Polymethyl methacrylate (PMMA) resin has been successfully used for denture base materials for years (1, 2). Several different kinds of denture base acrylic resins are used and named according to their production mode, such as auto-polymerizing acrylic resin, heat-polymerized acrylic resin, light cure resin, special form resins polymerized in microwave processing and computer-aided design/computer aided manufacturing (CAD/CAM) PMMA block resin (3, 4). These materials have many advantages, including their low cost, ease of manipulation, adequate mechanical and physical properties and satisfactory appearance. Despite these advantages, the following disadvantages exist for PMMA: hypersensitivity, color change, abrasion, and porosity (5-7).
Light-activated urethane dimethacrylate resins were developed to replace PMMA for eliminate long flasking procedure and contact allergies. Recently, a new light-activated denture base material named Eclypse (Eclipse, Dentsply, York, PA, USA) has been introduced. The denture base record is produced from the baseplate resin, and after light polymerization it becomes the permanent denture base of the final denture. Eclipse denture base has showed significantly higher impact and flexural strength when compared to PMMA denture bases (8).
In recent years, with advancements in CAD/CAM technology, manufacturers have produced CAD/CAM PMMA based polymer blocks as an alternative for denture base resins (4, 9). CAD/CAM PMMA block manufacturers claim that these materials will have better mechanical properties than conventional denture base resins (10). CAD/CAM PMMA based polymer blocks, that are polymerized under high temperature and high pressure conditions, reduce residual monomer release, improve optical properties, improve stability of color and facilitate the production of denture bases by easy milling (11, 12).
The appearance and color of denture base is an important property of the denture. In addition, denture base material should match the color and appearance of the underlying tissues (1). One of the most important clinical features of all dental materials is color stability and any color changes are indicators of aging or damaged materials (13-15). Additionally, the esthetic appearance of the prosthesis is one of the important factors in meeting the expectations of patients (16-18).
Various factors may affect the color change of denture base materials after prolonged use. These factors are: water absorption, stain accumulation, degradation of intrinsic pigments, dissolution of ingredients, foods, beverages and roughness of surface (1, 19, 20).
When assessing color alterations, visual examination is a subjective physiological and psychological procedure. On the contrary, when the spectrophotometer is used for a determination of color alteration, not only does it eliminate subjective interpretations but also allows identification of minor color alterations (21). A color system the name of which is The Commission Internationale de l’Eclairage (CIE) L*a*b is a constant color scale that includes all the colors visible to the human eye. Hence, it is appropriate for perceptual studies of color changes in dental materials (22).
Even though the current scientific data promote CAD/CAM- fabricated complete dentures clinical superiority, the data about their material properties are still limited (23). Therefore, the aim of the present study was to compare the influence of various storage media on the color changes of an autopolymerizing resin, heat polymerized resin, urethane dimethacrylate resin and a CAD/CAM PMMA block resin. The null hypothesis was that that different storage media does not affect the color changes in various denture base resins.
Materials and Methods
Four kinds of denture base resins were used in this study; an auto-polymerizing resin (A), a heat polymerized resin (H), a light-activated resin (L) and a CAD/CAM block resin (C). All of the materials were used according to the manufacturer’s recommended procedures. The denture base resins used in this study and their manufacturers are summarized in Table 1.
Sixty disc-shaped specimens were prepared for each group. The CAD/CAM PMMA denture base materials were designed as STL files and milled by CAD-CAM milling system (Ceramill Motion 2; Amann Girrbach). Then specimens were sliced with a cutting machine (IsoMet 1000; Buehler) and diamond-wafering blade (IsoMet Blade 15 LC; Buehler) to obtain disc-shaped specimens 2 mm in thickness. Previously prepared CAD/CAM specimens were coated with high viscosity polyvinylsiloxane (Silagum Putty, DMG, Hamburg, Germany). They were first invested in conventional flasks by Type 3 dental stone (Moldano; Heraeus Kulzer, Hanau, Germany). Acrylic resins were mixed and applied in according to the manufacturers' instructions. Heat polymerized acrylic specimens (Paladent 20) were polymerized at 74°C for 9 hours in the automatic polymerization unit (Kavo EWL 5501, Kavo Electrotechnisches Werk GmbH, Germany). Polymerizations of Weropress specimens were performed in a pressure pot heat cure unit (Ivomat IP3, Ivoclar Vivadent AG, Schaan, Lichtenstein) at 45°C for 12 minutes under pressure of 2 bars.
