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 Table of Contents  
Year : 2018  |  Volume : 10  |  Issue : 5  |  Page : 224-228

Nickel, copper, and chromium release by CuNi-titanium orthodontic archwires is dependent on the pH media

Department of Dental Materials, São Leopoldo Mandic Research Institute, Campinas, São Paulo, Brazil

Date of Web Publication24-Oct-2018

Correspondence Address:
Prof. Roberta Tarkany Basting
São Leopoldo Mandic Research Institute, Department of Dental Materials, Rua José Rocha Junqueira, 13, Bairro Swift, Campinas, São Paulo
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jioh.jioh_186_18

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Aim: The aim of this study was to evaluate the concentrations of nickel (Ni), copper (Cu), and chromium (Cr) ions released by Ni archwires with the addition of Cu to their composition CuNi-titanium in neutral and acid media. Materials and Methods: The 0.016” cross-section heat-activated archwires evaluated were Ni titanium (Ti) Memory Wire (American Orthodontics), Damon Optimal-Force Cu Ni-Ti (Ormco), Tanzo Cu NiTi (American Orthodontics), and Flexy NiTi Cu (Orthometric). Ten archwires were used from each commercial brand, obtaining two segments measuring 2 cm (centimeters) from the most posterior portion of the archwires. The archwires remained immersed in neutral or acid solutions for a time interval of 7 days. The samples were analyzed by Graphite Furnace Atomic Absorption Spectrometry to evaluate Cr and by Inductively Coupled Plasma Atomic Emission Spectrometry to evaluate Ni and Cu. Two-Way Analysis of Variance (ANOVA) and the Tukey tests were applied to the data. Results: The immersion of the NiTi Memory Wire and the Flexy NiTi Cu archwires in both solutions presented significantly higher mean Ni concentration values than the other archwires (P = 0.0001). The immersion of the Tanzo Cu NiTi and the Flexy NiTi Cu archwires in the neutral solution yielded a higher concentration value for detectable Cu than that of the other groups (P = 0.0055). In the acid solution, only Damon Optimal-Force Cu Ni-Ti presented no experimental segment with detectable Cu concentration. Cr release could not be quantified. Conclusions: Metal ion release by archwires is dependent on the commercial brand and the immersion solution.

Keywords: Chromium, copper, copper NiTi archwires, nickel, orthodontic wires

How to cite this article:
Furlan TP, Barbosa JA, Basting RT. Nickel, copper, and chromium release by CuNi-titanium orthodontic archwires is dependent on the pH media. J Int Oral Health 2018;10:224-8

How to cite this URL:
Furlan TP, Barbosa JA, Basting RT. Nickel, copper, and chromium release by CuNi-titanium orthodontic archwires is dependent on the pH media. J Int Oral Health [serial online] 2018 [cited 2022 Aug 17];10:224-8. Available from:

  Introduction Top

The mechanical properties, superelasticity, and shape memory of nickel (Ni) titanium alloys in the composition of orthodontic archwires have made them widely used, especially in the initial stages of treatment for leveling and alignment of the teeth.[1],[2] These alloys contain Ni and titanium (Ti) ions in a relatively similar ratio by weight: 44% Ti and 56% Ni.[1]

Orthodontic heat-activated archwires containing copper (Cu) in their composition (CuNi-titanium [CuNiTi]) promote more biological tooth movement.[2] The clinical advantages of CuNiTi over NiTi archwires include the generation of more constant forces, longer periods of activation, greater resistance to permanent deformation, more stable characteristics of superelasticity, and lower hysteresis.[1],[2]

The chemical composition of CuNiTi archwires corresponds to approximately 50.7% Ni, 42.4% Ti, and 6.9% Cu.[1] However, the CuNiTi archwires from different manufacturers do not necessarily have similar properties, because manufacturing conditions are not consistent.[1],[2] The degree of manufacturing defects and the composition of the orthodontic devices are factors also known to accelerate the process of corrosion and ion release by the archwires,[3],[4],[5],[6] considering that unstable chemical structures may cause ion release and affect the biocompatibility of these materials.[7]

