Corrosion of Nickel-Titanium Orthodontic Archwires in Saliva and Oral Probiotic Supplements.

OBJECTIVES: The aim of the study was to examine how probiotic supplements affect the corrosion stability of orthodontic archwires made of nickel-titanium alloy (NiTi).
MATERIALS AND METHODS: NiTi archwires (0.508x0.508 and having the length of 2.5 cm) were tested. The archwires (composition Ni=50.4%, Ti=49.6%) were uncoated, nitrified and rhodium coated. Surface microgeometry was observed by using scanning electron microscope and surface roughness was measured by profilometer through these variables: roughness average, maximum height and maximum roughness depth. Corrosion was examined by electrochemical method of cyclic polarisation.
RESULTS: Rhodium coated alloy in saliva has significantly higher general corrosion in saliva than nitrified alloy and uncoated alloy, with large effect size (p=0.027; η2=0.700). In the presence of probiotics, the result was even more pronounced (p<0.001; η2=0.936). Probiotic supplement increases general and localised corrosion of rhodium coated archwire and slightly decreases general corrosion and increases localised corrosion in uncoated archwire, while in the case of nitrified archwire the probability of corrosion is very low. The differences in surface roughness between NiTi wires before corrosion are not significant. Exposure to saliva decreases roughness average in rhodium coated wire (p=0.015; η2=0.501). Media do not significantly influence surface microgeometry in nitrified and uncoated wires.
CONCLUSION: Probiotic supplement affects corrosion depending on the type of coating of the NiTi archwire. It increases general corrosion of rhodium coated wire and causes localised corrosion of uncoated and rhodium coated archwire. Probiotic supplement does not have greater influence on surface roughness compared to that of saliva.

Introduction

The oral cavity is a complex environment in which teeth and surrounding tissues are under constant influence of various forces caused by swallowing and chewing. Saliva composition and pH differ from person to person. Saliva contains a mixture of inorganic salts (mainly chlorides and phosphates), organic acids, enzymes, bacteria and gastric secretions (). Since the environment is very aggressive, an essential property of dental materials is their biocompatibility with surrounding tissues and the entire body. In consequence, they are expected to be resistant to mechanical stress and degradation caused by forces and the corrosive environment. Corrosion in the oral cavity causes tooth discoloration, and in some cases allergic reactions as a consequence of metal ions being released into the organism (-).

Apart from the abovementioned consequences, corrosion of metal parts of orthodontic appliances can affect their biomechanical properties, whereby it can affect appliance efficiency (). Lately, it has been recommended that, in addition to improved oral hygiene, orthodontic patients should use probiotic supplements to decrease the incidence of caries and gingivitis as the most frequent side effects of long-term orthodontic therapy. Probiotics are microorganisms that have a beneficial effect on human health (). The use of probiotic bacteria in the oral cavity has shown good results in treatment of candidiasis, halitosis, periodontitis, gingivitis and cavities (-). Orthodontic appliances are designed in such a way that the incidence of the abovementioned conditions is significantly increased in orthodontic patients when compared to general population. Orthodontic appliances make maintenance of adequate oral hygiene more difficult, whereby conditions are created for the development and maturation of biofilm and for propagation of pathogenic microorganisms around the appliance. Research of influence of Lactobacillus paracasei and Bifidobacterium animalis subsp. lactis has shown a significant reduction in the number of S. mutans in orthodontic patients (-).

Most common probiotic bacteria used for oral health are lactobacilli (). However, there is no relevant research to indicate how probiotic supplements affect corrosion stability of orthodontic archwires. It is known that different bacteria and fungi can cause metal corrosion, either directly through their metabolic influence, or by creating locally corrosive conditions. For this reason, the objective of this paper was to examine how the use of probiotic supplements affects corrosion stability of orthodontic archwires made of nickel-titanium alloy (NiTi). It has been hypothesised that, in addition to the corrosive effect of saliva on orthodontic archwires, the use of probiotic supplements during orthodontic therapy can further propagate corrosion, either generally or locally. It has been expected that the effect would manifest itself in a higher surface roughness of archwires that have been exposed to the effect of probiotic supplement. Also, it was expected that the type of coating would change susceptibility to corrosion and that nitrification would decrease, while rhodium coating would increase susceptibility to corrosion.

