Metal Release and Surface Degradation of Fixed Orthodontic Appliances during the Dental Levelling and Aligning Phase: A 12-Week Study

Abstract

The purpose of the present study is twofold: (i) to assess the salivary nickel, chromium, and iron concentrations and (ii) to characterize the surface microstructure of the typical commercially available Ni-containing metallic appliances during the first 12-week orthodontic treatment period. A total of 85 unstimulated saliva samples were collected from patients before treatment, after 2 days, and after 1, 4, and 12 weeks. Salivary ion concentrations were determined by inductively coupled plasma optical emission spectroscopy, and data were analyzed with the Statistical Package for Social Sciences (IBM SPSS) software. The recorded mean metal concentrations were in the ranges of 132–175 µg/L for Ni, 171–192 µg/L for Cr, and 826–1023 µg/L for Fe. No statistically significant variations were observed between the different study times, and the null hypothesis (the concentrations of metallic ions in patients’ saliva did not significantly change after the placement of the orthodontic appliances) was accepted (p > 0.05). Mean salivary metallic ions were below toxic levels, and no adverse clinical reactions were registered. The intraoral surface degradation of the fixed components was corroborated by optical microscopy, scanning electron microscopy, and energy dispersive spectrometry. Microstructural analysis after complete orthodontic procedure confirmed different corrosion types, from pitting to biocorrosion.

Corrosion of Fixed Orthodontic Appliances: Causes, Concerns, and Mitigation Strategies

“…Recovered SS brackets [110] and archwires [106] unequivocally revealed highly adherent biofilm deposits (Figure 9a), under which pitting or crevice corrosion occurred [106,116], as presented in Figure 7 for brackets after two years of intraoral use. Calcium-precipitating bacteria on SS orthodontic wires cause surface calcium deposits, into which chlorine ions penetrate and induce pitting [130].…”

“…Pitting corrosion is a localized type of corrosion that initiates on metallic surfaces when the protective passive film disrupts due to mechanical and/or electrochemical attack [93,100], leading to the formation of pit holes and/or cavities. This attack has been associated with other corrosion types [114,116], such as the one caused by the welladherent biofilm that forms during orthodontic treatments (Figure 7)-discussed further in Section 2.4.9. Aggressive ions in saliva, such as chlorine and fluoride [29,42,117]-especially under acidic conditions [101,104,118]-and food additives, such as certain spices [119,120], effectively damage surface protective oxides.…”

“…Accordingly, oral biofilms create differential concentration cells on the metallic surfaces of three main types: oxygen concentration cells, metal concentration cells, and active-passive cells (Figure 7). [116]; (b,c) pitting corrosion induced by sulphate-reducing bacteria: confocal scanning microscope images after a 56day in vitro test using D. nigrificans bacteria on 316LV steel; and (d) representation of the proposed corrosion mechanism by [128] (reproduced from [73]).…”

“…and optical micrographs coupled to energy dispersive (EDS) chemical compositions of two distinct zones: SS alloy and pitting morphology underneath a biofilm layer. Adapted from [116]; (b,c) pitting corrosion induced by sulphate-reducing bacteria: confocal scanning microscope images after a 56-day in vitro test using D. nigrificans bacteria on 316LV steel; and (d) representation of the proposed corrosion mechanism by [128] (reproduced from [73]).…”

“…Calcium-precipitating bacteria on SS orthodontic wires cause surface calcium deposits, into which chlorine ions penetrate and induce pitting [130]. Abundant biofilm layers were also observed on NiTi archwires [116] (Figure 9b), in spite of the good resistance against MIC corrosion [131,132] of the Ti-based alloys. Still, dental plaque accumulates and might enhance crevice corrosion in the welding gaps of Ti-based brackets [50,98].…”