Effects of fixed orthodontic treatment using conventional (two-piece) versus metal injection moulding brackets on hair nickel and chromium levels: a double-blind randomized clinical trial

Introduction: Although nickel and chromium are known as allergen and cytotoxic orthodontic metals, very few and controversial studies have assessed the effect of orthodontic treatment on their systemic levels especially those reflected by their best biomarker of exposure, hair. Additionally, metal injection moulding (MIM) brackets are not studied, and there is no study on systemic ion changes following their usage. Methods: In this double-blind randomized clinical trial, scalp hair samples of 24 female and 22 male fixed orthodontic patients [as two groups of conventional (two-piece) versus MIM brackets, n = 23 x 2] were collected before treatment and 6 months later. Randomization was carried out using a computer-generated random number table. The patients, laboratory expert, and author responsible for analyses were blinded of the bracket allocations. Hair nickel and chromium levels were measured using atomic absorption spectrophotometry. The effects of treatment, bracket types, gender, and age on hair ions were analysed statistically (alpha = 0.05, beta = 0.02). Results: In both groups combined (n = 46), nickel increased from 0.1600 +/- 0.0890 mu g/g dry hair mass (pre-treatment) to 0.3199 +/- 0.1706 (6th month). Chromium increased from 0.1657 +/- 0.0884 to 0.3066 +/- 0.1362 mu g/g. Both of these increases were significant (paired t-test, P = 0.0000). Bracket types, age, and gender had no significant influence on ion levels (P > 0.05). ANCOVA indicated different patterns of chromium increases in different genders (P = 0.033) and ages (P = 0.056). Limitations: Sample size determination should have accounted for the grouping as well. Conclusion: Hair nickel and chromium levels might increase about 185-200% after 6 months. They might not be affected by bracket types. Gender and age might not influence the baseline or 6thmonth levels of both metals. Gender might however interact with orthodontic treatment, only in the case of chromium. Registration: The research is registered offline (thesis) and online (IR. AJUMS. REC. 1394.516). Protocol: The protocol was pre-determined before any experiments begin.