Effect of cigarette smoke on aesthetic brackets: an in vitro study

ABSTRACT Objective: The objective of this study was to evaluate the effect of cigarette smoke (CS) on physical and mechanical properties of ceramic, polycarbonate and alumina ceramic brackets. The null hypothesis tested was that aesthetic brackets would not be influenced by CS. Methods: Ninety aesthetic brackets were allocated to three groups (n = 30): ceramic (GCE), polycarbonate (GCO) and alumina ceramic (GPS). Ten samples of each group were assigned to color and surface roughness analysis, performed before (T0) and after (T1) exposure to CS; and twenty samples were allocated into control and experimental groups (n = 10) (not exposed and exposed to CS, respectively) for shear bond strength test (SBS). Exposure to CS followed an adaptation of the method described by Le Mesurier. Colorimetric reading, surface morphology and roughness, SBS and adhesive remnant index (ARI) were assessed. Statistical analysis comprised independent and paired t-tests, ANOVA/Tukey and Fisher’s exact tests (α = 0.05). Results: Changes were observed in brackets’ color (NBS: GCE = 2.4; GCO = 1.9; GPS = 2.1), surface roughness (ΔRa: GCE = 1.1 ± 0.8 µm; GCO = 1.9 ± 1.5µm; GPS = -0.3 ± 0.1 µm / ΔRz: GCE = 1.4 ± 1.0 nm; GPS = -0.5 ± 0.1 nm); and SBS (GPS - experimental = 221.8 ± 48.6 N) after exposure to CS (p< 0.05). Conclusions: Exposure, in vitro, of aesthetic brackets to CS resulted in changes of color to darker and more opaque shades, surface roughness alterations, and higher SBS values. ARI scores were not associated with exposure to CS.


INTRODUCTION
Orthodontics aims to achieve functional efficiency of the stomatognathic system, facial and dental aesthetics, periodontal health, as well as the stability of treatment outcomes. Among these, the search for aesthetic improvement is one of the main reasons why patients seek orthodontic treatment. 1 Aesthetic brackets may be translucent, made of plastic or monocrystalline ceramic (called sapphire); or non-translucent, made of plastic or polycrystalline ceramic (machined or injected). However, the aesthetic property is also directly related to good color stability. 2 Monocrystalline and polycrystalline ceramic brackets have been reported to resist staining or discoloration caused by substances that can be found in the mouth, 3 and that plastic brackets have undesirable effects of darkening (staining) after a short time in the oral cavity. 4 However, ceramic and plastic brackets may be affected by endogenous and exogenous factors when exposed to the oral environment. 5 As exogenous factors, the aesthetic brackets are susceptible to changes in its optical properties due to pigment staining in food and beverages. 6 However, the endogenous discoloration might be caused by UV irradiation and thermal energy. 4 Borges L, Castro ACR, Elias CN, Souza MMG -Effect of cigarette smoke on aesthetic brackets: an in vitro study 5 The effect of aging and chemical agents on the color stability of aesthetic brackets may vary according to their constitution, morphology, and surface characteristics, and may influence their aesthetic performance. 3,7 In the case of smoking patients, the oral cavity is susceptible to cigarette smoke, which is constituted of toxic substances, as carbon monoxide, ammonia, nickel, arsenic, tar, lead and cadmium. 8 According to the World Health Organization (WHO), there are In the oral cavity, the cigarette smoke compounds impregnates tooth and resin composites surfaces , incorporating yellow and blackish pigments. 10,11 Thus, it is also questioned whether aesthetic brackets may suffer color changes from exposure to cigarette smoke, and the heat generated by smoking. Therefore, this study aimed to analyze the color change, surface roughness and resistance to debonding of aesthetic brackets when exposed to cigarette smoke. The null hypothesis of this study was that physical and mechanical properties of aesthetic brackets would not be influenced by cigarette smoke. A previous sample calculation, as described by Pandis, 12 considering an alpha of 0.01 and power of 90%; required a minimum of 9.3 samples, in order to detect a color change of 1 in ΔE measurement, with a standard deviation of 0.56 mm. 13 Then, a minimum of 10 samples for each group was considered in the present study. Ninety aesthetic brackets were allocated to three groups (n=30) according to the type of bracket material: GCE -ceramic brackets (Ceramic); GCO -polycarbonate brackets (Composite); and GPS -ceramic of alumina brackets (PolySafira) (Morelli ® , Sorocaba/SP, Brazil). Samples from each group were further divided according to experimental tests, as follows: 10 samples were assigned to color and roughness analysis, evaluated before (T 0 ) and after (T 1 ) exposure to cigarette smoke; the remaining 20 samples from each group were allocated to the shear bond strength test, of which 10 samples were not exposed to cigarette smoke (control group) and 10 samples were exposed to cigarette smoke (experimental group).

