Evaluation of methods for stain removal in acrylic resin denture teeth: in vitro study

Introduction: The staining of artificial teeth can be related to the acrylic resin abrasion caused by brushing, resulting in higher deposition of dyes from the beverage, and consequently higher aesthetic damage. Objective: The aim of this in vitro study was to evaluate methods for removal of stains from acrylic denture teeth using spectrophotometric analysis. Material and method: Artificial teeth were divided into twelve groups (n=10) according to the type of treatment (re-polishing Re or immersion in Corega Tabs Sp), staining solutions, coffee (Cf) and Coca-Cola (Cc) or water (W) and with/without toothbrushing (B). The Sp specimens were submitted to seven immersion cycles (5 min each). The Re specimens were polished with pumice stone followed by Spain white paste. Color differences (ΔE) were captured by a spectrophotometer: T0 (baseline), T1 (after brushing/immersion in solutions) and T2 (after Re or Sp). Result: Statistically significant color change between T1 and T2 (paired T-test; α =.05) was observed for the group CfSp (p=.032); and for the groups BWRe (p=.000), BCfRe (p=.049) and CcRe (p=.042). Higher color changes were observed for the specimens submitted to toothbrushing (ANOVA two way; p<.001). Conclusion: It could be concluded that the immersion in sodium perborate (Corega Tabs) can be used for removal of coffee stains from denture teeth, and re-polishing for removal of Coca-Cola stains. Still, toothbrushing produced greater color changes on denture teeth, regardless of the immersion solution. Descriptors: Tooth artificial; denture complete; toothbrushing; color stability. Cassiano, Leite, Policastro et al. Rev Odontol UNESP. 2016 July-Aug; 45(4): 195-200 196


INTRODUCTION
Acrylic resin artificial teeth are frequently used for dentures fabrication due to several advantages, such as the chemical bond to the denture base, improved aesthetics and the ease of adjustment 1,2 . However, resin teeth have the disadvantage of high susceptibility to stains [2][3][4] .
Clinical discoloration and extrinsic staining in acrylic resin teeth can be affected by many factors, including the type of denture tooth, dietary habits and oral hygiene of patients 2 . Different beverages, accelerated aging, light activation, bleaching, cleaning agents, and smoking affect the color of such materials 5 . The extrinsic stain is time dependent and associated with eating habits, such as consumption of tea, red wine, cola and coffee that act as an extrinsic factor for color alteration due to the absorption and adsorption of these stains 1,6 . Furthermore, improper brushing techniques could potentially cause wear of denture teeth and acrylic resins and adversely affect the esthetic of the denture, providing an increased roughness and increasing the accumulation of dyes derived from the diet 6 . Therefore, it is possible to suppose that the surface topography affects the amount of staining of a material, and an increased surface area is more prone to be stained than a flattened one 6 . Acidic solutions promoted greater material surface degradations; therefore the pH of the staining solution may be another factor affecting the material color change 1 . Bagheri et al. 7 demonstrated the pH of different staining solutions, among them coffee (pH=5.01), red wine (pH=3.70) and cola (pH=2.70).
The staining of acrylic resin denture teeth may be removed by mechanical or chemical denture cleansing regimens 8 . The abrasive action of polishing has been used to remove extrinsic stains on the surface of acrylic resin 9 , and it is considered a basic procedure for obtaining good aesthetics as well as smooth and polished surfaces 10 , facilitating the hygiene and comfort of users of removable dentures 11 . Alkaline peroxides could also contribute to stain removal by means of the mechanical action of the bubbles of oxygen resulting from the effervescent reaction of this product 12 .
Many studies have investigated the effects of staining pigments in denture teeth 2,5,6 . However, few studies evaluated methods for removing stains from beverages in acrylic resin denture teeth 8 . The aim of this study was to evaluate, by means of spectrophotometric analysis, methods for removal of stains from acrylic denture teeth. The null hypothesis was that color stability of the denture teeth would not be influenced by the procedures for removing stains during the proposed period. In addition, it was hypothesized that toothbrushing would not affect the color change of the denture teeth, regardless of the coloring beverage.

Experimental Design
The specimens consisted of artificial acrylic resin tooth, maxillary right central incisors; shade A2 (Biotone, Dentsply Ind. e Com, Rio de Janeiro, RJ, Brazil). Artificial teeth were divided into twelve groups (n=10). Table 1 describes the groups of the study.

