The study of corrosion behaviour of NiTi orthodontic wires in different oral cavity locations

Abstract

For two reasons corrosion of NiTi wires is important: biocompatibility concerns due to high nickel contents and effect of corrosion on unique mechanical properties of them. Wires in different intra-oral locations serve in various biologic and mechanical conditions: forces from occlusion and engagement in slots, saliva composition and flow, and even temperature. And none of rare retrieval studies had studied the effects of these parameters. We decided to be more specific about these factors and to the study corrosion behavior on NiTi wires used under different conditions at different locations of buccal cavity. Materials and Methods: After meticulous criteria, 15 participants demanding orthodontic treatment were entered the study, four NiTi SE(G&H) wires were used for each one: two inserted in bracket slots and band tubes and two passively in auxiliary tubes of molars. After 2 months wires were retrieved and were compared with as-received wires regarding clinical conditions (tarnish, deformation). Then they were cut in half from midpoint and electrochemical impedance spectroscopy test was performed on distal segment of wires. Rp (polarization resistance) values were evaluated. Non-parametric tests of independent samples were performed in order to compare these groups. Results: All retrieved wires showed signs of tarnish that was more intense in lower arch. Among all retrieved wire, 5 wires showed deformation. Mean Rp in all retrieved groups were significantly lower than as-received wires also wires had significantly lower Rp when they were engaged in lower arches(p0.4). Again, difference between upper and lower arch was not statistically significant (p=0.08). Conclusion: 1) Retrieved wires showed clinically & electrochemically significant corrosion compared with as-received wires, especially engaged wires and in lower arches. 2) Corrosion in all groups of retrieved wires was significantly higher than as-received wires (p0/05). 5) Wires in upper arch will have less corrosion from lower arch so expensive surface treatment methods might not be so critical for arch wires produced for upper arches. Alternatively, retrieved wires from upper arch they can be recycled and used for engagement in lower arch without extra concern; especially in the same patient.