Nickel Release from Ni-Ti-based Orthodontic Wires in Acidic Test Solution

Aim: The aim was to evaluate the amount of Ni released from three different NiTi-based orthodontic wires during corrosion in an acidic test solution. Occasional Ni allergy has been observed with the clinical use of wires with similar compositions.

Methods: The following Ormco rectangular, 0.019” x 0.025,” arch wires were evaluated: (1) NiTi, (2) 27˚C NiTiCu, and (3) 40˚C NiTiCu. Wire 1 contains equiatomic proportions of Ni and Ti. Ni is replaced with 5-6% of Cu in 2 and 3 (Brantley, 2001). Segments of each wire with a nominal surface area of 5.4 cm² (n=3 for each wire) were immersed in a chloride/lactic acid solution (pH=2.2) at 37˚C according to the ISO static corrosion test procedure. Ni released was measured by the ICP-OES technique. Pre- and post-corrosion specimen surfaces were examined by SEM.

Results: The concentrations of Ni released (µg/cm², mean ± st. dev.) from different wires were: (1) 3.209 ± 0.022, (2) 2.274 ± 0.088, and (3) 2.416 ± 0.019. The mean value for 1 was significantly (p<0.001) higher than those for 2 and 3. The means for 2 and 3 were not statistically different from each other. Surface defects such as scratches, pits, and inclusions were present in all wires in their as-received condition; they were more numerous in 2 and 3. Post corrosion wire surfaces showed very mild dissolution effects but no evidence of localized corrosion. Gradual build up of gas bubbles occurred on the surface of each wire during corrosion.

Conclusion: The ternary NiTiCu wires, is spite of their larger surface defects, released less Ni in an acidic solution than the binary NiTi wire with fewer defects.