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 (8 mm thick) and cortical bone 1, 2, 3 and 6 mm 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 1–2 mm cortical bone, COS, CON and CYT had torque values statistically higher, but CON had higher torque values compared with the others when 3–6 mm 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.