Materials and Methods

Testing was performed on NiTi archwires having dimensions 0.58x0.508 mm (0.020x0.020 inch), and the length was 2.5 cm (composition Ni=50.4%; Ti=49.6%), uncoated surface (BioForce Sentalloy®), nitrified surface (IonGuard®) and rhodium coated (High Aesthetic®) (Dentsply GAC, Bohemia, USA). A total of 45 wires, 15 of each type were used. They were distributed in two experimental groups and one control unexposed group. First experimental group consisted of five samples of each type of wire which were exposed to artificial saliva while the second group, also consisting of five sample of each type of wire, was exposed to artificial saliva with addition of probiotic supplement. Unexposed archwires (five samples of each archwire type) served as absolute controls. Data from the pilot study were used to calculate the size of the sample Assuming a difference in the roughness average parameter of 0.021 between two experimental conditions and taking into account a standard deviation of 0.005 for one condition and 0.013 for the other, with power of 80% and significance of α=0.05, a size of five samples per group was obtained. The calculation was done by using statistical software MedCalc 14.8.1 (MedCalc Software bvba, Ostend, Belgium).

Artificial saliva had the following composition: 1.5 g/L KCl, 1.5 g/L NaHCO3, 0,5 g/L NaH2PO4xH2O, 0,5 g/L KSCN, 0,9 g/L of lactic acid, pH 4.8 (). A pH of 4.8 was measured in one and two days old plaque, which served as a simulation of a patient with very poor oral hygiene (). Oral probiotic supplement containing bacteria Lactobacillus reuteri Prodentis DSM 17938 and ATCC PTA 5289 (BioGaia, BioGaia AB, Sweden) was added to artificial saliva of the second experimental group Each archwire sample was immersed into 1 mL of experimental solution (pure artificial saliva or artificial saliva containing dissolved probiotic supplement at the ratio of 1 lozenge to 1 mL of saliva) in plastic 1.5 mL Eppendorf test-tubes (Sigma-Aldrich, St. Louis, USA). The immersion lasted for 28 days, and solutions were changed once a week. During the first five days of testing and in order to simulate temperature variations in the oral cavity when consuming hot and cold beverages, thermocycling was performed in 2500 cycles at temperatures from 5oC to 50oC by using the Thermo Haake Willytech (SD Mechatronik Feldkirchen-Westerham, Germany) machine. A wide range of cycles and temperatures for thermocycling have been reported in the literature. Also, it has been stated that the setting simulated longer exposition of the material to the temperature extremes (). Test-tubes containing samples and experimental solution, one after another, were immersed in thermal baths for 30 seconds, with two seconds at air temperature between immersions. After that, the test-tubes with samples were stored in an incubator where they were kept at a temperature of 37oC until the end of the experiment. Unexposed archwires served as absolute control for comparison of mechanical properties. To minimise the effect of substances added to the tablet after dissolution of BioGaia in artificial saliva, the solution was filtered and the presence of probiotic bacteria in the filtrate was tested by inoculating them onto MRS agar (Sigma-Aldrich, St. Louis, USA).

The shape and depth of corrosive damage on the surface of the archwires before and after exposure to the media were determined by using a scanning electron microscope (SEM FEI Quanta-200, FEI Company, Hillsboro, USA) at 1000x magnification with the secondary electron imaging. Measurement of surface roughness was performed using the contact profilometer Talysurf CLI 1000 (Taylor Hobson Ltd., Leicester, UK). Traced profiles of the real surface were acquired with a diamond stylus of 5 µm radius. During the measurement, the stylus was moved at a constant speed across the samples with a measuring force of 1.3 mN. Five specimens from each wire type were measured, and on each sample, three variables: roughness average (Ra), maximum height (Rz) and maximum roughness depth (Rmax) were measured on three profiles, using a Gaussian filter with a cutoff value of 0.8 mm and the evaluation length of 4 mm. The arithmetic mean of two repeated measurements was used for statistical analysis. For the sake of precision control, two measurements were taken for each wire and the result of each measurement was read two times.

