Fracture resistance of orthodontic mini-implants: a biomechanical in vitro study

Sufficient primary stability is of importance for the survival of orthodontic mini-implants. This means that adequate torque has to be achieved during insertion. However, as moments exceeding the fracture resistance of a mini-implant may result in their fracture, the maximum torque load capacity should be known. In this study, the threshold torque values resulting in the fracture of various mini-implant types and diameters were evaluated. Forty-one different mini-implants with diameters ranging from 1.3 to 2.0 mm (Aarhus screw, Abso Anchor, Ancora, Bone screw, Dual Top, Lomas, MAS, O.S.A.S, Ortho Easy, Spider Screw, and Tomas pin) were inserted in acrylic glass by a robot system. Ten specimens of each mini-implant type were tested. The insertion torque was measured and the maximum torque at the time of mini-implant fracture was evaluated. Significance of the mean value differences was evaluated by Kruskal-Wallis tests. Fracture moments varied depending on the diameter of the mini-implants. The measured values ranged from 108.9 Nmm (MAS 1.3×11 mm) to 640.9 Nmm (Lomas 2.0×11 mm). The differences were highly statistically significant (P<0.001). The risk of mini-implant fracture should be borne in mind at the time of insertion, especially if mini-implants with a small diameter are employed. To minimize the risk of fracture, pre-drilling should be carried out if the mini-implants are to be inserted at a site with a high bone density.