An osteosynthesis system includes a pedicular screw and a bar of longitudinal axis, the screw having a U-shaped head with two branches and a base for defining an inter-branch space in which the bar is received, a screw shank with a bone thread, and elements for blocking the bar in the space and including at least a tab and movement elements for moving the tab relative to the branches in a plane substantially parallel to the axis, the movement elements being arranged so that the tab occupies two positions, a first position where the tab does not co-operate with the bar, and a second position where the tab co-operates with at least one of the branches in such a manner that at least a portion of the tab is situated in the space and applies on the bar a pressure force that is directed towards the base of the U-shape.
