Incorporation of metal and color alteration of enamel in the presence of orthodontic appliances

Obtaining beautiful and harmonious smiles is one of the objectives of orthodontic treatment. For a pleasant smile, it is important that the teeth are not colored and are stain-free at the end of orthodontic treatment. This is a critical condition for satisfaction with the appearance of the smile.1 

Clinical reports have pointed out iatrogenic color alteration of tooth enamel after the removal of orthodontic appliances.2 Frequently, this alteration is attributed to the resin tags remaining after orthodontic bracket debonding,2,3 which have no color stability.

Metal bracket corrosion may be another possible cause of tooth enamel staining. In the oral environment, metal ions released by the corrosion of brackets and bands may diffuse through the adhesive system and may even penetrate into the tooth enamel in a process denominated metalosis.4 Should there be intense bracket oxidation, tooth pigmentation may occur, and in extreme cases, it may be necessary to restore the compromised area with composite resin.5,6 

In patients with poor oral hygiene, there is frequently plaque accumulation on the vestibular surface of teeth, around the brackets, leading to a process of enamel demineralization.7,8 In these cases, the presence of acidogenic microorganisms also favors biodegradation of the brackets, with the consequent release of metal ions9 that may be incorporated into the tooth enamel during a possible remineralization cycle.

The aim of this study was to evaluate the possible incorporation of metal ions into tooth enamel when it is submitted to cyclic processes of demineralization and remineralization and its possible influence on tooth color alteration. For these purposes, the null hypotheses assumed that it is not possible to incorporate metal ions arising from orthodontic appliance corrosion into tooth enamel with resulting tooth color change.

Seventy bovine incisors were used, which were washed after extraction and conserved in distilled water with the addition of 0.1% thymol. They were stored under refrigeration at 5°C until the experiment was performed.10 At this time, a fragment was removed from the vestibular surface of each tooth, using a diamond disk fitted to an Isomet machine (model No. 11-1280-170, Buehler, Lake Bluff, Ill). Each fragment obtained measured 5-mm long by 5-mm wide, and its enamel surface was polished first with 400- and then 600- and 1200-grit abrasive papers in a Politriz (grinder and polishing machine model PLF, Fortel Indústria e Comércio LTDA, São Paulo-SP, Brazil) until a flat, polished surface was obtained. To standardize the test specimens, the dentin surface was also worn to obtain equal fragments 5-mm long, 5-mm wide, and 3-mm high (5 × 5 × 3).11 An adhesive tape was placed on the enamel surface, and the other surfaces were covered with an acid-resistant uncolored varnish (Elke, Nasha LTDA, São Paulo, Brazil), so that a 25-mm² enamel surface remained exposed.

Of the 70 obtained test specimens, 10 were randomly selected to be submitted to repeated cycles of demineralization and remineralization and constituted Group DesRe. Another 10 fragments, also randomly selected, underwent demineralization and remineralization cycles associated with orthodontic appliance corrosion and constituted group Corrosion + DesRe. The 50 remaining specimens were not submitted to any type of treatment and formed the control group (Table 1). The test specimens of the experimental groups were numbered, the numbers being engraved on the dentin surface with a spherical diamond bur 1012 (KG Sorensen). Afterward, this surface was again covered with acid-resistant varnish.

Group DesRe was submitted to pH cycling and remained alternately in the demineralizing solution, pH 5.0 (Table 2), for 4 hours and in the remineralizing solution, pH 7.0 (Table 3), for 20 hours for a period of 8 days.12 The proportion of demineralizing and remineralizing solutions per area of enamel was 6.25 mL/mm² and 3.12 mL/mm², respectively.12 

The cycling was performed in two subgroups with five test specimens in each to diminish the quantity of liquid necessary in each receptacle. The cycling process was performed in plastic receptacles at 37°C (±1°C), with constant agitation in a Dubnoff metabolic shaker incubator, Model 145 (Fanem Ltda, Guarulhos/SP, Brazil). After the eighth cycle, the test specimens remained in the remineralizing solution for another 24 hours until the analyses were performed.13 

The group Corrosion + DesRe was also submitted to the same cycling process as group DesRe. In this group, however, 7 days before cycling began, orthodontic stainless-steel brackets, bands, tubes, and wires were added to each receptacle containing the demineralizing and remineralizing solutions, to simulate standard orthodontic appliances in mandibular and maxillary arches (Table 4). The tubes were welded to the bands, and the brackets were attached to the wires with steel ties. At the beginning of the cycling process, the orthodontic appliances immersed in the solutions presented clinical signs of corrosion such as color change and deposits of corrosion products on the surfaces of the brackets.

