[Popliteal cyst].

A Baker’s cyst or popliteal cyst is a fluid filled mass located behind the knee. It can vary in size and shape based on the amount of fluid in the cyst. It is common in children four to eight years of age and is usually painless. A popliteal cyst does not affect the child’s daily activities, but some children may complain of the cyst being irritated after riding in a car seat for an extended period of time or with prolonged running.