What causes Baker's cyst?

The knee normally makes a small amount of lubricating fluid. When the joint lining is irritated, it can produce more fluid. Pressure may then move that fluid through a one-way opening into a soft sac behind the knee, creating a popliteal cyst. Knee osteoarthritis is a frequent underlying driver because worn and inflamed joint surfaces can cause ongoing irritation. A meniscus tear can do the same, particularly when the knee also catches, clicks, or swells. The cyst itself may feel like fullness or a lump that becomes more noticeable when the knee is straight.

Management usually focuses on the knee problem rather than the cyst alone. Reduce activities that repeatedly provoke swelling, use gentle movement, and follow a clinician's advice about strengthening the thigh and hip. Ice wrapped in cloth may ease swelling after activity. Assessment can clarify whether the knee has osteoarthritis, a meniscus injury, or another source of fluid. A clinician may discuss medication, drainage, or other treatment when symptoms are persistent; draining the sac without addressing the joint may not prevent it from returning.

Get urgent help if the calf suddenly becomes painful, swollen, warm, or red, or if you develop chest pain or breathlessness. A ruptured cyst can irritate the calf, but these symptoms can also signal a clot and should not be self-diagnosed. Arrange review for a growing lump, repeated knee swelling, locking, inability to bear weight, or symptoms that limit walking.

What is causing the fluid in my knee? Could arthritis or a meniscus tear be involved? Do I need an ultrasound or another test? Which activities and exercises are suitable while the knee is swollen?