A Baker’s cyst is a localized collection of fluid (cyst) behind the knee usually caused by an increase in synovial (joint lubricant) fluid. The increase in synovial fluid causes pressure to build up and pushes the fluid into the back of the knee. As fluid collects in this space, the body builds a soft tissue envelope to contain the fluid, which results in the balloon-like cyst.
Baker's Cyst Misinterpretation
Many of the conditions that result in a Baker’s cyst cause pain in the back (posterior) of the knee joint, so that it is a common misconception that the cyst is actually causing the pain. However, a Baker’s cyst rarely causes pain and usually is considered a sign of other pathology (meniscus tear, arthritis, etc) in the knee joint. For this reason, it is very uncommon that the cyst itself requires treatment.