EXPOSURE TO CIGARETTE SMOKE
The method used for brackets exposure to cigarette smoke followed an adaption of Le Mesurier et al. 14 The exposure was performed in a hermetically sealed acrylic device, that contained two chambers (Fig 2A and 2B), separated by a partition containing ten holes (Fig 2C), an air inlet (Fig 2D), and two air outlet holes ( Fig 2E). into holes E to create a negative pressure and cause air to pass through the cigarette filter barrier so that the smoke came into contact with the specimens (Fig 3). The value obtained for each specimen (L*a*b*) corresponded to the mean of these measurements.
The color was analyzed according to the International Commission of I'Eclairage (CIE) color scale, concerning the lighting pattern D65, that distributes the color by a mathematical process of the colorimetric curve into three fields, as follows: "L" or "ΔL", which represents brightness, or color values from black to white; "a" or "Δa", comprising colors from green to red; and "b" or "Δb", comprising colors from yellow to blue. 16 The color change (ΔE) was calculated by the following equation: Data normality was verified with Shapiro-Wilk test. Evaluation of color and roughness variables before and after exposure to smoke was performed with paired t-tests. Comparisons between control group (not exposed) and experimental group were performed with ANOVA/Tukey test. The adhesive remnant index was evaluated by a Fisher's exact test. The significance level of 0.05 was adopted in all analyzes.

RESULTS
Descriptive statistics of before and after exposure to smoke results and intergroup comparisons for L*, a*, b*, ΔE and their conversions to NBS scale parameters are presented in Table 1.
Intergroup differences were observed before and after exposure to smoke in both Ra and Rz parameters (p < 0.05). Changes in bracket's surface morphologies are illustrated in Figure 4.    Values of the shear bond strength test and intergroup comparisons are presented in Table 3. Polysafira brackets presented a significant difference between control and experimental groups, as higher SBS values were observed in the group exposed to

DISCUSSION
Changes in color can be detected visually or with the aid of a colorimeter. Nevertheless, the human eye is not able to observe slight color differences, and therefore, visual color interpreting and comparison is performed subjectively. Thus, in order to reproduce the results of color assessment, colorimetric measurements became necessary. 21 The color assessment performed in the present study indicated that after exposure to smoke, brackets became visually darker and opaque (Fig 6). Despite the lack of intragroup differences for L* parameter, values of b* decreased in Ceramic and Polysafira groups, and values of a* increased in Composite group, after exposure to smoke.
Ruyter et al. 22 reported in 1987 that a color change (ΔE) of

is visually perceptible and thus clinically unacceptable.
The ΔE values obtained from the present study did not exceed this limit and did not show a statistical significant difference (p > 0.05) among the groups. In order to relate the value of ΔE to clinical standards, ΔE was converted to the National Bureau of Standards (NBS) scale ( Table 1). The values of ΔE were converted using the equation 17 : NBS = ΔE * x 0.92, and all resulted in noticeable color changes (1.5 < NBS < 3.0). Previous researches have investigated the color stability of dental materials in vivo and in vitro, 23,24 and observed the influence of cigarette smoke in color changes and surface texture of dental composites; 13 and also changes in color stability (ΔE > 3.3) of aesthetic brackets after exposition to UV irradiation, and to different food dyes, as red wine, coffee and tea. 4 With the production of polycarbonate brackets in the 1970s, deficiencies such as wing fractures, distortion and staining 25 were surpassed with ceramic reinforcement and metal slot brackets. Ceramic brackets, in general, still have a rough surface when compared to stainless steel brackets. 26 Additionally, polycrystalline brackets have increased coefficient of friction when compared to monocrystalline ceramic and stainless steel brackets, and this is attributed to their rougher and more porous surface. 27 In the present study, these characteristics could also be observed before exposure to smoke, as polysafira group had increased roughness values, compared to ceramic group.
However, after exposure to smoke, ceramic group presented higher values of roughness, in relation to polysafira group.
Despite composite group had higher initial roughness (Ra) and greater roughness variation (ΔRa), it was the least affected by color variation (ΔE). In contrast, ceramic group presented lower values of initial roughness (Ra and Rz), and was the group most affected by color variation (ΔE).
Borges L, Castro ACR, Elias CN, Souza MMG -Effect of cigarette smoke on aesthetic brackets: an in vitro study 21 An efficient ceramic bracket must provide adequate bond strength to dental enamel, but also offer easy debonding, without causing damage to tooth surface. A great advantage of ceramic brackets compared to metallic brackets is that, ceramic translucency allows the transmission of light during photopolymerization and thus, provides a higher shear bond strength 28 . According to the present results, polysafira brackets presented significant higher SBS values in specimens exposed to cigarette smoke. Composite brackets' control group and ceramic brackets' experimental group presented the lowest and the highest debonding values, respectively.
Despite there was no association of ARI scores' distribution between control and experimental groups, it is worth noting that before exposure to smoke, no case of ARI score 3 was observed, but after exposure to smoke, a certain frequency of this score was noticed. ARI score 1, in which less than half of the adhesive remains adhered to enamel surface after debonding, was predominantly frequent in the majority of the groups, which may represent more tension in the enamel-adhesive interface, and thus increase the chance of dental damage. 29 The main limitations of this study consists of its in vitro design,

CONCLUSIONS
The exposure of aesthetic brackets to cigarette smoke, in vitro, revealed that: » Brackets color changed to darker and more opaque shades.
» Ceramic and composite brackets presented increased average roughness (Ra) and medium depth roughness (Rz), whereas polysafira brackets presented a decrease in surface roughness parameters.
» Polysafira brackets exposed to CS presented higher SBS values, compared to non-exposed brackets.
» ARI scores were not associated to exposure to cigarette smoke.
Thus, the null hypothesis of the present study was rejected.