The invention relates to a guide element (100) for arranging on a device for the penetrating extension of a blind hole created in hard tissue, in particular in the jawbone (24), wherein said device comprises a hollow body (1) that forms a pressure chamber (7) and that has a distal working opening (2) and an inlet opening (3) opposite the working opening (2). According to the invention, the guide element (100) can be inserted into the inlet opening (3) with a tight fit and the inlet opening (3) can be closed by the guide element (100), the guide element (100) has a through-hole (101), through which a shaft (5) of a working tool (6), for example, of a milling tool, can be guided and inserted into the hollow body (1), and the guide element (100) has a connection (108) for a working medium for applying an internal pressure in the pressure chamber (7) or in the hollow body (1).
