Fixing unit with an end imprint in a threaded terminal portion

A fixing unit comprising a threaded terminal portion with an end face having a hollow imprint able to receive a maneuvering tool of said unit, said imprint including a peripheral edge in the shape of a continuous curved line including at least three lobes, distributed on said edge, wherein the distance between the peripheral edge of the imprint and an external peripheral surface of the threaded terminal portion varies between a maximum distance and a minimum distance, whereby the ratio between said maximum and minimum distances is between about 1.25 and 1.75, wherein each lobe comprises two side faces forming between them an angle, the apex of which is substantially located on the external peripheral surface of the threaded terminal portion.