Effect of fluoride toothpaste on the physical, chemical, and mechanical properties of NiTi orthodontic archwires: an in vitro study

Abstract

Background

Fluoride is used to prevent both primary and secondary dental caries. Using fluoride-containing products is strongly correlated with the corrosion of orthodontic archwires. The aim of this study was to examine the physical, chemical, and mechanical properties of nickel-titanium (NiTi) orthodontic archwires affected by exposure to fluoride-containing toothpaste.

Methods

According to the study protocol, sixty round 0.014″ diameter archwires were tested. The study sample consisted of three groups, each containing twenty archwires. One experimental group used toothpaste without fluoride, while the other used toothpaste containing 1448 ppm fluoride. The control group was not exposed to any toothpaste.

Both study and control groups were divided into two subgroups, incubated for 120 min and 240 min at 37 °C. Study groups were incubated in artificial saliva mixed with toothpaste, while the control group was immersed in distilled water. Surface topography analysis was performed by scanning electron microscopy (SEM), and images were taken at 2,500×, 5000×, and 10,000× magnification. The chemical properties were evaluated using energy-dispersive X-ray spectroscopy (EDS). The mechanical properties were assessed using a universal testing machine with a tensile test. Statistical analysis was performed using SPSS 29.0.

Results

Compared with the non-fluoride and control groups, the fluoride group archwires increased the amount of corrosion and caused more surface degradation with deeper dark scratches. Compared with non-fluoride and fluoride groups, the surfaces of unused wires are less prone to surface irregularities. The strength analysis revealed that the applied force remained consistent across all study groups at the point of archwire breakage, with no statistically significant differences. However, after 240 min of incubation, the fluoride toothpaste group exhibited a significantly greater decrease in elongation compared to the other groups (42.2%) (p < 0.001). Moreover, chemical analysis after 240 min of exposure revealed the greatest decreases in titanium ions (19.23%) and nickel ions (15.97%) in the fluoride group compared to the other groups (p > 0.05).

Conclusions

Fluoride group archwires exhibited more corrosion, surface degradation, the weakest elasticity, and fewer titanium and nickel ions. Prolonged exposure to toothpaste also affects the mechanical properties of NiTi archwires, significantly reducing elongation, suggesting that archwires should be replaced more frequently during treatment.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12903-026-08056-5.

Introduction

Malocclusions are noticeable deviations from ideal occlusions that are considered unsatisfactory from an aesthetic or functional perspective [1]. In addition to dental caries and periodontal diseases, malocclusion is one of the most prevalent dental problems, with a frequency ranging from 39% to 93% [2]. Untreated malocclusion has a negative impact on quality of life, as severe cases are often associated with pain, social disability, and functional limitations, which affect emotional and social well-being [3–5]. The objectives of orthodontic treatment are to improve the appearance and function of teeth, improve psychological well-being, and minimize the risk of potential future issues linked to malocclusion, such as tooth wear and gum problems [6].

Orthodontists choose from a wide range of orthodontic appliances [7], but braces are widely used for orthodontic treatment [8]. During orthodontic treatment with braces, maintaining proper oral hygiene is essential; therefore, the selection of orthodontic appliances should be guided by the patient’s initial oral health condition [8]. Orthodontic archwires align maloccluded teeth by exerting mechanical forces [9]. Due to their advantageous mechanical and clinical characteristics, nickel-titanium alloys are among orthodontic wires’ most widely used materials [10, 11]. NiTi archwires offer a wide range of low-force activation and deflection due to their spring-back properties and pseudoelasticity, which are their most prominent features [10].

NiTi alloys have a passivation layer that shields the metal from external fatty agents and corrosion. However, fluoride ions can dissolve this layer, reducing corrosion resistance and releasing metal ions into the physiological environment [12]. Because of the destruction of the passivation layer, the friction between the wire and the bracket accelerates the ion release process and encourages electrochemical corrosion [12–15]. Moreover, Qassar et al. found that the use of hydrogen peroxide mouthwash on NiTi archwires increases nickel ion release over time, likely due to the peroxide’s ability to degrade the protective surface layer [16]. The use of fluoride-containing products, such as toothpastes and mouthwashes, appears to be strongly correlated with the corrosion of orthodontic archwires [17]. On the other hand, Qasim et al. reported that fluoride toothpaste can reduce medication-induced tooth erosion by forming calcium fluoride, which gradually releases fluoride ions during acid exposure. This emphasizes the importance of fluoride use, especially in young children undergoing fixed orthodontic treatment [18].

