Influence of different pilot hole sizes on torque measurements and pullout analysis of osteosynthesis screws

When screws are inserted in thick cortical bone, a small pilot hole size, corresponding to the core diameter of the screw, can result in high torsional stress, leading to screw fracture. The aim of this study was to enlarge the drill size up to a critical pilot hole size (CPHS) which, if exceeded, means a rapid decrease in the screw holding power. 1.5 and 2 mm titanium screws were inserted in discs of polyvinylchloride (PVC), wood and porcine mandibular bone with thicknesses differing between 2 to 4 mm, using an increasing pilot hole size between 66% and 95% of the screw external diameter. Torque measurements and pullout tests were performed and the CPHS was calculated. In torque measurements, the CPHS of microscrews ranged between 83% and 85% of the screw external diameter (SED). The CPHS of miniscrews lay between 80% and 90% of SED. In pullout analysis, the CPHS of microscrews ranged between 83% and 89% of SED; the CPHS of miniscrews lay between 79% and 91% of SED. The mean of the CPHS was calculated to be approximately 85% of the SED. Up to this critical point, the pilot hole size may be increased without affecting the holding power of the screws.