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Year : 2020  |  Volume : 23  |  Issue : 3  |  Page : 304-309

Effect of staining and denture cleaning on color stability of differently polymerized denture base acrylic resins

Department of Prosthodontics, Faculty of Dentistry, Karadeniz Technical University, Trabzon, Turkey

Date of Submission24-Jun-2019
Date of Acceptance27-Nov-2019
Date of Web Publication5-Mar-2020

Correspondence Address:
Dr. E A Ayaz
Department of Prosthodontics, Faculty of Dentistry, Karadeniz Technical University 61080 Trabzon
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/njcp.njcp_330_19

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Background: Denture hygiene is an important factor to remove dental plaque and discoloration, to provide optimal oral health, and to prevent denture stomatitis. Aim: The purpose of this study was to evaluate the effect of coffee staining and denture cleaner on the color stability of two differently polymerized (heat and microwave) denture acrylic resin in vitro. Materials and Methods: Two commonly used denture base acrylic resins and effervesecnt tablet form denture cleaner were used in this study. Disc-shaped specimens from each acrylic resin were polymerized according to the manufacturer's instructions (n = 7). Color values were recorded before and after immersion in distilled water (I), denture cleaner (II), coffee (III), and coffee + denture cleaner (IV) with a tristimulus colorimeter using CIEDE2000 color difference formula. Two-way analysis of variance (ANOVA) and Fisher's least significant difference tests were used for multiple comparisons at a 95% confidence level (P < 0.05). Surface topography of the acrylic resin denture base specimens before and after immersion were taken under 500 × magnification. Results: The two-way ANOVA revealed significant interactions between denture base resins and solutions (P < 0.001). Immersion in denture cleaner and coffee solution caused significant color changes in acrylic resin denture bases. Conclusion: Heat- and microwave-polymerized acrylic denture base resins can show different color stability. Immersion in coffee and denture cleaner solutions can cause noticeable color changes.

Keywords: Acrylic resin, color stability, denture base, denture cleaner

How to cite this article:
Ayaz E A, Ustun S. Effect of staining and denture cleaning on color stability of differently polymerized denture base acrylic resins. Niger J Clin Pract 2020;23:304-9

How to cite this URL:
Ayaz E A, Ustun S. Effect of staining and denture cleaning on color stability of differently polymerized denture base acrylic resins. Niger J Clin Pract [serial online] 2020 [cited 2020 Apr 7];23:304-9. Available from:

   Introduction Top

Oral hygiene and denture cleaning is very important in prosthodontics to remove plaque and discoloration and to provide good oral health. Dentures with poor cleanliness cause mucosal diseases such as denture stomatitis.[1] Denture cleaning techniques are either mechanical methods with toothbrush, tooth paste, soap, and water or chemical methods with disinfecting solutions or tablets and chemical agents. Commercial denture cleaners are classified as neutral peroxides with enzymes, acids, hypochlorites, peroxides, crud drugs, and mouth rinses for dentures. Denture cleaners were shown to be an acceptable method for denture cleaning although some denture cleaners are known to damage denture base resins.[2],[3] Hong et al.[4] reported that the influence of denture cleaners on the color stability of denture base acrylic resins varies according to the type of denture cleaner used. The effervescent tablet forms are commonly used to remove calculus and debris from denture surfaces effectively, but the alkaline peroxide solution may alter the resin properties if not correctly used.[5] These effervescent tablets containing sodium perborate and alkaline peroxide are reported to change the color of dental resins due to the release of oxygen by loosening debris via mechanical way.[6],[7]

Denture base resins must have the color stability and match with the oral tissue appearance during the clinical use time for optimal esthetic results. In addition, color stability of a denture resin is important because it is related to the durability of the material.[8],[9] Denture base materials and denture teeth are subjected to intrinsic and extrinsic factors that cause color change.[10],[11] It has been reported that drinking tea, coffee, and wine cause discoloration of acrylic resins.[12],[13],[14] The most frequently prefered color difference formulas are CIELab and CIEDE2000. Ren et al.[15] reported that the CIEDE2000 color difference formula provided a better fit than the CIELab formula for evaluating the color difference, perceptibility, and acceptability thresholds for denture base acrylic resins under controlled conditions in vitro.