Since metal corrosion in the oral cavity occurs as an electrochemical mechanism, wire corrosion was examined through the electrochemical method of cyclic polarization by using a PAR263A potentiostat and a frequency analyser PAR FRD 1025 (Princeton Applied Research, Oak Ridge, USA) (). The measurements were performed in 300 ml of artificial saliva with addition of 6 probiotic tablets at a temperature of 37±2oC. Test wires were cut out and insulated with lacquer so that exposed surface was 0.61 cm2.

Polarisation curves of dependency of potential on logarithmic value of corrosion current density were used to determine corrosion current density, Icor, which is an equivalent to general corrosion rate i.e. speed at which a metal deteriorates. The incidence of local corrosion damage on the material was determined based on the shape of the polarisation curve, i. e. based on the ratio between corrosion potential, Ecor –the potential at which the lowest current density is recorded, passive oxide film breakdown potential Ebd –the potential at which a sudden increase in current density occurs, and repassivation potential Erp –the potential at which the reverse part of the curve shows that the currents become equal to those in the early stages of the cycle. At Ebd potential, the protective passive film becomes damaged and intensive dissolution of the alloy occurs, which can result in a significant release of nickel ions. The bigger the difference between Ecor and Ebd values, the lower the probability that significant damage to the passive film can occur under real conditions. Damaged passive film can be recovered (repassivated) at lower potential values in the reverse part of the polarisation curve. If repassivation takes place at potentials close to Ebd, the probability of occurrence of localised corrosion is negligible. Upon completion of electrochemical measurements, the surface of test samples was scanned by using a scanning electronic microscope with the secondary electron signal (Tescan Vega 3, Tescan, Brno, Czech Republic) at magnification 1000x.

In statistical analysis, t-test was used. Variance analysis with a Students-Newman-Keuls post-hoc test was also used. The effect size was assessed by η2. Cohen criteria were used for interpretation: η2=0.02-0.13 = small effect size, 0.13-0.26 = medium and >0.26 = large. Reproducibility of measurement was assessed by the intraclass correlation coefficient (ICC), and differences in measurements were determined by means of dependent t-test for paired samples. Statistical software IMB SPSS 22 (IBM Corp, Armonk, USA) was used.

Results

Surface roughness

Measurements of surface roughness in two places of the same wire showed a significant correspondence of all three parameters (ICC=0.693-0.875; p<0.001) and an excellent reproducibility of measurement result readings (ICC=0.997-0.999; p<0.001).

The differences between NiTi wires before corrosion are not significant. Exposure to the saliva decrease Ra in rhodium coated wire (p=0.015; η2=0.501; Figure 1). Media do not significantly affect surface microgeometry in nitrified and uncoated wires.

Figure 1

Comparison of surface roughness between NiTi archwire types and experimental conditions. Horizontal lines connect the media that produce significant differences for the same wire type.

SEM at 1000x magnification confirms the results of profilometer (Figure 2 and 3). It was observed that the surface of all wires before exposure was not entirely homogenous and that there were microdefects in the surface structure, which was most pronounced in rhodium coated wire (Figure 2). Roughness and microcracks in the form of nicks were also observed in certain places, which could be a consequence of mechanical damage during thermocycling, or of localised corrosion. After exposition to artificial saliva, rhodium coated wire looked somewhat smoother in some places (Figure 3).

Figure 2

SEM magnification 1000x of unexposed material

Figure 3

SEM magnification 1000x material after being exposed for 28 days to artificial saliva (first row), and material after being exposed to artificial saliva and probiotic supplement (second row)

Electrochemistry

Figure 4 shows polarisation curves of dependency of potential on the logarithmic value of corrosion current density. In uncoated NiTi material immersed in saliva it is possible to observe more positive values of corrosion potential and passive film breakdown potential than those recorded in saliva containing probiotic supplement. Also, in the case of pure saliva, wire repassivation occurs easily, which is not the case with the saliva containing probiotic. As is the case with uncoated wire, nitrified surface samples in saliva containing probiotic also have a more negative corrosion potential and smaller repassivation ability than those immersed in pure artificial saliva. By comparing rhodium coated wire immersed in pure saliva and the wire immersed in saliva containing probiotic, one can note that addition of probiotic increases passive film breakdown potential, but there is no possibility of repassivation in solutions.

