Geometrical design characteristics of orthodontic mini-implants predicting maximum insertion torque

OBJECTIVE To determine the unique contribution of geometrical design characteristics of orthodontic mini-implants on maximum insertion torque while controlling for the influence of cortical bone thickness. METHODS Total number of 100 cylindrical orthodontic mini-implants was used. Geometrical design characteristics of ten specimens of ten types of cylindrical self-drilling orthodontic mini-implants (Ortho Easy®, Aarhus, and Dual Top™) with diameters ranging from 1.4 to 2.0 mm and lengths of 6 and 8 mm were measured. Maximum insertion torque was recorded during manual insertion of mini-implants into bone samples. Cortical bone thickness was measured. Retrieved data were analyzed in a multiple regression model. RESULTS Significant predictors for higher maximum insertion torque included larger outer diameter of implant, higher lead angle of thread, and thicker cortical bone, and their unique contribution to maximum insertion torque was 12.3%, 10.7%, and 24.7%, respectively. CONCLUSIONS The maximum insertion torque values are best controlled by choosing an implant diameter and lead angle according to the assessed thickness of cortical bone.