Chromium release from new stainless steel, recycled and nickel-free orthodontic brackets

To test the hypothesis that there is no difference in the amounts of chromium released from new stainless steel brackets, recycled stainless steel brackets, and nickel-free (Ni-free) orthodontic brackets. This in vitro study was performed using a classic batch procedure by immersion of the samples in artificial saliva at various acidities (pH 4.2, 6.5, and 7.6) over an extended time interval (t(1) = 0.25 h, t(2) = 1 h, t(3) = 24 h, t(4) = 48 h, t(5) = 120 h). The amount of chromium release was determined using an atomic absorption spectrophotometer and an inductively coupled plasma atomic emission spectrometer. Statistical analysis included a linear regression model for repeated measures, with calculation of Huber-White robust standard errors to account for intrabracket correlation of data. For post hoc comparisons the Bonferroni correction was applied. The greatest amount of chromium was released from new stainless steel brackets (0.52 +/- 1.083 mug/g), whereas the recycled brackets released 0.27 +/- 0.38 mug/g. The smallest release was measured with Ni-free brackets (0.21 +/- 0.51 mug/g). The difference between recycled brackets and Ni-free brackets was not statistically significant (P = .13). For all brackets, the greatest release (P = .000) was measured at pH 4.2, and a significant increase was reported between all time intervals (P < .002). The hypothesis is rejected, but the amount of chromium released in all test solutions was well below the daily dietary intake level.