A Teflon mold was designed with a transparent Plexiglas lid to produce the Eclipse specimens. The Eclipse specimens were cured in specific unit (Enterra VLC Curing Unit; DeguDent GmbH, Hanau, Germany) using 15-minute polymerization cycle. The polymerization residue materials were then removed with tungsten carbide burs using a handpiece at low speed. Smoothing process was used with a 400-grit silicon carbide abrasive paper (English Abrasives) on a machine (Phoenix Beta; Buehler). The specimens were polished by a conventional pre-polishing technique using slurry of coarse pumice (IMIPOMZA; Imıcryl), water and a bristle brush on a polishing lathe (P1000; Zubler) at a rate of 1500 rpm for 90 seconds. Polishing process was conducted with a conventional pre-polishing technique using a bristle brush on the rough pumice water slurry and a polishing lathe at a speed of 1500 rpm at 90 rpm and then fine polishing were done with using a polishing paste (chalk plus alcohol) and lathe flannel wheel for 90 seconds. All specimens were ultrasonically cleaned (Araysonic; Array) in distilled water for 10 minutes and dried with a paper towel.
A total of 240 disc-shaped specimens were thermal cycled for 5,000 cycles between 5°C and 55°C with a 30-second dwell time and a 20-second transfer time from one bath to the other. The specimens were divided into four main groups (n=15), and each group was divided into 4 subgroups (n=10) according to storage media: coffee, coke, red wine and distilled water. The distilled water group was used as a control.
All process steps were accomplished by the same operator at 23ºC room temperature to ensure standardization and avoid change due to temperature.
All operations were carried out at 23ºC to ensure standardization by the same operator and to avoid changes due to temperature. In accordance with the recommendations of the coffee manufacturer, a coffee solution was prepared by mixing 15 g of instant coffee powder (Nescafé Classic; Nestlé, Vevey, Switzerland; pH 5.56) with 200 mL of hot water and sugar free. After the preparation, the coffee solution was allowed to cool down to room temperature. There was no special preparation for the coke (Coca Cola Co, Atlanta, GA; pH 2.37) or red wine (Vinkara Winehouse, Ankara, Turkey; pH 3.6) groups. The specimens were kept in storage media for 15 minutes twice per day, the solution media were refreshed on a daily basis for up to a 30 days. The pH values of the storage media were verified by a pH meter (HI 221; Hanna Instruments Inc., Woonsocket, RI) before each storage. After the storage periods had been completed, the specimens were washed with and then stored in distilled water. This procedure was followed for 30 days. The specimens were kept in distilled water at 37°C between storage periods.
The color data was recorded before and after storage (7 and 30 days) in different media according to the CIE L*a*b* color scale using a spectrophotometer (Data color CHECK 3, USA). The color difference (ΔE) between the color coordinates was calculated by applying the formula ΔE* = [(ΔL*)2 + (Δa*)2 + (Δb*)2]1/2 in order to compare values before and after the storage treatment. Each sample was subjected to color measurement four times and the average value was recorded.
Statistical analysis was performed using Statistical Package for Social Sciences (SPSS) for Windows software (IBM Corp. Released 2013.The IBM SPSS Statistics for Windows, Version 22.0. Armonk, NY, USA). The Shapiro–Wilk test was used to identify if the measured parameters met the assumptions of normal distribution. The results showed that the data were not normally distributed. Between the groups, the color differences (ΔE) were analyzed by the Kruskal- Wallis test at 7-day and 30-day periods, while median and range values were used in the descriptive statistical analysis. Between the groups, pairwise multiple comparisons were performed using the Mann-Whitney U test at the 0.05 level of significance.
Table 2 shows the color change values after storage in the four different storage media for four denture base materials after the period of 7 and 30 days. The Weropress denture base resins demonstrated the highest color change in red wine, which represents a significant difference when compared to the other storage media both at 7 and 30 days (p˂0.001). The Paladent denture base resins demonstrated the highest color change in distilled water, which represents a significant difference when compared to the other storage media after 7 days, after 30 days it demonstrated the highest color change in red wine, which represents a significant difference when compared to the other storage media (p˂0.001). The Eclypse denture base resins demonstrated the highest color change in coffee, which represents a significant difference when compared to the other storage media after 7 days, after 30 days it showed the highest color change in red wine, which represents a significant difference when compared to the other storage media (p˂0.001). The CAD-CAM acrylic denture base resins demonstrated the highest color change in red wine, which represents a significant difference when compared to the other storage media at both 7 and 30 days (p˂0.001).
Table 3 shows the discoloration effects of storage media on the denture base resins after 7 and 30 days. The coffee solution, 7 and 30 days, affected the Eclypse denture base the most (p˂0.001). The coke solution affected the Paladent denture base the most after 7 days, at the end of 30 days the Eclypse denture base was the most affected (p˂0.001). The red wine solution affected the Eclypse denture base the most both at 7 and 30 days (p˂0.001). The distilled water affected the Paladent denture base the most 7 days, at the end of 30 days the Eclypse denture base was the most effected (p˂0.001).
In the current study, the previously introduced CAD/CAM PMMA block resins were compared with an autopolymerizing resin, a conventional heat polymerized resin, a urethane dimethacrylate resin, as storage media using red wine, coke, and coffee because basically that these beverages are frequently consumed by people. The denture base materials which were tested showed significantly different discoloration after storage in the different media at both evaluation stages. Hence, the null hypothesis of the study was rejected.
