Effect of fixed orthodontic therapy on urinary nickel levels: a long-term retrospective cohort study

Nickel constitutes about 8-60 % of orthodontic alloys. It is known as an allergenic/cytotoxic trace metal. Therefore, it should be investigated in patients undergoing orthodontic treatment which might last for 2 or 3 years. However, no controlled studies have assessed the influence of orthodontic treatments of longer than 5 months on its systemic levels. Thus, the aim of this retrospective cohort study was to evaluate systemic nickel in patients undergoing orthodontic therapy for a minimum period of 1 year. In this study, urinary nickel concentrations in 20 female and 10 male patients being treated with stainless steel appliances were measured using atomic absorption spectrophotometry. The same procedure was done on a control group of the patients' same-gender near-age siblings (n = 30). The effect of treatment and gender on urinary nickel levels were assessed using a repeated-measures two-way analysis of variance (ANOVA) and a Tukey test (α = 0.05). The mean treatment duration was 17.1 ± 6.4 months (range, 12-21). The mean nickel concentrations in male and female patients were 9.67 ± 3.25 and 9.9 ± 3.83 μg/L, respectively. These statistics for male and female control subjects were 6.65 ± 2.57 and 8.43 ± 2.94 μg/L, respectively. The ANOVA showed a statistically significant difference between the urinary nickel levels of the treatment and the control groups (P = 0.009) but not between the genders (P = 0.194). The interaction between gender and treatment was also nonsignificant (P = 0.337). The Tukey test indicated that the increase in nickel was higher in male patients, in comparison to their brothers (P < 0.05). It could be concluded that orthodontic therapy for longer durations with stainless-steel archwires might elevate slightly, but significantly, urinary nickel levels.