One of the most common side effects associated with orthodontic treatment is a significantly increased risk of enamel demineralization, which often presents as white spot lesions or dental caries [19–13]. Fluorides can enhance the process of tooth remineralization because fluoride ions favor the creation of calcium fluoride globules. For this reason, it is essential to use oral products containing fluorides regularly [20–21]. In orthodontic patients, the regulation of salivary pH is important, as studies have demonstrated a significant decrease in pH within the first month of initiating fixed orthodontic therapy. This acidic oral environment has been shown to increase archwire surface roughness, which may contribute to delays in orthodontic treatment [22]. Some studies have examined how fluorinated compounds can alter some of the main mechanical properties of orthodontic alloys based on their acidity. Daily fluoride application at a relatively acidic pH of 4 was shown to have an impact on the load-deflection characteristics of NiTi orthodontic wires [20].

Mimicking intraoral conditions in vitro is difficult, and more research is needed to confirm these changes [23]. Having knowledge of possible changes in archwire mechanical properties and surface topography in the oral environment would help clinicians use these wires more effectively [24].

This study aimed to analyze the changes in the physical, chemical, and mechanical properties of NiTi orthodontic archwires with and without exposure to toothpaste containing fluoride. Our hypothesis is that prolonged exposure of NiTi orthodontic archwires to fluoride-containing toothpaste will result in a decrease in elongation and force at break, alterations in surface topography, and increased release of nickel and titanium ions.

Materials and methods

The study sample consisted of 60 round 0.014″ diameter archwires (Dentaurum rematitan^® LITE ideal arch, round).

The sample size for this study was calculated according to the following formula:

𝑛 = required sample size

𝑁 = total number of specimens

𝑍 = Z-score for 95% confidence level (1.96)

𝑝 = estimated proportion of the specimens

𝐸 = margin of error, set at 0.05 (5%)

Calculations were performed using G*power software, version 3.1.9.2. The minimal sample size required is 10 archwires for each group. Based on this calculation, a total of 60 archwires were divided evenly among six groups for analysis.

According to the study protocol, archwires were divided into three groups: two study groups and one control group. The first experimental group (Group 1) (n = 20) was placed in toothpaste without fluoride (Ecodenta expert sensitive whitening toothpaste with Omyadent), and the second (Group 2) (n = 20) was placed in toothpaste with 1448 ppm fluoride (Ecodenta whitening toothpaste with mint oil and sage extract). The control group (Group 3) (n = 20) consisted of unused archwires not exposed to toothpaste. Each wire sample was placed in 10 ml of experimental solution (composed of 8 ml of toothpaste and 2 ml of artificial saliva) or distilled water (control group) [25]. The wires were placed in different toothpastes at a constant temperature of 37.5 °C. Half of the archwires were incubated for 120 min, while the other half were incubated for 240 min. An exposure duration of 120 min is equivalent to 1 month of 4 min of daily toothbrushing, whereas 240 min corresponds to 2 months of 4 min of daily toothbrushing, as described by Walker et al. [26]. After exposure to the toothpastes, the experimental group archwires were washed with deionized water and placed in clean and individually color-coded containers. Each evaluation of the surface topography and chemical properties was repeated three times to reduce the risk of test errors.

Surface topography analysis

The surface topography was analysed using an SEM (Cal Zeiss EVO MA10 scanning electron microscope, United Kingdom). The wire specimens were fixed on SEM specimen stubs and then placed in the SEM with a standardized tilt angle. Photographs were taken at 2,500×, 5000×, and 10,000× magnification.

Chemical properties testing

The chemical properties were analysed using X-ray spectrometry. The study was performed using energy dispersive X-ray spectroscopy (EDS) with an XFlash^®6 silicon drift detector (SDD) (Bruker QUANTAX 200 EDS with XFlash^®6 SDD, United Kingdom) with nickel (Ni) and titanium (Ti) target radiation at 20 kV. Energy dispersive X-ray spectrometry evaluated the mass percentage (wt%) of the chemical composition of all archwires surfaces.

Mechanical properties testing

The mechanical characteristics of the NiTi orthodontic archwires were evaluated using tensile tests to determine the maximum tensile strength. The maximum elongation (until the sample breaks) during stretching was determined with a universal 10 kN tension-compression machine (Tinius Olsen H10KT). A 250 N sensor from the same manufacturer was used. Two grips were used to secure each wire at both ends. A single wire length of 40 mm was established between the grips.

