1.

Introduction

The importance of artificial tooth staining is clear considering that a significant part of the population needs rehabilitation treatment with removable dentures. As aesthetic demand increases and patients’ expectations are high, stain resistance plays a significant role in the selection of denture teeth.¹

Acrylic resin denture teeth are used more frequently because they form a chemical bond with the acrylic resin denture base, are lighter, have a lower propensity to fracture, and have greater ease in occlusal adjustment.^2–4 However, acrylic teeth are inferior to porcelain teeth in regard to maintaining color stability,⁵ and they do not remain aesthetic over an adequate long-term period and have less durability.²

Denture discoloration can be caused by many factors, which can be divided into intrinsic factors, such as the characteristics of the material composition (e.g., type of photoinitiator and inorganic material), and extrinsic factors relating to wear and exposure to staining substances causing absorption and adsorption of stains.^1,2,4–6

Several authors have shown the effect of beverages on the color stability of direct and indirect restorative materials^7–22 and acrylic resin denture bases.^23–28 The authors^7–9 observed that both composite and glass ionomer cement are susceptible to staining by the action of various beverages. Gawriołek et al.¹⁴ evaluated the color stability of composites and ceramics after immersion in tea, red wine, coffee, and distilled water. The composites demonstrated lower color stability than the ceramic materials. Leite et al.²⁴ evaluated the color stability and durability of two soft liners using direct and indirect techniques after thermocycling and immersion in coffee, tea, soda, and red wine. In general, coffee was the solution that most altered the color of these materials. Polished specimens had lower color changes than nonpolished specimens.

There are few in vitro studies^1–3,5,29 evaluating the color stability of artificial teeth used in dentures. Studies^1,2,5 on the staining of acrylic resin denture teeth after exposure to beverages such as tea, coffee, cola, juice, or wine are restricted to a few brands. In these studies, the authors observed that all evaluated teeth had good color stability, but that the degree of coloration varied between them.

As the dental market offers a wide variety of denture teeth, a quality evaluation of denture teeth properties that is based on scientific findings is necessary to allow the best choices by clinicians. Thus, the aim of this study was to evaluate the color degradation of 10 different brands of acrylic resin denture teeth when exposed to different beverages (coffee, orange juice, and cola) for four immersion times (1, 7, 15, and 30 days) using spectrophotometry. The null hypothesis being tested was that there would be no significant color change in any of the denture teeth brands irrespective of staining solution and measurement period.

2.

Material and Methods

3.

Results

Table 2 shows the mean and standard deviation of $\mathrm{\Delta }E$ for each type of denture tooth for the different measurement periods and solutions. The obtained $\mathrm{\Delta }E$ values showed no clinically significant change in color ($\mathrm{\Delta }E>3.3$) (Table 2). The largest color change occurred for the BI denture that was immersed in cola for 30 days, and the lowest color change was observed for the BX denture that was immersed in cola for 1 day.

Table 2

Mean values (SDs) of color change (ΔE) in denture teeth.

Factors for dentures, solution, measurement period, $\text{tooth}\times \text{solution}$, and time of $\text{measurement}\times \text{tooth}$ interactions were statistically significant (ANOVA) (Table 3). Considering the different brands of denture teeth, regardless of the measurement period and solution used, BI ($\mathrm{\Delta }E=1.82\pm 0.95$) and TR ($\mathrm{\Delta }E=1.78\pm 0.72$) had a higher degree of color change with no significant difference between them ($P=1$). Meanwhile, BX ($\mathrm{\Delta }E=0.88\pm 0.43$) and MD ($\mathrm{\Delta }E=1.09\pm 0.44$) had the lowest values with no significant difference between them ($P=0.396$); however, they were significantly different from BI and TR ($P<0.05$) (Fig. 1).

Table 3

Results of three-way repeated measures analysis of variance (ANOVA).

Fig. 1

$\mathrm{\Delta }E$ mean values of denture teeth regardless of solution and measurement period.

In comparing solutions, regardless of the brand of denture tooth and measurement period, cola was the solution that caused the greatest degree of color change ($\mathrm{\Delta }E=1.51\pm 0.83$), but there was no significant difference between cola and coffee ($\mathrm{\Delta }E=1.48\pm 0.74$) ($P=0.944$). However, cola was significantly different from orange juice ($\mathrm{\Delta }E=1.36\pm 0.73$) ($P=0.048$) and artificial saliva ($\mathrm{\Delta }E=1.18\pm 0.59$) ($P<0.0001$). Artificial saliva showed the lowest values of color change (Tukey HSD test).

In evaluating the $\mathrm{\Delta }E$ values across the different measurement periods, regardless of the type of denture tooth and solution, 30 days ($\mathrm{\Delta }E=1.68\pm 0.84$) of immersion produced statistically higher values of color change in the denture teeth ($P<0.05$), which was followed by 15 days ($\mathrm{\Delta }E=1.40\pm 0.72$). However, 1 day ($\mathrm{\Delta }E=1.20\pm 0.62$) and 7 days ($\mathrm{\Delta }E=1.25\pm 0.66$) of immersion produced similar color change values to each other ($P=0.833$).