Toothbrushing
The toothbrushing test was performed in a testing machine (MAVTEC, comércio e serviços de desenvolvimento para laboratório, Ribeirão Preto, SP, Brazil), where 11,000 brushing cycles were performed on the buccal face of the artificial tooth, simulating one year of brushing 13,14 .
A solution of distilled water with toothpaste (Colgate, Colgate-Palmolive, São Bernardo do Campo, SP, Brazil) at a ratio of 1:1 by bulk, and Classic Colgate brushes (Colgate, Colgate-Palmolive, São Bernardo do Campo, SP, Brazil) with soft bristles, were chosen for testing. The brushes and the solution were replaced every 2,750 cycles, simulating three months of use.

Preparation of Staining Solutions
Coffee (Nescafé tradição instantâneo, Nestlé Brasil Ltda, Araras, SP, Brazil) and cola (Coca-Cola, Curitiba, PR, Brazil) were the beverages selected in this study, and distilled water was used as a control group. Coffee solution was prepared by adding 3.6 mg of coffee to 300 mL of boiling distilled water and this solution was used only after its complete cooling.
The artificial teeth were immersed in 20 mL of each beverage and remained immersed for 12 days at 37 °C, simulating 1-year daily consumption of beverages 15 . The beverages were replaced every 3 days, and stirred once a day to avoid particles precipitation 1 .

Protocols Proposed for the Removal of Stains
Chemical method: immersion in sodium perborate For Groups 1 to 6, the effervescent solution of sodium perborate (Corega Tabs, Stafford Miller Ind., Rio de Janeiro, RJ, Brazil) was prepared according to the manufacturer's instructions, by adding one tablet to 200 mL of warm tap water (40 °C). The specimens were submitted to seven immersion cycles (5 min each), simulating a 7-day use 16,17 . After each cycle, the soaking solution was discarded and the specimens were thoroughly washed in running water. After having been subjected to the seven cycles of immersion in sodium perborate, the specimens were washed for 1 minute in an ultrasonic device containing deionized water and 1%detergent, and were then dried with paper towels prior to the color readings 15 .
Mechanical method: re-polishing The specimens from Groups 7 to 12 were subjected to a re-polishing procedure according to the routine laboratory procedures used for polishing dentures. For this, a micromotor coupled to a handpiece was fixed on a device to standardize this procedure. This device was also used to adequately position the specimen ( Figure 1).
Each specimen was polished in a two-step procedure: 1) use of a soft bristle brush (Bordente, São Paulo, SP, Brazil) and pumice stone (SSWhite Artigos Dentários Ltda., Rio de Janeiro, RJ, Brazil) and; 2) use of a felt wheel (Bordente, São Paulo, SP, Brazil) and Spain white paste (Asfer, São Caetano do Sul, SP, Brazil). For each step, each specimen was polished for 10 seconds, six times per specimen, at 3,000 rotations per minute 18 . The pumice stone and Spain white pastes were prepared in the proportion of 5.0 g to 1 ml of water. After the re-polishing procedures, the specimens were washed for 1 minute in an ultrasonic bath containing deionized water and 1% detergent and were dried with paper towels prior to color readings 15 .

Spectrophotometric Analysis of Color Stability
Color stability readings were performed at different times: T0, T1 and T2, according to the experimental diagram (Figure 2), using a portable spectrophotometer (BYK Gardner, São Paulo, SP, Brazil).
Color measurements were obtained using the CIELAB (Commission Internationale de L'Eclarirage) system, which positions the color in a coordinate system 19 . For each artificial tooth, three color measurements were made on each occasion (T0, T1 and T2) on the buccal surface of the tooth, obtaining an average. The color comparison between the readings was determined by the color difference or ΔE.

Statistical Analysis
In the present study, two separate analyses were performed to compare the values corresponding to ΔE. For comparing the T0T1ΔE and T1T2ΔE, data were submitted to a paired T-test, which showed significant effects of variation factor periods. For the comparison between the proposed stain removal treatments (Sodium perborate and re-polishing), the T1T2 ΔE data were compared by an independent T-test. In addition, ANOVA two way for independent samples was performed to compare data of T0T1ΔE with and without toothbrushing. All analyses were performed with α=.05.    Table 2, for the groups treated with sodium perborate (Groups Sp), the results showed a statistically significant color change between T0T1ΔE and T1T2ΔE, only for group CfSp. For the groups submitted to re-polishing (Groups Re), significant differences between T0T1ΔE and T1T2ΔE were found for groups BWRe, BCfRe and CcRe.
The comparison between the proposed methods for stain removal demonstrated that the type of treatment was significant for all groups except for the group of teeth immersed in water (p =.052). Treatment with Corega Tabs was more effective for the brushing + immersion in cola, brushing + immersion in coffee and immersion in coffee groups. Nevertheless, re-polishing was more effective in teeth submitted to brushing + immersion in water and immersion in cola (Table 3). Figure 3 shows the comparison of ΔE, for the groups with and without toothbrushing, after immersion protocols in the different solutions (water, cola or coffee). It was observed that toothbrushing produced higher color changes on the denture teeth (ANOVA two way, p<.001), regardless of the solution.

