Roughness profile and surface roughness after toothbrushing and degree of conversion of bulk-fill resinsAbstract
The aim of this study was to assess roughness profile and surface roughness after simulated toothbrushing cycles, as well as the degree of conversion (DC) of bulk-fill resin composites at different depths. Forty nine composite discs were made from three low-viscosity bulk-fill resins (Filtek Bulk-Fill Flowable/3M Oral Care - FBF, Beautifil-Bulk Flowable/Shofu Inc. - BBF and Surefill SDR Flow/Dentsply Caulk - SDR), three high-viscosity bulk-fill resin composites (Filtek Bulk-Fill Restorative/3M Oral Care - FBR, Beautifil-Bulk Restorative/Shofu Inc. - BBR and Tetric EvoCeram Bulk Fill/Ivoclar Vivadent - TEB) and one conventional composite (Filtek Supreme Ultra/3M Oral Care - FSU) (n=7), using addition silicon molds (2 mm thick x 10 mm diameter). The roughness profile (Rv) and surface roughness (Sa) were analyzed after 30,000 brushing cycles using confocal laser scanning microscopy. Five samples (4 mm thick x 2 mm diameter) were produced per resin composite and light-cured for 20 seconds using a LED-curing unit. After 24 hours at 37oC, DC of samples was evaluated at four depths, using a confocal Raman microscope. The data were analyzed by ANOVA and Tukey post hoc multiple-comparison tests (α = 0.05). The Rv values of FSU were lower than those obtained for BBR and BBF. The toothbrushing significantly decreased the Sa for all composites. The DC was significantly reduced at 4 mm depth. Toothbrushing changed composite surface of all materials, especially for BBR and BBF. Composites were not able to maintain the DC values shown at the top compared with those at the bottom (4 mm depth).
Composite Resins; Polymerization; Dental Restoration Wear
Introduction
Two advantages gained with the use of bulk-fill resin composites (BFR) are that larger amounts of these can be applied in dental preparations than those used in traditional technique and the reduced chair time.1 These restorative materials have been developed with the intention of providing more depth of cure and lower stress, shrinkage and cusp deflection.2-4 Traditionally, composite restorations were performed by inserting multiple layers and the thickness of each increment was around 2 mm,5 while the manufacturers’ recommendations for BFR have indicated at least 4 mm.6
Occlusal and proximal wear of BFR, and the effects of toothbrushing on these materials have been the purposes of some studies.7-16 Although BFR of low viscosity are not indicated for the reconstruction of large areas such as occlusal surfaces and cusps, some new rehabilitation techniques using injectable resins have recommended the use of the material for establishing vertical dimension and for altering occlusal schemes (anterior guidance and posterior disocclusion) prior to performing the final restorations.17Some BFR contain lower amounts of filler particles than conventional resin composites, which could make them more susceptible to surfaces alterations.18 The maintenance of surface smoothness is an important characteristic to be evaluated and various tools can be used to assess it.19
The replacement of Bis-GMA by UDMA or even the combination of these two monomers in BFRs have improved their degree of conversion.20 UDMA has lower molecular weight, higher concentration of double bonds, and lower viscosity than Bis-GMA, so that the co-polymerization of UDMA and Bis-GMA improves cross-linking.20 Additionally, the BFRs have higher translucency,21 reduced filler size, high molecular weight monomers, and novel initiator systems that favor the polymerization reaction. In general, BFR depth of cure values are higher than those of conventional resin composites, with low-viscosity BFR showing better results.6
In this context, the aim of this study was to assess the roughness profile (Rv) and surface roughness (Sa) after toothbrushing cycles, and the degree of conversion (DC) of BFR at different depths, when compared with a conventional composite. The following null hypotheses were tested: a) The Rv for BFR would not differ from that of a conventional composite; b) the Sa of composites tested after toothbrushing cycles would not change; c) the DC of composites would not change irrespective of the measurement of depth.
Methodology
Three low-viscosity BFRs (Filtek Bulk-Fill Flowable, Beautifil-Bulk Flowable, and Surefill SDR Flow), three high-viscosity BFRs (Filtek Bulk-Fill Restorative, Beautifil-Bulk Restorative, and Tetric EvoCeram Bulk Fill) and a high-viscosity conventional composite (Filtek Supreme) that served as Control were tested. Information on the manufacturers, compositions, lot numbers, and shades of composites are presented in Table 1.
