Biomechanical Properties of Orthodontic Miniscrews. An In-vitro Study

Abstract

Comparing five commercially-available miniscrew types for skeletal anchorage with regard to the biomechanical properties influencing their primary stability. Included in this study was a total of 196 self-tapping and self-drilling miniscrews having a diameter of 2 mm (or the largest possible diameter of the manufacturer in question), a thread shaft length of 10 mm, or the longest miniscrew supplied by the manufacturers. The screw types tested were the FAMI 2, Orlus mini-implant, T.I.T.A.N. Pin, tomas-pin and Vector TAS. Insertion and loosening torque measurements, and pullout tests in axial (0 degrees), 20 degrees and 40 degrees directions, as well as test series with and without pilot hole drilling were performed. Bovine femoral heads having the same bone mineral density (BMD) were used as bone-testing material. Higher insertion torques were found for the cylindrical FAMI 2 screw, the conical Orlus mini-implant and the Vector TAS screw (with mean values of 39.2 Ncm, 32.1 Ncm and 49.5 Ncm) than for the cylindrical T.I.T.A.N. pin and tomas-pin. Insertion without predrilling led the insertion torques of all five screws to rise significantly. We noted statistically significant differences among the five screws in the pullout tests. Those highly significant differences at axial (0 degrees) and 20 degrees angles were not apparent at the 40 degrees pullout angle. Compared with the pullout forces (load) in the axial direction, the cylindrical screws' load values decreased markedly according to the angle (by up to -46.6%). The reduction in pullout force in conjunction with an increasing angle was much less pronounced in the conical screws (-0.8% to -29.0%). The tomas-pin demonstrated the highest pullout force and stiffness values throughout the tests. A total of five tomas-pins, two Orlus mini-implants and one FAMI 2 screw fractured during the pullout tests. Results from our insertion torque measurements suggest that a conical screw design will provide greater primary stability than cylindrical screw types. The cylindrical screw design's superiority was evident in the pullout tests. All the miniscrews' primary stability rose after drill-free insertion. The tomas-pin screws, although biomechanically superior to the other screws, were most prone to fracture.