Definition of Baker's cyst:

Baker's cyst is an accumulation of joint fluid (synovial fluid) that forms behind the knee.

Causes, incidence, and risk factors:

A Baker's cyst may form by the connection of a normal bursa (a normal lubricating fluid sac) with the knee joint. This type is more common in children.

The condition can also be caused by the herniation of the knee joint capsule out into the back of the knee, which is more common in adults. This commonly occurs with a tear in the meniscal cartilage of the knee.

In older adults, Baker's cysts are frequently associated with knee arthritis.