u00b0 the angle a 2 is 15 u00b0 while b 1 is fixed at 50 u00b0 and b 2 at 40 u00b0 to allow immediate anchoring that is to say a 1 b 1 56 u00b0 and a 2 b 2 55 u00b0 in cross section the cutting ridges are obtained by the machining of a closed l by the external diameter of the thread p of the screw this l is formed by a long side l 1 and a short side l 2 fig3 finally so as not to weaken the screw the position of the point of intersection of the straight line sides l 1 and l 2 point l that is to say the depth of the cut in no circumstances bites into the main barrel in the advantageous embodiment more particularly described the depth of the cutting ridges ar is less than or equal to the depth of the screwthread finally in fig3 a the position of the plane defined in cross section by the straight line l 2 where it intersects the thread represents the cutting ridge ar is offset rearward or negatively in the screwing in direction to a radial axis of the screw axr so that when turning as can be seen in fig3 b these ridges ar attack the bone with an angle c defined in cross section between the radial axis passing through ar and the straight.