Roughness profile and surface roughness analysis
Seven resin composite discs with dimension of 10 mm in diameter x 2 mm thick were produced for each restorative material (n = 7), using addition silicone molds (Express XT, 3M Oral Care, St. Paul, USA). The resin composites were inserted into the molds, then a polyester strip and a glass slip were lightly pressed onto the molds to allow excess flow. The specimens were light activated with a LED-curing unit (Valo, Ultradent Product Inc., South Jordan, USA) for 20 seconds. The light-curing unit delivered an irradiance of 1,001 mW/cm2 and an emission spectrum from 380 nm to 490 nm, with three emission peaks at 396, 447, and 466 nm. The light output from the light-curing unit was measured using a 6-inch integrating sphere (Labsphere, North Sutton, USA) that was attached to a spectrometer (USB-4000, Ocean Optics, Dunedin, USA).
All samples were prepared at controlled room temperature (24oC), under orange light and polished in a polishing machine (Aropol VV-PUD, Arotec, Cotia, Brazil), using a 1200-grit silicon carbide paper under water lubrication with distilled water and felt with 1 μm grit paste (Buehler Ltd., Lake Buff, USA). Afterwards, the composite discs were sonicated for 10 minutes (Thornton USC 1400, Unique Group, Indaiatuba, Brazil).
An adhesive tape (Scotch Duct Tape 471, 3M of Brazil, Sumaré, Brazil) was applied onto half of the composite disc surface, separating a protected area from the area that would be subjected to toothbrushing cycles, therefore, each sample had a half-area that served as its own control. The composite discs were submitted to 30,000 cycles of brushing, using a soft toothbrush (Indicator Plus 35 Oral-B, Procter & Gamble, Seropédica, Brazil) and 20 g of toothpaste (Oral-B Pro-Health, Procter & Gamble, Seropédica, Brazil) mixed with 20 mL of distilled water in a brushing machine (MSEt - Biopdi, São Carlos, Brazil). The toothbrushes were constantly loaded with 200 g throughout all the toothbrushing simulation cycles, and an individual toothbrush was used for each composite sample. After the brushing procedures, the adhesive tapes were removed, and samples were thoroughly washed under running water, cleaned in the ultra-sonic device for 10 minutes in order to remove any debris and toothpaste residue, and air-dried.
The Rv was the maximum valley depth of roughness profile and this was the largest valley depth deviation from the mean line within a given length (5 readings at each image). The Sa parameter corresponded to the two-dimensional parameter Ra, which measures surface roughness by detecting the maximum peak to valley heights of a specific surface profile. The composite discs were analyzed using confocal microscopy (LEXT 3D Measuring Laser Microscope OLS4000, Olympus Corp., Tokyo, Japan). A 5x magnification lens was used, which corresponds to a real 107x magnification. Rv and Sa data were provided in micrometers (µm) and obtained by using the specific software of confocal microscopy, which also provided two- and three-dimensional images of the unbrushed and brushed areas of samples. Rv data were analyzed by one-way ANOVA (factor: restorative composite) and Tukey post hoc multiple-comparison tests (α = 0.05), while the Sa data were analyzed by two-way ANOVA (factors: restorative composite and toothbrushing) and Tukey post hoc multiple-comparison tests (α = 0.05) (SigmaPlot 12.0 program).
Degree of conversion
Five cylindrical samples measuring 4 mm high x 2 mm diameter were made for each composite resin by inserting only one 4 mm-increment of the restorative material, using a stainless steel matrix. Specimens were light activated with the same LED-curing unit (Valo, Ultradent Product Inc., South Jordan, USA) for 20 seconds, removed from the matrix and then kept in an oven at 36°C for 24 hours. The opposite side to the one that received the light was marked with a pen to identify the correct reading position.
A confocal Raman microscope (Senterra, Bruker Optik GmbH, Ettlingen, Germany) configured with a laser excitation at 785 nm and nominal power of 20 mW 20x objective and 50x1000 µm slit, was used to assess the DC of the resin composites (BFR and Control). Starting from the top of the cylinder surface that was in contact with the light-curing tip to the base of the cylinder, Raman spectrum was taken at every 0.2 mm along the lateral surface of the composite cylinder, totalizing 20 readings performed on each composite cylinder. Scans were individually analyzed, and each spectrum consisted of an average of 20 coadditions, with 3 seconds of time integration and 3-5 cm-1 of spectral resolution. All spectra were corrected at baseline, and normalized by total spectrum area. The Raman band intensities at 1610 and 1640 cm-1 were selected to assess the DC, calculated according to the equation (1). OPUS software (7.2, Bruker Optik GmbH, Ettlingen, Germany) was used for mathematical analysis.
Where R was the ratio between the band intensities 1640 and 1610 cm-1, R cured was light activated and R uncured non light-cured samples. DC data were analyzed by one-way ANOVA (factor: measurement depth) and Tukey post hoc multiple-comparison tests (α = 0.05) (SigmaPlot 12.0 program).
