An assembly for augmenting hard tissue and/or hard tissue replacement material, the assembly comprising: - a thermoplastic augmentation element having an augmentation element contact face; - a tool with a contact face, the tool being pressable against the augmentation element contact face; and - an auxiliary element; - wherein the auxiliary element or the tool has having a guiding portion; - wherein the augmentation element at least partially encompasses the guiding portion; - wherein the tool is a sonotrode equipped for being coupled to a source of mechanical vibrations and is equipped for coupling a pressing force and mechanical vibration energy into the augmentation element to liquefy material of the augmentation element and to cause it to penetrate into structures of the hard tissue and/or hard tissue replacement material; and - wherein at least one of the following conditions is fulfilled: A. the assembly further comprises an outer protection element at least partially encompassing the tool and locally preventing the tool from being in contact with the hard tissue and/or hard tissue replacement material; B. the augmentation element is generally sleeve-shaped and comprises at least one indentation (61, 62) or hole (63, 64) in a sleeve wall; C. in a telescoping region, a portion of the tool encompasses a portion of the auxiliary element or a portion of the auxiliary element encompasses the tool, wherein at least one of the tool and of the auxiliary element comprises at least one protrusion (21, 54, 55, 81, 82) facing to the other one of the tool and the auxiliary element, whereby in the telescoping region a contact between the tool and the auxiliary element at locations different from the at least one protrusion is prevented; D. the tool comprises a distal broadening forming an salient feature that prevents a contact between the tool and the hard tissue and/or hard tissue replacement material at locations proximally of the salient feature; E. the augmentation element is connected to the tool by an axial positive-fit connection. - a thermoplastic augmentation element having an augmentation element contact face; - a tool with a contact face, the tool being pressable against the augmentation element contact face; and - an auxiliary element; - wherein the auxiliary element or the tool has having a guiding portion; - wherein the augmentation element at least partially encompasses the guiding portion; - wherein the tool is a sonotrode equipped for being coupled to a source of mechanical vibrations and is equipped for coupling a pressing force and mechanical vibration energy into the augmentation element to liquefy material of the augmentation element and to cause it to penetrate into structures of the hard tissue and/or hard tissue replacement material; and - wherein at least one of the following conditions is fulfilled: A. the assembly further comprises an outer protection element at least partially encompassing the tool and locally preventing the tool from being in contact with the hard tissue and/or hard tissue replacement material; B. the augmentation element is generally sleeve-shaped and comprises at least one indentation (61, 62) or hole (63, 64) in a sleeve wall; C. in a telescoping region, a portion of the tool encompasses a portion of the auxiliary element or a portion of the auxiliary element encompasses the tool, wherein at least one of the tool and of the auxiliary element comprises at least one protrusion (21, 54, 55, 81, 82) facing to the other one of the tool and the auxiliary element, whereby in the telescoping region a contact between the tool and the auxiliary element at locations different from the at least one protrusion is prevented; D. the tool comprises a distal broadening forming an salient feature that prevents a contact between the tool and the hard tissue and/or hard tissue replacement material at locations proximally of the salient feature; E. the augmentation element is connected to the tool by an axial positive-fit connection. A. the assembly further comprises an outer protection element at least partially encompassing the tool and locally preventing the tool from being in contact with the hard tissue and/or hard tissue replacement material; B. the augmentation element is generally sleeve-shaped and comprises at least one indentation (61, 62) or hole (63, 64) in a sleeve wall; C. in a telescoping region, a portion of the tool encompasses a portion of the auxiliary element or a portion of the auxiliary element encompasses the tool, wherein at least one of the tool and of the auxiliary element comprises at least one protrusion (21, 54, 55, 81, 82) facing to the other one of the tool and the auxiliary element, whereby in the telescoping region a contact between the tool and the auxiliary element at locations different from the at least one protrusion is prevented; D. the tool comprises a distal broadening forming an salient feature that prevents a contact between the tool and the hard tissue and/or hard tissue replacement material at locations proximally of the salient feature; E. the augmentation element is connected to the tool by an axial positive-fit connection.