For all denture teeth, regardless of the measurement period, no significant differences were found between the staining solutions and artificial saliva, except for the BI teeth immersed in cola ($\mathrm{\Delta }E=2.18\pm 1.13$, $P<0.0001$) and orange juice ($\mathrm{\Delta }E=2.01\pm 0.91$, $P=0.01$) and for the TB teeth immersed in cola ($\mathrm{\Delta }E=1.84\pm 0.8$, $P=0.002$). All teeth showed similar color change values when immersed in artificial saliva ($P>0.05$). For teeth immersed in coffee, the MG teeth showed the highest color change ($\mathrm{\Delta }E=2.06\pm 0.92$), which was statistically significant in relation to BX ($\mathrm{\Delta }E=1.06\pm 0.37$, $P<0.0001$), TB ($\mathrm{\Delta }E=1.08\pm 0.62$, $P<0.0001$), SR ($\mathrm{\Delta }E=1.30\pm 0.48$, $P=0.021$), MD ($\mathrm{\Delta }E=1.15\pm 0.46$, $P=0.0004$), and AP ($\mathrm{\Delta }E=1.25\pm 0.58$, $P=0.007$) teeth. For the cola solution, the BI teeth showed the highest color change ($\mathrm{\Delta }E=2.18\pm 1.13$), but there were similarities among TB ($\mathrm{\Delta }E=1.84\pm 0.8$, $P=0.999$), VD ($\mathrm{\Delta }E=1.86\pm 0.82$, $P=1$), and TR ($\mathrm{\Delta }E=1.88\pm 0.77$, $P=1$) teeth. When immersed in orange juice, the BI teeth showed the highest color change ($\mathrm{\Delta }E=2.01\pm 0.91$), which was significantly different from BX ($\mathrm{\Delta }E=0.71\pm 0.30$, $P<0.0001$), SR ($\mathrm{\Delta }E=1.15\pm 0.39$, $P=0.002$), MD ($\mathrm{\Delta }E=1.08\pm 0.38$, $P=0.0003$), MG ($\mathrm{\Delta }E=1.24\pm 0.64$, $P=0.019$), and AP ($\mathrm{\Delta }E=1.07\pm 0.44$, $P=0.0002$) teeth (Fig. 2).

Fig. 2

$\mathrm{\Delta }E$ mean values of denture teeth as a function of solution type, regardless of measurement period. Uniform standard deviations of $\mathrm{\Delta }E$ averaged $\pm 0.5$ for AP, BX, MD, and SR teeth; $\pm 0.6$ for MG, TB, and VD teeth; and $\pm 0.8$ for BI, PR, and TR teeth.

Regardless of the staining solution, there were no statistically significant differences between the measurement periods for all denture teeth except for TB ($\mathrm{\Delta }E=1.73\pm 0.88$), PR ($\mathrm{\Delta }E=1.96\pm 1.10$), and BI ($\mathrm{\Delta }E=2.52\pm 0.89$) teeth, where the 30-day period produced the highest values of $\mathrm{\Delta }E$ ($P<0.05$). For the 1-day period, the BX tooth ($\mathrm{\Delta }E=0.75\pm 0.35$) had the lowest amount of color change, and this value differed from BI ($\mathrm{\Delta }E=1.50\pm 0.67$, $P=0.026$) and TR ($\mathrm{\Delta }E=1.59\pm 0.63$, $P=0.003$) teeth. For the 7-day period, the TR tooth had the greatest color change ($\mathrm{\Delta }E=1.75\pm 0.66$), and this value was significantly different from MD ($\mathrm{\Delta }E=0.97\pm 0.52$, $P=0.015$) and BX ($\mathrm{\Delta }E=0.76\pm 0.35$, $P<0.0001$) teeth. For the 15-day period, the TR tooth again had the highest value of $\mathrm{\Delta }E$ ($\mathrm{\Delta }E=1.81\pm 0.90$), and this value was significantly different from SR ($\mathrm{\Delta }E=1.09\pm 0.35$, $P=0.04$) and BX ($\mathrm{\Delta }E=0.90\pm 0.36$, $P=0.0004$) teeth. In the 30-day period, the BI tooth showed the highest color change ($\mathrm{\Delta }E=2.52\pm 0.89$), but there was no significant difference with the TR ($\mathrm{\Delta }E=1.98\pm 0.66$, $P=0.589$), PR ($\mathrm{\Delta }E=1.96\pm 1.10$, $P=0.484$), and MG ($\mathrm{\Delta }E=1.85\pm 0.78$, $P=0.127$) teeth (Fig. 3).

Fig. 3

$\mathrm{\Delta }E$ mean values of denture teeth as a function of measurement period, regardless of solution. Uniform standard deviations of $\mathrm{\Delta }E$ averaged $\pm 0.4$ for BX, MD, and SR teeth; $\pm 0.6$ for AP, TB, and VD teeth; and $\pm 0.7$ for BI, MG, PR, and TR teeth.

4.

Discussion

The null hypothesis was rejected, as statistically significant differences were found among the color changes of the denture teeth according to the solution and measurement period. However, all color changes were within the clinically acceptable range ($\mathrm{\Delta }E\le 3.3$); thus indicating a satisfactory color stability of the evaluated teeth.