Orthodontic archwires may remain in the patient's oral cavity for a prolonged period, lasting from 18 to 24 months; therefore, degradation of these wires is likely, bearing in mind the influence of changes in pH (hydrogen ionic potential), temperature, and constant archwire fatigue.[3],[4],[5],[6],[7],[8],[9] Changes in pH have been recognized as having a significant influence on ion release, and acid conditions produce a higher rate of ion release due to the reduced unoxidizable acid pellicle needed to provide resistance to corrosion.[3],[4],[5],[8] Among the ions released, those of Ni has been far more extensively investigated[3],[4],[5],[6],[7],[9],[10],[11],[12],[13],[14] than those of Cu.[3],[14],[15] Not only is the Ni released by orthodontic devices considered toxic[10],[11],[12],[15],[16],[17] but Cu also tends to be susceptible to galvanic corrosion, in addition to being capable of causing allergic reactions and toxic effects on cells when its concentration in a tissue exceeds the recommended level.[15]

Considering the long orthodontic treatment time and the incidence of allergic and toxic effects of metals from orthodontic appliances,[12],[17],[18] which may be dependent of pH media,[8] further studies are important to evaluate the concentration of ions released by archwires containing Cu, and the influence of the ions released by the storage media. Therefore, the aim of this study was to evaluate the concentration of Ni, chromium (Cr), and Cu ions in artificial saliva, released by CuNiTi archwires into different (neutral and acid) storage media.

  Materials and Methods Top

This study was dispensed from evaluation by the Research Ethics Committee from the São Leopoldo Mandic Research Institute (#2016/0614) since no studies were conducted with the use of human subjects. The materials used in the experiment, and their respective characteristics and manufacturers are shown in [Table 1]. The orthodontic archwires were obtained from round, precontoured maxillary archwires, 0.016” in diameter. Ten archwires from each commercial brand were used. This sample size was based on Ortiz et al.[12] Two archwire segments measuring 2 cm were obtained from the most posterior straight extremities of each arch, as used by Huang et al.[10] The two segments were immersed in a closed plastic receptacle containing titanium of each neutral or acid solution (n = 10). The solutions were prepared as those used by Serra and Cury.[19] The neutral solution contained 1.5 mM Ca (Calcium), 0.9 mMP (Phosphorus), 20 mM Tris buffer, and 150 mM potassium chloride and were pH 7.0. The acid solution contained 2 mM Ca, 2 mM P, and 74 mM acetate buffer and were pH 4.3.
Table 1: Materials used in research and their respective specifications

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The archwires remained immersed in the solution for a time interval of 7 days, on an agitator table (DragonLab, SK0330-Pro, DragonLab Laboratory Instruments Limited, Beijing, China), at a speed of 120 rpm (rotations per minute), within an acrylic tub with a thermostat that kept the temperature at 37°C. After this, the archwires were removed from the tubes and the samples were frozen in a freezer at −18°C (Consul, CRM 32 A, Consul do Brasil, São Paulo, SP, Brazil) until the chemical analyses were performed.

The samples were analyzed by the Inductively Coupled Plasma Atomic Emission Spectrometry (Model ICAP 6300, Thermo Scientific) to determine the Cu and Ni concentrations. The minimum detection limit of the equipment for each chemical element is 3 μg/L for Cu and 2 μg/L for Ni. Afterward, the solutions were analyzed for Cr concentration, by the Graphite Furnace Atomic Absorption Spectrometry technique (model 6.500, Thermo Jarrel Ash). The minimum detection limit of the equipment for Cr is established at 0.5 μg/L.

The data were submitted to statistical analysis using frequency distribution tables, descriptive statistics, and the Fisher's exact test. Exploratory Ni data analysis indicated performing logarithmic transformation so that the data would meet the presuppositions of a parametric analysis. After transformation, the analysis of variance (ANOVA) with a 4 × 2 factorial scheme (archwire vs. solution) was applied, followed by the Tukey test, adopting a 5% level of significance (SAS Institute Inc., Cary, NC, USA, Release 9.2, 2010).

  Results Top

The concentration of the chemical element Cr was below the inferior limit of detection of the equipment, and concentrations lower than 0.05 μg/L were verified for all the archwires analyzed in both neutral and acid solutions.