A color change that is more than detectable (ΔE˃1.0) is considered acceptable up to a ΔE value of 3,3 in dentistry; above this value it is considered unacceptable (24, 25). Only the Eclypse denture base groups, stored in red wine, showed that the discoloration was clinically unacceptable after the 30 days (ΔE 3, 59).
The color change was determined for all the acrylic denture base resins and they all increased over time. There are intrinsic and extrinsic factors that can cause discoloration of denture base materials (15, 26). These factors include: physical chemical change, stain accumulation, the residual monomer used, water absorption and, degradation of intrinsic pigments, dissolution of the ingredients and the surface roughness. It is well known that beverages such as coffee, coke and red wine enhance the discoloration of all denture base resins (18, 22, 26).
Zuo et al. examined the discoloration of different denture base resins after immersion in different cleaners and different beverages (26). The conclusion of the study points to the fact that color change of the Eclipse denture base resin was much higher than the clinically acceptable value of ΔE 3.3. This is in line with the present study results which showed that the Eclipse denture base groups had the most defined color change. This result could be due to the tendency of high water absorption in light activated denture base materials when compared to the other materials. Kerby et al. reported that Eclipse is also sensitive to hygroscopic expansion; this is caused by the 2 hydrophilic urethane groups within its molecular structure, but less than PMMA (27). Further studies, which take both water absorption and color change of different denture base materials into account, are needed.
CAD/CAM fabricated acrylic has achieved a better color stability, better mechanical properties, prevention of porosities and a better fit than the conventional PMMA resins. Polymerization methods and composition of a resin matrix may have a great effect on its stability of color (28-30). Conventionally fabricated PMMA resins are dependent on the
technician, mixing proportions of the resin components, polymerization device and duration of the polymerization, among others (31). According to the results of the present study, the least color change was observed in CAD-CAM denture base resins in all of the beverages. This is due to the fact that it can be better polished, there is no porous structure, less water absorption and less wear as proved in the literature.
Alp et al. examined the effect of coffee solutions on the discoloration of different CAD-CAM acrylic resins, likewise the current study, researchers reported that clinically admissible color changes did not occur in different denture base acrylic resins due to coffee staining (32). However, researchers observed that the color change and surface roughness in heat-polymerized and different pre-polymerized CAD-CAM PMMA specimens were not significantly different. This may be attributed to the 8 hours of heat polymerization of the heat-polymerized control group, which enhance its physical features.
Presently, there are varieties of options available including the new generation of PMMA-based self-polymerizing denture base materials. These materials have a shorter production process but the residual monomer can cause an enhance risk of tissue reactions and it has decreased mechanical properties (33). According to a previous study, Weropress have acceptable flexural properties when compared to traditional heat polymerized resins and light-activated resins (34). The present study reported that Weropress had a clinically acceptable color change and this material needs further study in order to determine residual monomer degree and cytotoxicity.
In the previous study, similar to other studies, a minimum color change was detected in the specimens which were immersed in distilled water (1, 35). The reason for this situation is that there are no substances which may cause discoloration in materials, and pH of the distilled water does not cause roughness on the surface due to neutrality.
Alcohol content in beverages has proven to be the reason of surface degradation, expansion and therefore low physical properties in the resin. Coloring effects of red wine can be caused by its alcohol content that causes denture base surfaces to become rough (35). In the current study, a higher color difference for all denture base resins was observed in the red wine group than the coke or coffee group. Although red wine has a relatively low acidic pH (3.6) when compared to coke, it still caused greater discoloration.
Other explanations of the coloring effects of red wine may be the softening of the materials from the absorption of alcohol molecules into the organic matrix and following change in surface smoothness (36-38). The most probable cause is that the acidic pH and alcohol content of red wine affected the surface roughness of the prosthetic base. Red wine contains anthocyanin that is a water-soluble pigment which provides the grapes their color (39, 40). The color change of the resin denture base which was stored in red wine was presumably a result of the concentration of the red color coming from its pigments, along with the higher absorption of the red pigments due to alcohol, which has a plasticizing effect on the organic matrix during the storage period, thus causing a significant color change in the denture base.
The present study has a number of limitations. First, the present in vitro result prerequisites need to be tested in vivo trials. However, in vivo studies are more challenging to be carried out. Besides, standardization in vivo studies is less likely to be accomplished using the methods applied in the current study. Although, the discoloration of denture base materials evaluated in vitro methods may not be as accurate or valid as those obtained through in vivo methods, they can provide useful guidance for clinical applications. Despite the limitations of the current study it presents beneficial evidence regarding the color change of recently introduced CAD-CAM denture base resins and the discoloration of some beverages, which are frequently used up in daily life.
Within the limitations of this in vitro study, the following conclusions could be drawn: The color stability of CAD-CAM denture base resins is better than that of some other kinds of denture base resins. All the changes in the color values of the groups, except those in Eclypse which was stored in red wine, were under the clinically perceptible value. The color stability of the Eclipse denture base resin was lower compared to other denture base groups. All beverages used in the study had an effect on color change.