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Journal of Oral Biology and Craniofacial Research logoLink to Journal of Oral Biology and Craniofacial Research
. 2020 Aug 5;10(4):435–440. doi: 10.1016/j.jobcr.2020.07.006

Effect of fluoride agents on surface characteristics of NiTi wires. An ex vivo investigation

Prasad Chitra a,∗, GS Prashantha b, Arun Rao c
PMCID: PMC7426570  PMID: 32817814

Abstract

Aim

To analyze the degree of corrosion of nickel titanium arch wires in patients with and without exposure to fluorides.

Material and methods

This was an ex vivo study comprising of 60 subjects undergoing fixed orthodontic treatment. Group 1(controls) comprised of 30 sets of new unused NiTi wires and unused 11, 15 brackets, Group 2(patients) comprised of 30 sets of non fluoridated NiTi wires and 11, 15 brackets and Group 3(patients) had 30 sets of fluoridated NiTi wires and 11, 15 brackets. NiTi wires were used over 6 months of treatment(0.014″,0.016″, 16 × 22”, each wire was used for 2 months and replaced with the next size). All wires were retrieved, stored and analyzed. At 6 months, brackets from 11 to 15 were debonded in both treatment groups. Archwires and brackets in 3 groups were subjected to SEM analysis at 500 and 1000X to observe differences. Additionally, EDX Spectroscopy was undertaken to evaluate surface elemental compositional differences in groups.

Results

Significant differences among groups were evident in brackets and archwires tested. Maximum degradation, cracks and dark spots were seen in wires and brackets exposed to fluoride agents. EDX spectroscopy revealed least Ni% in fluoridated wires and brackets.

Conclusions

Increased leaching of metal ions was evident when wires and brackets are exposed to fluoride agents during treatment. Use of non fluoridated mouthwash and toothpastes may be considered in orthodontic patients without risk of caries to mitigate such effects

Keywords: NiTi, SEM, Corrosion, EDX Spectroscopy, Fluoride

1. Introduction

Fixed appliances for correction of malocclusion are the preferred method for most orthodontic patients. Brackets used in orthodontics are predominantly metal alloys of which iron, cobalt, chromium, nickel and manganese predominate.1, 2, 3 Brackets have also been manufactured using titanium alloys in order to reduce the nickel content in conventional stainless steel alloys.

Arch wires used in orthodontics are available in several types and compositions ranging from alloys of stainless steel, chrome cobalt, nickel titanium, titanium molybdenum etc. In recent years, alloys made with nickel titanium started to get popular in orthodontics due to their favorable properties enabling rapid alignment of teeth with minimal force. NiTi alloys used in orthodontics generally are composed of 55% Ni and 45% Ti.4 Various types of NiTi alloys have been introduced over time with additions of Cu and Cr to enhance certain mechanical properties. Their exact composition is generally not available. Brackets used in orthodontics remain till treatment end, and are removed after treatment. Orthodontic arch wires in contrast, are replaced every few months as alignment of teeth is accomplished. During the period of treatment, both brackets and arch wires are exposed to the intra oral environment which is humid due to saliva and of variable pH and temperature. Additionally, food and drinks consumed are of very variable pH. The saliva in the mouth is an electrolyte, with metal orthodontic appliances behaving like a cell. The whole system undergoes electrochemical processes resulting in corrosion. The corrosion process is further enhanced due to the presence of microorganisms which cause rapid degradation of metals.5,6 Corrosion can occur due to loss of metal ions into solution directly or due to the protective surface oxide layer degrading slowly. Corrosion occurs due to simultaneous oxidation and reduction reactions taking place, which are called redox reactions. In general, once the corrosion process begins, it continues till the metal gets fully consumed, or a surface protective layer due to passivation is formed or till full consumption of reactant. The solvent type in which the metal is immersed(in this case saliva) will determine the amount of corrosion that can take place. Alloys commonly used in orthodontics resist corrosion by forming passive surface oxide films. These are formed on both brackets as well as arch wires. This surface protective layer can be damaged on account of several factors like mechanical stresses, acidic conditions etc. Corrosion is again of several types like uniform attack, pitting corrosion7, 8, 9, crevice corrosion10,11, galvanic corrosion12, fretting corrosion, fatigue induced corrosion13 and microbial corrosion14

