Abstract
Stainless steels (SS) are the most-used alloys for manufacturing fixed orthodontic appliances due to their attractive set of mechanical properties, biocompatibility, and high corrosion resistance. Nevertheless, during regular orthodontic treatments–taking at least around 2 years–the intraoral environment inevitably degrades these bioalloys, releasing metallic ions into the oral cavity. In the first part of this in vitro study, the corrosion resistance of commercial SS appliances (brackets, tubes, and bands) was evaluated in Fusayama-Meyer artificial saliva at pH values of 2.3 and 6.8 over the course of 30 days. As expected, the results corroborated that salivary pH highly influences corrosion behaviour. Released Ni, Cr, and Fe were within dietary intake values. In the second part, a novel approach for oral corrosion prevention based on the chemical inertness of DLC materials is presented. SS surfaces were functionalized with biocompatible a-C:H-sputtered coatings and submitted to the same experimental conditions. The anticorrosion ability of this system was demonstrated, preventing the pitting corrosion that occurred on the SS substrates. Despite the galvanic coupling effect due to the presence of the Cr-based interlayer, this study enhanced the potential use of the reactive sputter-deposited a-C:H coatings in orthodontics.
Highlights
The increased rate of metallic element release is evident in the decreasing pH of the saliva; Ni is released in a higher rate than Cr for all orthodontic components; Ni release during the sampling period is significantly higher for brackets in comparison to tubes and bands at pH = 2.3
Orthodontic appliances may be an additional source of potentially toxic elements that should not be disregarded [39], for patients hypersensitive to Ni
The results showed that: (i) salivary pH clearly influenced corrosion behaviour; (ii) brackets were, by far, the main source of metallic ions into the saliva; (iii) Ag welding found on bands may have influenced the corrosion behaviour, regardless the pH value of the saliva; (iv) released Ni was within daily dietary intake values
Summary
The European Union (EU) forbids the use of Nickel (Ni) [1]: Accepted: 22 October 2021. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in (b) published maps and institutional affiliations. «in any post-assemblies which are inserted into pierced ears and other pierced parts of the human body unless the rate of nickel release from such post-assemblies is less than 0.2 μg/cm2 /week (migration limit)»;. «in articles intended to come into direct and prolonged contact with the skin
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