Abstract
Abstract Aim To characterise the mechanical properties and galvanic coupling of Ni-free orthodontic brackets with stainless steel (SS) and Nickel-Titanium (NiTi) orthodontic wires. Methods Three Ni-free bracket types (Topic [TOP], Equilibrium [EQU] and Orthos [ORT] made of Ni-free alloys), one conventional (Mini 2000 [MIN]) made of SS alloy and an SS and a NiTi wire were examined in the present study. All brackets and wires were embedded in epoxy resin and, after metallographic grinding and polishing, the Martens hardness (HM), the indentation modulus (EIT), and the elastic index (ηIT) were recorded, employing Instrumented Indentation Testing (IIT) by monitoring force over indentation depth curves during a loading-unloading cycle. The galvanic coupling of all bracket-wire combinations was tested in 0.1M NaCl-0.1M lactic acid and 0.3% (wt.) NaF solutions by noting the potential differences over 48 hours. The mechanical properties were statistically analysed by one-way ANOVA and Tukey multiple comparison tests at alpha = 0.05. Results Significant differences were identified in the mechanical properties of the materials tested. The TOP (2372 ± 182 N/ mm2), ORT (wing) (2418 ± 164) and SS wire (2302 ± 85) showed significantly higher HM compared with all other materials tested. The MIN (base) (1115 ± 81) and ORT (base) (1237 ± 101) showed the lowest HM while MIN (wing) (1520 ± 138), EQU (1620 ± 139) and NiTi wire (1526 ± 42) demonstrated intermediate HM values. The ORT (wing) (101 ± 6 GPa) displayed the highest EIT while NiTi wire (24 ± 5) showed the lowest. The latter had the highest elastic index (59 ± 5%) with MIN (base)(15 ± 3) possessing the lowest. The potential difference for all bracket wire combinations was found below the threshold for the initiation of galvanic corrosion (200 mV) apart from MIN coupled with NiTi wire in the NaF solution. Conclusions The mechanical properties of Ni-free brackets are significantly different compared with the SS bracket assessed. Galvanic testing revealed that conventional and Ni-free brackets are compatible with both SS and NiTi wires in media containing chloride and fluoride ions.
Highlights
The extensive usage of Nickel (Ni)-containing stainless steel (SS) orthodontic brackets in clinical practice and related biocompatibility issues are well documented and a highly investigated area of concern.[1,2,3] Manufacturers have attempted to overcome nickel reactivity by developing Ni-free fixed appliances based on Cobalt-Chrome (CoCr) and Titanium (Ti) alloys
Different force– indentation depth curves were recorded for Mini 2000 (MIN) (Figure 1A) and ORT (Figure 1D) for base and wing regions, while similar curves were acquired for TOP (Figure 1B), EQU (Figure 1C) and the orthodontic wires (Figure 1E and 1F)
The indentation modulus and elastic index were included; the former is the elastic modulus derived by the unloading curve and the elastic index is indicative of the relative ductility of the alloy tested
Summary
The extensive usage of Nickel (Ni)-containing stainless steel (SS) orthodontic brackets in clinical practice and related biocompatibility issues are well documented and a highly investigated area of concern.[1,2,3] Manufacturers have attempted to overcome nickel reactivity by developing Ni-free fixed appliances based on Cobalt-Chrome (CoCr) and Titanium (Ti) alloys. The replacement of SS brackets by other alloys has overlooked the effect of galvanic corrosion as a contributing factor to intraoral metal deterioration and ion release This is a biological concern related to all heavy metals used in orthodontics including brackets, wires and soldering alloys.[8,9,10,11] Galvanic coupling is generally associated with a pair of dissimilar metallic materials capable of acting as an electron source when a current flows from one material to the other through an electrolytic medium. The corrosive environment as well as the corrosive resistance and stability of an orthodontic alloy play a major role.[16,17,18]
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