A vertebral anchor device comprising a bone anchor screw having a screw body and a screw head for coupling a link element to the screw body, the screw head being mounted to move on the screw body； the screw body comprising a threaded shank of a longitudinal axis AA for anchoring the screw body in a vertebra, and provided at one of its ends with a coupling head for coupling the screw body to the screw head； the screw head comprising a head body of a longitudinal axis BB and through which a channel passes longitudinally that is suitable for receiving, in the head body bottom portion, the coupling head； said vertebral anchor device further comprising assembly means for assembling together the coupling head and the screw head, the coupling head having a portion of cylindrical shape and of axis perpendicular to the threaded shank.