At the end of the experiment, each test specimen from the two experimental groups received prophylaxis with fine-grained pumice stone and water, applied with a rubber cup at low speed for 5 seconds. Following this, each one was washed with a jet of water for 60 seconds.

Before the experiment began and after completing cycling and performing prophylaxis, the color of the enamel was measured in the two experimental groups using the portable digital spectrophotometer Vita Easyshade Compact (model DEASYC220, Germany).

The system used to measure color was that of the Commission Internationale de I'Eclairage (CIE) LAB, which divides color by means of a mathematical colorimetric curve process, into three fields: L or ΔL, which represents luminosity or the color values from black to white; a or Δa axis, which measures the color from green to red; and b or Δb, which measures the axis from yellow to blue.14,15 

The measurements were taken under the same ambient light with the tooth dried with a clean cloth, and the value obtained for each test specimen at each time interval corresponded to the mean of three measurements.15 The color changes (ΔE) in the experimental groups after the cycling process were calculated by the equation ΔE_ab  =  [(ΔL)² + (Δa)² + (Δb)²]^1/2, when values from L, a, and b were different before and after the experimental period.15 Only readouts whose standard deviation for ΔE was a maximum of 1 were accepted, and after three measurements, the spectrophotometer was calibrated in accordance with the manufacturer's instructions.

The test specimens of both experimental groups and the control group were analyzed by atomic absorption spectrophotometry (spectrophotometer Varian, model AA-1475, Varian Indústria e Comércio Ltda, São Paulo/SP, Brazil) to quantify the content of metal ions of nickel, chromium, and iron at the end of the experiment. To make the analysis possible, the test specimens were previously dissolved in 3 mL of 65% nitric acid P.A. (HNO₃; Vetec Química Fina LTDA, Rio de Janeiro/RJ, Brazil).

The concentrations of nickel, chromium, and iron were evaluated statistically, applying the analysis of variance test with Tukey's post hoc test for intergroup evaluation. For color alteration, the Student's t-test was used. The data were statistically analyzed using SPSS 17.0 software (Statistical Package for Social Sciences, SPSS Inc, Chicago, Ill), and the level of significance adopted was .05.

The content of nickel, chromium, and iron in tooth fragments after the experiment was measured in all groups (Table 5). There was a significantly higher concentration (P < .05) of metal elements in the group Corrosion + DesRe when compared with the control group.

Color quantification of the experimental groups before and after the experimental time interval demonstrated color alteration (ΔE) in both groups, with a significantly greater extent (P ≤ .001) in the group Corrosion + DesRe (Table 6).

There are clinical reports of dark stains on teeth after the removal of metal orthodontic accessories, such as brackets and bands, which possibly occur because of the diffusion and incorporation into the enamel of products arising from the corrosion of these accessories.4,6,9,16–18 However, there were no in vitro studies proving the incorporation of these metals into tooth enamel. A search for articles published until the second week of September 2011 in all languages was performed in the Ovid, VHL, Scopus, and PubMed databases. There were no in vitro studies evaluating the incorporation of metals into enamel that included the descriptors corrosion, orthodontic appliances, and color.

