Methods: Twenty-eight identical samples were used, each set corresponding to one half-maxillary arch. Samples were divided in 2 groups (14/group). The first group was ligated with Ormco NiTi archwires (USA) and the second with Smart NiTi archwires (China) with elastomeric ligatures. Different compositions and surface characteristics of the two archwires were noted. Half sets of each group were immersed in 50 ml artificial saliva at pH 5.14, and the other half at pH 6.69. Ni release was quantified with the use of flame atomic absorption spectrophotometry. Analysis of variance was determined on days 1, 4, 7, 9, 14, 21 and 28 comparing Ni release between the groups and Student t-test determined the difference between pH.
Results: Both NiTi Ormco and Smart continuously increased Ni release at time intervals at both pH levels. The NiTi Ormco had more Ni release at pH 6.69 than pH 5.14 but NiTi Smart had more Ni release at pH 5.14 than 6.69. At 4 w, the Ni release of one half-maxillary arch was 1.221 ppm (1221 µg/l) at pH 5.14, 1.267 ppm (1267 µg/l) at pH6.69 for NiTi Ormco and 2.175 ppm (2175 µg/l) at pH 5.14, 0.676 ppm (676 µg/l) at pH 6.69 for NiTi Smart. No significant difference was found in Ni release from Ormco and Smart NiTi at pH 5.14 and pH 6.69, except on days 14, 21 and 28, Smart NiTi showed a significant difference (p<0.05) at pH 6.69.
Conclusions: Ni release depends on compositions and surface characteristics of archwires and pH.