An anchorage in tissue is produced by holding a vibrating element and a counter element against each other such that their contact faces are in contact with each other, wherein at least one of the contact faces comprises a thermoplastic material which is liquefiable by mechanical vibration. While holding and then moving the two elements against each other, the vibrating element is vibrated and due to the vibration the thermoplastic material is liquefied between the contract faces, and due to the relative movement is made to flow from between the contact faces and to penetrate tissue located adjacent to outer edges of the contact faces. For liquefaction of the thermoplastic material and for displacing it from between the contact faces no force needs to act on the tissue surface which is to be penetrated by the liquefied material. For this reason anchorage in tissue which has little mechanical stability is possible.