Poly methyl methacrylate (PMMA) is the most commonly used polymer in prosthodontics as denture base resin, which was introduced in the late 1930s by Walter H. Write. In the following years, microwave polymerization was introduced by Nishii and has been used since then.[13] Considering polymerization types, acrylic denture base resins may alter the physical properties due to the polymerization degree, water sorption properties, and residual monomer content variations.[4],[16] Microwave polymerization was reported as a better technique than heat-polymerized denture resins and it was related to the decrease in the residual monomer ratio providing optimal physical properties.[17] In another study, it was reported that the color change in denture resin affected not only the polymerization technique but also the material chemical content.[16]

Many studies reported that the daily use of denture cleaners may affect the denture base resin physical property such as color, gloss, hardness, and surface roughness.[4],[7],[16],[18] Although some authors have evaluated color change and staining of acrylic denture base resins submitted to storage of oral fluids and immersion in denture cleaner, studies evaluating the color change in acrylic resin bases as affected by polymerization methods and denture cleaners are lacking. The purpose of this study was to evaluate thein vitro color stability of two differently polymerized (heat and microwave) acrylic denture base resin when subjected to coffee and denture cleaner. The first null hypothesis was that denture cleaner and coffee staining would not affect the color stability of denture base acrylic resins. The second null hypothesis was that different polymerization methods would not affect the color stability of denture base resins.

   Materials and Methods Top

Two commonly used denture base resins – one heat-polymerized (Meliodent) and one microwave-polymerized (Acron MC), are listed in [Table 1].
Table 1: Manufacturers and compositions of the materials used in this study

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Specimen preparation

Wax patterns were prepared in a stainless steel metal mold of 2 mm thickness and 10 mm diameter. Twenty-eight disc-shaped specimens from each denture base material (n = 7) were prepared according to the manufacturer's recommended monomer: liquid ratio. Heat-polymerized resin was fabricated at 100°C for 30 min and then the flasks were allowed to bench cool down in room temperature before opening. Microwave-polymerized resin was fabricated in a microwave oven (Vestel, Goldstar, Turkey) at 500 W for 3 min.

After the polymerization process was completed, residual acrylic resin was carefully removed with a tungsten carbide bur at low speed. One of the surfaces was finished with waterproof carbide papers using an automatic polishing machine (Grin PO 2V grinder-polisher; Metkon A.Ş., Bursa, Turkey) The specimens were then stored in distilled water at 37 ± 1°C for 24 h before testing.

Immersion procedures

A total of 56 acrylic resin specimens were divided into four groups: the first group was kept in distilled water and called as control, the second group was immersed in denture cleanser (Corega Tablets) prepared in enough warm water (not boiling) for 15 min and then rinsed throughly under running water (DC), the third group (CF) of specimens were kept in 7.5 g coffee solution (Nescoffee Classic; Nestle) to 500 mL boiling distilled water for a period of 4 h, and the last group of specimens were kept in coffee solution and then immersed in denture cleanser (CF + DC). These procedures were repeated daily for a period of 30 days. Fresh solutions were prepared every day. After the immersion procedures, each specimen was washed and dried before color measurement.

Color measurements

The color measurements, based on the International Organization of Standardization standards (ISO 7491), were obtained with a tristimulus colorimeter (ShadeEye NCC; Shofu Inc., Kyoto, Japan) under D65 standard illumination on a gray background. The resins were dried with absorbent paper, and the colorimeter was calibrated before the color analysis. The colorimeter was positioned in the middle of each resin during the measurements. The L*a*b* color notation of each resin was measured three times consecutively, and the average of the three readings was calculated to yield the final color of the specimen. The color change values of the denture base materials were determined using the CIEDE2000 color difference formula.

