Abstract: The cavity of a screw body presents a portion with an enlarged cross-section forming a chamber, positioned such that it is set back from the proximal surface of said head, over a sufficient distance so that first slots and first expandable portions which define these first slots constitute a gripping element to drive the screw in rotation. The expansion rod comprises a proximal head equipped with gripping element to drive it in rotation, intended to be found in the chamber in the non-expanded position of the screw head: The cavity has a first portion with a limited cross-section located at the expandable portions, this limited cross-section portion being intended to cooperate with the head to achieve expansion of the expandable portions while also defining a passageway having a cross-section sufficient to allow access, from the outside of the screw, to the gripping means comprised by the expansion rod.