INTRODUCTION

During orthodontic treatment various types of wires, brackets, bands, and attachments are used. These can include, but are not limited to Kobayashi ties, transpalatal and lingual arches, fixed expansion appliances, quad helices and palatal expanders, and removable appliances. In some cases, patient-specific orthodontic appliances that have soldering and welding parts are used. In addition to the functional benefits of these appliances, there is a drawback that these are prone to corrosion in the varying conditions of the oral cavity. In previous studies, the release of elements from orthodontic appliances as a result of corrosion was investigated. Iron, nickel, and chromium ions were found to be the main elements that were released from stainless steel orthodontic appliances.^1–5 The released ions can be allergic, locally cytotoxic, and carcinogenic.⁶

Among orthodontic appliances, soldered appliances are the most responsive to corrosion and have toxic effects.^3,7–10 Some in vitro studies have reported that the metal ion release and corrosion of arch wires increase when silver soldering and heat treatment are applied.^3,9 Grimsdottir et al.³ reported that facial arches prepared with silver soldering were corrosion sensitive. This outcome was thought to be related to the 300°–500°C heat applied during soldering. Hwang et al.¹¹ reported that silver soldering causes surface roughness and decomposition of the crystal structure and leads to increased susceptibility to corrosion. Animal studies and toxicity tests performed using cell cultures revealed that silver soldering causes toxic effects on mucosa cells.^12,13

With advances in dental technology, lasers have begun to be used for soldering. Researchers proposed using a laser welding method because it would be less toxic compared to silver soldering. The results of the few existing toxicity studies that compared laser welding and silver soldering methods support this result.^10–18 Sestini et al.¹⁸ noted that laser welding was the only well-tolerated bonding method, whereas silver soldering was highly toxic to all types of cells. Bagatin et al.¹⁹ used scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy in an in vivo study in which they investigated the corrosion of silver soldered Haas expanders in patients using antimicrobial agents. They found that the amount of metal ions increased in areas of corrosion, especially in regions where soldering was performed.

Previous studies indicate that the application of orthodontic bands and brackets increases plaque formation in the oral cavity as well as the density of bacteria inside the plaque.^20,21 Bacteria in dental plaque that are deposited on the surface of the orthodontic appliance cause corrosion of metal surfaces.²² Additionally, many studies revealed that fixed orthodontic treatments cause mild to moderate gingivitis, dental caries, and decalcification as a result of the accumulation of nutrients and increased bacterial plaque formation together with gingival growth due to the acidic metabolites produced by the bacteria.^23,24 For this reason, patients undergoing orthodontic treatment should be educated about mechanical cleaning and given instructions to use additional protective methods such as antibacterial mouthwashes when needed. The effects of mouthwashes on oral hygiene have been previously examined in numerous studies.^25,26 It has been reported that mouthwashes increase the risk of corrosion and cause the release of some metals from the surface of orthodontic appliances.^27–30

There are no prior studies in the literature that investigate the relationships between various mouthwashes, metal ion release, and various soldering methods. The aim of the present study was to compare the release of nickel, chromium, iron, silver, and copper ions; the corrosion rate; and the amount of corrosion for two different soldering methods used in the preparation of orthodontic appliances—laser welding and silver soldering—in three different mouthwash solutions.

MATERIALS AND METHODS

In the present study, 72 samples were prepared. Half of the samples were silver soldered and the other half were laser welded. Left upper molar bands without attachments (3M Unitek, Victory Series, Monrovia, Calif), size 42.5, were used to prepare the samples. They were bonded to pieces of Lewa Dental (75196, Remchingen, Germany) stainless steel wire with a diameter of 1 mm and a length of 5 mm. Silver soldering or laser welding techniques were used for bonding. For silver soldering, pieces of solder wire (Dynaflex Silver Solder, Silvaloy A56T, Wolverine Joining Technologies, LucasMilhaupt, Cudahy, WI, USA) with a length of 5 mm and a diameter of 0.45 mm were melted and spread on the region using a micro torch (Proxxon Micromot MFB/E mircoflame burner, Proxxon, Hickory, NC, USA) (Table 1). In the laser welding process, assembly was performed with a laser welding device (Laser XXS EVO 100, Orotig, Verona, Italy) without using any solder wire. A one-kilowatt power shot was set to remain on the surface of the solder for 1.1 milliseconds, and seven shots were carried out per second. The diameter of the shots was set at 0.3 mm. After leveling and polishing the soldered samples, small pieces were obtained, leaving 1.5-mm solder-free band regions on the left and right sides of the soldered areas.

