Pivotal securing system at a bone screw

A pivotal securing system between a bone screw having a spherical head part and a reception part which at the end of a passage bore encloses the head part with a shoulder at its lower side and which receives a clamping screw in the passage bore by means of which the head part can be pressed on against the shoulder in a selectable pivotal position, where the spherical head part is executed as a separate part which can be screwed together with the bone screw in order to be able to place on the reception part and to connect it to the bone screw after the implantation of the bone screw and where a press-on surface of the clamping screw is provided with an inner thread which is directed the same as an outer thread of the clamping screw and has a similar pitch in order that the inner thread forms in a groove when the clamping screw is drawn on to the spherical head part which improves the torque which can be transmitted to the spherical head part.