INTRODUCTION

Resin-based composites (RBCs) have evolved significantly in terms of their use of filler content, resin matrix, and initiator systems.^1-3 These efforts have focused on improving the strength and wear resistance and decreasing the polymerization shrinkage stress of the RBCs. The features should enhance the clinical longevity of restorations, decrease the complexity of the restorative procedure, and decrease chairside time.^2,4,5

Incremental filling and light curing have been used successfully for many years and are intended to both improve the margin adaptation and reduce the shrinkage stress after photocuring the RBC.^6,7 The introduction of bulk-fill RBCs has meant that instead of using the traditional 2-mm increment composite filling and curing technique, a 4- or 5-mm increment of bulk-filled RBC can now be successfully light cured.^4,8

Bulk-fill composites have been classified according to their viscosity.⁴ Low-viscosity bulk-fill composites (flowable composites) generally have a lower filler content and are best used as a base or for small restorations. High-viscosity bulk-fill RBCs generally have higher filler content and can be used to cover the softer flowable RBCs or they can be used to fill entire restorations because they have better wear resistance and improved mechanical properties compared with flowable RBCs.^4,9 Differences in filler content are also found within conventional RBCs, mostly with respect to particle size and shape. Smaller particle sizes will often produce RBCs that have greater surface gloss retention and improved wear resistance.^10,11 Besides the filler size and shape, the hardness of these fillers, the strength of the bond between the inorganic content and polymer matrix, and the light curing of the RBC can also affect the wear resistance.¹² Consequently, surface properties such as gloss, roughness, and wear resistance will vary among different RBCs. The surface gloss and smoothness of the composite surface are factors involved in the esthetic appearance of the restoration; glossier restorations may provide a better match to the surrounding tooth structure.¹³ In addition, it has been reported that there is a significant relationship between the gloss and the surface roughness of RBCs.^13,14

One of the main reasons for replacing an RBC restoration is the recurrence of tooth decay.⁵ Surface roughness plays a crucial role in the amount of bacterial adhesion, biofilm accumulation, and surface staining.¹⁵ Microbial colonization begins in the surface irregularities in which the bacteria can grow protected from the hydrodynamic shear forces and in which cleaning is also more difficult. This results in increased bacterial growth^16,17 and an increase in the risk of caries or periodontal disease.¹⁸

Therefore, it is clinically relevant to investigate the effect of toothbrushing with a dentifrice on the surface gloss, surface roughness, roughness profile, and the wear resistance of different commercially available RBCs. The null hypotheses of this study were as follows:

1. Toothbrushing would not affect the gloss retention of RBC specimens.

2. Toothbrushing would not affect the surface roughness of RBC specimens.

3. The roughness profile after brushing would not vary among different types of RBCs.

4. Surface wear after brushing would not vary among different types of RBCs.

5. There would be no correlation between the investigated properties.

METHODS AND MATERIALS

RBCs

To measure the effect of toothbrushing with a dentifrice on the change in gloss, surface roughness, roughness profile, and wear, a wide range of commercially available RBCs was chosen for this in vitro study. Table 1 describes the RBCs that were tested and their classifications. Sonic Fill 2 was delivered using two different sonication settings on the handpiece: 1 (slowest delivery) and 5 (fastest delivery).

Table 1 RBC Manufacturer, Classification, Lot Number, and Shade

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RESULTS

Gloss Retention

The mean gloss data before and after brushing and the mean percentage of gloss decrease are reported in Table 2. Before brushing, the initial gloss of the materials that had been light cured against the Mylar ranged from 81.6 GU (Admira Fusion X-tra) to 90.6 (Mosaic Enamel). After brushing, the gloss decreased for all the RBCs tested (p<0.05). The mean percentage of gloss decrease ranged from 96.4% (Admira Fusion X-tra) to 16.4% (Mosaic Enamel). Mosaic Enamel and Filtek Supreme Ultra showed the greatest gloss retention after brushing, with only a 16.4% and 22.6% decrease, respectively (p<0.05). There was no statistical difference between the specimens made on the two sonication settings (1 and 5) for Sonic Fill 2 (p≥0.05).

Table 2 Mean Gloss (±SD) of the RBCs Before and After Brushing^a

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Surface Roughness and Roughness Profile

Table 3 shows the mean surface roughness before and after brushing and the magnitude of the increase in surface roughness after brushing compared with the unbrushed side. Before brushing, the initial surface roughness ranged from 0.08 μm for Filtek Bulk Fill Flowable to 1.14 μm for Tetric EvoCeram Bulk Fill. After brushing, the surface roughness showed an increase for all RBCs (p<0.05), with values ranging from a low of 0.99 μm (Filtek Bulk Fill Flowable) to 2.67 μm (Filtek Bulk Fill Posterior). Filtek Bulk Fill Posterior showed the greatest surface roughness after brushing, although it was not statistically different from the results for Filtek Supreme Ultra and Tetric EvoCeram Bulk Fill. On the other hand, Filtek Bulk Fill Flowable had the least surface roughness after brushing (although not statistically different from Mosaic Enamel). This was despite the fact that the surface roughness of Filtek Bulk Fill Flowable increased 12.4× after brushing. No statistical differences were observed in the roughness between the specimens made on the two different sonication settings (1 and 5) of Sonic Fill 2 either before or after brushing (p≥0.05).