Fluorides in mouthwashes and toothpaste are frequently recommended during treatment for orthodontic problems to reduce risk of dental decay.15 Fluoride is a known corrosive agent where sodium fluoride from toothpastes and mouthwashes reacts with bacterial products with formation of hydrofluoric acid(HF) which causes dissolution of the surface protective oxide layer causing corrosion in both brackets and wires.16 Evidence showing decreased corrosion resistance of metals containing titanium when exposed to fluoride environments is available.17,18 Fluoride causes a breach in the protective titanium oxide layer enabling the corrosion process to begin. Cytotoxicity of metals is an important factor in overall biosafety. Metallic ions, when released intra orally, are absorbed in saliva, gingival crevicular fluid and also ingested. Local effects could be damage to DNA.19 Hypersensitivity to chromium and nickel may also be caused due to such exposure to metal ions.20

Exposure to fluorides on brackets and arch wires in clinical conditions and their likely effects has not been evaluated adequately. In vitro experiments on reaction of metals to fluoride agents have been carried out with variable results, due primarily to the nature of the experimental set up. Mimicking intra oral conditions in vitro is difficult and caution is required when interpreting results of such studies. Fluorides are known to cause more release of metal ions in clinical situations during orthodontic treatment.21 The objective of the study was to make comparisons of brackets and arch wires in different groups of patients using non fluoridated and fluoridated oral hygiene products. Detailed analysis of wires and brackets in both clinical situations, when compared to unused brackets and wires, would throw light on any differences. The findings of such a study would be of immense clinical importance where technical guidelines on safe concentrations of fluoridated oral hygiene agents could be decided.

2. Material and methods

The sample size was finalized using G Power software version 3.1.9.2 after performing a power analysis. Considering the effect size to be measured (f) at 40%, power of the study at 80% and the margin of the error at 5%, the total sample size needed was 66. The final sample size is rounded off to 75. So, each study group comprised of 25 samples. An additional 5 samples per group were included to consider drop outs if any.

Inclusion criteria was patients requiring non extraction therapy with all teeth present till second molars without restorations. Patients taking medications, smokers and those who had previous orthodontic treatment were excluded. The institute ethical board approved the design of the study and written informed consent was obtained from all participants prior to starting treatment.90 sets of NiTi archwires were used. There were 3 groups as under:

Group I - 30 sets of new unused NiTi archwires and brackets( 0.014″, 0.016″, 16 × 22” wires and unused 11 and 15 maxillary arch brackets.

Group 2–30 sets of NiTi archwires and brackets used in non fluoridated toothpaste patient group.

Group 3–30 sets of NiTi archwires and brackets used in fluoridated toothpaste and mouthwash patient group.

All patients had similar fixed appliances placed in a predetermined wire sequence till the 6 month study period was completed, in order to make suitable comparisons. Fixed orthodontic appliances were bonded in all patients in both arches simultaneously. Mini Twin 0.022 slot steel brackets(Ormco Corporation, Glendora, CA) with Enlight light cured adhesive(Ormco Corporation, Glendora, CA) were placed. Only Nickel Titanium archwires were used in the study. The arch wires used were 0.014″ NiTi, 0.016″ NiTi and 16 × 22″ NiTi(Tru-Arch Align NiTi; Ormco), with each wire being used for two months before being replaced with the next higher dimension archwire to eliminate confounders. Oral hygiene maintenance protocols were individually taught to all patients before commencement of treatment. Group 2 patients used only non fluoridated toothpaste(Dabur Red, Dabur India Ltd) for oral hygiene maintenance twice daily. Patients in Group 3 used fluoridated mouthwash twice daily(Colgate Plax, 225 ppm fluoride, Colgate Palmolive Co, India) and a fluoridated toothpaste twice daily(Colgate Strong Teeth, 1000 ppm fluoride, Colgate Palmolive Co, India).

All NiTi archwires were retrieved carefully after two months of intraoral use, rinsed in distilled water and preserved till analysis. After 6 months of treatment, one bracket from the maxillary right central incisor and second premolar was debonded and used for analysis. New brackets were immediately rebonded and treatment continued as usual. The brackets were cleaned using distilled water and stored in labeled plastic bags till they were analyzed.

Scanning Electron Microscopy was used to evaluate all 3 groups of wires and brackets to observe surface characteristics at 500X and 1000X magnification. Comparisons were made between new unused wires and brackets with those in Groups 2(non fluoridated) and 3(fluoridated) which had been used intraorally. Additionally, Energy Dispersive X-ray Spectroscopy(EDX) was also performed to evaluate surface qualitative elemental microanalysis of each wire and bracket. The EDX spectrum, mass(Wt) and atomic(At) percentage of chemical composition was evaluated for all surfaces of samples analyzed.

