Equiatomic and near-equiatomic nickel-titanium alloys exhibit a shape-memory effect and superelasticity. However, the properties of such alloys are extremely sensitive to the precise nickel-titanium ratio and the addition of alloying elements. High corrosion resistance is necessary for biomedical applications, especially orthodontic. The purpose of this study was to investigate the effect of silver addition to nickel-titanium alloys for dental and medical application. Arc melting, homogenization, hot rolling, and solution heat treatment were performed to prepare the nickel-titanium-silver (NiTi-Ag) specimens. The properties of the ternary NiTi-Ag alloys such as phase-transformation temperature, microstructure, microhardness, corrosion resistance, and cytotoxicity were investigated. The NiTi-Ag alloys showed low silver recovery rate for the cast alloy, due to silver's low evaporation temperature, and low silver solubility in nickel-titanium. Silver addition to nickel-titanium increased the transition temperature range to 100 degrees C and stabilized the martensitic phase (monoclinic structure) at room temperature, because the martensitic transformation starting temperature (Ms) was above room temperature. Martensitic and austenitic phases existed in X-ray diffraction patterns of solution-annealed NiTi-Ag alloys. The silver addition was considered to improve the corrosion resistance and form a stable passive film. Significantly, the mechanical properties of the silver-added alloys were dependent upon the amount of alloying addition. There was no toxicity in the NiTi-Ag alloys, as the response index showed none or mild levels.
Objective: To evaluate the structural stability of anodic oxidation treatment of miniscrews during a self-drilling procedure and an initial loading period. Materials and Methods: Eight orthodontic miniscrews with a machined surface and an anodic oxidized surface were placed in the mandible of two beagle dogs. With all miniscrews, an orthodontic force was applied immediately after placement and was continued for 12 weeks. After beagle dogs were sacrificed, the miniscrews were carefully removed from decalcified bone fragments. Miniscrews were evaluated by comparing and quantitatively analyzing changes in surface roughness of unused and used miniscrews (machined surface vs anodic oxidized surface) utilizing both scanning electron microscopy (SEM) and atomic force microscopy (AFM). Results: SEM revealed that only a thread edge close to the tip of the used anodic oxidized miniscrew became smooth by smearing, compared with the unused anodic oxidized miniscrew. No definite changes were observed in the thread valleys of the two groups after placement. AFM measurements demonstrated that all surface roughness parameters of thread edges of the used anodic oxidized miniscrews were significantly reduced compared with the unused anodic oxidized miniscrew (P , .05). A middle thread edge of the used anodic miniscrew surface was rougher than the unused and used machined surface miniscrews (P , .05). Conclusion: Anodic oxidized miniscrews had improved surface characteristics compared with machined surface miniscrews, even if the surface texture was changed by the self-drilling procedure and during the initial loading period. (Angle Orthod. 2012;82:522-528.)
Heat treatment is applied to orthodontic wires to inhibit the fracture after the orthodontist manipulates them to make loops, helical springs, and arch forms. We tried to investigate the effect of heat-treatment conditions on the surface properties of orthodontic wires. Four types of wires were heattreated in air, argon, or a vacuum, and were either cooled in a furnace or water. The mechanical and surface properties of the heat-treated wires were investigated. Heat treatment increased the microhardness, strength, and Young's modulus of the orthodontic wires. The heat treatment of stainless steel wires in air formed very thick and rough iron and chromium oxides on the surface, but the heat treatment in a vacuum or argon inhibited surface oxidation. After heat treatment, furnace cooling inhibited the surface oxidation of the wires better than water cooling did. Water-cooled wires had a rough surface, where oxide or hydroxide layers were formed by reactions with the cooling water. It is thought that water cooling formed layers that were not dense enough to play a key role in protecting the metals from corrosion. Although heat treatment can improve the mechanical properties of orthodontic wires, it reduces corrosion resistance when performed in air. Therefore, the orthodontic wires should be heat-treated in either a vacuum or inert gas and furnace-cooled to inhibit oxidation and minimize ion release.
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