Effect of Commonly Consumed Beverages on Color Stability of Polymethyl Methacrylate Denture Base Material

Many resins have been used for denture base construction. These materials include heat-activated resins (polymerized using a water bath or microwave oven), chemically activated resins (cold polymerized), and light-activated denture base acrylic resins (eg, Eclipse^®, Dentsply Sirona, dentsplysirona.com).^1,2 Polymethyl methacrylate (PMMA) has been the most popular material used for denture base fabrication since its introduction in 1937.^1-6 Most PMMA resin systems include powder and liquid components.⁷ The powders are pure polymers that are clear and adaptable to a wide range of pigments. Such pigments, which are used to obtain various tissue-like shades, are, in fact, compounds such as mercuric sulfide, cadmium sulfide, cadmium selenide, and ferric oxide.⁸

PMMA has been used in dentistry for various purposes such as manufacturing denture bases, artificial teeth, provisional restorations, surgical splints, stents, and orthodontic appliances.² Acrylic resins have been widely used due to their acceptable esthetics and desirable characteristics such as easy handling, good thermal conductivity, low permeability to oral fluids, and color stability.^2,5,9,10 Problems with acrylic resins, however, include polymerization shrinkage, weak flexural strength, lower impact strength, and low fatigue resistance,^2,4,6,9 as well as the possibility of hypersensitive patients experiencing allergic reactions to residual monomer.^1,6 Staining of acrylic resins can lead to poor esthetics, resulting in patient dissatisfaction and the potential additional expense of replacement.^1,6,10-13

Denture discoloration may be due to many factors, which can be divided into intrinsic factors, such as the characteristics of the material composition, and extrinsic factors related to wear and exposure to staining substances causing absorption and adsorption of stains.^3,9,11,13,14 Exposure of denture base resin to commonly consumed beverages, oral fluids, and denture cleansers has demonstrated color changes in these materials. This occurs as a result of the entry of colored mixtures into the resin matrix, causing oxidation of amine accelerators. The ability of the resins to resist color changes can be affected by the structure and physical and chemical characteristics of inorganic fillers present in the resin.^1,3,11,13,15

The discoloration of resin restorations can result in an esthetic problem. Minimization of color change is a property that should be considered in the selection of materials and techniques to achieve the optimal esthetic effect. Color stability and translucency should be maintained during processing, and these resins should not stain or change color while being used. The color stability criteria may provide important information on the serviceability of the materials. It has been reported that certain beverages, including tea, coffee, and wine, can cause discoloration of acrylic resins.¹¹

Evaluation of staining can be measured visually and by using certain instrumentation. Instrumental technique greatly reduces the chances of error during visual interpretation of color measurement. Colorimeters and spectrophotometers are commonly used to evaluate change in color of dental materials.^1,3,15

In 1976, the International Commission on Illumination (Commission Internationale de l'Eclairage [CIE]) developed a 3-dimensional color space based on axes "L" ("black" [0] to "white" [100] values), "a" ("green" [negative] to "red" [positive] values), and "b" ("blue" [negative] to "yellow" [positive] values)called the CIE L*a*b* scale.¹² The scale covers all colors visible to the human eye and allows for studies in color differences in dental materials.¹² The amount of color change may be described by the mean color difference (ΔE), which shows the distance between two given colors in this space. This scale has been used in several studies assessing acrylic resin color stability when in contact with pigmented liquids.^{3,5,6,9,13,14,16,17}

The commercial availability of different acrylic resins along with the wide variety of beverages consumed by different populations prompts the need for investigation of color stability of denture materials with regard to daily-consumed beverages in various communities. The purpose of this study was to evaluate the color stability of two types of heat-cured acrylic resins (Hiflex-H, Prevest Denpro Limited, prevestdenpro.com; and DPI^® Heat Cure, Bombay Burmah Trading Corporation Ltd, bbtcl.com) as denture base materials by subjecting them to the most highly consumed beverages on a daily basis in Sudan.

In this experimental in vitro comparative study, two types of heat-cured acrylic resin dental materials available in Sudan and three beverages consumed daily by Sudanese citizens, specifically, tea, coffee, and Pepsi^®, were tested. The sample size was determined by the formula presented in Figure 1. Based on this formula, the total sample size was (N) = 2 × 4 × 17 = 136 specimens (68 samples for each type of acrylic resin). The number of treatments (beverages) wasfour (three staining beverages comprising tea, coffee, and Pepsi, and distilled water as a control group).

Two types of heat-cured denture base acrylic resins were used (Hiflex-H and DPI Heat Cure). Total samples of 136 specimens (68 specimens for each group of acrylic resin) were processed in gypsum molds made by investing wax discs in a denture flask. Dimensions of the acrylic discs (specimens) were: 20 mm diameter and 3 mm thick, ± 0.1 mm.¹

The specimens (discs) were finished and wet-polished with abrasive paper. Then, the discs were immersed in distilled water for 24 hours.¹ All the specimens (discs) were prepared by the same laboratory technician to minimize technical differences/errors. Each specimen then was suspended with dental floss so it was not in contact with the container or other specimens.¹ Finally, the specimens were divided and put into four containers holding 50 ml of each beverage (tea, coffee, Pepsi, and distilled water).

