10.25241/stomaeduj.2019.6(4).art.1

  • DENTAL MATERIALS

Effects of acIdıc/alcoholıc beverages on the surface roughness of composıte resıns lıght-cured for two dıfferent perıods of tıme

DOI: https://doi.org/10.25241/stomaeduj.2019.6(4).art.1

Authors:

Günçe Ozan1a , Meltem Mert Eren2b , Sevda Ozel Yildiz3a , Hande Șar Sancakli1c* , Esra Yildiz1d

1Department of Restorative Dentistry Faculty of Dentistry, Istanbul University, TR-34390 Fatih / Istanbul, Turkey
2Department of Restorative Dentistry, Faculty of Dentistry, Altınbaș University, TR-34218 Bakirkoy / İstanbul, Turkey
3Department of Biostatistics, Faculty of Medicine, Istanbul University, TR-34093 Fatih / Istanbul, Turkey

aDDS, PhD, Research Assistant
bDDS, PhD, Assistant Professor
cDDS, PhD, Associate Professor
dDDS, PhD, Professor

 

Abstract
Introduction The purpose of this study was to evaluate the surface roughness values of various resin-based composites (RBC) regarding exposure time and immersion in alcoholic and acidic beverages.
Methodology A total of 240 disc-shaped specimens (8 mm x 2 mm) were prepared from two microhybrid, one nanofilled and one nanohybrid RBC. Specimens were divided into two groups, according to the exposure time; 20 or 40 seconds and immersed for 10 min/day during one month in either non-alcoholic (Coca Cola), alcoholic (red wine) beverages, or distilled water (n=10). Surface roughness was measured after 24 hours, one week, and one month. Results were analyzed statistically using parametric and nonparametric test.
Results The roughness values (Ra) measured at 1-month immersion were signi-ficantly higher than those measured at 24 hours. There was no statistically significant difference due to exposure time (20 or 40 seconds) (p>0.05). Structure of RBC, and presence of alcohol, and phosphoric acid in the immersion solutions caused a statistically significant difference among baseline and 1-month immersion intervals (p<0.05). Among all RBCs, lowest Ra was observed in the microhybrid RBC Charisma Classic group.
Conclusion Immersion in both acidic and alcoholic beverages altered the surfaces of all RBCs and generated significant surface roughness changes. All analyzed RBCs showed unacceptable changes in surface roughness.

Keywords
Resin-based composites; Exposure time; Surface roughness; Aging.

1. Introduction
Resin-based composits (RBC) are successfully used for the direct restoration of anterior and posterior teeth due to their simplified adhesive protocols, improved esthetic and adequate physical properties. Moreover, patients’ priorities have shifted to highly esthetic restorations in both the anterior and posterior region. Considering that the posterior region has its own characteristics regarding the masticatory loads, chewing forces and possible parafunctional habits, RBCs challenge certain shortcuts when used in the posterior region. However, with the highlighted improvements in resin and filler technology, various types of RBCs have become available allowing for a clinically successful placement also in the posterior areas [1,2].
The mostly used RBC categories placed in the posterior areas include microhybrid [3], nanohybrid [4] and nano [5] RBCs [4]. To increase the esthetic aspect, progressively smaller particles have been incorporated in the monomer matrix leading to higher surface quality and superior polish retention [4,8], associated with low wear rates and increased wear resistance. Besides the filler, the degree of cure of the monomer matrix may also affect the polish ability of a RBC [9]. Under ideal polymerization conditions, less residual monomers are evidenced and consequently a lower monomer release.
On the contrary, monomers that were not involved in the polymerization reactions are able to alter the restorative material, due to a softening effect of the polymer matrix, thus making the RBC prone to wear and negatively affecting the surface qualities. Therefore, the curing units used for polymerization as well the exposure time and distance may have a direct effect on the properties of the RBCs and their surface. Apart from an improvement in the filler system, modifications in the chemical composition of the monomer system are identified as well in modern RBCs. Besides traditional monomers such as Bisphenol A glycidylmethacrylate (Bis-GMA), Urethane-dimethacrylate (UDMA), or Triethylene glycol dimethacrylate (TEGDMA), novel monomers such as modified aromatic (AUDMA = aromatic urethane dimethacrylate) and aliphatic (Bis-EMA = ethoxylated bisphenol A glycol dimethacrylate) methacrylates have been included in organic matrices [10,11] to reduce viscosity [11], water sorption, and solubility [10], thus preventing resin matrices from being softened and degraded [5]. Apart from the content of the resins, the surfaces of RBCs are expected to degrade according to the dynamics of the oral environment [12]. Excessive surface degradation by means of oral conditions may not only lower the physical properties of RBCs but also cause plaque accumulation, discoloration, and secondary caries [13]. Therefore, maintaining a smooth surface is important to the long-term success of restorations. Consuming certain types of beverages may change the surface texture of RBC restorations. The chemical properties of beverages, such as their acidity, may affect the surface properties leading to wear, softening, severe degradation, and staining [14]. Likewise, ethanol (present in alcoholic beverages) has the potential to plasticize the organic matrix of RBCs, thus lowering the physical and mechanical properties [15]. Thus, the effects of alcoholic and acidic beverages on the surface properties of RBCs need to be analyzed. The purpose of this study was to evaluate the surface roughness values of various RBCs as a function of the exposure time and the immersion in alcoholic and acidic beverages.
The null hypotheses were that (1) the exposure time would not affect the surface roughness of the various analyzed RBCs and (2) the analyzed RBCs would have similar surface properties after exposure to alcoholic and acidic beverages.