The staining solutions used in this study act as an extrinsic factor for color alteration due to the absorption and adsorption of these stains.^1,2,4–6 Increase in solubility induces greater damage to the surface, affecting the surface roughness and the color of an acrylic resin.^28,32 As previously stated, specimens of soft liners with polished surface exhibited less color alteration than nonpolished ones.²⁴ Therefore, it is possible to infer that the surface profile/topography affects the amount of staining of a material. The pH of the staining solution may be another driving force toward material color change. Acidic solutions promoted greater material’s surface degradations. Increased surface area is more prone to be stained than the flattened one.²⁴

Color changes can be evaluated clinically or by instrumental techniques by means of a spectrophotometer or calorimeter.^2,16 These devices minimize subjective errors and are more accurate than visual measurement.^10,17 In the present study, a spectrophotometer was used to measure the amount of reflected light, and the color was then evaluated by the CIE $L*a*b*$ color system. The $L*$ value is a measure of the whiteness or brightness of an object. The $a*$ value is a measure of the redness (positive $a*$) or greenness (negative $a*$). The $b*$ value is a measure of the yellowness (positive $b*$) or blueness (negative $b*$). The advantage of the CIE $L*a*b*$ system is that color differences may be expressed in units that can be related to visual perception and have clinical significance.^6.

To avoid factors such as superficial color and clarity of the sample influencing color determination,^16,17 a D65 lighting standard and black background were adopted. Other factors that may influence the evaluation of color are the smoothness and thickness of the sample; however, the sample thickness had no influence in this study, because the color determination was performed in two three-dimensional reference positions in color space $L*a*b*$.¹⁷

In addition to the light source of the spectrophotometer, the position of the specimen affects the color measurement. A silicone rubber device was manufactured in order to provide the maintenance of each denture tooth in the same position during all measurement periods. After each color measurement of each group, the spectrophotometer was calibrated with a 100% white color device, as an effort to avoid errors during color measurement.

Overall, the BI and TR teeth exhibited the highest values of $\mathrm{\Delta }E$ with no significant difference between them. These data partially corroborate the results found by Assunçao et al.,³ in which the greatest color change was found for the BI tooth. Kanie et al.²⁷ noted that the luminosity of the acrylic resin decreases as the material thickness increases. Therefore, it is believed that the BI and TR teeth are transversely thicker and have a high proportion of dark resin (less light transmission), which could explain the greater color change.

The lowest $\mathrm{\Delta }E$ values were found for the BX and MD groups, corroborating previous results.³ Acrylic resin teeth are essentially formed of polymethylmethacrylate and have a high conversion rate and low levels of dibenzoyl peroxide. Dibenzoyl peroxide remains after the conversion reaction and can cause deterioration of the color stability.³⁰ According to Assunção et al.,³ BX and MD teeth have small quantities of dibenzoyl peroxide, explaining their reduced color change after staining for up to 30 days. In addition, hydrophilic materials generally exhibit a greater degree of color change than hydrophobic materials.²⁸ The use of hydrophobic monomers, such as 2-hydroxyethyl methacrylate, for the production of denture teeth can increase the color stability of these teeth. This result could explain why the BX and MD teeth had the lowest color change.

Cola and coffee were the staining solutions that caused the greatest degree of color change with no significant difference between them. This finding was consistent with several studies^1,2,5,16,18 that also concluded that coffee was the most chromogenic solution for different restorative materials. In the study by Mutlu-Sagesen et al.,⁵ cola produced a slight color change in conventional acrylic resin, reinforced acrylic resin, and porcelain denture teeth.

In this study, the degree of staining of the denture teeth was time dependent, and a longer immersion in chromogenic solutions caused a greater color change of the denture tooth. These findings are in line with those of previous studies.^1,2,12,23 For the purposes of this study, an immersion period of 30 days was established. The use of this period was based on the fact that most in vitro studies^{2,4,5,12,19,21,22} have used a time similar to 30 days to obtain a cumulative staining effect.

One limitation of this study may be the protocol used for staining because, clinically, saliva is present, and the teeth do not stay in contact with the staining solutions constantly or for over a long period. Furthermore, teeth are generally cleaned using mechanical means such as brushing, and the use of chemical agents such as mouthwashes and toothpaste that can alter topography and also interfere with staining.

The solutions were stored at 37°C, which simulated the oral cavity. However, coffee is normally ingested when hot, while cola is ingested when cold, and thus these temperature changes could also affect the color change as they can create more porosity, thereby changing the topography.³

The present study sought to evaluate the influence of chromogenic beverages on denture tooth staining; however, other factors may also influence color changes. Future studies evaluating the combined effect of brushing and staining solutions are warranted. Additionally, the effect of chemical cleaning agents on the color stability of artificial denture teeth is needed. For more practical information, longitudinal studies monitoring the color change of denture teeth of removable denture users is desired. This is important to evaluate how these different variables interact in vivo.

5.

Conclusions

Notwithstanding the limitations of this in vitro study, the following conclusions were drawn: