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, called the popliteal space. The cyst itself isn’t a separate growth. It’s a fluid-filled sac that develops because something inside the knee is producing more lubricating fluid than normal. Understanding what triggers that overproduction is the key to understanding what causes the cyst.
How Fluid Builds Up Behind the Knee
Your knee joint is lined with a membrane that produces synovial fluid, a slippery liquid that reduces friction when you bend and straighten your leg. When something irritates or damages structures inside the knee, this membrane responds by making more fluid than usual. The extra fluid increases pressure inside the joint.
At the back of the knee, there’s a natural opening between the joint capsule and a small sac called the gastrocnemius-semimembranosus bursa. Fluid can flow out through this opening, but the tissue acts like a one-way valve: fluid moves into the bursa but can’t easily flow back into the joint. As more fluid accumulates, the bursa stretches and bulges outward, forming the soft, sometimes egg-sized lump you can feel behind the knee. The cyst is really just a consequence of whatever is happening inside the joint.
Osteoarthritis Is the Most Common Trigger
In adults, the most frequent cause of a Baker’s cyst is osteoarthritis. The gradual breakdown of cartilage irritates the joint lining and stimulates chronic fluid production. In one ultrasound study of 399 patients with knee pain, about 73% had signs of osteoarthritis and roughly 26% had a Baker’s cyst. The connection is straightforward: the more inflamed and irritated the joint lining becomes from cartilage wear, the more fluid it generates, and the more likely that fluid is to migrate behind the knee.
Because osteoarthritis is progressive and tends to affect people over 50, Baker’s cysts in this age group often persist or recur unless the underlying arthritis is managed. The cyst may shrink when inflammation is controlled and swell again during flare-ups.
Meniscus Tears and Cartilage Injuries
A torn meniscus is another very common cause, particularly in younger and middle-aged adults. The meniscus is a C-shaped pad of cartilage that cushions the space between your thighbone and shinbone. When it tears, whether from a sports injury, an awkward twist, or gradual degeneration, it creates ongoing irritation inside the joint. That irritation drives fluid production the same way arthritis does.
Other structural damage can have the same effect. Torn ligaments, damaged articular cartilage (the smooth coating on the ends of bones), and loose fragments of cartilage floating inside the joint all provoke inflammation and excess fluid. In many cases, the Baker’s cyst is actually the first noticeable sign that something inside the knee has been injured. People sometimes discover the cyst before they realize they have a meniscus tear.
Inflammatory Arthritis and Other Conditions
Rheumatoid arthritis and other inflammatory joint diseases cause the immune system to attack the joint lining itself, producing significant swelling and fluid buildup. Baker’s cysts are common in these conditions because the inflammation tends to be persistent and widespread within the joint. Gout, which causes crystal deposits inside the knee, and other crystal-related joint diseases can trigger the same cycle of inflammation and fluid overproduction.
Any condition that results in a knee effusion (the medical term for fluid accumulating inside the joint) can potentially lead to a Baker’s cyst. Even infections inside the knee, though less common, can generate enough excess fluid to push through into the popliteal space.
Baker’s Cysts in Children Are Different
In children, Baker’s cysts typically have no identifiable cause. Unlike adults, kids who develop these cysts usually have no underlying joint damage, no arthritis, and no cartilage tears. The cysts are considered idiopathic, meaning they arise on their own, possibly from minor mechanical stress or simply from how the bursa develops during growth.
Parents often notice the cyst as a painless lump behind their child’s knee, or it shows up incidentally during a medical exam. In most pediatric cases, the cyst resolves on its own over months to a couple of years without any treatment. The standard approach for children is observation and reassurance, as long as imaging confirms there’s no underlying joint problem.
What Makes a Baker’s Cyst Grow or Rupture
The size of a Baker’s cyst fluctuates with the amount of fluid your knee is producing. Activities that stress the joint, periods of increased inflammation, or new injuries can cause the cyst to swell noticeably over days or weeks. Conversely, rest and reduced inflammation can allow it to shrink.
In some cases, the cyst ruptures. When this happens, synovial fluid leaks into the calf, causing sudden sharp pain, swelling, and redness in the lower leg. A ruptured Baker’s cyst can closely mimic the symptoms of a deep vein thrombosis (a blood clot in the leg), which is why sudden calf swelling warrants prompt medical evaluation to distinguish between the two. The fluid from a rupture is eventually reabsorbed by the body, but the process can take several weeks and the calf may remain sore during that time.
Why Treating the Underlying Cause Matters
Because a Baker’s cyst is a symptom rather than a standalone problem, draining the cyst without addressing the source of inflammation inside the knee usually leads to recurrence. The fluid simply builds up again. Effective management focuses on whatever is driving the excess fluid production: reducing arthritis-related inflammation, repairing a torn meniscus, or managing an autoimmune condition.
Physical therapy that strengthens the muscles around the knee can help stabilize the joint and reduce irritation. Compression wraps and periodic icing can ease discomfort from the cyst itself. When a cyst is large enough to limit how far you can bend or straighten your knee, or when it causes persistent pain in the back of the knee, your doctor may drain it with a needle and inject a corticosteroid to temporarily reduce inflammation. But lasting relief depends on identifying and treating the root cause inside the joint. In cases where a meniscus tear or cartilage damage is the trigger, surgical repair of that damage often resolves the cyst as well.