drilled in the bone to take the screw this central section sits tightly in the bone bore providing additional lateral support there is an additional advantage obtained by the presence of the blank central section of the screw of fig4 compared with that of fig3 as is known in the art screw fatigue failure generally takes place near the center of the screw the machining or grinding of the screw thread generates microscopic stress raisers in the surface of the screw thread from which fatigue cracks can propagate the elimination of the screw cutting procedure at the central section of the screw of fig4 contributes to an increase of the fatigue breaking stress of the screw as compared to a screw with a thread all the way along its length fatigue texts were performed on otherwise identical screws having the form of that of fig3 and that of fig4 while the continuous thread screws failed after approximately 20 000 stress cycles the screw with the clear unmachined center withstood almost 100 000 cycles before failing furthermore unlike most orthopedic screw fixations where the screw is fully supported along its length within the bore in the bone in an oblique entry intervertebral fusion process the screw is not supported by bone in the disc space or at least not until bone graft material has fully grown in the disc space consequently.