[Preformed round-section NiTi arch wires: the mechanical and clinical aspects]

The authors emphasize the need to set up several products proposed by the manufacturers. Regarding to orthodontic wires, a suitable evaluation of their mechanical properties to fit them for clinical use is needed. After a short outline of the mechanical properties of the wires on the market, the authors focus their attention on the NiTi Japanese alloy superelastic wires. Therefore, an evaluation of the failure aspect of NiTi preformed arch wires with round section of 0.018" of diameter is reported. The tested wires underwent to clinical use as first arch wires, and were evaluated after 8 weeks from the beginning of the therapy, when they have shown failure at the control in the 8th week. After having cleansed and prepared the specimens, their failure zones have been observed by the SEM. The failure aspect was evaluated to check the reasons that produced it, basing on its surface characteristics. The aspect of the surface of the failure and the absence of permanent deformations showed that it is a fragile failure. The fragile failure occurs when materials are not able to tolerate plastic deformations and, typically concerning NiTi alloys constituent the tested wires, it joins the superelastic behaviour and the high wire resilience. It means that this wire is able to produce constant, continuous and light forces for a given stress range, and, furthermore, it is able to absorb energy without permanent deformations. Since 1971 NiTi wires have been on the market but without superelastic and crystallographic shape memory properties possessed by the wires produced by Furukawa El. Co. in 1978.(ABSTRACT TRUNCATED AT 250 WORDS)