Long-term microwaving of denture base materials: effects on dimensional, color and translucency stability

Abstract While the combined effect of microwave irradiation with cleansing solutions on denture base materials has been investigated, the effects of only using microwave irradiation and, more importantly, in a long-term basis, was not studied yet. Objective The purpose of this study was to evaluate the effect of a long-term repeated microwaving on the dimensional, color and translucency stability of acrylic and polyamide denture base materials. Material and Methods Thirty two specimens (32 mm x 10 mm x 2.5 mm) from polyamide (Valplast) and PMMA (Vertex Rapid Simplified) denture base materials were made. Eight specimens from each material were immersed in distilled water (control) and 8 were subjected to microwave exposure at 450 W for 3 minutes for a period simulating 224 days of daily disinfection. Linear dimension, color change (ΔE*) and translucency parameter (TP) were measured at baseline and after certain intervals up to 224 cycles of immersion, using a digital calliper and a portable colorimeter. The results were analysed using two-way repeated measures ANOVA to estimate possible differences among predetermined cycles and material type. Regression analysis was also performed to estimate the trend of changes with time. Statistical evaluations performed at a significance level of 5%. Results Data analysis showed significant changes in length at baseline with an increasing number of cycles (p<0.05) and a significant interaction of cycle-material (p<0.001). The ΔΕ* parameter was significantly higher with a higher number of cycles (p<0.001), but it did not vary between materials (p>0.05). TP decreased similarly in both materials following microwave action but in a significantly higher level for Valplast (p<0.001). Conclusions The results indicated that long-term repeated microwaving affects linear dimensional, color and translucency changes of both materials. Differences between PMMA and polyamide material were noted only in dimension and translucency changes.


Introduction
Contamination of removable prostheses with microorganisms (e.g. bacteria, fungi or viruses) turns them into sources of infection risk that may affect denture wearers and dental professionals. It is imperative that such prostheses be disinfected, at the dental surgery or laboratory, to reduce risks of crosscontamination and to comply with infection control guidelines 3 .
Several methods of disinfection have been suggested 3,5,28 , mainly including immersion of dentures in chemical solutions of sodium hypochlorite, alkaline glutaraldehyde, 4% chlorhexidine, chlorine dioxide, and denture cleansers such as alkaline peroxides, herbal and photodynamic therapy 1, 11,13,15 . However, past studies have indicated that such solutions affect the physicomechanical properties of the materials used to construct removable prostheses 19,25 . Furthermore, bleaching of dental base materials, unpleasant taste to patients and oral tissue reactions are some of the adverse effects of chemical disinfectants 15 . The use of such solutions has also been considered to be time-consuming 15 .
To overcome the above drawbacks, microwave energy was proposed as a simple, safe, low-cost and effective alternative method of disinfection [26][27][28] . Microwave irradiation of dentures is performed by either the wet (placed in a water bath) or dry disinfection method.
The microwave energy ranges between 450-650 W for a period of 2 to 10 minutes. However, temperature developing during microwave disinfection may have a negative impact on polymer structure. The fact that water starts boiling after 90 seconds of irradiation 22 and the appliance remains at this temperature until the end of the disinfection cycle may further enhance an acrylic resin polymerization reaction, which in turn may result in denture distortion 14 . Furthermore, it is welldocumented that temperature levels exceeding 77°C may distort the base of the denture due to the release of internal stresses trapped within the material during the polymerization procedure 7 . In order to minimize undesirable side effects of exposure to excessive temperature during microwaving, some researchers have recommended adding alkaline peroxide to the plain water to reduce the time that the denture base is exposed to high temperatures 28 .
The effects of repeated microwave energy on physico-mechanical properties of denture base materials have been investigated in many studies 2,4,6,14,23,24,27,29,31 . However, the maximum number of disinfection cycles in the above studies was 36. Such number of cycles is considered as a short period of denture life in service according to the 5 year normal average life suggested by Dorner, et al. 8 (2010).
Dimensional stability of dentures during processing and while in-service is of great importance for denture fit and patient satisfaction. The effects of microwave disinfection on the dimensional stability of denture base materials have been extensively studied, and some of them showed significant dimensional changes 2,14,24,27,29,31 , while others reported dimensional stability 4,6 .
Color is considered a significant parameter for the aesthetic appearance of dentures since color change acts as an indicator of material aging or damage 21 . Previous studies examined the effects of denture cleansers on the color stability of denture base materials 10 Translucency is also an important property of denture aesthetics 30 . Denture base materials should have a color similar to normal soft tissues, but also a translucency that will allow the light to pass through and reflect back normal tissue shades for a more natural appearance. It is important that both color and translucency of denture base materials be maintained throughout their clinical use 17 . However, there are no studies focusing on the translucency of denture base materials and how much it is affected by their color changes. Computations for the required sample size indicated a maximum total (for all four groups) size of 32, with actual power 0.962, 0.999, and 0.977 for between, within and within-between factors effects, respectively.
In the case of the polyamide specimens, a wax sprue was attached to the metallic pattern before investing, while in the PMMA's case the conventional denture flasking technique was followed. The gap created after the boiling and removal of the patterns was filled with suitable material for the fabrication of specimens.