2. Materials and methods
240 disc-shaped samples (8 mm in diameter and
2 mm in thickness) from four RBCs were prepared with a custom made stainless steel mold: the microhybrid Charisma Classic (Kulzer, Wehrheim, Germany), the microhybrid G-eanial (GC, Tokyo, Japan), the nanohybrid Aelite Aesthetic Enamel (BISCO Dental Products, Schaumburg, IL, USA) and the nanofilled RBC Filtek Ultimate (3M, St. Paul, MN, USA). The composition of the materials is shown in Table 1.

2.1. Specimen Preparation
Specimens were prepared by inserting materials in one increment into a mold and placing a glass plate on the top and bottom of the mold, with a Mylar strip in between. A constant pressure (with 1 kg weight) was applied on the glass plate for 15 seconds to allow the excess material to escape, thus obtaining a flat specimen surface without bubbles. Sixty specimens were prepared for each of the four RBCs. After removal of the weight and the glass plate, the specimens were polymerized with a LED (Light Emitted Diode) LCU (Light curing unit, SmartLite Max, Dentsply, Pennsylvania, USA, 1600 mW/cm2) for 20 seconds and 40 seconds. The LCU’s tip was positioned perpendicularly to specimens’ surfaces and the distance between the tip and specimen was standardized using a glass microscope slide (1 mm thickness).
All specimens were stored in distilled water at 37±1°C for 24 hours to allow for post-polymerization. The top surface of each specimen was polished with flexible aluminum oxide discs (Sof-Lex; 3M) under running water for 30 seconds. Polishing was performed by one operator to eliminate operator-dependent variability, and the discs were renewed after their 5th use.

2.2. Exposure to alcoholic and acidic beverages
Specimens from each group (n = 10) were stored for 10 min/day during one month in one of the following media: alcoholic (red wine-RW), acidic non-alcoholic (Coca Cola-CC), and distilled water (DW) as control (Table 2). All of the beverages were used at room temperature and were renewed during every period. The specimens were kept immersed in distilled water at 37±1°C between cycles.

2.3. Surface Roughness Measurements
The roughness values for each specimen were measured with three consecutive readings, and mean Ra values were calculated. Before the measurement, the top surface of each specimen was blotted dry using tissue paper, and the contact guide of a surface profilometer (Taylor Hobson Surtronic 3+, Taylor Hubson, Leicester, UK) was positioned at the center of the specimen surface. The profilometer, which was calibrated against a standard after each measurement, was set to a cutoff value of 0.8 mm, a transverse length of 0.8 mm and a stylus speed of 0.1 mm/s. Measurements were taken directly after polishing the specimens (baseline) as well as after one week and one month.

2.4. Statistical Analysis
Statistical analysis was performed using Statistical Package for Social Sciences (SPSS) for Windows 21.0. The normal distribution of the values was verified by Shapiro-Wilks tests. Acording to data distribution, both parametric (repeated measures) and non-parametric (Friedmann) tests were used for statistical analysis. A percentage roughness increase (%) after one month of immersion was calculated using following formula:
Percentage roughness increase =
(R – one month a – R-Baseline a )/ R-Baseline a
With a = exposure time (20 or 40 seconds);
R = Roughness,
p values less than 0.05 were considered statistically significant.

3. Results
The mean values of the surface roughness (Ra) of the RBCs cured for 20 or 40 seconds and immersed in varied beverages are shown in Table 3.

