Baker’s Cyst (Popliteal cyst)
Baker’s cyst, named after the English doctor who first described it, presents as a soft bulge at the back of the knee, which may be painful and is associated with a feeling of tautness in the area.
A cyst is defined as a cavity lined by epithelium and filled with a liquid or gaseous content. It is an abnormal structure and may lead to swelling and pain in the area. Baker’s cyst is a fluid-filled sac that develops at the back of the knee, a space called the popliteal space; hence it is also referred to as a popliteal cyst.
How does a Baker’s cyst form?
Baker’s cyst, is not a true cyst, since in most cases the lining isn’t complete, rather it communicates with the joint cavity. The knee joint is surrounded by a fibrous capsule that encloses the articulating ends of the bones, the cartilages covering their surfaces and certain ligaments. The inner lining of this capsule is called the synovial membrane and it secretes a lubricating fluid called synovial fluid which bathes the whole interior of the joint cavity. This fluid not only serves to reduce friction between the articular cartilages but also provides them with essential nutrients and drains their waste (as cartilages lack proper blood supply).
In certain conditions, when the inside of the joint cavity is irritated, whether due to trauma or disease, the production of synovial fluid increases significantly. As the fluid accumulates in the joint space, the pressure rises and the fluid finally pushes to the back of the knee, accumulating in the popliteal space as a popliteal cyst. This cyst may remain associated to the main synovial sac or may pinch away, forming a separate cyst.