Results
Table 2 presents the means for Rv and Sa of composite surfaces (brushed and unbrushed areas). Rv values of the conventional composite (FSU: 3.31 ± 0.78 µm) were significantly lower than those obtained for BBR (5.1 ± 0.9 µm) and BBF (4.9 ± 0.6 µm). Rv of the other composites (FBR, FBF, TEB and SDR) did not differ from FSU, BBR and BBF (p > 0.05). The toothbrushing altered the surface and significantly increased the Sa. Sa value for unbrushed area of BBR was similar to that of BBF, SDR and TEB, and all of them were significantly rougher than FSU, the conventional composite. In the brushed area, FSU (2.19 ± 1.15 µm), FBR (2.45 ± 0.29 µm), FBF (2.13 ± 0.54 µm) and SDR (2.35 ± 0.36 µm) showed lower Sa values than that obtained for BBR (3.41 ± 0.77 µm). Toothbrushing increased the Sa for all composites tested. Figures 1 to 7 showed the aspect of the unbrushed and brushed surfaces of all composites after toothbrushing. 3D images demonstrated composite surface wear after toothbrushing compared with unbrushed side.
Table 3 presents the means for DC of tested composites. SDR and FBF were the composites that showed the same DC until 3 mm (p > 0.05). However, DC of all composites decreased at 4 mm (p < 0.05)
Discussion
The results of this study showed that the toothbrushing cycles altered the surface of all flowable or regular BFR and composite Control tested. The Rv values of FBR, FBF, TEB and SDR did not differ from that of FSU conventional composite (Control), but the Rv values of BBR and BBF were significantly higher than that of the Control. Thus, the first null hypothesis was rejected. The second null hypothesis was also rejected because the toothbrushing significantly increased the Sa of all composites. In this study, Rv and Sa were evaluated after toothbrushing, i.e, to test its abrasive effect. This methodology has been used in other studies9,11 and is recommended by ISO.22 It is estimated that 10,000 brushing cycles correspond to approximately one year of in vivo brushing,23 thus, this study simulated the period of three years. The effect of occlusal loading or erosion on composite resin surfaces lead to different consequences and these were not tested. Moreover, the simulation of clinical wear in laboratory models of localized and generalized wear has been developed, however. limited data of BFR composites are available in the literature.8
Some factors can influence the wear resistance of resin composites such as filler content, fillers size, shape and hardness, the bond between the inorganic content and polymer matrix, and their degree of conversion.19 The toothpaste used in this study (Oral-B Pro-Health, Procter & Gamble) had a relative dentin abrasivity (RDA) of 120 and could be considered to have medium abrasiveness. The filler loading by volume of the resin materials tested varied widely (Table 1); however, the alterations caused by the toothpaste and brushing were not similar among them. Only BBF and BBR resins showed significantly higher Rv values than that of conventional FSU resin, probably due to the presence of fluoro-alumino-silicate glass fillers in these BFR composites.
One study showed that FSU height loss or wear was 0.75 ± 0.14 µm after 25.000 toothbrushing cycles with use of whitening toothpaste with RDA of 101.11 The toothpaste was also considered to have medium abrasiveness, and cycling corresponded to approximately two years of in vivo toothbrushing. The authors also showed that the FSU height loss did not differ from that of FBF as found in this study relative showed a Rv value of 3.31 ± 0.78 µm that represented the maximum valley depth of roughness profile after toothbrushing, while for FBF the Rv was 3.86 ± 1.35 µm. Other studies found that some of the flowable composites7,16 exhibited abrasive wear resistance comparable with that of the universal composites or those indicated for posterior restorations. Thus, the fact that flowable BFR generally have a lower filler content would apparently not compromise the wear resistance.
