Modelling of nickel release dynamics for three types of nickel-titan orthodontic wires: nickel release dynamics assessment.

Rationale: The main goal of the present study is to evaluate the optimal duration of safe intraoral use (based on Nickel Release hypothesis) for austenitic NiTi, heat-activated NiTi, heat-activated CuNiTi types of orthodontic wires, analysing the dynamics of Nickel Content (NC) as a function of intraoral usage duration. Materials and Methods: The analysed dataset contains several wires of each type, divided into four states: S0 as-received-autoclaved, S1 as-received, S2/S3 retrieved after (less than six weeks/more than eight weeks) intraoral use. The NC is assessed via analysis in several visually different areas along each wire. To quantify the NC for statistical analysis a global (average over all samples along the wire) and a local (over most corroded areas) measures are used. To model NC dynamics, the results are approximated in time domain by sigmoid functions. Results: Statistical analysis performed on the global measure has not shown significant changes in the NC between S0, and S1 states as well as between S1, and S2/S3 states. Thus, the global measure has confirmed and extended previously published results to the examined types of wires. In contrast, the local measure analysis has revealed statistically significant changes between S1, S2, S3 states. A model of NC dynamic behaviour has been constructed over the local measure, and group-level optimal durations of use have been proposed. Conclusions: If further extended, the obtained model may become a tool in the orthodontists' hands to decide about the wire's optimal duration of use on a patient-specific level.