Corrosion susceptibility and nickel release of nickel titanium wires during clinical application.

BACKGROUND AND OBJECTIVE: Orthodontic wires are exposed to a corrosive intraoral environment and are subject to mechanical and thermal load. This could affect how nickel titanium (NiTi) wires corrode, as they possess temperature- and load-dependent characteristics. It was the scope of this study to determine whether the clinical application of NiTi wires would lead to corrosion defects on the wire surfaces, and whether an influence on the patients' salivary Ni ion concentration would become apparent.

MATERIAL AND METHODS: A total of 115 wires of different manufacturers (Forestadent Titanol 'Low Force' und 'Martensitic', Ormco Copper Ni-Ti 35 degrees C, Ortho Organizers NiTi) was retrieved after intraoral application lasting 1 to 12 months. The wires were examined after cleaning with a scanning electron microscope. We also analyzed the salivary Ni ion concentration in 18 patients at predefined intervals following a detailed orthodontic treatment protocol during the initial phase of orthodontic therapy. The intervals were: 1) a saliva sample before treatment commenced, 2) after bonding of brackets and bands, 3) 2 weeks after bonding, immediately before and 4) immediately after fitting the archwires, and 5) 4 and 6) 8 weeks after placing the archwires. 16 to 20 brackets and bands were bonded in the upper and lower jaws, while NiTi leveling arches (ODS Euro Arch Opto Therm, 0.40 mm round) were fitted. The saliva samples were quick-frozen and subsequently dried under red light. Dried residuals were dissolved in aqua regia and filled up to 3 ml. The solutions were analyzed using a mass spectrometer (Perkin Elmer Elan 5000).

RESULTS: Surface analysis revealed no differences in the degree of corrosion of the different products. In fact, we observed only extremely small and isolated corrosion defects. No statistically-significant differences were noted in the Ni ion concentration at time points 1 (reference value), 3, 5 and 6 (34, 34, 28 and 30 microg/l, respectively). The samples taken immediately after bracket bonding or the NiTi wire application however displayed a significant increase in the salivary Ni ion concentration (2: 78 and 4: 56 microg/l). It was significantly higher after bonding of the steel brackets than after NiTi wire application.

CONCLUSIONS: Increased Ni ions are released initially after the orthodontic devices have been fitted, but they decay quickly. This is reflected in miniscule corrosion defects as pitting. It is unlikely that orthodontic nickel titanium wires are a relevant additional Ni load for the patient.