Table 1. Percentage of Metal Elements of Molar Band, Laboratory Wire, and Silver Soldering Wire

View Fullscreen

The samples of each weld group were divided into four mouthwash test groups (Tables 2,3). The mouthwashes used in the study contained sodium fluoride (NaF; Colgate Total Professional Dental Gum Health®, Palmolive, Guildford Surrey, UK), sodium fluoride + alcohol (NaF + alcohol; Listerine®, Johnson and Johnson, Skillman, NJ, USA), and chlorhexidine (CHX; Andorex®, Istanbul, Turkey). Artificial saliva (AS) was the negative control solution (Table 2). Seventy-six 2000-µL laboratory tubes were used to contain the solutions. Each tube was filled with 1500 µL of 9% (135 µL) artificial saliva and 91% (1365 µL) of either mouthwash. The remaining 19 tubes were filled with 100% artificial saliva (Table 3).

Table 2. Ingredients of Mouthwashes and Artificial Saliva^a

View Fullscreen

Table 3. Distribution of the Samples to the Groups^a

View Fullscreen

Next, the samples were kept in the solutions and shaken (Stuart, 3D gyratory rockers - SSL3, Bibby Scientific Limited, Staffordshire, UK) for 24 hours with an agitation rate of 60 rpm. The amounts of metal ions released from the samples and dissolved in the solutions were subsequently measured using inductively coupled plasma mass spectrometry (ICP-MS) (Thermo Scientific, XSeries 2, Thermo Fisher Scientific Inc, Waltham, MA, USA). The obtained values were recorded in parts per million, and the metal release values of the two soldering methods and three mouthwashes and AS were compared. NCSS (Number Cruncher Statistical System) 2007 Statistical Software (Kayesville, UT, USA) for Windows was used for the statistical analysis. The Kruskal-Wallis and analysis of variance (ANOVA) tests were used for the group comparisons, and post hoc Dunn multiple comparison test was utilized for the two group comparisons. Eight samples—four from each subgroup—were chosen randomly for examination of the surface changes by scanning electron microscopy (SEM) (EVO 40 Carl Zeiss AG, Dresden, Germany) at 200× magnification (Figure 1).

View Figure

• Download as Powerpoint
• Download Figure

RESULTS

Greater amounts of silver and copper ions were found in all solutions containing silver soldering samples than in those containing laser welding samples (Tables 4 and 5). Silver soldered samples released higher amounts of nickel in CHX, chromium in NaF, chromium and nickel in AS, and all analyzed elements in NaF + alcohol (Table 6).

Table 4. Comparison of Silver Ion Release From Silver Soldering and Laser Welding Groups in Different Test Solutions

View Fullscreen

Table 5. Comparison of Copper Ion Release From Silver Soldering and Laser Welding Groups in Different Test Solutions

View Fullscreen

Table 6. Comparison of Chromium Ion Release From Silver Soldering and Laser Welding Groups in Different Test Solutions

View Fullscreen

In CHX, the chromium release was found to be lower than in the other solutions. The chromium release in AS was found to be lower than in NaF and NaF + alcohol (Table 6).

The iron release in the CHX and NaF groups was lower than in the AS and NaF + alcohol groups. Released iron in the AS group was lower than in the NaF + alcohol group (Table 7).

Table 7. Comparison of Iron Ion Release From Silver Soldering and Laser Welding Groups in Different Test Solutions

View Fullscreen

The nickel release in the CHX and NaF groups was lower than in the AS and NaF + alcohol groups, while the nickel release in the NaF + alcohol group was higher than that of the AS group (Table 8). Among the solutions in which laser welded samples were placed, the nickel release in the CHX group was found to be lower than in the AS and NaF + alcohol groups.

Table 8. Comparison of Nickel Ion Release From Silver Soldering and Laser Welding Groups in Different Test Solutions

View Fullscreen

Silver release from silver soldered samples in the NaF group was higher than that in the AS group (Table 4).

The copper release from silver soldered samples in the CHX group was found to be lower than the other groups (Table 5). The copper release was lower in the NaF group than in the AS and NaF + alcohol groups. Among the solutions in which laser soldered samples were placed, the values of the CHX group were lower than those of the NaF, NaF + alcohol, and AS groups (Table 5).

The representative SEM images (at 200× magnification) in Figure 1 show the silver soldered and laser welding samples kept in different mouthwashes for 24 hours. The topography of the samples immersed in AS shows no difference compared to the samples that were not placed in any solution. Slight cracks formed on the samples placed in NaF. Color changes can be observed on the surface of the specimens placed in NaF + alcohol and CHX. These images support the results of ICP-MS.

DISCUSSION

In studies where the amounts of metal ions released from orthodontic appliances were analyzed, different types and numbers of orthodontic appliances were used such as molar bands, brackets, arch wires, and maxillary expansion devices, or pieces of these items.^4,16,19 In order to evaluate the net corrosion of the welded or the soldered parts, the whole appliance was not tested in this study.

