Comparing biomechanical properties of "Self tapping" and "Self drilling" screw systems on rabbit facial bones

Nowadays, open reduction and internal fixation with plaque and screw systems is the most preferred system for maxillofacial traumatology and orthognathic surgery. Absolute indications about used systems are not mentioned. Purpose of this study is to compare biomechanic properties of ?self drilling? and ?self tapping? screws on rabbits different face bones.Sixteen New Zealand male weighting between 2800-3000 grams were used in this study. On all test subjects for the left side ?self drilling? and for the right side ?self tapping? screws are applied. Two mm diameter, 7 mm length screws were placed mandible bicortically, 2 mm diameter, 5 mm length screws were placed mandible monocortically, maxilla and orbita lower rim. 8 of the rabbits are sacrificed instantly and the rest 8 rabbits are sacrificed after the 6 week recovery period. To compare the screws initial torque forces and axial pull-out tests are applied. Cortical bone thickness measurements are done for each screw. Dependent sample t test and Pearson correlation analysis are used for the statistical study. Cortical thickness means are found as 630,74µm on mandible angle, 445,00µm on mandible front part, 275,05µm on maxilla and 335,25µm on orbita lower rim. For all areas in both 0. week and 6. week recovery period groups axial pull-out forces of ?self drilling? screws are found higher than ?self tapping? screws. But this difference was not statistically significant .On all areas initial torque forces of self drilling screws are found higher than self tapping screws. This difference was statistically significant for mandible bicortically (p<0,001), mandible monocortically (p=0,002), and orbita lower rim (p=0,008) but wasn?t for maxilla (p=0,021). 0. week and 6. week test subjects are compared for investigating bone recovery effect. For ?self drilling? screws in 6. week group on all areas pull-out forces are found higher but the difference was not statistically significant. For self tapping screws 6. week pull-out forces are found decreased on maxilla and increased on the other areas but the difference was not statistically significant. In 0. week group for both ?self tapping? and ?self drilling? screws pull-out forces of the screws that are placed on mandible bicortically are found higher than the monocortically placed screws and this difference was statistically significant. In 6. week group for both self tapping and self drilling screws pull-out forces of the screws that are placed on mandible bicortically are found higher than the monocortically placed screws but this difference was not statistically significant. For all screws initial torque forces of bicortically placed screws are found higher than the monocortically placed screws and this difference was statistically significant. For all screws a linear relationship between cortical bone thickness and both pull-out and initial torque forces was foundAs a result, it may be advantageous to use self drilling screws on thin face bones like maxilla and orbita as they don?t form thermal damage, decrease the surgery time, provide enough holding force and are easy to be placed on thin bones.