Mechanical characteristics of various orthodontic mini-screws in relation to artificial cortical bone thickness

Objective: To evaluate the effect of cortical bone thickness on the maximum insertion and removal torque of different types of self-drilling mini-screws and to determine if torque depends on the screw design. Materials and Methods: Three different types of self-drilling mini-screws (cylindrical type [Cl], taper type [Ta], taper type [Tb]) were inserted with the use of a driving torque tester at a constant speed of 3 rotations per minute. Experimental bone blocks with different cortical bone thicknesses were used as specimens. Results: Differences in the cortical bone thickness had little effect on the maximum insertion and removal torque in Cl. However, with Ta and Tb, the maximum insertion torque increased as the cortical bone thickness increased. The maximum insertion torque of Tb was highest in all situations, followed by Ta and Tb, in that order. Cl showed less torque loss in all cortical bone thicknesses and a longer removal time compared to Ta or Tb. There were significant relationships between cortical bone thickness, maximum insertion and removal torque, and implantation time in each type of self-drilling mini-screw. Conclusion: Since different screw designs showed different insertion torques with increases in cortical bone thickness, the suitable screw design should be selected according to the cortical thickness at the implant site.