Valplast was the polyamide material used and Vertex
Rapid Simplified was the PMMA's one. Before injecting polyamide into the gap, the material was heat-melted at 280°C for 11 minutes in a digital melting Valplast furnace. The flask was placed in the press for 3 minutes and then bench-cooled before opening. Following opening, the specimen was removed from the stone mold and the sprue was removed. PMMA specimens were fabricated according to the manufacturer's instructions using a conventional pressure-pack technique and a 20 minutes water bath procedure at 100°C. After polymerisation, flasks were bench-cooled before specimens were removed from them.
Following deflasking, all specimens were stored in dry conditions. Initial finishing was performed with 600 grit, 800 grit and 1200 grit waterproof silicon carbide paper in an Ecomet III polishing equipment (Buehler Ltd, Evaston, III, USA) while polishing was performed using a high gloss agent (KMG, Candulor AG, Zurich, Switzerland) on a white cotton yam wheel polishing brush (Bur Dental, Guangzhoo, China). From each material, sixteen specimens were constructed, which were divided into two groups (n=8).

Linear dimensional change measurements
The length of each specimen was measured using a digital caliper calibrated to 0.03 mm (Mitutoyo Inc., Tokyo, Japan). Each measurement was repeated 3 times and the mean value was calculated and recorded.

Color measurements
The primary color parameters (lightness-L*, red/ green-a* and yellow/blue-b*) of all specimens in the  Measurements were performed at a right angle with the surface of the specimen, at three different sites in the middle area of the specimen for obtaining a mean value. The instrument was initially calibrated following manufacturer's instructions, and then once every 20 measurements. The secondary parameters (ΔL*, Δa*, Δb*) were also estimated, and color differences were based on equation [1].
Translucency measurements Translucency of the specimens was estimated by calculating the TP (Translucency Parameter) using the equation [2], where "B" and "W" refer to color tristimulus values against a black (B) and white (W) background, respectively.
Trend analysis also indicated a logarithmic regression line of change with the number of cycles (Table 2)

Translucency changes
Calculations of TP for the groups at the specified cycle intervals, their difference from the translucency at the baseline are given in Figure 7 with their trend lines, and R 2 following a logarithmic regression ( Table 2). The  Table   2).

Optical interferometric profiles
Representative profiles of PMMA and polyamide materials at the end of immersion cycles are given in nm and 79.83 nm respectively. The above indicates a two times higher value for microwaved materials and ten times higher for the polyamide.

Discussion
The hypothesis that no differences existed between denture base materials with respect to the effect of longterm microwaving on their dimensional and translucency stability was rejected, whereas accepted for the effects on color. This study's results showed that microwave disinfection cycles did not significantly alter the color of both denture materials in relation to the control group, while significant differences were found between materials in dimensional and translucency.
In in vitro studies assessing dimensional changes of denture base materials, specimens are usually made of various shapes 2,27,31 , involving within changes numerous other factors besides processing and microwave disinfection. Palate shape, denture base thickness and teeth presence are some of the factors that lead to the final effect. In order to eliminate the influence of such factors, simple-shaped specimens instead of complex ones, such as denture base, were fabricated for the purpose of this study 12 .

Length changes
Control groups showed only a slight expansion within water baths (less than 0.05 mm), probably due to water sorption by the hydrophilic acrylic resin, as a result of the polarity of its molecules 2 . However, both denture     Although color changes were not statistically significantly different between the two materials, they behaved differently. Looking at the graph of Figure 3, it is seen that the change for the Vertex was initially close to that of Valplast, but in two weeks reached the level of its total change and remained there until the end of 224 cycles (7.5 months). The change for Valplast was increasing with cycles, but the change was evident after 112 cycles (4 months) and continued to increase until the end of the experiment. This is why the trend line for Valplast was much steeper than Vertex's. This model of color changes was also followed by the materials in the control baths. Valplast reached the level of 1.5 ΔΕ* at 7 cycles (one week) and remained there until the end of the experiment, while Vertex reached the same level after 14 days and, continuing to increase, it finally reached a higher level of change than Valplast's.
The change of the irradiated Valplast group was due to the decrease in a* and b* parameters, while that of irradiated Vertex was due to the decrease in L* and b*.
The change in Valplast control group was mainly due to the decrease in L* and in Vertex to the decrease in L* with a small increase in a* (Figures 4-6). This means that polyamide becomes less red and yellow retaining its