3.1. Comparisons of alterations in Ra values of RBCs by time and beverages.

Table 3 shows that the groups exhibit higher roughness values (Ra) after one week immersion compared to the baseline, except for the nanohybrid RBC (Aelite – 20 seconds polymerization) immersed in distilled water and the microhybrid RBC (Charisma Classic – 40 seconds polymerization) immersed in red wine. The mentioned specimens of the nanohybrid RBC show no surface roughness change during storage, and the microhybrid specimens showed smoother surfaces after one – week of immersion in red wine.
Regarding the one month results, except for the groups microhybrid (G-aenial – 20 seconds polymerization), microhybrid (Charisma Classic
– 40 seconds polymerization) and nanohybrid (Aelite – 20 seconds polymerization) RBCs, all other groups exhibit highest roughness (Ra) values. Microhybrid (G-aenial) specimens immersed in distilled water showed no changes in surface roughness and microhybrid specimens had a slightly rougher surface after one month immersion in distilled water.
Microhybrid specimens immersed in red wine had higher roughness values compared to the one week evaluation, but still showed better surfaces than the baseline. In addition, regarding the change of Ra values of the RBCs overall, there are no statistically significant differences between 20 or 40 seconds of curing (p>0.05). Statistically significant differences between Ra values measured after one month immersion in different beverages are shown in Table 4 and Table 5. The tables indicated that the RBCs cured for 20 seconds and immersed in Coca Cola exhibit all statistical similar roughness. Likewise, specimens of the microhybrid RBC (Charisma Classic – 40 seconds polymerization) immersed in red wine had statistically significant lower roughness change than the nanofilled (Filtek Ultimate) and the nanohybrid RBCs (Aelite). After immesion in distilled water, specimens of microhybrid RBC (G-aenial) had hardly changed their surface roughness values. Therefore, specimens of microhybrid (Charisma Classic) and nanofilled (Filtek Ultimate) RBCs showed statistically higher roughness than the microhybrid RBC (G-aenial). All of the RBC were affected by immersion in distilled water but the specimens had statistically significant lower roughness changes compared to immersion in Coca Cola and red wine.