Figure 4

Graphic presentation of curves of cyclic polarisation measurements of NiTi archwires in saliva and in saliva containing probiotic supplement

By comparing corrosion parameters in artificial saliva to those in saliva containing probiotic, we can see that the values of corrosion currents in saliva containing probiotic are lower in uncoated NiTi, equal in nitrified material and higher in rhodium coated wire than those in pure saliva. In uncoated wire, probiotic causes local corrosion. In rhodium coated wire, damage is less likely to occur than in pure saliva, but once it does occur, the protective layer cannot recover any more. In nitrified wire, damage is less likely to occur, but once it does, it is more difficult for the oxide layer to recover (Table 1).

Table 1

Values of corrosion parameters

archwire	corrosion parameter	media	- mean± SD	95% CI*	p**	η2***
NiTi	corrosion current density (Icor/ nAcm-2)	saliva	31.3±16.3	-9.1-71.7
		probiotic	19.9±8.7	-1.6-41.4	0.343	0.224
	korocorrosion potential (Ecor / mV)	saliva	76.3±20.4	25.6-127.0
		probiotic	-149.3±37.7	-243.0-(-55.7)	0.001	0.954
	repasivation potential (Erp / mV)	saliva	1205.0±11.5	1176.2-1233.8
		probiotic	-	-	-	-
	brakedown potential (Ebd / mV)	saliva	1272.7±23.7	1213.9-1331.5
		probiotic	469.7±124.6	160.0-779.3	<0.001	0.968
NNiTi	corrosion current density (Icor/ nAcm-2)	saliva	24.3±20.5	-26.7-75.3
		probiotic	28.0±20.0	-21.8-77.8	0.834	0.013
	corrosion potential (Ecor / mV)	saliva	-31.7±65.0	-193.1-129.8
		probiotic	-247.4±59.3	-394.7-(-100.0)	0.013	0.818
	repasivation potential (Erp / mV)	saliva	1193.0±29.8	1118.9-1267.1
		probiotic	729.9±14.9	692.9-767.0	<0.001	0.993
	brakedown potential (Ebd / mV)	saliva	1328.7±82.1	1124.7-1532.6
		probiotic	1317.6±76.4	1127.8-1507.4	0.873	0.007
RhNiTi	corrosion current density (Icor/ nAcm-2)	saliva	100.7±40.1	1.2-200.2
		probiotic	120.4±12.7	88.8-152.1	0.461	0.142
	corrosion potential (Ecor / mV)	saliva	162.0±34.0	77.6-246.4
		probiotic	137.0±3.7	127.9-146.1	0.273	0.287
	repasivation potential (Erp / mV)	saliva	-	-
		probiotic	-	-	-	-
	brakedown potential (Ebd / mV)	saliva	764.7±60.4	614.5-914.8
		probiotic	1033.0±283.1	329.7-1736.3	0.184	0.392

* CI: confidence interval; p: level of significance; η2: effect size.

SEM scan shows an increase in the number of microcracks in the shape of a nick on the surface of uncoated and nitrified wire after exposure to artificial saliva containing probiotic supplement, which can be linked to localised corrosion (Figure 5). In rhodium coated wire, precipitation of visible corrosion products has taken place, and one can also observe densely distributed white spots.

Figure 5

SEM magnification 1000x of unexposed wire (first row) and wire after electrochemical testing in saliva containing probiotic supplement (second row)

Discussion

Our research shows that probiotic supplement used orally for the purpose of improving oral health and maintaining favorable microbial flora influences corrosion to some extent. It was assumed that, in addition to the corrosion effect of saliva, probiotic supplement would further increase general and localised corrosion. However, this assumption is not entirely correct. In NiTi alloys with uncoated surface, probiotic supplement leads to a greater propensity for localised corrosion and smaller propensity to general corrosion. General corrosion is the most common type of corrosion. It affects all metals and develops on the entire metallic surface (). Metal is susceptible to oxidation and reduction reactions in the medium surrounding it, and, depending on the type of metal, corrosion will vary in intensity. Local corrosion is characterized by corrosion intensity being increased at the local level. It will take place if there is a nonhomogeneity in the composition of the material or of the environment. Grain boundaries in metal can be the places where corrosion starts because of their state of elevated energy. Cracks are also sensitive to corrosion considering that chemical composition in a crack is different than the one in the surrounding medium.