3. Results

3.1. Scanning Electron Microscopy(SEM)

The surface morphologies of unused 0.014 NiTi, non-fluoridated 0.014 NiTi and fluoridated 0.014 NiTi are shown in Fig. 1. The topography of all unused 0.014" (Fig. 1a) wires shows characteristic striations derived from the drawing process. The surfaces of unused NiTi arch wires show reduced surface irregularities as compared to non-fluoridated (Fig. 1b) and fluoridated arch wires (Fig. 1c). Plaque deposits were noted on superficial surfaces of non fluoridated arch wires. Maximum degradation was noted in fluoridated arch wires in form of increased pitting, striations and crack formation. Also of note, is the appearance of dark areas on the surfaces of fluoridated arch wires which depict higher amounts of surface degradation.

Fig. 1.

Fig. 1

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SEM surface morphology of a) unused, b) non fluoridated & c) fluoridated 0.014″ NiTi.

The surface morphologies of unused 0.016″ NiTi, non fluoridated and fluoridated 0.016″ NiTi are shown in Fig. 2. The surfaces of unused NiTi arch wires (Fig. 2a) show less surface irregularities as compared to non fluoridated (Fig. 2b) and fluoridated arch wires (Fig. 2c). Superficial irregularities were noted on the surface of non-fluoridated arch wires. Increased amount of corrosion was noted in fluoridated arch wires in the form of globular defects on the wire surface and crevice formation.

Fig. 2.

Fig. 2

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SEM surface morphology of a) unused, b) non fluoridated & c) fluoridated 0.016″ NiTi.

The surface morphology of unused 0.016 × 0.022″ NiTi, non fluoridated and fluoridated 0.016 × 0.022″ arch wires are shown in Fig. 3. The topography of all unused 0.016 × 0.022" (Fig. 3a) wires show characteristic striations derived during the manufacturing process. In non fluoridated 0.016 × 0.022″ arch wires (Fig. 3b), pitting was noted along the entire length. Surface striations were increased in number in fluoridated arch wires (Fig. 3c) depicting increased degradation. Increased amount of deposits and crack formation were noted in fluoridated arch wires. White spots were noticeable in fluoridated arch wires which appear to be inclusions that were revealed by action of fluoride agents.

Fig. 3.

Fig. 3

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SEM surface morphology of a) unused, b)non fluoridated & c) fluoridated 16 × 22″ NiTi.

The surface morphology of unused incisor brackets, non-fluoridated and fluoridated brackets are shown in Fig. 4. The topography of all unused incisor brackets (Fig. 4a) show characteristic white dots on the surface derived during the manufacturing process. In non fluoridated incisor brackets (Fig. 4b), pitting was noted on the entire surface. Surface striations and dark black spots were observed in fluoridated brackets (Fig. 4c) depicting increased degradation due to exposure to fluoride.

Fig. 4.

Fig. 4

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SEM surface morphology of a) unused, b) non fluoridated & c) fluoridated incisor bracket.

The surface morphology of unused premolar brackets, non fluoridated and fluoridated brackets are shown in Fig. 5. The topography of all unused premolar brackets (Fig. 5a) show characteristic white dots on the surface derived by manufacturing processes similar to that seen on incisor brackets. In non fluoridated premolar brackets (Fig. 5b), surface irregularities were reduced as compared to fluoridated brackets (Fig. 5c). Surfaces of fluoridated brackets depicted a mixture of white and black spots indicating increased degradation and irregular surfaces.

Fig. 5.

Fig. 5

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SEM surface morphology of a) unused, b)non fluoridated & c)fluoridated premolar.

Energy-dispersive X ray spectroscopy or EDX is an analytical method for determining the chemical characterization of a sample. It works on the principle that each element has a unique atomic structure which shows up as peaks on the electromagnetic emission spectrum which is also the defining principle for spectroscopy. EDX Spectroscopy of unused NiTi arch wires depicted Nickel and Titanium as major chemical elements in alloys with uniform distribution throughout the surfaces analyzed, ensuring similar wire properties at any point on the surface. Non -specific elements like carbon, oxygen, silicon, chlorine, aluminum and calcium in smaller percentages were also observed. On fluoridated and non fluoridated arch wire surfaces, additional elements like sodium and phosphorus were observed, confirming presence of organic deposits after intra oral exposure. The percentage of nickel was least in fluoridated arch wires indicating leaching of Ni ions intra orally on exposure to fluoridated mouthwashes and toothpastes(Table 1).

