Treating Ti containing dental orthodontic wires with nitrogen plasma immersion ion implantation to reduce the metal ions release and bacterial adhesion

Many dental orthodontic wires are produced from alloys that are prone to the release of metal ions and the adhesion of bacteria during orthodontic treatment. To reduce these effects, this study implanted nitrogen ions into the surface of four different commercial Ti containing dental orthodontic wires via nitrogen plasma immersion ion implantation (NPIII) treatment. The wires contained Ti-Mo-Zr-Sn, Ni-Ti-Cu, Ni-Ti and Ti-Nb alloys respectively. Wires with and without nitrogen ions implantation were then immersed in artificial saliva with added bacteria (Streptococcus salivarius or clinical plaque). Following a 28 day immersion period, we analysed the release of metal ions into the immersion solution using atomic absorption spectrometry. We also determined the quantity of bacteria attached to orthodontic wires using the turbidity measurement method. Results revealed that NPIII treatment led to the formation of titanium nitride (TiN) in the surface film of the orthodontic wires. The presence of the TiN containing film on the NPIII treated wires reduced both the release of metal ions and the adhesion of bacteria, compared to the untreated wires. Regardless of whether NPIII treatment was performed, Ni-Ti-Cu and Ni-Ti wires had more metal ions release but less bacterial adhesion than did Ti-Mo-Zr-Sn and Ti-Nb wires. Our results demonstrate that implanting nitrogen ions into Ti containing dental orthodontic wires is an effective way to block the release of metal ions and retard bacterial adhesion.