An implant or endoprosthesis suitable to be implanted in human or animal tissue includes two (or more than two) parts to be joined in situ. Each one of the parts includes a joining location, the two joining locations facing each other when the device parts are positioned for being joined together, wherein one of the joining locations includes a material which is liquefiable by mechanical vibration and the other one of the joining locations includes a material which is not liquefiable by mechanical vibration and a structure (e.g. undercut cavities or protrusions) suitable for forming a positive fit connection with the liquefiable material. The joining process is effected by pressing the two device parts against each other and by applying ultrasonic vibration to one of the device parts when the two parts are positioned relative to each other such that the two joining locations are in contact with each other.