A guide element is constructed for arrangement on a device for the penetrating extension of a blind hole created in hard tissue, in particular in the jawbone, wherein the device includes a hollow body that forms a pressure chamber and that has a distal working opening and an inlet opening opposite the working opening. The guide element can be inserted into the inlet opening with a tight fit and the inlet opening can be closed by the guide element, the guide element has a through-hole, through which a shaft of a working tool, for example, of a milling tool, can be guided and inserted into the hollow body, and the guide element has a connection for a working medium for applying an internal pressure in the pressure chamber or in the hollow body
