Water on the knee, or knee effusion, is swelling that could be caused by injuries, arthritis, or infections. Learn more about ...

A: A Baker's cyst is a collection of fluid behind the knee. It was named after a British surgeon, William Baker, who described these cysts in the late 1800s. It's sometimes called a popliteal cyst.

A Baker’s cyst will not cause long-term harm, but it can be annoying and painful. A cortisone injection can reduce the inflammation, though it doesn’t always prevent recurrence.