Mechanical Effects of Micro-thread Orthodontic Mini-screw Design on Artificial Cortical Bone

This study evaluates the mechanical effects of the micro-thread design of a mini-screw using in vitro insertion/removal torque, pull-out tests, and numerical simulation on artificial bone samples with various cortical bone thicknesses. Single-threaded (ST) (0.8-mm pitch) and dual-threaded (DT) (0.25-mm pitch in the cortical bone contact region) mini-screws (diameter: 1.6 mm; length: 9.5 mm) were inserted into, removed from, and pulled out of artificial cortical bones with 0, 1, and 2 mm thicknesses. Maximum insertion/removal torque (MIT/MRT), insertion/removal angular momentum (lAM/RAM), and maximum pull-out force (Fmax) were recorded and statistically analyzed for evaluating the micro-thread mechanical retention in cortical bone with various thicknesses. A parallel finite element (FE) analysis was performed to determine the mini-screw micro-motion at the mini-screw/bone interface and the surrounding bone strain development. The MRT, RAM, and F max tests show that DT screws exhibit higher retention than that of ST screws when cortical bone existed, but there was no significant difference (p＜0.05) for only medullary bone. The maximum pull-out force increased with increasing cortical bone thickness for both screw types. High MIT and IAM values were found for DT screws inserted into cortical bone with thicknesses of I mm (48 N∙cm/1529 N∙cm∙s) and 2 mm (54 N∙cm/1582 N∙cm∙s). The FE simulation results show that screw micro-motions were smaller than 5 μm for all cases, far from the critical bone remodeling threshold. Bone strain values for DT screws were lower than those for ST screws. DT screws had better mechanical stability with higher MRT, RAM, and pull-out force in the experimental tests and lower bone strain value in the FE analysis compared to those for ST screws. However, the DT design may need improvement to reduce the insertion torque and angular momentum to decrease the stress to the surrounding tissue.