Table 1.

EDX Spectroscopic analysis of unused, non fluoridated & fluoridated NiTi wires.

Elements
 Unused 0.014 NiTi
 Unused 0.016 NiTi
 Unused 16*22 NiTi
 Non-fluoridated 0.014 NiTi
 Non-fluoridated 0.016 NiTi
 Non-fluoridated 16*22 NiTi
 Fluoridated 0.014 NiTi
 Fluoridated 0.016 NiTi
 Fluoridated 16*22 NiTi
Wt% At% Wt% At% Wt% At% Wt% At% Wt% At% Wt% At% Wt% At% Wt% At% Wt% At%
Ni 40.02 18.92 47.45 16.96 46.23 27.68 35.77 12.45 43.54 15.67 45.58 18.26 31.54 12.54 34.52 23.62 38.62 26.94
Ti 31.46 18.23 29.87 16.58 37.13 27.24 25.4 12.29 25.4 12.29 31.41 18.22 28.73 15.42 34.67 23.07 36.49 26.41
C 23.18 53.57 22.59 50.01 12.41 36.31 28.66 55.28 28.66 55.28 45.76 45.76 23.44 50.17 15.82 41.96 12.01 34.68
O 5.34 9.27 9.72 16.16 3.64 8 14.05 19.71 12.97 18.78 10.22 17.76 11.12 17.87 5.22 10.39 5.12 11.1
Al 0.59 0.77 0.16 0.12 1.08 0.93 0.38 0.37 0.31 0.37 0.5 0.64
Si 0.15 0.14 0.19 0.16 0.55 0.51 0.16 0.18
P 0.16 0.12
Cl
Ca 0.22 0.14 0.26 0.22
K 0.19 0.16
Fe
Na 0.3 0.41
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EDX spectroscopy of NiTi wires depicted Ni and Ti to be specific elements. Ni % was highest in unused wires (Fig. 6a) followed by non fluoridated NiTi (Fig. 6b) with least % noted in fluoridated NiTi (Fig. 6c). The EDX spectroscopy findings were similar for all groups of NiTi wires tested. Other elements like potassium and aluminum were noted in non fluoridated arch wires indicative of organic adhesions. There was no change in % of titanium content in all the 3 tested arch wire types proving that titanium remains stable and does not degrade on exposure to fluoride.

Fig. 6.

Fig. 6

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EDX Spectroscopy elemental analysis of 0.014″ NiTi wires used in the study. Maximal %.

4. Discussion

Biosafety of orthodontic materials is of importance due to the relatively long period of time such products are used in the mouth. The mouth is a harsh environment for most materials since they are constantly subjected to the forces of mastication, foods with variable acidic concentrations and pH, saliva, temperature variations and microorganisms. Saliva additionally behaves like an electrolyte making the entire orthodontic appliance a cell and encouraging electrochemical processes that predispose the appliance to corrosion and metal ion release. Nickel and chromium are known carcinogens and are primary constituents in orthodontic brackets and arch wires due to the favorable properties they impart.22,23 Understanding the complex in vivo environment and effects on such biomaterials is difficult. Majority of studies for this reason have been in vitro or animal based. Dental materials used in the mouth undergo gradual corrosion causing degradation in their mechanical properties with release of metal ions into the mouth.24 When brackets and wires corrode, surface roughness also increases with increase in friction, ultimately affecting the efficiency of orthodontic treatment. Long term consequences of increased surface roughness on archwires may not be significant as most wires are replaced within a 3 month time frame. Leaching of metal ions from these wires constitutes a more serious problem.

Several methods have been employed by manufacturers to reduce the phenomenon of metal ion release from brackets and arch wires. Alloy substitution where certain specific metals are added to reduce the corrosion potential have been tried. Brazing of orthodontic brackets has been replaced with welding using lasers to reduce corrosion potential.25 Coating wires and brackets with titanium nitride or epoxy resin was carried out.26 Titanium aluminum nitride(TiAIN) was used as a coating on β Ti arch wires which were more susceptible to corrosion. Coated and uncoated wires were immersed in fluoride solutions where the coated wires showed superior properties and reduced corrosion.27