In the literature, three is little information available on the wear resistance of BFR, which continues to be cause for a concern, mainly when used in large restorations or for patients with bruxism and clenching behavior.24 The results of the studies that have evaluated the wear resistance of BFR have shown that: a) there was a negative correlation between gloss and surface roughness;11 b) in general, bioactive materials showed significantly more wear than the flowable composites;12 and c) the structure and composition of resin composites had an effect on the wear resistance.9
All specimens tested received the same type of finishing and polishing before undergoing brushing cycles. After brushing, the Sa showed a significant increase for all resin-based composites (p < 0.05), with values ranging from a low of 2.13 µm for FBF to 3.41 µm for BBR. The roughness value of 0.2 µm is recommended and clinically acceptable25 for not allowing bacterial accumulation. and humans can perceive the roughness of a restoration when the Ra value is 0.5 µm.26 Therefore, the Sa value of the materials tested was very high and could probably lead to biofilm accumulation. After brushing the BBR composite showed higher Sa values than those of FSU, FBR, FBF and SDR. The filler content of BBR might have contributed to this result. Whereas, BFR appeared to be capable of resisting erosion from extrinsic and intrinsic acids, considered a good clinical behavior, particularly in individuals who have problems with endogenous erosion or with exogenous erosive habits.24
Studies that have evaluated the BFR roughness11,28-30 found that they seemed to be more prone to staining by cigarette smoke when compared with the conventional microhybrid resin composites.30 The variations in the filler size and quantity had significant influence on surface roughness29 and physico-mechanical properties of resin composites.31 The finishing and polishing methods, control of the oxygen-inhibited layer, and the type of BFR, significantly improved the surface roughness and surface free energy.15,28Other parameters should be analyzed to confirm the adhesion of microorganisms such as the free surface energy and hydrophobicity because the surface roughness of BFR had no effect on bacterial adhesion; however, bacterial adhesion increased with higher surface free energy values.32 Furthermore, factors other than the inorganic filler content of the restorative material can influence the roughness such as duration of the brushing cycles and load applied on the toothbrush bristles. The organic matrix, spatial conformation, hardness and distribution of bristles(?), may also play a role in the wear pattern caused by toothbrushing.
BFRs have opened other good perspectives. Apparently, they can be indicated for patients who have gastroesophageal reflux disease since their exposure to an acid environment had no effect on the state of their surface.13
In this study, SDR had a DC higher than 90% and the other restorative materials obtained DC values below 90% at the deepest portion. SDR and FBF showed a decrease in DC in the deepest portion (4 mm), because DC values of up to 3 mm did not differ from those at the top (Table 3). Both composites are flowable BFRs that seem less affected by the material thickness than high-viscosity BFR.21,33A systematic review concluded that BFR reached an acceptable depth of cure (at 4 mm) depending on the light curing conditions and showed that an amount of ≥20 J/cm2 should be the minimum energy dose required to polymerize 4 mm increments of BFR.34The DC of FSU conventional resin met with the manufacturer’s instructions since an increment of 2 mm thick is indicated. On the o ther hand, the FSU were made with a thickness of 4 mm as did for BFR and at the depth of 3 mm, significant reduction in DC was already detected. Because the DC of all restorative materials tested (BFR and the conventional type) decreased at 4 mm depth, the third null hypothesis was rejected.
Some BFRs contain alternative photoinitiators that are activated with polywave light-curing units, in order to match the correct wavelength at which those photoinitiators are excited.35 Moreover, light curing units with larger tip diameters, with more homogeneous light beams were found to be more convenient for light-curing large MOD restorations.36These authors also observed that the polywave unit did not improve the polymerization of composites in the proximal boxes, even for composites that had a photoinitiator that required a shorter wavelength (i.e. violet light). The light-curing unit used in this study delivered an irradiance of 1,001 mW/cm2 with emission of violet (396 nm) and blue lights (447 and 466 nm), however it was not able to maintain the DC of composites at 4 mm depth.
Other studies have reported a decrease in microhardness with increased depth.37 However, the use of different curing units and restorative composites must be considered when comparing the results of different studies.38 According to the methodology adopted to investigate the DC, small variations in results between studies could occur, given that the technique of measurement significantly influences the DC values reported. In this study, a Confocal Raman microscope was used; this collects spectra that are used to calculate the DC at the different composite depths.33In this study, a metal mold measuring 4 mm high and 2 mm in diameter also used to evaluate the DC of composites. Composition of the mold could influence the light transmission and not reproduce the light activation of the BFR in vivo. Future studies should be performed on vital teeth, containing a complex class II cavity to better simulate clinical conditions. Studies have found that both the position of the tooth in the dental arch and position of the operator have an influence on the result of the complete polymerization of a resin composite restoration.39,40
Another aspect to be taken into account is that clinically, the height of the cusp can increase the distance between the light-curing unit tip and the resin composite inserted into the cavity, thereby further reducing polymerization in the deeper layers. These precautions are essential for a good clinical outcome of the restoration especially for BFR. Considering the possibility of saving time and the increase in indication of the injectable resin composite technique, the clinical use of BFR must consider the surface changes after toothbrushing when it is used in anterior teeth and reduce the increment thickness in the case of restorations performed in posterior teeth.
Conclusion
Toothbrushing changed the composite surface roughness of all restorative materials and two bulk-fill composites (Beautifil-Bulk Restorative and Beautifil-Bulk Flowable) showed a higher roughness profile than that of conventional resin. Composites were not able to maintain the DC values found at the top when compared with those at the bottom (4 mm depth).
Acknowledgements
The authors would like to thank the both the Programa Nacional de Cooperação Acadêmica (Procad) and the Brazilian funding agency, Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (Capes) - Grant No. 88881.068416/2014-01 for the support they provided to conduct this study.
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