An assembly for fixating a prosthesis in body tissue such as bone of a mammal, comprising a mechanical compacting device and a mass comprising at least granules of a biocompatible material having a sponge-like structure, wherein the mechanical compacting device is designed for compacting the granules during use by insertion of the device in a hole in bone and subsequently inducing relative movement of at least a first part of the compacting device relative to a second part thereof, such that the volume of at least one space between at least part of an outer surface of the device and the adjacent inner surface of said hole is reduced.