[Effect of the diameter of various bore holes on retention of osteosynthesis screws].

The standard drill size for most osteosynthesis screws is about 75% of the screw's external diameter. When screws are inserted in thick cortical bone, a small pilot hole size can result in high torsional stress, leading to screw fracturing. The aim of this study was to enlarge the drill size up to a critical pilot hole size, exceeding of which leads to a rapid decrease of the screw's holding power. Titanium screws of diameter 1.5 and 2 mm were inserted in discs of PVC, wood and porcine mandibular bone with different thickness between 2 to 4 mm, using pilot hole sizes of 66-95% of the screw's external diameter. The maximum torque was recorded and pull-out tests were performed. Ten trials were conducted for each screw-pilot hole size-material combination, yielding a total of 1560 tests. A multiphase regression analysis was performed to calculate the critical pilot hole size (CPHS). In torque measurements, the CPHS of microscrews were between 83% and 85% of screw outer diameter (SOD) and the CPHS of miniscrews were between 80% and 90% of SOD. In pull-out analysis the CPHS of microscrews were between 83% and 89% of SOD and the CPHS of miniscrews were between 79% and 91% of SOD. The CPHS was thus found to be around 85% of the screw's external diameter. Up to this critical point the pilot hole size may be increased without affecting the holding power of the screws.