Three staining solutions (tea, coffee, and Pepsi) were used, and distilled water served as a means of control.

For preparation of tea: One tea bag (Lipton, Egypt by Unilever Mashreq Tea Company, unileverme.com) to each 100 ml of boiling tap water, for 2 minutes (according to manufacturer's instructions).

For coffee preparation: One tablespoon of coffee (Alfungal, Cofftea Factory for Tea & Coffee Packing, cofftea.net) to each 200 ml of boiling tap water. Coffee was added to boiling tap water and mixed with spoon until creamy bubbles appeared.

For Pepsi preparation: A ready-made Pepsi cola (a registered trademark of PepsiCo, Inc) was used.

For distilled water preparation (for control): The water was ready-made available in bottles.

The solutions (beverages) were renewed daily for 30 days¹⁰ because of the hot climate in Sudan, which could cause evaporation of water and change the concentration of the examined solutions as well as the level of the examined solutions in the containers. All solutions were prepared by the same operator for the 30 days to minimize variances/errors in methodology.

The aforementioned CIE system was used with the aid of a computer analysis program (Adobe^® Photoshop^® CS5 Extended version 12.0, Adobe, adobe.com). All color measurements of the specimens (acrylic discs) were taken by the same investigator to minimize technical variations/errors. The specimens were scanned using an HP scanner (HP DeskJet F300 scanner, HP Inc, hp.com) three times¹ at the following intervals:

• 0 days (T0): before immersion in the discoloring solutions
• 15 days (T1): after 15 days of immersion in the discoloring solutions
• 30 days (T2): after 30 days of immersion in the discoloring solutions

The total color differences were calculated by the formula:

 ΔE*ab = [(Δ L*)² + (Δa*)² + (Δ b*)²]

where ΔE*ab = mean color difference,L* = white-black, a* = redness-greenness, b* = yellowness-blueness. Statistical analysis was performed with two-way ANOVA (SPSS statistic version 20.0) to compare results from each denture base resin brand and beverage. The mean color difference (ΔE) was computed according to the National Bureau of Standards (NBS) using the formula [NBS units = ΔE × 0.92], where NBS = National Bureau of Standards system and ΔE = mean color differences.

A pilot study was carried out to: test the reliability and validity of scanning sample acrylic discs; practice suspending the sample acrylic resin discs in the staining beverages; deal with the color measurement system; assess whether the research protocol was realistic and workable; and assess the proposed data analysis technique and avoid any problems. A total of 40 specimens (20 specimens for each type of acrylic resin) were recruited for the pilot study.

When using the CIE system, the greatest staining was apparent with tea on DPI Heat Cure (ΔE = 96.52, P < .05) compared to distilled water (ΔE = 0.44, P < .05). The least staining was apparent with Pepsi on Hiflex-H (ΔE = 2.4, P < .05) compared with distilled water (ΔE = 0.385, P < .05). Hiflex-H acrylic resin (group A) had more color stability than DPI Heat Cure acrylic resin (group B). When applying the NBSsystem, very much change in color was shown with tea compared to trace change in color showed with distilled water (Table 1). A noticeable color change was shown with Pepsi compared to trace change in color with distilled water.

For reliability, two readings were recorded with 14 days in between. The intraclass correlation coefficient (ICC) test was applied, and the results revealed significant correlation with an overall reading of 0.87.

The mean color changes (∆E) between the tested beverages through the test periods showed considerable differences. After 30 days of immersion (T0-T2), it was noted that specimens in group B (DPI Heat Cure) showed the highest color difference with tea (∆E = 39.21), followed by specimens immersed in Pepsi (∆E = 15.22), followed by those in coffee (∆E = 11.95). For specimens in group A (Hiflex-H), the mean color difference was highest with tea (∆E = 17.17), followed by specimens in coffee (∆E = 7.77) and then Pepsi (∆E = 3.96). All specimens immersed in the tested beverages showed considerable difference in color than those immersed in the control beverage (distilled water). More details about mean color differences for the different beverages and materials are illustrated in Figure 2.

When computing the mean color difference to the NBS system, the results showed very much change in color in both groups with tea for the immersion periods T1-T2 and T0-T2. For the same immersion periods there was much change in color in both groups with coffee. With Pepsi, there was only one very much change in color in group B for the period T0-T2, while there was much change in color in the same group for the first immersion period (T0-T1). For the control group (distilled water), the majority of changes were slight. More details are shown in Table 2.