Statistical analysis

The statistical analysis was conducted using the Statistical Package for Social Science (SPSS) software. Prior to statistical analysis all data were tested for normality (Shapiro-Wilk test). The data are presented as numbers, means, and standard deviations. The averages of 3 independent groups were compared using the Kruskal−Wallis test. A p-value of ≤ 0.05 was considered statistically significant.

Results

Surface morphology

The surface characteristics are shown in Figs. 1 and 2. The surface topography of the control group with unused 0.014 NiTi wires (1 A-1 C) shows an irregular surface with few lines and pits. Unused NiTi archwires have less surface irregularities than both non-fluoride and fluoride group archwires. After 120 min of exposure (Fig. 1), SEM analysis showed notable surface degradation in both non-fluoride and fluoride groups of NiTi archwires. The non-fluoride group (2 A–2 C) exhibited increased crack formation, deeper striations, and the presence of surface pits. In comparison, the fluoride group (3 A–3 C) demonstrated more extensive surface degradation, characterized by a greater density of striations, more pronounced pitting, and increased crack formation. After 240 min of exposure (Fig. 2), the non-fluoride group wires (2 A-2 C) surface had even more irregular surface, containing white spots, deeper lines and dark round pits. Maximum degradation was observed in the fluoride group archwires after 240 min of exposure (3 A–3 C), which showed a highly irregular surface with dark and white spots, as well as the greatest pitting and crack formation. At 5.000x magnification, the photograph of the fluoridated wire shows increased surface degradation and corrosion, whereas, at 10.000x magnification, it shows deeper dark scratches than those in the non-fluoride and control groups.

Fig. 1.

The surface topography: unused (1A-1C), non-fluoride (2A-2C), fluoride group archwires (3A-3C) after 120 min of exposure

Fig. 2.

The surface topography was as follows: unused (1 A-1 C), non-fluoride (2 A-2 C), and fluoride group archwires (3 A-3 C) after 240 min of exposure

Chemical properties analysis

The compositions of the chemical elements in the archwires across the different study groups are presented in Fig. 3. After 120 min of exposure, the concentration of titanium ions in archwires exposed to fluoridated toothpaste decreased by 16.54%, while a smaller reduction of 5.51% was observed in the non-fluoride group, compared to the control (p = 0.112). Similarly, nickel ions decreased by 12.66% in the fluoride group and by 5.02% in the non-fluoride group (p = 0.127). The greatest decrease in titanium ions was observed after immersing the NiTi orthodontic archwires in toothpaste containing 1448 ppm fluoride for 240 min. Compared with those in the control group, the titanium ions of archwires exposed to fluoridated toothpaste decreased by 19.23%, whereas in the non-fluoride group, it decreased by only 7.73% (p = 0.099). Furthermore, a decrease in nickel ions in fluoride group NiTi orthodontic archwires was also observed. Compared with those in the control group, the nickel ion content of the archwires exposed to fluoridated toothpaste decreased by 15.97%, whereas in the non-fluoride group, it decreased by only 7.00% (p = 0.670).

Fig. 3.

EDS analysis after 120 min (B, C) and 240 min of exposure (D, E): unused (A), non-fluoride (B, D), and fluoride (C, E) group archwires

Mechanical properties analysis

The mechanical properties of each archwire were assessed by evaluating the force at break and elongation. Each study group measured the force at break in Newtons (N). The mean force at break and elongation values are presented in Table 1. Elongation significantly differed between the two time periods, with a greater reduction following extended exposure (p < 0.001). After 120 min of incubation, archwires exposed to non-fluoridated toothpaste presented a significant decrease in elasticity of 20.8% compared with those in the control group.

Table 1.

A comparison of force at break and elongation among the three groups was performed using the Kruskal‒Wallis test

Exposure time	Groups (n = 10)	Mean force ± SD (N)	P-value	Mean elongation ± SD (mm)	P-value
After one month of exposure (120 min.)	Control	115.06 ± 2.77	0.874	8.77 ± 0.35	< 0.001
	Fluoride	114.75 ± 2.89		6.39 ± 0.29
	Non-fluoride	114.27 ± 4.35		6.95 ± 0.11
After one month of exposure (240 min.)	Control	115.06 ± 2.77	0.772	8.77 ± 0.35	< 0.001
	Fluoride	114.42 ± 2.87		5.05 ± 0.21
	Non-fluoride	112.67 ± 5.40		5.55 ± 0.31

In contrast, those treated with fluoridated toothpaste presented a 27.1% decrease (p < 0.001). After 240 min, the elasticity of the archwires exposed to non-fluoridated toothpaste decreased significantly by 36.5% compared with that of the control group. In contrast, the elasticity of archwires treated with fluoridated toothpaste decreased even more by 42.2% (p < 0.001).

