The bursae of the knee are the fluid sacs and synovial pockets that surround and sometimes communicate with the joint cavity. Thin-walled and filled with synovial fluid, they represent the weak point of the joint, but also produce enlargements to the joint space. They can be grouped into either communicating and non-communicating bursae or, as below, after location. The bursae of the knee are the fluid sacs and synovial pockets that surround and sometimes communicate with the joint cavity. Thin-walled and filled with synovial fluid, they represent the weak point of the joint, but also produce enlargements to the joint space. They can be grouped into either communicating and non-communicating bursae or, as below, after location.