An in-bone implantable shaft for prosthetic joints or for direct skeletal attachment of external limb prostheses, comprised of a central body fitted in the zone of the bone's medullary cavity conventionally prepared for implantation, and of side elements attached to the central body and fitted in the slots specially made in the bone's walls surrounding said medullary canal; said side elements have spaces between them, arranged to be filled by bone cells to provide a natural and safe osseolocking of the shaft. A method of preparing the bone for implantation of a prosthetic shaft, comprising the steps of: placing a cylindrical guide with slots made in the longitudinal direction of said guide inside said bone's canal which is conventionally prepared for implantation; cutting said bone's walls by progressing a saw along the edges of the slots of said guide; removing the guide; fitting the shaft in the bone's canal, provided that the side elements are fitted to the slots in the bone's walls.