A cushion is disclosed for insertion in a patients knee joint to serve as a buffer between a patients femur and tibia. The cushion is filled with a substantially incompressible fluid, which can be a foam or a gel or a liquid, like water, which is capable of supporting the entire weight of the patients body. The cushion is preferably made from a flexible material that allows relative movement between the patients femur and tibia, while resisting rupture as it supports these two bones as they are moving relative to one another as the knee joint is flexed, bent or extended.