The ANOVA test showed that the interaction of archwire and solution was significant (P = 0.0001). The concentration of Ni was higher in the acid solution, for all the archwire brands [Table 2]. In both solutions, the NiTi Memory Wire and Flexy NiTi Cu archwires presented significantly higher mean Ni concentration values than the other archwires. In the neutral solution, the Tanzo Cu NiTi archwire presented a significantly higher mean value than the Damon Optimal-Force Cu Ni-Ti archwire. The Cu concentration presented a significant difference between the wires and the solutions (P = 0.0055) [Table 3]. A higher percentage of experimental segments with detectable Cu concentration was observed for the Cu-containing archwires of the Tanzo Cu NiTi and the Flexy NiTi Cu brands in the neutral solution than the percentage for the other groups (P = 0.0055). Among the Cu-containing archwires, Damon Optimal-Force Cu Ni-Ti presented no experimental segment with detectable Cu concentration in the acid solution.
Table 2: Mean value (standard deviation) of nickel concentration (μg/L), considering archwire and solution

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Table 3: Distribution of frequencies of groups according to detectable copper concentration (μg/L)

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  Discussion Top

This results of this study showed differences in the amount of ions released among the archwires and immersion media. Among the archwires with the addition of Cu, Flexy NiTi Cu was the archwire that presented the highest release of Ni and Cu, followed by Tanzo Cu NiTi archwire and Damon Optimal-Force Cu Ni-Ti. It was not possible to quantify Cr release from any of the archwires. The highest Ni release occurred from archwires immersed in an acid medium, whereas the highest release of Cu occurred in a neutral medium.

These results may be explained by some aspects of the methodology employed, as the immersion time used in this study. The immersion time was 7 days, based on various studies[6],[20] that demonstrated that peak ion release occurred on the 7th day, after which it diminished over time. This may be attributed to two probable factors: (1) the Ni present on the archwire surface rapidly undergoes corrosion during the first 7 days and (2) after this period, corrosion products are formed on the archwire surface, thus substantially hindering Ni ions release in the subsequent periods.[14],[21]

As a limitation, the Cr concentration could not be quantified, because it was below the detection limit of the graphite furnace atomic absorption spectrometer. The authors could consider that the amount of Cr present in the NiTi archwires was low in comparison with that of Ni, based on an estimate of approximately 50% Ni and 18% Cr.[1] This low Cr result can be attributed not only to the detection limit of the appliance but also to the immersion time applied in this study, in both the acid and the neutral media. Although this immersion time was sufficient for evaluating the release of other chemical elements, especially Ni, several studies have shown an increase in the levels of Cr from 4 to up to 8 weeks of treatment.[3],[22] Barret et al.[21] showed that Cr release increased progressively up to day 28, but in 7 days, the amount of Cr released was 236 times lower than that of Ni, which may justify the results of the present study. Nayak et al.[23] found a significant increase in Ni and Cr after the final stage of alignment. After 10–12 months of treatment, the Ni values diminished; however, the concentration continued to increase. Kumar et al.[24] also showed that Ni presented a gradual increase during the first 10 days, and from then on presented a gradual decline up to day 30, whereas Cr presented a gradual and statistically significant increase during the 30 days of the experiment.

As regards Ni, a higher concentration was verified in the acid solution for all the archwire brands. This result corroborates those of other in vitro studies that evaluated ion release into different solutions, showing that the acid immersion media increased the release of ions.[4],[8],[9],[10],[11],[25] The low pH intensified the cathodic reaction to corrosion. The postulated mechanism is that the passive protective layer of titanium oxides formed on the archwire surfaces is dissolved by the action of protons in an acid pH, leading to the release of metal ions.[4],[10]

In both solutions, the NiTi Memory Wire and Flexy NiTi Cu archwires presented significantly higher mean values for Ni concentration than those of the other archwires. The values for Ni released from the Ni-titanium archwire were higher than those for Ni released from the archwires with the addition of Cu, as expected. This could be attributed to the fact that the addition of a small quantity of Cu to the structure of the archwires reduced the reactivity of titanium and increased the resistance to corrosion and biocompatibility of the CuNiTi archwires.[5] Kuhta et al.[25] also verified that the NiTi superelastic archwires released a higher concentration of Ni than the heat-activated NiTi archwires. Nevertheless, because Flexy NiTi Cu is a type of Cu-added archwire, it presented Ni release in an amount similar to that of the Ni-titanium archwire. The chemical composition of the CuNiTi archwires corresponds to approximately 50.7% Ni, 42.4% titanium, and 6.9% Cu.[1] However, the manufacturing process of archwires containing Cu in their composition uses a sensitive and not very consistent technique,[2] which could accelerate the process of archwire corrosion.[6]

In regard to the other brands of Cu-containing archwires, the authors verified that the Damon Optical Force Cu Ni-Ti presented a lower value of Ni release into both the neutral and acid solutions. The wire with the 2nd lowest Ni release values was the Tanzo Cu NiTi archwire in both neutral and acid solutions. This result could also be attributed to the archwire manufacturing process. Resistance to corrosion is a fundamental aspect of biocompatibility and may be affected by different factors.[8] The first depends on the manufacturing process, the type of archwire, and the characteristics of the archwire surface.[8],[13],[24],[26] The studies by Huang et al.[4] and Huang et al.[10] concluded that the NiTi orthodontic archwires from different manufacturers would have different degrees of resistance to corrosion.

