Assessment of corrosion processes in steel orthodontic archwires – in vitro studies

Introduction
Both clinicians and researchers studying biocompatibility of medical materials have been interested in corrosion of elements of orthodontic appliances made of metal alloys. The oral cavity environment is highly moist, subject to temperature changes and pH changes depending on beverage and food consumed, and it favours degradation of dental materials.

Aim
The aim of the study was to assess the susceptibility of steel orthodontic archwires to corrosion processes.

Material and methods
The study evaluated orthodontic archwires with the cross section of 0.017 by 0.025 inch (0.04 by 0.06 cm) by Dentaurum (Germany) and Ortho Classic (USA), made of alloy steel. Topography of surface of study specimens was assessed before and after tests using a scanning microscope. Corrosion resistance of materials was assessed with impedance and potentiodynamic testing. Specimens of steel orthodontic archwires were placed in a solution of artificial saliva at 37°C, in which corrosion processes were induced using a system of three electrodes. The obtained results were subject to a statistical analysis at the significance level of p = 0.05.

Results
The analysis of impedance test results indicates that orthodontic archwires supplied by the American manufacturer were characterised by slightly higher corrosion resistance. For Dentaurum archwires, the average resistance of the electrical double layer is 4.43 × 106 Ω/cm2, and is slightly lower compared to the resistance observed for Ortho Classic wires (Rt = 6.12 × 106 Ω/cm2). However, this trend has not been confirmed by potentiodynamic studies.

Conclusions
1) Orthodontic archwires made of stainless steel evaluated in the study were subject to corrosion, regardless of the material manufacturer. 2) The process of assessing the physical properties of medical materials allows to use products with the lowest biological damage potential in clinical practice.