Baker's cyst is a fluid-filled cyst that leads to a bulge and degree of tightness behind your knee. The pain can worsen when you bend or straighten your knee or when you're going about your day to day activities. It is also referred to as a popliteal cyst which stems from issues in your knee joint such as a meniscal tear or arthritis. These can both lead to your knee producing excess synovial fluid resulting in a Baker's cyst. Treatment: Needle aspiration Corticosteroid injection Rest ...