A Baker’s cyst forms when excess joint fluid pushes through a one-way valve at the back of the knee and pools in the space behind it. The underlying cause is almost always a problem inside the knee joint itself, most commonly arthritis or a torn meniscus, that triggers the overproduction of lubricating fluid. Understanding what’s driving that fluid buildup is key, because the cyst is a symptom rather than a standalone condition.
How the Cyst Actually 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 lining responds by producing more of this fluid than it normally would. That extra fluid has to go somewhere.
At the back of your knee, there’s a small channel between two tendons (the semimembranosus and the medial gastrocnemius) that connects the joint cavity to a natural pocket called the popliteal bursa. This channel acts like a one-way valve: fluid can flow out of the joint and into the bursa, but it can’t easily flow back. As fluid accumulates in that pocket, it swells into the soft, fluid-filled lump known as a Baker’s cyst (also called a popliteal cyst). Cysts vary widely in size, with an average volume around 10 cubic centimeters, though some grow several times larger.
Osteoarthritis Is the Most Common Cause
Osteoarthritis is by far the leading trigger. In one ultrasound study of 328 knees with chronic osteoarthritis, 27% had a Baker’s cyst. Among 54 knees without osteoarthritis, only 2% did. The wear-and-tear damage of osteoarthritis creates persistent, low-grade inflammation in the joint lining, which keeps synovial fluid production elevated. The more inflamed the lining, the more likely a cyst will develop and the larger it tends to grow.
Rheumatoid arthritis can cause the same problem through a different mechanism. Instead of cartilage breakdown driving the inflammation, the immune system attacks the joint lining directly, producing large volumes of inflammatory fluid. People with rheumatoid arthritis often develop Baker’s cysts that fluctuate in size alongside their disease flares.
Meniscal Tears and Other Knee Injuries
A torn meniscus, the C-shaped cartilage that cushions each side of the knee, is the second most common cause in adults. The tear irritates the joint, triggering extra fluid production in the same way arthritis does. Ligament injuries, cartilage damage from sports, and even repeated minor trauma can have the same effect. In these cases, the cyst often appears weeks after the original injury, once enough fluid has accumulated to stretch the popliteal bursa.
Gout and other crystal-related joint conditions can also produce sudden surges of synovial fluid and lead to cyst formation, though this is less common.
Baker’s Cysts in Children
In children, Baker’s cysts behave differently. Most arise without any identifiable knee problem. These idiopathic cysts are thought to result from a natural variation in the anatomy of the popliteal bursa rather than from joint damage or inflammation. They typically appear between ages 4 and 7, cause little discomfort, and often resolve on their own within one to two years without treatment. In adults, a Baker’s cyst almost always signals an underlying joint issue that needs attention.
Symptoms and What They Feel Like
A small Baker’s cyst may cause no symptoms at all. Many are discovered incidentally on imaging done for other reasons. As a cyst grows, you’ll typically notice a firm, rounded bulge at the back of the knee that’s easiest to feel when your leg is fully straight. Bending the knee deeply can produce a sensation of tightness or pressure, and some people feel an aching pain that worsens with activity.
If the cyst is large enough, it can limit how far you can bend or straighten the knee. Stiffness tends to be worst after long periods of sitting or first thing in the morning, then eases as you move around.
When a Cyst Ruptures
A Baker’s cyst can rupture, releasing its fluid into the calf. This causes sudden, sharp pain behind the knee, followed by swelling, redness, and warmth spreading down the lower leg. The problem is that these symptoms closely mimic a deep vein thrombosis (DVT), a blood clot in the leg that requires urgent treatment. In clinical series, ruptured Baker’s cysts have been found in up to 80% of patients initially suspected of having a DVT who turned out not to have one. Nearly a third of those ruptured cysts had fluid tracking upward into the thigh, further complicating the picture.
Because the two conditions look so similar, an ultrasound is typically the first step to check for a clot and evaluate the popliteal fossa at the same time. If you develop sudden calf swelling and pain, getting this imaging quickly matters, since the treatment paths are completely different.
How It’s Diagnosed
A physical exam is often enough to suspect a Baker’s cyst, but imaging confirms it. Ultrasound is the fastest, cheapest option and is remarkably accurate. When fluid is visible in the characteristic location between the two tendons at the back of the knee, the diagnosis is essentially certain. Studies using ultrasound to identify Baker’s cysts have shown 100% accuracy when that specific finding is present. MRI provides more detail about what’s happening inside the joint, which helps identify the underlying cause, whether that’s a meniscal tear, cartilage loss, or inflamed joint lining.
Treatment Targets the Underlying Cause
The most important principle in treating a Baker’s cyst is addressing whatever is producing the excess fluid. If a meniscal tear is driving the inflammation, repairing the tear often allows the cyst to resolve as fluid production returns to normal. If osteoarthritis is the cause, managing the arthritis with physical therapy, anti-inflammatory medications, or injections into the joint can reduce fluid output enough for the cyst to shrink.
Draining the cyst with a needle (aspiration) provides quick relief but has a high recurrence rate when used alone. Published recurrence rates after aspiration range from 5% to 70%, depending on whether the underlying joint problem is also treated. A cyst that’s drained without fixing the source of irritation will often refill within weeks or months.
In persistent cases, surgery can address the one-way valve mechanism directly. Using an arthroscope inserted into the knee, a surgeon can widen the valve opening so fluid flows freely in both directions, preventing it from becoming trapped in the bursa. This is typically combined with treatment of the intra-articular problem, such as removing a torn piece of meniscus or smoothing damaged cartilage, to reduce fluid production at its source.
For many people, especially those with mild cysts linked to early arthritis, a combination of regular low-impact exercise, ice after activity, and compression wraps is enough to keep symptoms manageable without any procedure. The cyst may never disappear entirely, but it can remain small and painless for years if the underlying joint condition stays under control.