The effect of insertion angle on orthodontic mini-screw torque

Background: Primary stability is an important factor for the clinical success of orthodontic mini-screws. The present study made an attempt to evaluate the effect of insertion angle changes on the maximum insertion and removal torque of orthodontic mini-screws. Materials and Methods: In this experimental study, 72 mini-screws (Dual Top Anchor System, Jeil, 1.6 mm diameter, 8 mm length) were used. They were randomly divided into four equal groups and inserted in poly-carbonate plates with 3 mm thickness. Then, their maximum insertion torque (MIT) and maximum removal torque (MRT) were recorded using a digital torque tester/screwdriver. Each group had a different insertion angle (90o, 75o, 60o and 45o). The data were analyzed by SPSS software (version 18) using one-way ANOVA and post-hoc Tukey’s tests. The level of signifi cance was set at 0.05. Results: The maximum MIT was observed in 45o insertion angle (14.84 Ncm) and the minimum MIT was reported in 75o insertion angle (12.66 Ncm). The maximum MRT was observed in 45o insertion angle (23.21 Ncm) and the minimum MRT was reported in the 90o insertion angle (17.43 Ncm). Conclusion: Oblique insertion of the mini-screws results in higher insertion and removal torques and probably more primary stability compared to the vertical insertion.