4. Discussion
The surface quality of the RBC materials is dependent on the physical characteristics of the materials as well as the techniques used for finishing and polishing them. The application of nanotechnology in dental materials enables the incorporation of smaller particles with increased filler loading and results in lower polymerization shrinkage and better physical and mechanical properties [4]. The polishability is also correlated, besides filler, to the polymerization quality [9]. The parameters affecting the quality of curing, such as the exposure time, the exposure distance or the curing characteristics of the used curing unit (radiant emittance, spectral distribution etc) may directly be related to the polishability. The effects of the polishing systems on the surface quality has already been analysed [7,18,19], however without involving the exposure time. Prolonged curing time may increase the degree of conversion, lower the residual monomers, and improving thus the surface quality of RBCs [20]. In order to obtain sufficient curing, the type of the curing unit used, either LED or QTH (Quartz Tungsten Halogen), was proved to have no direct influence on the surface roughness of the RBCs [16]. This must also be the case for the curing conditions in the present study, since no significant difference in surface roughness was identified in any of the analysed materials when increasing the exposure time from 20 seconds to 40 seconds. Therefore, the first null hypothesis is accepted. Regardless of the curing time, the various types of RBCs analyzed in the present study reacted differently to storage in beverages with regard to their surface quality. This confirms the results of previous studies published for the same materials [16]. The RBCs tested in the present study were selected due to their specific filler properties. Regarding the differences in the filler content and size of the RBC that affect the physical characteristics, polymerization quality and surface roughness values were assumed to be different. Moreover, microfilled RBCs have adequate polishability, thus they could mimic the surface smoothess of enamel greatly. However, these types of RBCs had lower mechanical strength, so they are recommended for low-stress regions [3]. On the contrary, microhybrid RBCs had higher mechanical strength but lower polishability [19]. Apart for the above mentioned RBC categories, nano RBCs could be developed with not only excellent polishability but also with better mechanical and physical properties, including resistance to different media. This general remarks stay in contradiction to the results of the present study, since the analyzed microhybrid RBC, Charisma Classic, showed smo-other, while the nanohybrid RBC, rougher surfaces. Thus, the second hypothesis was rejected.
The present study was intended to ensure standardization by selecting the ideal pair of materials and methods. For instance, the size of specimens was designated with a diameter of 8 mm to match the tip of the curing unit to use the light-curing unit only once and eliminate the tip location from being a polymerization variable. All of the RBCs were chosen in the same shade to avoid different curing time requirements. Varying particle sizes were also enrolled to evaluate the different resistance and responses to acidic/alcoholic beverages effects in terms of surface roughness. Physical, thermal, and chemical factors in the oral environment play fundamental roles in the degradation process of RBC surfaces. The influence of these processes reflects a change in the surface roughness values [21]. In the present study, lower Ra values were reached not only by aging specimens in acidic and alcoholic beverages but also in distilled water. According to the results, all of the groups and subgroups were similarly affected by distilled water. Exposure to water could result in the hydrolytic degradation of fillers’ silane coating, loss of chemical bonding between fillers and plasticizing and swelling of resin matrices [22,23]. Consequently, fillers may be pulled out from the specimen’s surface after the organic matrix absorbs water, which could increase the surface roughness [23]. In this context, RBCs containing TEGDMA, a hydrophilic monomer, are more susceptible to water degradation following water uptake [24]. In the present study, all of the RBCs except the microhybrid (G-aenial) contain TEGDMA in their organic matrix. The RBC G-eanial is advertised as a microfilled RBC, but according to its particle sizes, it needs to be classified as a microhybrid RBC. In the present study it showed the significantly lowest surface changes among the analysed RBCs when cured for 20 seconds (Table 4). Evaluating the acidic (CC) and alcoholic (RW) beverages, the ethanol concentration of red wine and the phosphoric acid in Coca Cola may lead to surface degradation. Add-itionally, ethanol could penetrate the organic matrix, alter the polymeric structure [12], and eventually affect the mechanical and physical properties of the RBCs. The effects of ethanol are thought to be more significant than the prolonged exposure to water [25]. In the present study, there is no significant difference among RBCs cured for 20 seconds. However, after 40 seconds of curing, the nanofilled and nanohybrid RBCs showed significantly rougher surfaces than the microhybrid RBCs. This could be attributed to the higher amount (68 vol%) of smaller fillers (0.01-0.07 µm) in the microhybrid RBC. The present data confirmed thus the study of Tantanuch et al [23] which found out that nanofilled RBCs showed better surface properties than nanohybrid RBCs after being immersed in red wine. Furthermore, the acidity of the immersing solution may have a direct effect on softening resin matrices, allowing filler to be pulled out and creating thus voids over the surface, that may enhance roughness [23]. Red wine contains not only ethanol but may also act as an acidic solution. Both analysed beverages are characterized by a low pH; however, in the present study, regardless of the curing time, there were not statistically significant differences between the roughness values of RBCs after immersion in Coca Cola and red wine. On the other hand, Ra scores above 0.2 μm have been reported to increase the biofilm formation [26]. Ra values higher than 0.3 μm can be physically perceived by patients, which could lead to patient’s dissatisfaction and an extra clinic sessions for polishing the restoration’s surface [27]. Although all of the specimens were polished with discs (Sof-Lex), described as one of the best protocols to create low roughness scores in resin materials [28], even at the baseline results, all of