Two-way ANOVA testing revealed significant differences for the first immersion period (T0-T1) between materials (acrylic resins) (P < .001) and treatments (beverages) (P = .003). Also, there was a significant difference in the interaction between materials and treatments (P = .001). For the second immersion period (T1-T2), there was only significant difference in the interaction between materials and treatments (P < .001). For the 30 days of immersion (T0-T2), no significant difference was found between the different types of materials (P = .882). However, significant difference was found between the different types of beverages (P = .017), and a highly significant difference (P < .001) was noticed in the interaction between materials and treatments (Table 3).

Color change in prosthodontic materials may result in patient dissatisfaction and require additional expenses for their replacement.^1,6,9-13While denture base resins may undergo color changes over time due to intrinsic and/or extrinsic factors, extrinsic factors are considered the more frequent causes of color changes.¹² The aim of this study was to assess the color stability of denture base material using three extrinsic staining beverages (tea, coffee, and Pepsi) and one control solution (distilled water). The two tested heat-cured acrylic resin denture base materials were the most commonly used in denture base construction in Khartoum state, Sudan. The most color change was apparent with tea after 30 days when compared to specimens stored in distilled water. This result is in agreement with the study of Hatim et al where they found tea caused the highest color change compared with coffee and Pepsi.⁹ Other studies conducted by Turker et al and Waldemarin et alfound the same result.^18,19 The second-most discoloring beverage in the present study was Pepsi, followed by coffee. The present authors' result, however, is not comparable with some other studies in which the highest color change was observed with coffee over tea and Pepsi.^11,20,21 One study conducted by Amin et al found that Coca-Cola^® was the most discoloring beverage over coffee and tea.³ The discoloration caused by tea and coffee is thought to be due to the tannic acid (C14H10O9) contained in them, which is water-soluble and known to trigger brown pigmentation.¹² The extremely low pH value in Pepsi may be a contributing factor to the dramatic change in the color of denture base acrylic resin materials caused by this beverage.⁹ Although the two denture base acrylic resins used in this study have the same base chemical structure (PMMA), Hiflex-H acrylic resin material showed more color stability than DPI Heat Cure acrylic resin material.

Increasing the immersion time of specimens in the discoloring beverages caused an increase in the staining of the acrylic resin denture base material; this result agrees with many other studies, including Duymus et al,²² Hipólito et al,¹⁴ and Ribeiro et al.¹³ Dentists should take the initiative to increase patient awareness about discoloration with certain beverages that might affect the denture base and potentially lead to additional expense for replacement. Patients may, for example, wash their mouths and clean their dentures after drinking staining beverages using tap water or any commercial mouthwash to minimize the discoloration effect. Dental laboratories/suppliers should use high-color-stable acrylic resin denture base materials to ensure high-quality dental service to patients.

To the best of the authors' knowledge, this study is the first in Sudan to investigate the effect of daily-consumed beverages on color stability of denture base materials. Some limitations should be acknowledged, however. First, this is an in vitro study and, therefore, might not duplicate the oral environment, which includes saliva and other contents that may interfere with the effect of the discoloring beverages. Second, the study was carried out on acrylic resin denture base materials only; no acrylic resin for artificial teeth was included. Finally, no other gradients were added to the beverages (eg, sugar), which may also interfere with the discoloring effect of these beverages. Further research on the staining effects of other locally popular beverages made with ingredients such as hibiscus, Tamarindus indica (Tamarind), and sweet and sour (traditional Sudanese juice) is highly recommended and may offer considerable more information.

Within the limitations of this study, the following conclusions can be drawn: (1) Tea showed the greatest effect as a staining beverage, followed by Pepsi, then coffee. (2) The group B acrylic resin showed greater color changes (more staining) compared to the group A acrylic resin, which was more color stable. (3) Increased immersion time of acrylic resins in beverages led to increased staining.

Mohamed Osman Babikir, BDS, GBOI, MSc
Department of Oral Rehabilitation, Faculty of Dentistry, University of Khartoum, Khartoum, Sudan

Magdi Wadie Gilada, BDS, MSc
Department of Oral Rehabilitation, Faculty of Dentistry, University of Khartoum, Khartoum, Sudan

Faisal Fahmy, BDS, MSc, PhD
Department of Prosthetic Dental Science, Faculty of Dentistry, King Saud University, Riyadh, Saudi Arabia

Ibrahim A. Ismail, BDS, DDS, MS
Department of Oral Rehabilitation, Faculty of Dentistry, University of Khartoum, Khartoum, Sudan

Mohammed Nasser Alhajj, BDS, MSc
Department of Oral Rehabilitation, Faculty of Dentistry, University of Khartoum, Khartoum, Sudan; Department of Prosthodontics, Faculty of Dentistry, Thamar University, Dhamar, Yemen

Abdelaal Abdellatif Fadul, BSc, MSc, PhD
Economic and Social Research Bureau, Khartoum, Sudan

Ahmed Elasyouti, BDS, MSc
Sudan Council of Medical Specialties, Khartoum, Sudan