Discussion

The physical, chemical, and mechanical properties of the materials used during orthodontic treatment are important because of the extended duration of the treatment. The present study aimed to evaluate the changes in nickel-titanium orthodontic archwires after exposure to non-fluoridated and fluoride-containing toothpastes. The 0.014 round nickel-titanium archwire was selected based on its established clinical use during the initial alignment phase of fixed orthodontic treatment, during which it typically remains in place for up to two months.

This in vitro study evaluated the surface morphology of NiTi orthodontic archwires after exposure to fluoridated and non-fluoridated toothpastes. The results from the SEM test revealed that, compared with non-fluoridated and unused archwires, fluoride group wires have more surface irregularities and dark and white spots. In addition, increased pitting and cracks formation, corrosion, and more surface degradation were observed. Ogawa et al. [27] analyzed the corrosion of NiTi orthodontic archwires of patients using mouthwash with fluoride and demonstrated that exposure to fluoride influences the corrosion of NiTi archwires, mainly through the use of acidulated (pH 5.1) fluoride mouthwashes. Kedia et al. [28] compared the effects of sodium fluoride (NaF) (0.2%), chlorhexidine (CHX) (0.2%), and acidified phosphate fluoride (APF) (1.23%) agents on the surface morphologies of different types of orthodontic archwire. After 1.5 h at 37 °C and 45 °C, the titanium‒molybdenum alloy (TMA) wires exhibited the maximum corrosive effect, whereas the SS wires presented minimal changes. CHX and NaF mouthwash caused the least amount of corrosive damage to the different orthodontic archwire surfaces; therefore, they were more effective. Farrag et al. [29] analyzed the surface topography of NiTi orthodontic archwires after immersion in sodium fluoride or chlorhexidine and compared it to that of the control group. Compared with the other groups, the NiTi archwires immersed in sodium fluoride presented more pits, fissures, and corrosion.

The duration of fluoride exposure can significantly influence the mechanical properties of orthodontic Ni-Ti archwires. In this study, the elongation of the Ni-Ti wire decreased by 27.1% after only 120 min of fluoride exposure, equivalent to 1 month of daily toothbrushing for 4 min. Moreover, the elongation decreased even further, by 42.2%, after 2 months of daily 4-minute fluoride exposure. Walker et al. [26] incubated Ni-Ti archwires for 240 min in high-concentration fluoride gels (1.1% sodium fluoride) and reported a significant reduction in the archwire unloading modulus and unloading yield strength. Similarly, Sukhpreet et al. [30] exposed Ni-Ti archwires to the same fluoride gel for only 120 min. They reported a notable decrease in the mean loading force, from 682.6 MPa in the control group to 596.1 MPa in the fluoride gel group. These results indicate that prolonged exposure to fluoride-containing products can further decrease the mechanical properties of Ni-Ti alloys. Clinically, the actual fluoride exposure may be even longer, as archwires are sometimes used for several months to up to a year. Therefore, replacing 0.014 Ni-Ti archwires after 1 month of treatment would be advisable rather than extending their use to 2 months.

The concentration of fluoride-containing products can also affect the mechanical properties of orthodontic Ni-Ti archwires. Belasic et al. [13] found that saliva together with toothpaste containing 1450 ppm fluoride reduced the elasticity of NiTi archwires compared with wires that were exposed to chlorhexidine and a high (6150 ppm) concentration of sodium fluoride gel. Sufarnap et al. [31] studied the effects of NaF on the mechanical properties of 0.016 × 0.025-inch copper‒nickel-titanium orthodontic archwires and reported significant differences in the unloading force overtime only when the samples were exposed to 0.15% NaF compared with 0.05% NaF. Kassab et al. [32] investigated the influence of corrosion on the fracture of NiTi superelastic wires and reported sufficient corrosion resistance in body fluids such as saliva, but exposure to fluoride (NaF 0.1%) significantly increased their susceptibility to localized corrosion. When exposed to corrosive electrolytes, NiTi exhibits a brittle fracture mechanism; therefore, using these archwires in oral environments, particularly in the presence of fluoride, poses a significant risk of corrosion-induced fracture.

