A Baker’s cyst is a collection of fluid in the diamond shaped region at the back of the knee (popliteal fossa). They are most commonly on the inner side of the popliteal fossa. They are present in many people (up to 50% in one study) and have a direct connection to the knee joint interior. They are rarely problematic however if they become enlarged they can be troublesome. They can become enlarged in patients with arthritis or following an injury to the knee that causes the knee to swell up.
The swelling can recur even after surgery to remove it due to the connection between the cyst and the knee joint which can be difficult to close.