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Year : 2012 | Volume
: 23
| Issue : 5 | Page : 591-595 |
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Influence of fluoride-containing acidic artificial saliva on the mechanical properties of Nickel-Titanium orthodontics wires |
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Jun Lin1, Shujuan Han1, Jiajun Zhu1, Xiaoxiang Wang2, Yuan Chen3, Oliver Vollrath4, Huiming Wang1, Christian Mehl5
1 Department of Stomatology, First Affiliated Hospital, Hangzhou, China 2 Department of Material Chemistry, Zhejiang University, Hangzhou, China 3 Department of Dentistry, Zhejiang Yongjia Hospital, China 4 Institute of Medical Informatics and Statistics, University Hospital Schleswig-Holstein - Campus Kiel, Germany 5 Department of Prosthodontics, Propaedeutics and Dental Materials, Christian-Albrechts University at Kiel, Germany, Now at: Private Practice, 10 Brook Street, W1S 1BG, London, United Kingdom
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Date of Submission | 22-Feb-2011 |
Date of Decision | 27-Jun-2011 |
Date of Acceptance | 05-Jul-2012 |
Date of Web Publication | 19-Feb-2013 |
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Abstract | | |
Objective: This study aimed to investigate the influence of fluoride and an acidic environment on the mechanical properties of NiTi orthodontic wires (NiTiW) in artificial saliva. Design: A prospective laboratory investigation. Setting: Department of Stomatology, First Affiliated Hospital, Zhejiang University, Hangzhou, China. Materials and Methods: Commercial, round 0.016-inch NiTiW were immersed in 0% or 0.05% Natrium-Fluoride-containing artificial saliva at a pH of 4 or 6 for one or three days, respectively. NiTiW were examined with a three-point bending test, Vickers' microhardness tests and surface morphology observation (SEM). Results: A pH of 4 increased microhardness and decreased the three-point bending strength significantly (P≤0.05), whereas immersion time and fluoride concentration had no significant influence on the microhardness or on the three-point bending strength (P>0.05). When examining the test group NiTiWs after three days of immersion at a pH of 4 the SEM revealed a rough surface morphology, a damaged oxide layer and signs of corrosion. Conclusions: The most influential factor for decreasing the unloading force and increasing the hardness seems to be the pH value, whereas immersion time and NaF addition do not have a major influence. Keywords: Corrosion, fluoride, microhardness, nickel, orthodontic wires, surface morphology, three-point bending test, titanium
How to cite this article: Lin J, Han S, Zhu J, Wang X, Chen Y, Vollrath O, Wang H, Mehl C. Influence of fluoride-containing acidic artificial saliva on the mechanical properties of Nickel-Titanium orthodontics wires. Indian J Dent Res 2012;23:591-5 |
How to cite this URL: Lin J, Han S, Zhu J, Wang X, Chen Y, Vollrath O, Wang H, Mehl C. Influence of fluoride-containing acidic artificial saliva on the mechanical properties of Nickel-Titanium orthodontics wires. Indian J Dent Res [serial online] 2012 [cited 2023 Mar 23];23:591-5. Available from: https://www.ijdr.in/text.asp?2012/23/5/591/107332 |
The most commonly used alloys for orthodontic purposes are heat-activated NiTi or stainless steel wires. NiTi alloys are stoichiometric compounds of 55 wt% Ni and 45 wt% Ti. [1] These near-equiatomic NiTi alloys have many excellent properties such as adequate springback, low stiffness, superelasticity, and shape memory, which are essential for orthodontic purposes. [2],[3],[4] Additionally, heat-activated NiTi wires (NiTiWs) combine good biocompatibility with living tissue and corrosion resistance [5],[6],[7],[8] with the application of a constant and light force which is ideal to move teeth over a period of weeks. [9],[10] The properties originate from the proportion of elements, characteristics of their microstructural phases and the transformation of the crystal structure from austenitic form to martensitic form due to the change of temperature or stress. [11]
A great focus in the literature in recent years has been the remineralization of the enamel with fluoride to avoid non-developmental white spot lesions (WSLs), which occur in nearly 50% of the patients undergoing fixed orthodontic treatment. [12],[13],[14],[15] Methods of delivering fluoride to the oral cavity are homecare products or dental materials. [1],[14],[16],[17] Hydroflourid acid seems to be considered as one of the main causes for the destruction of the passivation film of titanium when the concentration is higher than 30 ppm. [18],[19]
An additional factor influencing the properties of NiTi wires is the pH. The pH of human saliva normally ranges from 6-8, [20] but can vary from 2 to 11 while eating different food and beverages. [21] Since the recent literature mainly focused on the corrosion resistance, enamel remineralization and mechanical properties, this study aimed to investigate and summarize the influence and interactions of a low fluoride concentration and an acidic pH on the microhardness, mechanical properties (three-point bending test) and corrosion.
The null hypotheses to be tested was
An acidic ph, higher fluoride content and a longer immersion time reduce the three-point bending strength and increase corrosion.