Our study evaluated ion release from NiTi archwires and reported an increase in the release of titanium and nickel ions after fluoride exposure. However, this difference was not statistically significant. Pastor et al. [14] evaluated the ion release of superelastic NiTi archwires using three types of mouthwash with fluoride concentrations of 130, 200, and 380 ppm. The release of Ni and Ti ions from the NiTi archwires surpassed 220,000 ppb and 180,000 ppb, respectively. The release of ions alters the chemical composition of the wires, leading to the formation of martensite plates in the austenitic matrix and resulting in the loss of superelastic properties. Immersion in 380 ppm fluoride mouthwashes for more than 7 days causes nickel-rich precipitates to form, which embrittle the wire and eliminate its tooth-correcting ability. Additionally, the release of Ni ions can cause hypersensitivity, especially in women [14]. Chantarawaratit et al. [25] analyzed three different types of orthodontic archwires: stainless steel, NiTi, and titanium molybdenum. The APF gel induced nickel ion release in all the experimental groups. In addition, the maximum amount of nickel ions released was found in the NiTi orthodontic archwires. Researchers have reported that APF gel is not recommended for patients receiving treatment with fixed-metal orthodontic appliances because of the release of metal ions. Chitra et al. [33] investigated the amount of metal ions in gingival crevicular fluid from patients undergoing orthodontic treatment, both with and without fluoridated toothpaste and mouthwash, after six months. The results of this study revealed that fluoride-containing products led to greater metal ion release than products without fluoride, with a statistically significant increase in nickel release. A peak in Ni and Ti ion release was observed after 30 days in the fluoride group; however, fluoride had a minimal effect on the release of chromium and manganese ions. Overall, the findings of this study indicate that the release of metal ions into the mouth can lead to the absorption of these elements into the body and their associated consequences.

Changes in the mechanical characteristics of orthodontic NiTi archwires are important clinically because they may influence the forces delivered to teeth and treatment efficiency. Therefore, our study confirmed our hypothesis, suggesting that prolonged exposure to toothpaste may extend orthodontic treatment by reducing the mechanical properties of archwires and affecting their surface topography. Moreover, attention should be given to the release of metal ions from NiTi archwires to avoid reaching a toxic state. However, further research on the effects of fluoride on orthodontic archwires is needed for more reliable conclusions. This study has some limitations. One limitation is that fluoride application was performed by immersing the wires in toothpaste suspensions rather than simulating clinical toothbrushing. While immersion ensured standardized exposure conditions, it does not fully replicate the mechanical environment of brushing. Further research using brushing simulation models is needed to better reflect clinical practice and enhance the reliability of the findings. Additionally, this study focused only on NiTi wires; future studies including stainless steel archwires are recommended to provide a better understanding of material-specific responses to toothpaste exposure. Furthermore, in vivo studies are necessary to evaluate corrosion behaviour and surface changes under actual oral conditions and to confirm the clinical relevance of the present in vitro findings. Another limitation of this study is that elastic modulus and unloading forces, which are more directly related to clinical tooth movement, were not evaluated. Although force at break and elongation limit provide valuable information regarding material degradation, future studies should include Young’s modulus measurements and three-point bending tests to better reflect the mechanical behavior of NiTi archwires under clinical conditions.

Conclusions

Fluoride group archwires exhibited more corrosion, surface degradation, the weakest elasticity, and decreased titanium and nickel ions. Prolonged exposure to toothpaste also affects the mechanical properties of NiTi archwires, significantly reducing elongation, suggesting that archwires should be replaced more frequently during treatment. Using fluoride toothpaste could affect the surface, decrease the mechanical and chemical properties of the NiTi archwire, and potentially contribute to prolonged orthodontic treatment.

Abbreviations

NiTi

Nickel-titanium

SEM

Scanning electron microscopy

EDS

Energy Dispersive X-ray Spectroscopy

NaF

Sodium fluoride

CHX

Chlorhexidine

N

Newtons

SD

Standard deviation

TMA

Titanium‒molybdenum alloy

APF

Acidated phosphate fluoride

Authors’ contributions

Smiltė Paldauskaitė: experimental analysis, data analysis, statistical analysis, writing the original draft. Roberta Lekavičiūtė: experimental analysis, data analysis, statistical analysis, writing the original draft. Rita Kriūkaitienė: experimental analysis, data analysis. Giedrė Trakinienė: editing. Kristina Lopatienė: supervision, editing. All the authors contributed to the study’s conception and design.

Funding

This study did not receive any external funding.

Data availability

The data analyzed in this study are available from the corresponding author upon reasonable request.

Declarations

Ethics approval and consent to participate

The current study was approved by the Kaunas Regional Biomedical Research Ethics Committee (No. P1-BE-2-5, 5 November 2024). Informed consent was waived by the Kaunas Regional Biomedical Research Ethics Committee.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.