Comparison of Chemical Properties and Ni Release of Stainless Steel and Nickel Titanium Wires

Objective: This study aimed to determine the composition and corrosion resistance of stainless steel and nickel titanium (NiTi) archwires and to compare Ni released from simulated standard fixed orthodontic appliances ligated with stainless steel and NiTi wires in artificial saliva at pH 5.14 and pH 6.69. in 4 weeks at 37 degrees C. Materials and Method: Two commercial rectangular wires, 0.016 x 0.022 " in size were studied, one Ormco stainless steel wire and another Ormco nickel titanium wire. Their composition was analyzed by an energy disporsive X,ray spectrometer (ESC) and their corrosion resistance was evaluated by a potentiodynamic polarization technique. For Ni release, the twenty-eight simulated fixed appliance sample sets were used, each corresponditg to one half-maxillary arch. The samples were divided in 2 groups (14 sets per group). The first one was ligated to Ormco stainless steel archwires and the other one to Ormco NiTi archwires with elastomeric ligatures. Half sets of each group were immersed in 50 ml artificial saliva at pH 5.14 and the other half were immersed at pH 6.69. Ni release was quantified using flame atomic absorption spectrophotometry. Statistical analysis of variance (t-test) was determined on days 1, 4, 7, 9, 14, 21 and 28 comparing the amount of Ni released between groups. Results: Stainless steel wire was composed of Ni, Cr, Si, Mn and Fe while the nickel titanium wire was composed of Ti, Ni and Cr. NiTi wire has more percentage of Ni and therefore has less corrosion resistance than stainless steel wire. For Ni release,. stainless steel and NiTi continuously increased at the time intervals for both pH levels. For group one, stainless steel wire had more Ni release at pH 5.14 and for the other group, NiTi wire had more Ni-release at pH 6.69. At 4 weeks, the Ni release of one half-maxillary arch was 1.383 ppm (1383 mu g/l) at pH 5.14 and 1.079 ppm (1078 mu g/l) at pH 6.69 for stainless steel wire while it was 1.221 ppm (1221 mu g/l) at pH 5.14 and 1.267 ppm (1267 mu g/l) at pH 6.69 for NiTi wire. No significant difference was found in the amount of Ni release from stainless steel and NiTi wires at pH 5.14 and pH 6.69 in the different time intervals except on day 1 wherein the stainless steel archwire was significantly different (p<0.05) at pH 5.14 and pH 6.69. Conclusions: The NiTi wire had more Ni contain and less corrosion resistance than stainless steel wire. Stainless steel had more Ni release at pH 5.14 than pH 6.69 but NiTi had greater Ni release at pH 6.69 than pH 5.14.