cysts are almost always secondary to an existing knee problem. The two most common triggers are arthritis (particularly osteoarthritis and rheumatoid arthritis) and cartilage tears, especially meniscus injuries. Both conditions cause chronic inflammation that drives the overproduction of synovial fluid.
In children, the picture looks quite different. Baker cysts in kids typically appear without any associated joint disease or injury history, which suggests a fundamentally different cause than in adults. Pediatric cysts also tend to resolve on their own over time, while adult cysts usually persist until the underlying knee condition is addressed.
What a Baker Cyst Feels Like
Some Baker cysts cause no symptoms at all and are discovered incidentally during imaging for another issue. When symptoms do appear, the most common ones include a visible bulge or tight sensation behind the knee, stiffness that worsens with activity, and swelling that may extend into the upper calf. Many people notice that bending the knee fully becomes difficult or uncomfortable, since the fluid-filled sac gets compressed during flexion.
Symptoms often fluctuate. You might notice the swelling increases after periods of activity and improves with rest. The cyst itself can range from marble-sized to several centimeters across.
When a Cyst Ruptures
The most notable complication is rupture. When a Baker cyst bursts, the synovial fluid leaks into the surrounding tissue of the calf, causing sudden, sharp pain behind the knee that spreads downward. The calf swells, becomes tender, and may develop bruising. Walking often becomes painful, and some people develop a noticeable limp.
A ruptured Baker cyst closely mimics a deep vein thrombosis (DVT), which is a blood clot in the leg. Both conditions produce calf pain, swelling, and tenderness, and even a clinical exam can’t reliably tell them apart. Pain when flexing the foot upward, a classic sign of blood clots, also occurs with ruptured cysts. This overlap means that calf swelling with sudden onset almost always requires imaging to rule out a clot before attributing symptoms to a ruptured cyst. An ultrasound of both the popliteal fossa and the leg veins is typically the first step.
How It’s Diagnosed
A physical exam can often detect a Baker cyst, especially when the bulge behind the knee is visible or easy to feel. But confirming the diagnosis and identifying the underlying cause usually requires imaging. Ultrasound is a quick, widely available option that can distinguish a fluid-filled cyst from a solid mass and check the leg veins at the same time. MRI provides a more detailed look, showing not only the cyst but also cartilage tears, arthritis changes, or other joint problems that may be driving the fluid production.
Treatment Without Surgery
Mild Baker cysts that cause minimal discomfort often need nothing more than activity modification. Avoiding movements that aggravate the knee, such as deep squats, prolonged standing, or high-impact exercise, can help reduce irritation and slow fluid production.
The RICE approach works well for flare-ups: rest the leg, ice the knee, use a compression wrap or sleeve, and elevate the leg when possible, particularly at night. Over-the-counter pain relievers like ibuprofen, naproxen, or acetaminophen can help manage pain and inflammation during these episodes.
When the cyst is large or persistent, a cortisone injection into the knee joint can reduce inflammation and shrink the cyst. This often provides meaningful relief, though it doesn’t always prevent the cyst from returning. In some cases, a doctor may drain the cyst directly using a needle (aspiration), sometimes combined with a steroid injection. Gentle range-of-motion exercises and strengthening work for the muscles around the knee can also improve mobility and reduce discomfort over time.
The Role of Surgery
Surgery is generally reserved for Baker cysts that don’t respond to conservative treatment or that cause significant functional problems. The key principle is that simply removing the cyst without addressing the underlying knee problem tends to lead to recurrence. If a torn meniscus is driving the excess fluid production, repairing the cartilage is more effective than draining or excising the cyst alone. Similarly, if arthritis is the root cause, managing the inflammatory process in the joint is essential for lasting results.
Arthroscopic surgery, which uses small incisions and a camera, is commonly used to treat both the intra-articular problem and the cyst. Recovery depends largely on the underlying procedure performed rather than the cyst removal itself.
Why Treating the Underlying Problem Matters
A Baker cyst is a symptom, not a standalone diagnosis. The cyst exists because the knee is producing too much fluid, and the knee is producing too much fluid because something inside the joint is inflamed or damaged. Draining the cyst or reducing it with injections provides temporary relief, but unless the arthritis, cartilage tear, or other joint condition is managed, the fluid will keep accumulating and the cyst will keep reforming. This is why your doctor will focus as much on what’s happening inside the knee joint as on the cyst itself.