Torsional Properties and Micro-XRD of Miniscrew Implants with Different Compositions

Titanium miniscrew implants are popular in orthodontics, but little information exists about their torsional performance. Objective: Study torsional properties and determine metallurgical phases in three experimental miniscrew implants with the same design and different compositions. Methods: Experimental miniscrew implants with 1.4 mm diameters were manufactured by cold working from pure titanium Grade 2 (Implant A) and two different titanium alloys (Ti-4Al-4V, Implant B; Ti-33Nb-15Ta-6Zr, Implant C). The miniscrew implants were loaded to failure in torsion, and the mean moment and twist angle were determined for each group (n=5). Data were compared by the Kruskal-Wallis test, followed by the Bonferroni multiple comparison test. Micro-XRD diffraction (Rint-2000, Rigaku) was performed at room temperature (25°C), using CuKα radiation at 40kV and tube current of 300 mA to identify phases in the implants. The diameter of a Micro-XRD analysis site was approximately 100 µm. Results: The Micro-XRD pattern from Implant A contained three peaks, which could be indexed to the 100, 101 and 110 planes of alpha-titanium. The Micro-XRD pattern for Implant B contained eight major peaks (100, 002, 101, 102, 110, 103, 112 and 201) for alpha-titanium. There were five peaks for Implant C, which could be indexed to the 110, 200, 211, 220 and 310 planes of beta-titanium. From the Kruskal-Wallis and Mann-Whitney tests, Implant B (1.94 kgf•cm) and Implant C (1.80 kgf•cm) had significantly higher mean torque at fracture than Implant A (1.23 kgf•cm). Implant C had significantly higher mean twist angle at fracture (608.7 ± 114.5 degrees) than the other implants (Implant A: 267.4 ± 126.9 degrees; Implant B: 76.6 ± 22.6 degrees). Conclusions: The Ti-4Al-4V and Ti 33Nb-15Ta-6Zr miniscrew implants had higher mean torque at failure compared to the pure titanium implant. The Ti-33Nb-15Ta-6Zr implant was single-phase beta titanium and had significantly higher torsional moment at failure.