What Is a Subchondral Bone Cyst? It's a fluid-filled sac that forms in one or both of the bones that make up a joint. They're especially common at the knee or hip. The cysts show up just under the ...

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.