Materials and Methods | |  |
Materials and specimen preparation
As specimens commercially available round heat-activated orthodontic wires (55 wt% Ni, 44 wt% Ti) with a diameter of 0.016 inch (0.41 mm) were used (Nitinol HA, 3M Unitek, Monrovia, USA). Before immersing the specimens in artificial saliva for one or three days [1],[22] the straight ends of the wires were cut into 20-mm pieces. After cleaning the cut wire ends ultrasonically in distilled water for 10 min the specimens were randomly assigned to the experimental groups.
Fusayama-Meyer's artificial saliva with a pH of 7.1 was used as a replacement for human saliva. It contains: NaCl (0.400g/l), KCl (0.400g/l), CaCl 2•H 2 O (0.906g/l), NaH 2 PO 4·2H 2 O (0.690g/l), Na 2 S·9H 2 O (0.005g/l) and Urea (1g/l). [23] In order to investigate minor pH changes the artificial saliva was only adjusted to the pH of 4 and 6 with lactic acid under constant monitoring of the pH (pH meter, model 818, Orion, China). [1],[22],[24] Additionally, the NaF concentration was only slightly adjusted by initially measuring the fluoride concentration of the artificial saliva (control at 0% NaF) and then adding NaF using ultraviolet spectrophotometry (Cary 100, Varian Inc., USA) to increase the concentration to a level of normal toothpaste (0.05 wt%). The experimental groups were assigned to one of the following parameters: NaF (0 or 0.05 wt%), pH (4 or 6) or immersion time (IT, 1d or 3d). Thus eight wire specimens (n=8) were randomly assigned to eight experimental and one control groups.
Microhardness measurement
To measure the surface microhardness of the wires, a Vickers hardness tester (MVK-H1, Akashi Co., Japan) was used. After completing the experimental cycle of each respective experimental subgroup the specimens were polished with 2000-grit SiC paper and diamond paste (1.5 μm) and then cleaned ultrasonically in distilled water for 10 min. The specimens were placed in epoxy resin using an acrylic ring mold (20 x 20 x 10 mm). Specimens were light-cured (Curing Light XL3000, 3M, St. Paul, USA) for 40 sec with 700 mW/cm 2 light intensity, as measured using a radiometer. Five points (n=5) were selected randomly for Vicker`s microindentations on the surface of each specimen using a 9.8 N (1000 gf) load for a dwell time of 10 sec. The length of the microindentations was measured and the resulting Vickers hardness calculated (n=40 per experimental subgroup).
Three-point bending test
A three-point bending test was used to simulate the oral environment and measure the activation (loading) and deactivation (unloading) force-deflection behavior [Figure 1]. The length between the two bearings was 12 mm imitating the distance of two brackets in vivo. Each specimen was loaded to a deflection of 3 mm and then unloaded to zero deflection at 37 °C at a crosshead speed of 1 mm/min. The three-point bending test of each group was conducted three times per specimen with eight specimens assigned to each experimental subgroup (n=8). The data were recorded according to the load and unload deflections of NiTi wires at a deflection ranging from 0 to 3 mm in 0.25-mm steps. [25]  | Figure 1: Schematic diagram of the three-point bending test used in this study
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Surface morphology Observation
After the microhardness test the specimens were air-dried for 24 h, gold-sputtered and observed by a scanning electron microscope (S-4200, Hitachi, Japan) operated at 10-25 kV.
Statistical analysis
The data was statistically analyzed using "SPSS for Windows" (Version 18, SPSS Inc., USA). Since all the data were distributed normally (Shapiro-Wilks test) the data from the microhardness test and three-point bending test were analyzed using three-way ANOVA and significant results were further analyzed using multiple t-test two sample comparisons for independent samples adjusted with Bonferroni-Holm [26] at a confidence level of 95%.
Results | |  |
All data for the microhardness and the three-point bending tests are shown in [Table 1]. When evaluating the influence of the experimental factors with three-way ANOVA a pH of 4 increased the microhardness and decreased the three-point bending strength significantly (P≤0.001, [Table 2]), whereas immersion time and fluoride concentration had no significant influence on the microhardness or on the three-point bending strength ( P>0.05). Significant interactions could be found between the fluoride concentration and storage time for the microhardness values and between pH and storage time for the three-point bending test data ( P≤0.05). Further t-tests for the three-point bending tests confirmed the significant differences when testing the influence of the pH, storage time and the interaction between the two variables (P≤0.05, [Table 2]). | Table 1: Means and Standard Deviations (SD) for microhardness (n=40, HV) and three-point bending test values (n=8, in cN) of the experimental subgroups
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 | Table 2: Statistically significant influences of the variables immersion time, fluoride concentration and pH tested with three-way ANOVA (n=8)
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Surface morphology - SEM evaluation
The surface morphology evaluation by SEM is shown in [Figure 2]a-e. The surface morphology of the control group just showed general groove patterns due to the grit paper grinding. The amount of corrosion and pitting seemed to have increased with decreasing pH and longer immersion time. | Figure 2: The SEM image of representative specimens. A high magnification (5000x, bar=10μm) exhibits grooves from the grit paper grinding in all groups. Images (a) and (c) with a pH 6 show similar surfaces. In contrast the images (b) and (d) show corroded and pitted surfaces (see asterisks). The image of the control group (e) exhibits no corrosion
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Discussion | |  |
This in vitro study tested the influence of the pH, fluoride and immersion time on NiTi heat-activated wires in artificial saliva. The null hypothesis that the immersion time, pH and fluoride have a significant influence on the Vickers hardness and the three-point bending strength had to be partially rejected, since only the pH showed a significant influence.
