An implantable medical device, for implantation in a mammal knee joint. The implantable medical device comprises an artificial contacting surface (1101, 1102) adapted to replace at least one contacting surface of the knee joint and to be lubricated when implanted in said joint. The implantable medical device further comprises a reservoir comprising a movable wall portion (1110) defining the volume of the reservoir and at least one inlet (1123) adapted to receive a lubricating fluid from the reservoir. The implantable medical device further comprises at least one channel (1105) at least partly integrated in said artificial contacting surface, wherein the channel is fluidly connected with said at least one inlet for distributing said lubricating fluid to the surface of said artificial contacting surface and an operation device (1007) adapted to non-invasively transport said lubricating fluid from said reservoir to said artificial contacting surface. The implantable medical device further comprises an implantable injection port (1107) for refilling said reservoir, wherein the movable wall portion is moved when the reservoir is refilled, such that the volume of the reservoir is increased.