In this study, the corrosion of standard fixed orthodontic appliances was associated with alternate enamel demineralization and remineralization cycles. This cycling model simulates the variation in pH that occurs in the oral environment in patients with deficient oral hygiene and favors the development of caries lesions.12,13,19,20 In these patients, there is also increased susceptibility to corrosion since the accumulation of acidogenic microorganisms around the bracket favors its biodegradation.4,7,16,17,21 

The atomic absorption spectrophotometry findings demonstrated that the concentration of nickel, chromium, and iron in the tooth fragments of the group with corrosion and cycling was higher than that found in both the control group and the group in which there were only demineralization and remineralization cycles (Table 5). Possibly, this higher concentration came from the additional incorporation of metal ions arising from the corrosion of orthodontic appliances.

The group submitted to cariogenic challenge only, without the presence of orthodontic accessories, also incorporated nickel and iron (Table 5). Possibly these metal ions arise from the solutions since the reagents used to obtain the demineralizing and remineralizing solutions did not present 100% purity. All of them accepted a maximum limit of impurities in their composition, and among these, there were iron and other heavy metals such as nickel. Only chromium was not present in the test specimens of this group.

The incorporation of metals was responsible for significant alteration in the color of the tooth fragments. Color quantification in the experimental groups demonstrated clinically perceptible (ΔE>3.7)22 color alteration in both groups (Table 6). However, in the group in which corrosion of orthodontic accessories was associated, the color measured at the end of the cycling process varied to a considerably greater extent (P ≤ .001; Table 6).

In both experimental groups, the test specimens darkened since the two groups incorporated metal ions. This can be proved by field L or ΔL of the CIE LAB color-measuring system, which represents luminosity, or the values of the color from black to white.2,14,15 In both groups, the values of this field were reduced, denoting alteration in the direction toward black, and in the group Corrosion + DesRe, this reduction was greater (Table 6).

The name chrome is of Greek origin (chroma) and means color. This metal was so called because all of its components are colored and frequently used for the fabrication of pigments used in the industry as coloring agents.23–27 In orthodontics, chromium is also pointed out as a factor responsible for the appearance of darkened stains in enamel associated with the corrosion of brackets.6,9,16 

In this study, chromium played a fundamental role. Its concentration in the group Corrosion + DesRe was significantly higher than that found in the other groups (P ≤ .001). There was practically no chromium incorporated into the test specimens of the control and DesRe groups (Table 5). Possibly, this presence of chromium was determinant in the significantly greater color alteration (P ≤ .001) in group Corrosion + DesRe (Table 6). The Pearson correlation test was performed to verify the degree of the relationship between the variables chromium and ΔE, and a coefficient of correlation of .772 (strong correlation) and a high significance (P ≤ .001) were found.

Nickel, also pointed out as being responsible for darkened stains in enamel,6,9,16 presented a significantly higher concentration in the group Corrosion + DesRe than that found in the group DesRe (P < .05; Table 5). Possibly, this additional incorporation of nickel was due to products arising from the corrosion of orthodontic accessories since both the experimental groups were submitted to the same demineralizing and remineralizing solutions. The control group presented a significantly lower concentration of nickel than that found in the other groups (P < .05).

There was a statistically significant incorporation of iron (P < .05) in the experimental groups when compared with the control group. The concentration of iron was higher in the group Corrosion + DesRe. However, it was not significantly higher than that found in the group DesRe (Table 5). This incorporation was probably more due to the ions of iron dispersed in the solution than those released by orthodontic appliance corrosion.

Although the oral environment is extremely complex and difficult to reproduce in vitro, this study represents an extreme situation and revealed, within its limitations, that the use of good-quality brackets with corrosion-resistant metals and efficient cleaning of the orthodontic appliances are fundamental.9,16,17,28 It was found that the incorporation of metal ions into tooth enamel submitted to a cariogenic challenge is possible and is responsible for the appearance of darkened stains, so the proper selection of the bracket and constant attention to oral hygiene is essential.

In addition, it is important to examine orthodontic accessories in routine consultations to seek out possible areas of corrosion. Should areas of biodegradation be perceptible, the accessory must be replaced immediately.5,9,16 

• The null hypothesis is rejected.

• There was chromium and nickel incorporation into enamel when a corrosion of orthodontic appliances was present.

• The incorporation of metal ions was capable of significantly altering color (ΔE).