In regard to Cu concentration, differences were also observed relative to release into different immersion media. The NiTi archwires had no Cu in their composition.[1] However, in the neutral solution, the authors verified that 10% of the Ni-Ti Memory archwire samples evaluated in the study presented release of Cu, which may have occurred due to some contamination of the sample. In regard to the Cu-containing archwires, 20% of the experimental segments with detectable Cu concentration were observed for the Flexy NiTi Cu and Tanzo Cu NiTi archwires in the acid solution. The Tanzo Cu NiTi and Flexy NiTi Cu archwires presented higher values of detectable Cu release (50% and 100% of the sample, respectively), which may be justified by both the manufacturing process and the Cu concentration in these archwires. In the CuNiTi archwires, the concentration of Cu ions ranged between 5.5 and 6.9%.[1]

The process of corrosion seems to affect the surface roughness and biocompatibility of these materials,[7] since the physical properties of orthodontic archwires are affected by corrosion, and cause a reduction in the clinical performance of the material. Superficial corrosion of NiTi arches may increase the friction that appears at the interface between the arch and bracket and that reduces the free sliding action during orthodontic treatment.[6] According to Suárez et al.,[20] CuNiTi archwires were the type that presented the most severe changes in terms of mean surface roughness, which changed from values considered smooth to values considered rough, as compared with the original archwires and those submitted to the immersion test in saline solution.

In relation to biocompatibility, the estimated lethal daily dose is known to be 300 μg (micrograms) for Ni, 200 μg for Cr, and 3 mg (milligrams) for Cu.[21] The results of the present study showed that the concentrations of Ni, Cr, and Cu released by the archwires were lower than those considered toxic, thus corroborating the findings of other studies.[3],[14] Although the amount of metals released from the archwires was low, appeared to represent no biological risk, and was within the limits of biocompatibility, orthodontic archwires are not used alone in mechanotherapy. They are always used with other orthodontic accessories that could release more metal.[3],[14] Although the large number of studies on the release of metal ions by orthodontic archwires has shown that the amount of ions released is far below the recommended daily doses, low levels of metal ions are known to be capable of changing cellular metabolism and morphology and even cause instability of DNA.[12],[15],[18] Furthermore, it must be considered that the most significant exposure of human beings to these ions occurs during food consumption, and that there are other sources of ions to which patients are exposed on a daily basis, such as drinking water, clothing, fine or costume jewelry, piercings, toothpastes, mouthwashes, foods rich in sugar, juices, and soft drinks.[6],[14],[15],[16],[26]

Considering that the archwires in the oral medium will be cyclically exposed to neutral and acid media, and to the clinical needs demanded by orthodontic treatment, the authors could suggest that, among the archwires with Cu addition analyzed in this study, the Damon Optimal-Force Cu Ni-Ti archwires were the type that presented the lowest values of metal ion release; however, all the archwires evaluated in the present study presented results that showed values below the levels considered toxic to human beings. Clinical studies are necessary to evaluate the metal ions concentrations in patients undergoing orthodontic treatment with these types of archwires associated with different dietary habits.

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There are no conflicts of interest.

  References Top

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Gopikrishnan S, Melath A, Ajith VV, Mathews NB. A comparative study of bio degradation of various orthodontic arch wires: An in vitro study. J Int Oral Health 2015;7:12-7.  Back to cited text no. 3
Huang HH. Surface characterizations and corrosion resistance of nickel-titanium orthodontic archwires in artificial saliva of various degrees of acidity. J Biomed Mater Res A 2005;74:629-39.  Back to cited text no. 4
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Nayak RS, Khanna B, Pasha A, Vinay K, Narayan A, Chaitra K, et al. Evaluation of nickel and chromium ion release during fixed orthodontic treatment using inductively coupled plasma-mass spectrometer: An in vivo study. J Int Oral Health 2015;7:14-20.  Back to cited text no. 23
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Lages RB, Bridi EC, Pérez CA, Basting RT. Salivary levels of nickel, chromium, iron, and copper in patients treated with metal or esthetic fixed orthodontic appliances: A retrospective cohort study. J Trace Elem Med Biol 2017;40:67-71.  Back to cited text no. 26


  [Table 1], [Table 2], [Table 3]

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