Characteristics of NITI-, TMA,- and Stainless Steel Wires in Three-point Bending Tests

Objectives: The purpose of this study was to investigate the distinctiveness and the force level of the superelastic plateau as well as the resulting deflection characteristics of 36 round and rectangular NiTi wires. Further the bending stiffness of 6 wire products with linear elasticity (TMA, stainless steel, cobalt chromium) was determined. Methods: The wires were subjected to symmetrical 3-point bending tests which were carried out to perform deflection tests closely adapted to the clinical situation in orthodontics. The reported data were derived digitally by a universal material testing machine (ZWICK/Z010, Ulm, Germany) in the unloading process at deflections of 0.5mm, 1mm, 2mm and 3 mm at 23°C and 37°C. Two symmetrical bending models were used: testing spans of wire between the supports of a) 12mm and b) 10 mm (radii of each support and of plunger 0.05-0.13 mm). Results: Variation of model design, wire alloy, deflection and temperature had a significant effect on measured unloading values of NiTi- and TMA wires. Force levels were higher when testing 10 mm spans of wire. Wires with non-linear elasticity displayed their plateaus at wire deflections of 1 and 2 mm. Intra-batch standard deviation was very low. Conclusions: Clinical inter-bracket distance varies from 5mm to 9mm. Both test designs with supports at a) 12mm and b) 10mm are reliable in determining unloading force level plateaus in 3-point bending tests. In case of lower incisors, force delivery characteristics concerning superelasticity obtained by an experimental setup with a support distance of 10 mm could be estimated to be closer to the clinical situation; however, concerning a full fixed appliance 12 mm seems to be the adequate design. Regarding the reported intra-batch force values, quality of manufacturing processes has been improved compared to results of former studies.