A ganglion cyst is a fluid-filled sac near a joint. The fluid from inside the joint leaks out through a channel and forms a pocket or cyst. This condition is slightly more common in men than women, but any adult can have a ganglion cyst.
The exact cause of the problem isn’t known yet. In the knee, it may start to form after the meniscus is torn or damaged. It does seem to be more common in middle-aged adults.