the RBCs showed surface roughness values higher as the mentioned thresholds. The possible reasons could be related to the inner characteristics, such as polishing responses of the materials. RBCs made of finer filler particles exhibit lower interspacing, less filler pullout and thus, smoother surfaces [29]. Research has shown that nanohybrid RBCs in par-ticular create smaller voids after finishing and polishing procedures [23,29]. However, in the present study, the specimens cured for 40 seconds consisting of a nanofilled (Filtek Ultimate) and a nanohybrid (Aelite Aesthetic Enamel) RBC showed statistically significant rougher surfaces than the microhybrid RBC (Charisma Classic) after immersion in all beverages. Moreover, after 20 seconds curing and immersion in red wine, Charisma Classic showed smoother surfaces as compared to the nano-RBCs. These results could be attributed to the type, size, number and distribution of fillers, which all have a significant impact on the mechanical and physical characteristics of the RBCs [29]. There is clear evidence that the size and irregularity of fillers is directly proportional to the surface roughness of a RBC [30]. Smaller fillers were thought to be less prominent on the surface; thus, they were more resistant to wear because of their homogeneity in the resin matrices [22]. Small fillers could reduce the spacing that provides resistance and protects the resin matrix [13]. In the present study, the microhybrid RBC (G-aenial) and the nanofilled RBC (Filtek Ultimate) had the largest fillers (16-17 µm and 0.6-10 µm, respectively) among the tested RBCs. That could be one of the reasons for those RBCs having the highest roughness scores of almost all groups.Besides filler size, a low inorganic content com-promises the surface smoothness [4]. The nanofilled RBC had the highest filler amount (63.3 vol%), which could be one of the reasons it has smoother surfaces than microhybrid (G-eanial) and nanohybrid RBCs, even having the largest filler particles overall. In general, the influence of the size and type of fillers on the surface quality has been thoroughly presented in the literature. Magdy et al [20] found that the RBCs with barium glass fillers showed higher surface roughness. In the present study, the microhybrid RBC was the only material containing Ba glass fillers, and results were not consistent with the above-mentioned study. It is reasonable to have conflicting results over fillers because the filler type may have an effect on surface roughness however, it is clearly not the only parameter. Besides size, amount and distribution of the fillers, their chemical composition may indirectly affect the roughness, since it determines the refractive index and thus may have an effect on the degree of conversion of the polymer matrix. At a lower refractive index mismatch between filler and matrix, less scattering occurs and the degree of conversion of the organic matrix may be increased. Marovic et al. [31] showed that Ba fillers could increase the degree of conversion of the organic matrix, while silica fillers may decrease it. However, it must be considered that scattering is not only dependent on the refractive index mismatch between filler and matrix but essentially also on the filler size and its relation to the wavelength used for curing a RBC. In the present study, the tested RBCs contained differently formulated filler particles and sizes, so the effect of the chemical composition and size on the Ra values cannot be properly discussed.The nanofilled RBC contains zirconia/silica clusters, the microhybrid RBC pyrogenic made silica, the microhybrid (Charisma Classic), RBC contains besides pyrogenic silica also prepolymerized filler based on silica while the nanohybrid RBC contains silica –glass particles. One study [4] found that, even though nano particles create smaller gaps after polishing, nanoclustered particles are not pulled out from the resin matrices neither. Among the tested RBCs, only the nanofilled RBC (Filtek Ultimate) had nano-clustered particles, which could explain the superior Ra results of the nanofilled RBC. The monomer type is one of the main component of the RBCs affecting surface quality, and it was demonstrated that the chemical composition of monomers is related to preserve surface smoothness against the tough oral conditions over time [22]. Bis-GMA and UDMA are the most common monomers used in RBCs. Monomers, such as Bis-EMA, have been developed to improve viscosity, lower the polymerization shrinkage and toughening the resin matrix. Bis-EMA has fewer carbon-carbon double bonds that lead to a softer, less cross-linked organic matrix that could be affected by acidic or alcoholic beverages [4]. The nanofilled and nanohybrid RBCs analyzed in the present study contain Bis-EMA and tended to be rougher than microhybrid RBCs based on Bis-GMA and TEGDMA. Another monomer with a hydrophilic character, TEGDMA, tends to degrade more quickly [4]. In the analyzed RBCs (nanohybrid and nanofilled), both TEGDMA and Bis EMA were used, thus inferior results of those RBCs could also be attributed to the softer matrices which might be degraded by acidic intake or water uptake. On the other hand, the nanofilled (Filtek Ultimate) and microhybrid (G-aenial) RBCs containe UDMA in their resin matrices. UDMA has lower water sorption and solubility than TEGDMA, Bis-GMA and Bis-EMA [4]. The microhybrid (G-aenial) contains only UDMA in the resin matrix that may lower the water uptake and may be responsible for the less changes in roughness when immersed in distilled water after 20 seconds of curing.
5. Conclusion
Nanofilled and nanohybrid RBCs (Aelite, Filtek Ultimate) as well as one of the analyzed microhybrid RBC (Charisma Classic) showed similar surface roughness after curing for 20 seconds.
Charisma Classic showed better surface quality than Aelite and Filtek Ultimate after 40 seconds of curing. Despite varying filler types and chemical composition of the monomer matrix, prolonged curing time had no significant effect on the surface roughness. Yet, immersion in both acidic and alcoholic beverages affected the surfaces of all RBCs and generated significant surface roughness changes. Thus, regardless of the curing time, all analyzed RBCs showed unacceptable changes in surface roughness.

Author Contributions
GO: had the idea of the hypothesis and MME designed the study. GO: collected the data and performed the experimental period with MME.
SOY: prepared the statistical analysis. HSS: revised the manuscript and corrected the language.
EY: editted the whole manuscript.

Acknowledgment
There is no conflict of interest.

 

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