In vitro evaluation of insertion and removal torques of orthodontic mini-implants.

This study evaluated the influence of bone cortical thickness on the maximum torque required for insertion and removal of orthodontic mini-implants of different shapes. Five different types of orthodontic mini-implants were examined Cylindrical 1 (CYM), Cylindrical 2 (CYI) and Cylindrical 3 (CYT) Conical 1 (CON), Conical 2 (COS). Insertion and removal torque tests were performed in mini-pig medullary bone (8mm thick) and cortical bone 1, 2, 3 and 6mm thick. A digital torque meter measured the torque forces; the maximum values of insertion and removal were obtained (N/cm). There were no statistically significant differences between the different implants in the torque forces required for insertion and removal from medullary bone (P>0.05). During insertion into 1-2mm cortical bone, COS, CON and CYT had torque values statistically higher, but CON had higher torque values compared with the others when 3-6mm cortical bone was used (P<0.05). The removal torque values were significantly lower for CYM and CYI. Conical type mini-implants require a greater torque force for insertion and removal compared with cylindrical types. Torque values were directly related to cortical thickness.