The manufacturing technique and finishing and polishing processes of brackets and arch wires can also affect corrosion properties. Brackets with similar composition have been shown to have different corrosion properties in some studies.28 Corrosion inhibitors in the form of salivary proteins, amylase and ɣ-globulin form protective films on surfaces of brackets and arch wires and act similar to corrosion inhibitors.29,30 Arch wires which were retrieved after use and analyzed had a proteinaceous surface biofilm comprising of organic compounds including amides, alcohols, and carbonates. Other constituents included crystalline precipitates of sodium chloride, potassium chloride and calcium phosphate. The mineralized layer was thought to impart greater resistance to corrosion and could also be an explanation as to why metallic orthodontic appliances show differing results in vivo as compared to in vitro test methods.6 Maximum leaching of metal ions from orthodontic brackets and NiTi wires was noticed at the 30 day interval(samples tested before, at 30 days, 3 months and 6 month intervals) in a previous study.21 A possible explanation for this phenomenon could be the formation of biofilms and crystalline precipitates on bracket surfaces providing an increased amount of protection.

Analysis of 0.014″, 0.016″ and 0.016 × 0.022″ NiTi wires in this study showed interesting results. In all 3 wire types, maximum degradation in the form of pitting, striations, crack formation, white spots and globular defects was observed in wires exposed to fluoride agents during the clinical treatment phase. The damaging effects of fluoride were evident under SEM evaluation. Changes in surface topography of NiTi arch wires exposed to commercial fluoride agents was observed in another study which also recommended that this should be taken into consideration when friction was a concern.31 SEM analysis of brackets and wires in another study evaluating fluoride effects, showed more lines and grooves in fluoridated wires as compared to other groups in similarity to findings in our study.32

Examination of unused, non fluoridated and fluoridated brackets under SEM showed fluoridated brackets with maximum signs of surface degradation. White and black spots with surface degradation changes were evident in this group, reinforcing the assumption of fluorides affecting brackets on long term exposure. In order to minimize the leaching of Ni and Cr from stainless steel brackets on fluoride exposure, evaluation of Ti brackets was carried out. However, results of an in vitro study of Ti brackets and their responses to long term fluoride exposure showed that resistance to corrosion decreased in the presence of fluoride in high concentrations.33

EDX spectroscopic analysis also served to reinforce findings of the SEM analysis. The percentage of Ni and Ti was highest in unused NiTi followed by non fluoridated and fluoridated wires. The fluoridated wires showed the least amount of Ni and Ti confirming leaching of these ions on exposure to fluoride agents. Other elements like silicon, calcium, potassium, aluminum etc were noted on some arch wires which were most likely from intra oral sources like saliva.

5. Conclusions

The study has proven release of metal ions from brackets and arch wires under clinical use conditions. The amount of metal ions released showed an increase when orthodontic appliances were exposed to fluoride agents during the period of use. Release of metal ions could cause oxidative stress and genotoxic damage to buccal mucosal cells if left unchecked. However, it should be kept in mind that several variables are responsible for overall effects in actual in vivo conditions. Type and pH of saliva, temperature variations, type of food and masticatory stresses the appliances are subjected to, biofilm formation in individual patients conferring additional corrosion resistance and the different manufacturing processes of NiTi arch wires. In cases where risk of caries developing during orthodontic treatment is low(reasonable arch alignment, good enamel quality, no pre existing carious lesions or restorations with good oral hygiene status), non fluoridated toothpaste and mouthwashes could be more beneficial in reducing the risk of metal ion release and change in surface properties of brackets and arch wires.

5.1. Study limitations

The amounts of metal ions released showed a definitive increase in the fluoridated wire and bracket group as compared to the non fluoridated group. In general, dietary effects on alteration of surface characteristics of orthodontic appliances are very difficult to identify. Food varies in type and composition and breaks down and deteriorates very quickly. Some studies investigating the effects of diet on corrosion properties and frictional resistance of orthodontic wires used food simulating liquids(FSLs) to mimic actual food types. Heptane, citric acid and ethanol solutions were used in place of fatty foods, citric type of acidic fruits and alcoholic drinks. It was found that citric acid food types showed the greatest changes affecting corrosion resistance and frictional properties of arch wires.34 This particular aspect was again difficult to verify in individual patients and may have to be investigated in greater detail for better understanding of the metal release process.

Declaration of competing interest

Nil.

Contributor Information

Prasad Chitra, Email: prasadchitra@yahoo.co.uk.

G.S. Prashantha, Email: prashantha.od.ds@msruas.ac.in.

Arun Rao, Email: raoar.od.ds@msruas.ac.in.

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