The oral cavity is an ideal environment for the development of corrosion, and it has been previously reported that certain species of bacteria could cause corrosion of materials containing titanium (-). Our research also showed that appearance of local corrosion could be promoted by the presence of probiotic bacteria from the BioGaia supplement. It is possible that corrosion is a consequence of absorption and digestion of metal from the alloy by bacteria, or of precipitation of insoluble components from the pastille on orthodontic archwire, in which case corrosion occurs under the precipitate. A research on the exposure of NiTi alloy to Lactobacillus reuteri culture could determine to what extent the bacterium itself is the cause of corrosion.

The structure of an orthodontic appliance provides perfect conditions for bacterial adhesion and development of a biofilm in which a complex mechanism of interaction between aerobic and anaerobic bacteria takes place, which is suitable for the occurrence of corrosion. Corrosive effect of probiotic supplements and L. reuteri on orthodontic arches has not been researched yet. Also, the mechanism of probiotic' action has not been fully explained, but it has been proven that probiotics prevent adhesion of pathological bacteria onto surfaces, change pH of the environment and the living conditions of pathogenic microorganisms, secrete antimicrobial substances and affect the host's immunological response (). The products of metabolism of probiotic bacteria could change environmental conditions and that could be another possible way of corrosion of a NiTi alloy ().

It has also been hypothesized that corrosive effect of probiotic supplements would manifest itself in the form of an increased surface roughness of uncoated NiTi wire. A correlation between corrosion of alloys and an increase in surface roughness, which directly affects mechanical properties of wires and can lead to an orthodontic wire fracture, has been proven by research (). Surface roughness affects the friction coefficient, which is an important factor of orthodontic tooth movement (-). Friction occurs as the wire slides through a bracket. The greater the friction between a bracket and the wire, the more force is required to overcome friction resistance, which eventually slows down tooth movement and extends the orthodontic therapy. But, contrary to our assumption, the parameters of micro geometric irregularities of the surface did not significantly change after exposing NiTi orthodontic archwires to saliva and probiotic supplement.

We had expected that the type of coating would change the propensity for corrosion and that surface nitrification would decrease, whereas rhodium coating would increase propensity for corrosion. As we had anticipated, this research confirmed that surface coating does change propensity for corrosion. Generally, nitrification improves, and rhodium coating decreases corrosion resistance. Probiotic supplement affects both general and localised corrosion and the effect is modified by the type of coating. In rhodium coated wire, an increase in both general and localised corrosion has been observed after probiotic supplement had been used. In uncoated wire, probiotic supplement decreases general corrosion but increases localised corrosion. In nitrified wire, probiotic did not change general or localised corrosion to a large extent when compared to the effect of saliva alone. Based on the results obtained, one can conclude that the tested probiotic supplements are safest for use with nitrified wire, while in the case of uncoated wire, there is a significant possibility of localised corrosive damage which would lead to nickel ions being released into the oral cavity. Previous research conducted on these types of wire showed similar results and reduced corrosion resistance of rhodium coated wires was explained through existence of micro galvanic cells between the noble coating and the less noble underlying surface (NiTi alloy), which occurs due to defects in the coating itself ().

Although we had assumed that corrosive effect of probiotic supplement would result in an increase in surface roughness of all the tested wires when compared to the effect of saliva alone, no significant difference in surface roughness of nitrified and rhodium coated wire was found after exposing the wire to saliva versus exposing it to probiotic supplement. Neither the medium nor the type of coating have significant effect on surface roughness of nitrified and rhodium coated wire, although previous research showed increased roughness of rhodium coated and polymer coated NiTi wires when compared to uncoated NiTi (, ).

Conclusion

Probiotic supplement affects general and local corrosion depending on the type of coating of the NiTi archwire. It increases general corrosion of rhodium coated wire and causes localised corrosion of uncoated and rhodium coated archwire. Surface nitrification improves corrosion resistance, while rhodium coating decreases it. Probiotic supplement does not have greater influence on surface roughness compared to that of saliva..