While storing the solder samples in the solutions, a 3D gyratory rocker machine was used to imitate the oral environment. The artificial saliva that is commonly used in in vitro studies may not be sufficient to create the desired dynamic oral environment; therefore, in their in vitro study, Kerosuo et al.² placed samples in an environment that provided a special artificial mouth stimulus. They demonstrated that while the fixed or dynamic nature of the environment had no effect on chromium release, nickel release was accelerated. Further studies can be planned where oral environment is simulated; however, it should be considered that different simulations complicate the comparison of the results of the studies.

Nickel, chromium, iron, copper, and silver levels were evaluated in our study, because they are present at high levels in bands, laboratory wire, and solder wire. In previous studies,^{2,4,5,16,19,29} the metal ion release from different types of orthodontic appliances in various solutions was examined for different time periods. In our study, corrosion was evident after the first 24 hours. In some of the previous in vitro studies, corrosion was tested on the first day, in others on following days. Because the methods of the experiments vary, the results of the studies differ from each other.

In our ICP-MS measurements, the silver soldered specimens showed higher amounts of metal ion release than the laser welded specimens. These results are similar to those from previous studies.^10,14,18 Sestini et al.¹⁸ reported that laser welding was the only bonding method that was well tolerated by cells, whereas silver soldering was evaluated as highly toxic for all cell types.

According to the results of the present study, despite the higher chromium content of the appliances, more nickel than chromium was released from the samples. In a study of Mikulewicz et al.,¹ the overall metal ion release from fixed appliances placed into pig snouts was investigated. The amount of nickel in the aorta was 4.8 times higher, and the amount of chromium in the hair was reported to be 3.4 times higher. These results are similar to our findings.

Previous studies have claimed that increased corrosion on the surface of the silver solder is caused by the high temperature and galvanic reaction in the region.³ Hwang et al.¹¹ reported that surface roughness caused by silver soldering led to decomposition of the crystal structure of the material and made the soldered surface more sensitive to corrosion. In addition, there are some technical features of silver solder application. Silver solder needs to be spread on a wide area to increase the resistance of the solder. As shown in the images of the samples prepared for the present study, silver solder covered a wider area than that of laser welding. Since no solder wire was used in the laser welding method, the soldering area was reduced; the region sensitive to corrosion became smaller, thereby decreasing the metal ion release. Heidemann et al.¹⁶ reported that soldered areas were problematic because they were sensitive to corrosion and released silver, copper, and zinc ions to the environment, so the surface area should be minimized as much as possible and they proposed the laser welding method.

In our study, despite the fact that the silver content of the silver solder wire was higher than the copper content, higher amounts of copper were released compared to silver. Previous studies on silver soldering yielded similar results.^15,17 Solmi et al.¹⁷ explained this result as being due to the more unstable elemental structure of copper compared to that of silver. On the other hand, even though no solder wire was used in the laser welding technique, some amount of silver and copper was detected in those solutions. This is considered a consequence of the release of the trace amounts of silver and copper present in some commonly used solutions, tubes, bottles, bottle caps, and clips.

Although there are several studies that have investigated corrosion on orthodontic appliances caused by mouthwashes,^19,28–30 there are not many studies that have investigated the effects of mouthwashes on different soldering applications. Our finding is that the greatest amount of metal ion release occurs in NaF + alcohol containing mouthwashes and may contribute to both the literature and to clinical practice. Schiff et al.²⁸ evaluated the corrosion effect of NaF containing mouthwashes with three different concentrations and concluded that 125 ppm NaF-containing mouthwashes should not be preferred for orthodontic appliances containing titanium, iron, chromium, and nickel. Danaei et al.,²⁹ concluded that NaF- and mostly the CHX-containing mouthwashes caused corrosion on stainless steel appliances. Dartar Oztan et al.²⁵ used 0.2% CHX on the root canal instruments and reported the corrosion effect of the mouthwash on stainless steel appliances. The results of our study indicated that CHX-containing mouthwash caused lower amounts of metal release compared to other mouthwashes. Green discoloration occurred on the CHX samples after 24 hours of immersion. Color pigments in the CHX solution are thought to adhere to the sample surfaces and constitute a type of passive surface. The pigments may be the reason for the lower metal ion release than expected. Further research is required to clarify the effects of CHX and pigment-free CHX-containing mouthwashes.

CONCLUSIONS

• Laser welding method can be preferred to silver soldering due to lower amounts of metal ion release.

• CHX-containing (Andorex) mouthwash can be offered to patients who have orthodontic appliances rather than NaF + alcohol–containing mouthwash.