The parametric factors of KL, KC, and KH were set to 1. In this study, the CIEDE2000 values were evaluated in terms of perceptibility and acceptability. Renet al.[15] reported that the 50% perceptibility threshold was 1.72 and that the 50% acceptability threshold was 4.08 CIEDE2000 units for denture base acrylic resins. These were considered perceptibility and acceptability thresholds in this study.

SEM analysis

Scanning electron microscopy (SEM) images (Carl Zeiss EvoLs10, USA) were taken to observe the surface topography of the acrylic resin denture base specimens before and after immersion in coffee and denture cleaner solutions. The acrylic resin specimens were sputter coated with Au-Pd to a thickness of approximately 5 nm (500 × magnification).

Statistical analysis

Statistical analysis was performed with SPSS for Windows 17.0. The results were tested regarding to the normality of distribution with Shapiro–Wilk test and the homogeneity of variance using Levene test. The data were normally distributed which indicates that a parametric analysis should be used. All analyses were performed at a 95% confidence level. Two-way analysis of variance (ANOVA) and Fisher's least significant difference tests were used for multiple comparisons. The significance level was set as P < 0.05.

   Results Top

The means, standard deviations (SD), and statistical significances are shown in [Table 2]. The two-way ANOVA revealed significant interactions between denture base resins and solutions (P < 0.001) [Table 3]. Immersion in denture cleaner and coffee solution caused significant color changes in acrylic resin denture bases (P < 0.001).
Table 2: Means (SDs) of color changes in denture base resins

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Table 3: Two.way repeated measures ANOVA for color change

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The greatest Δ E was observed in heat-polymerized acrylic in the CF + DC group, whereas the smallest Δ E was in the microwave-polymerized acrylic in the control group. Immersion in DC after coffee increased the Δ E values in all the groups (P < 0.05).

Heat- and microwave-polymerized acrylic resins showed significantly different color stability in all groups (P < 0.001).

The SEM images of the denture base resins seem to be parallel with the color change values. [Figure 1] and [Figure 2] represent the similar surface property before immersion, while increased roughness and damaged surfaces were observed after immersion in CF + DC solutions in [Figure 3] and [Figure 4]. Microcracks and cavities were observed in the SEM images of denture base acrylic resins after immersion groups.
Figure 1: Scanning electron micrograph analysis of Meliodent surfaces in the control group (500×)

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Figure 2: Scanning electron micrograph analysis of Acron MC surfaces in the control group (500×)

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Figure 3: Scanning electron micrograph analysis of Meliodent surfaces in the CF + DC group (500×)

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Figure 4: Scanning electron micrograph analysis of Acron MC surfaces in the CF + DC group (500×)

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   Discussion Top

The null hypothesis was rejected because coffee staining and using denture cleaner changed the color of heat- and microwave-polymerized acrylic resins significantly. After immersion in coffee and denture cleaner solutions, alone and along with, appreciable color changes were observed on both heat- and microwave-polymerized denture base resins. The SEM images also confirmed these results.

Immersion in denture cleaner alone showed significant color change in denture base resins. The SEM images cited to surface changes in the denture cleaner group. In accordance with the results of this study, Peracini et al.[5] reported color changes after immersion in Corega Tablets and concluded that alkaline peroxide effervescent denture cleaners should be used with caution. Effervescent tablets decompose to alkaline peroxide form when dissolved in water during cleaning. These alkaline peroxides perform micromechanical cleaning action by releasing oxygen.[19] Irregularities on the denture base acrylic resins resulting from these cleaners may be the possible reason for color changes. High amount of peroxide and oxygenation in alkaline solutions was reported to have damaging effects on denture acrylic resins.[20] Hong et al.[4] and Sarac et al.[6] reported that denture cleaners can change the color of acrylic resin materials.

Zilinskas et al.[21] reported that some cleaning methods may damage the denture surfaces and may form increasing rough surface areas that accumulate microorganisms and cause formation of dental calculi on the dentures. All these changes in dentures may result in mucosal irritation and halitosis. This causes psychological discomfort, nausea, and social problems.

