What it means
Behind the knee, between two tendons, sits a small cushioning sac called a bursa. It often connects to the knee joint through a narrow opening. When the knee makes extra fluid, for example from arthritis or a meniscal tear, some of it is pushed into this sac and it swells. Ultrasound and MRI show a smooth, fluid-filled sac in this typical spot, which is how radiologists recognize it.
When it matters
Baker's cysts are common and usually harmless in themselves. Many cause no symptoms, while larger ones can cause tightness or swelling behind the knee. They often shrink when the knee problem behind them settles. Sometimes a cyst leaks or bursts, and fluid runs down into the calf, causing pain and swelling that can look like a blood clot. An ultrasound can tell the two apart.
Questions to ask your doctor
- Which knee problem is behind the cyst?
- Is there any sign that it has leaked or burst?
- If my calf swells suddenly, should I have an ultrasound to check for a clot?
