Cyst behind the knee. A baker's cyst is classically a fluid filled cyst behind the knee. The cyst usually communicates with the knee via a small hole in the capsule or via the base of a tear in a meniscus. The knee makes fluid normally, but osteoarthritis or a mensical tear can cause an increase in fluid production, thus increasing pressure. The pathway of least resistance is out the back of the knee where a cyst forms.