Immersion in coffee solution caused great color changes in denture base acrylics. Coffee is selected as a staining material in this study because it is usually used inin vitro studies. Tannic acid in coffee has been reported as the primary staining material due to its yellow brown color.[22] Hollis et al.[9] and Lai et al.[23] reported color changes in acrylic resins after immersion in coffee solution. Staining mechanism of denture base materials has been explained by sorption of liquids and expansion of polymeric matrix and motion of staining agent toward polymeric chains.[24]

Using denture cleaner after immersion in coffee did not remove the denture stain caused by coffee; additionally it increased the color change. This increase may be explained by more irregular acrylic resin surfaces probably occurring with the cumulative effect of coffee and denture cleaner. This study results are in agreement with Hollis et al.[9] since they reported that common denture cleaners cannot remove denture base acrylic resin stains.

A previous study which evaluated the effect of denture cleaner after cigarette smoking showed that denture cleaner decreased the color change values after smoking and bleached the discoloration caused by smoking.[7] In this study, denture cleaner after coffee did not decrease the color change; nevertheless, it increased the color change. This situation might be explained as discoloration caused by cigarette smoke stayed on acrylic resin surfaces and caused extrinsic staining, while staining ingredients in coffee might move from the surfaces to inside the polymeric chains that denture cleaning tablets could not decrease the color change. Irregular and damaged acrylic resin surfaces were observed in scanning electron micrograph analysis after coffee and denture cleaner confirming these findings.

Heat- and microwave-polymerized denture base resins showed difference in color stability and heat polymerization promoted higher Δ E values. Color change observed in heat-polymerized denture acrylics in the control group stored in distilled water was already more than 50% of the acceptability threshold and 4.08 CIEDE2000 units for denture base acrylic resins. Microwave-polymerized acrylics showed minimal color change in the control group presenting under perceptibility and acceptability threshold. This significance difference is probably the result of polymerization methods, as polymer color stability is affected by polymerization method.[4] According to results of this study, microwave heating named as “dielectic heating” by electromanietic waves may be the reason of decreasing residual monomer content as it was reported that residual monomer related to porosityand color change has been attributed to residual monomers.[16] It has also been described that porosity related to the degree of polymerization and the residual monomer ratio.[25],[26]

In contrast to these findings, May et al.[27] reported that microwave-polymerized acrylic resins showed lower color stability than conventional materials after accelerated aging. Different chemical composition of the acrylic resins and different staining factors may cause variations in color stability.

Color change is an indicator of damage to dental materials and can be evaluated visually or using a colorimeter and spectrophotometer which is based on the digital color data.[4],[28] In this study, colorimeter was used for measuring and CIEDE2000 color difference formula was used to evaluate discoloration of denture base acrylic resin because it was reported to provide a better fit with visual assestments than CIELab in the evaluation of perceptibility threshold under controlledin vitro conditions.[15]

Thisin vitro study evaluated the effect of staining and denture cleaner on color change in acrylic resins. The limitations of this study include the lack of human saliva and denture biofilm that may affect the color change results. Future work should involve the polymerization degree and degree of conversion studies to explain the significant difference in color change in heat- and microwave-polymerized acrylic resins. In addition, repolishing effect should be investigated after these discolored acrylic surfaces.

   Conclusion Top

Based on the findings of thisin vitro study, the following conclusions were drawn:

  1. Heat- and microwave-polymerized acrylic denture base resins can show different color stability.
  2. Coffee and denture cleaner solutions can destroy the surface properties and polish of acrylic denture base resins and can cause color change.
  3. Using denture cleaner after coloring drinks cannot bleach the acrylic denture base resins.
  4. Patients with removable acrylic dentures should be informed about the detrimental effects of denture cleaner tablets and should be informed to use correctly.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

   References Top

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4]

  [Table 1], [Table 2], [Table 3]


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