An assembly (1) for fixating a prosthesis (3) in a body tissue such as bone (20) of a mammal, comprising a mechanical compacting device (4) and a mass (17) comprising at least granules (18) of a biocompatible material having a sponge-like structure, wherein the mechanical compacting device (4) is designed for compacting the granules (18) during use by insertion of the device (4) in a hole (15) in bone (20) and subsequently inducing relative movement of at least a first part (2) of the compacting device (4) relative to a second part (5, 30) thereof, such that the volume (Vo) of at least one space (16) between at least part of an outer surface (10) of the device and the adjacent inner surface (23) of said hole (15) is reduced.