Ion release and cytotoxicity of stainless steel wires

SUMMARY Heat treatment is generally applied to orthodontic stainless steel (SS) wires to relieve the stresses that result from their manipulation by orthodontists. The quality and thickness of the oxide fi lms formed on the surface of heat-treated wires can vary, and it is believed that these oxide fi lms can infl uence the properties of heat-treated wires. The aim of this study was to investigate the infl uence of heat treatment and cooling methods on the amount of metal ions released and to examine the cytotoxicity of heat-treated wires. In this study, four types of SS wires (Remanium, Permachrome, Colboloy and Orthos) with a cross-sectional area of 0.41 × 0.56 mm were investigated. These wires were heat-treated in a vacuum, air, or argon environment, and were cooled in either a furnace or a water bath. Four control groups and 24 experimental groups were classifi ed according to the type of wires, heat treatment conditions and cooling methods. In each group, the amount of nickel released as well as its cytotoxicity was investigated. The concentration of dissolved nickel ions in artifi cial saliva was measured for a period of up to 12 weeks. In all groups, the concentration of dissolved nickel ions in artifi cial saliva was lowest for the vacuum heat treatment-furnace cooling group and a signifi cant difference was shown compared with the other experimental groups. The concentration of dissolved nickel ions in artifi cial saliva was highest in the groups heat-treated in air (P < 0.05), while the amount of nickel released was highest in the Remanium and Colboloy (P < 0.05). The cytotoxicity was mild in all the experimental groups but the response index of the air groups was slightly higher than in the other groups. According to these results, SS wires retain their high corrosion resistance and low ion release rate when heat-treated in a vacuum and cooled in a furnace