The purpose of using fluoride-containing homecare products and dental restorative materials in dental clinics is to inhibit the growth of oral bacteria and caries. Several species of oral bacteria reside on the film of dental plaque, which can metabolize fermentable carbohydrates and then produce lactic, acetic and propionic acids. [27] If there is a small quantity of fluoride in the solution, hydrogen atoms in these acids readily dissociate and form hydrogen fluoride (HF) molecules. These molecules can react with the titanium oxide, dissolving the protective passivation (titanium-oxide) layer by forming titanium-fluoride on the surface. [28] In the current study the fluoride level chosen was deliberately set very low in order to copy a daily care situation with normal toothpaste for children. The slight increase of the fluoride concentration did not have a significant impact in this study, thus indicating that daily care does not reduce the mechanical properties. [19] In contrast higher fluoride levels applied through rinses or especially gels seem to alter the favorable mechanical wire properties and, hence, may prolong orthodontic treatment and negatively effect the aesthetical result. [29],[30],[31],[32]
Several studies report a loss of corrosion resistance, when titanium and titanium alloys are subjected to fluoride with varying pH values. [19],[22],[33],[34] The concentration limit of HF at which the corrosion resistance of titanium could be maintained was around 30 ppm. [18] Such an HF concentration can be obtained from a 0.05% NaF solution at pH of 4.0 or 0.1% solution of pH 4.3. [19] Any concentration higher than that will lead to a corroded passivation layer. The acids used to adjust the pH value are reported to being able to change the HF concentration due to different acidity. [19],[22] In the current study the HF concentration was around 30 ppm according to calculations in the literature [19] and to imitate a daily care situation for a child undergoing orthodontic treatment. Despite the HF concentration being on the border of not damaging the titanium alloy's passivation film, the amount of corrosion found seems to have increased with higher fluoride concentration, longer immersion time and most importantly a lower pH (for example, see [Figure 2]). [32] In the current study the results obtained when NiTi wires were subjected to a pH of 6 showed in parts higher three-point bending strength values than the control group. These differences might be subject to manufacturer's variations. Although all experimental subgroups subjected to a pH of 4 showed smaller three-point bending strength values than the control group it indicates that the statistically significant differences may not be of clinical importance.
The microhardness of all experimental groups increased in comparison to the control group, but was only significant for three subgroups. The increase in microhardness might be attributable to the degradation of the surface by the acidic fluoride-containing solutions. [32] The passivation film mainly consists of Ti 2 O 3 or TiO 2 and can be replaced with TiF 3, TiF 4 or TiOF 2 in a fluoride-containing solution. However, in the current study the fluoride concentration was not a significant factor so the alteration of the surface by replacing oxygen ions with fluoride ions cannot be the explanation of a different mechanical surface characteristic or microhardness. [1] A possible explanation might be the reduction of the purity content of titanium, which increases the microhardness and decreases the breaking elongation. [35] There are reports confirming that the microhardness is mainly influenced by an acidic environment. [1],[21],[22] It seems likely that an acidic environment causes a change in the crystal structure and therefore changes material characteristics like bending strength and mircrohardness. [36] The key to impede a change in the crystal structure seems to be the surface layer, which ideally has to be stable in an acidic environment. [37],[38],[39]
Conclusions | |  |
According to the limitations of this study the following conclusions can be drawn:
- Although a moderately low ph of 4 in artificial saliva is a significantly influential factor in ageing the NiTi orthodontic wires, the clinical effects of a pH of 4 might be negligible. A lower pH seems to corrode the surface layer, decrease the three-point bending strength and increase the microhardness.
- Immersion time and a low fluoride concentration do not have a significant influence on the microhardness, which indicates that the daily amount of fluoride in toothpaste does not reduce the mechanical properties of the NiTi wires.
- A fluoride-containing environment and a longer immersion time might enhance the corrosion of the surface layer of NiTi wires.
Acknowledgement | |  |
This work is financially supported by Qianjiang Talent project of zhejiang province(2011R10057), National Basic program of China(No.8127955) and Zhejiang Nature Program(Y2080338).
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Correspondence Address: Christian Mehl Department of Prosthodontics, Propaedeutics and Dental Materials, Christian-Albrechts University at Kiel, Germany, Now at: Private Practice, 10 Brook Street, W1S 1BG, London, United Kingdom
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/0970-9290.107332

[Figure 1], [Figure 2]
[Table 1], [Table 2] |
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