DISCUSSION
The null hypothesis was rejected, since significant differences were found among the color changes of the denture teeth according to the procedures for removing stains and periods evaluated   (T0, T1 and T2). The results indicated that there were differences in stain removal efficiency depending on the type of treatment of removal of stains. In addition, the hypothesis that toothbrushing would not affect the color change of the denture teeth was also reject, since higher color changes were observed for the groups submitted to toothbrushing, regardless of the coloring beverage.
Our results are in agreement with Hipólito et al. 6 and Leite et al. 21 . These authors inferred that the surface profile/topography affects the amount of staining of a material. Thus, the results of this study suggest that toothbrushing produced more rough surfaces favoring the accumulation of stains on the denture teeth.
Extrinsic factors such as adsorption or absorption of extrinsic stains are still a major problem for esthetic restorations 7 . The staining agents selected for this study are in common daily use and have a strong potential to stain acrylic denture teeth, as evaluated in many studies 1,2,4,6 .
The color change observed among the periods evaluated (T0, T1 and T2) suggests that the treatment of removing stains by immersion in Corega Tabs solution presented positive results only in the group of teeth submitted to immersion in coffee (Group CfSb). It could be demonstrated that the comparison between the T0T1 ΔE and T1T2 ΔE values for this group was statistically significant (p=0.032), and in T2, the ΔE value (1.13) was statistically lower compared to T1 (2.56). According to Jagger & Harrison 12 , this chemical method for denture cleaning performs a mechanical cleaning action induced by oxygen released during the effervescent reaction product, which could contribute to the stain removal.
In this study, the re-polishing produced a significant effect for removing stains for the brushing + immersion in water, brushing + immersion in coffee and immersion in cola groups. In these groups, the T1T2 ΔE values were statistically lower compared to the T0T1 ΔE values. The results of this study also suggest that the treatment with Corega Tabs was more effective than re-polishing for stain removal for the brushing + immersion in cola, brushing + immersion in coffee, and immersion in coffee groups. The re-polishing was only effective for stain removal for the group of teeth submitted to immersion in cola.
It could be supposed that re-polishing was more effective method for removal of stains from cola considering that immersion in this beverage produced greater surface degradations due to the acid pH of this beverage (pH=2.70) 6,7,21 . Therefore, re-polishing was more effective than Corega Tabs for removal of stains of cola because re-polishing is capable to alter the surface topography of the material, producing smoother surfaces 22 , whereas Corega Tabs used as denture cleanser did not alter the surface roughness of acrylic resin 17,23 .
On the other hand, in this study, the treatment with Corega Tabs was more effective for removal of stains of coffee. Coffee is considered the most chromogenic agent, which presents yellow dyes with chemical affinity with the acrylic resin 1,5,6,24 . It could be hypothesized that the mechanical action of the oxygen bubbles produced during the reaction of the sodium perborate solution was effective for removing coffee stains, as observed by Kurtulmus-Yilmaz, Deniz 8 . These authors showed that Corega tabs, among other denture cleansers, were efficient methods for removal of coffee, tea and red wine stains of artificial teeth.
In previous studies, it has been suggested that ΔE greater than 3.3 is considered clinically unacceptable 2,6 . In the present study, the artificial teeth submitted to immersion in cola and treated with Corega Tabs numerically showed a reduction in color change values, however, this average value reached 3.90 after this treatment. The group submitted to brushing associated with immersion in coffee treated with repolishing also obtained a reduction in the ΔE value from 7.61 to 4.67. Although the statistical test (paired t test, p=.049) has pointed out significant differences, the final ΔE value was greater than 3.3, indicating a clinically unacceptable color change 2,6 .
Thus, it could be suggested that the treatment with Corega Tabs could be more indicated for removing stains from coffee and re-polishing would be more appropriate for the removal of cola stains, since in these groups, the color change was significantly reduced and within clinically acceptable values.
Artificial brushing can be considered one of the limitations of this study because it may be vigorous and may be more abrasive than manual brushing 25 . Another limitation was the protocol used for simulation of methods of removing stains, and maybe other time periods or frequencies of re-polishing methods could provide more effective results. Further studies are needed to evaluate the efficacy of different types of treatments for the stain removal of artificial teeth.

CONCLUSION
Within the limitations of this study, the following conclusions were drawn: • Corega Tabs could be more indicated for removing stains from coffee and re-polishing would be more appropriate for the removal of cola stains. • Toothbrushing produced higher color changes on the denture teeth, regardless of the coloring beverage.