The mechanical properties of surgical bone screws and some aspects of insertion practice

The function of a surgical bone screw is to clamp together the bone and a bone plate or to fix bone fragments. This is achieved by the generation of a tensile stress along the length of the screw which is derived from the torsional moment introduced into the screw during the screwing process. This work shows that the strength of the commonly available surgical materials, assessed on a combined torque and tensile basis, is in the descending order cold-worked En58J stainless steel>cast Co-Cr-Mo>titanium 160. Core diameter is the major factor determining the strength of a screw in a given material. For maximum efficiency the highest tensile stress should be generated in a screw by the lowest torsional moment. It can be shown that under adverse circumstances only 5 per cent of the applied torque can be used to induce tension in the screw. By alteration of the method of insertion this value can be raised to about 65 per cent. The holding power of a screw is shown to be independent of the screw's material, but is dependent on the major diameter of the screw and also on the shearing strength of the bone. For any given level of bone strength there is optimum pilot hole size, which is related to the major diameter of the screw. The sizes of these holes have been determined for a range of screws.