Table 3 Mean (±SD) Surface Roughness (Sa) of the Unbrushed and Brushed Surfaces of the RBCs^a

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The mean roughness profile data are reported in Table 4. Admira Fusion X-tra had the greatest roughness profile (3.38 μm), although this was not statistically different (p≥0.05) from Tetric EvoCeram Bulk Fill (2.84 μm), Filtek Bulk Fill Flowable (2.44 μm), and Aura (2.38 μm). Mosaic Enamel had the lowest roughness profile (0.92 μm), which was not statistically different from Filtek Bulk Fill Posterior (1.24 μm). There were no statistical differences observed between the specimens made using the two sonication settings (1 and 5) of Sonic Fill 2 (p≥0.05).

Table 4 Mean (±SD) Roughness Profile (Rv) of the RBCs After Brushing^a

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Surface Loss (Wear) and Topographical Analysis

The mean surface height loss values are reported in Table 5. Admira Fusion X-tra had the greatest surface loss (3.51 μm) compared with the other RBCs (p<0.05). The surface loss ranged from a low of 0.75 μm (Filtek Supreme Ultra) to 3.51 μm (Admira Fusion X-tra). No statistical differences were observed between specimens made using the two sonication settings (1 and 5) of Sonic Fill 2 (p≥0.05). Figures 1 and 2 are representative images of the values described in Table 5 and Table 3, respectively. In Figure 1, the deepest areas, represented by the dark blue and purple colors, were observed in Admira Fusion X-tra, Aura, Tetric EvoCeram Bulk Fill, and Tetric EvoFlow Bulk Fill (Figure 1). Figure 2 shows representative microscopy images of all the RBCs on the unbrushed (left) and the brushed sides (right). After brushing, the filler particles of some RBCs were exposed, which can be seen in the images of Admira Fusion X-tra, SDR+, Sonic Fill 2, and Tetric EvoFlow Bulk Fill. The image of SDR flow+ after brushing shows filler particles that were greater than 10 μm in size.

Table 5 Mean (±SD) Loss in Height of the RBCs^a

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Correlation Coefficient

Figure 3 shows that the decrease in gloss after the toothbrushing for most of the samples tested was correlated with the increase in surface roughness (inverse linear correlation, p<0.05; R²=0.5504). Figure 4 shows the positive correlation (p<0.05; R² = 0.4617) between the percentage of gloss decrease and the surface loss (wear of the material). Figure 5 shows that there was a weak interaction between surface roughness and surface loss (p<0.05; R²=0.0802).

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DISCUSSION

RBCs have become the material of choice for direct restorations and are widely used in dental practice.⁵ Clinically, maintaining a smooth surface on the RBC is important because it may reduce plaque retention, surface discoloration, tissue inflammation, and secondary caries; improve the esthetics; and potentially add to patient comfort.^22,23

In this study, toothbrushing with a dentifrice decreased the surface gloss and increased the surface roughness for all tested RBCs, thus rejecting both the first and the second null hypotheses. The differences in roughness profile and wear after brushing among the RBCs meant that the third and fourth null hypotheses were also rejected.

Among many factors, the gloss can be affected by the particle size, chemical heterogeneity, and surface defects in the RBC.^22,24 Two of the RBCs tested, Admira Fusion X-tra (a nanohybrid Ormocer, organically modified ceramics) and Tetric EvoFlow Bulk Fill (a flowable bulk-fill composite), had more than a 90% decrease in gloss. The manufacturer recommends that Admira Fusion X-tra be used for class I, II, and V restorations and not in esthetic areas such as class III and IV, where the loss in gloss would be more important to the patient. Mosaic Enamel exhibited the greatest gloss retention (16.4% gloss decrease) followed by Filtek Supreme Ultra (22.6% gloss decrease); these RBCs are conventional nanohybrid and nanofilled composites, respectively. Herculite Ultra, also a conventional composite, had low gloss retention (66.5% gloss decrease). This RBC includes nanohybrid filler particles and was expected to have better gloss retention due to the spherical-shaped filler particles.²³ However, since the gloss decreased after 2 years of simulated toothbrushing, restorations may require repolishing or even replacement, to meet the esthetic requirements of the patient.²⁵

It is claimed that the larger size of the monomer molecules used in the Ormocer would reduce polymerization shrinkage and wear and leach fewer monomers.²⁶ However, this RBC exhibited the greatest surface loss, a large increase in surface roughness, a rough surface profile after brushing, as well as a large decrease in surface gloss. Another study has also reported low gloss retention and an increase in surface roughness for Admira Fusion X-tra, even after only 5000 reciprocal strokes.¹⁹

Some studies report that the surface roughness of composites is a key factor in biofilm formation,^17,27 and this increases the risk of caries around dental restorations.²⁸ Conversely, other studies have failed to report any significant relationship between the surface roughness and an increased biofilm formation.¹⁶ However, this result may be due to the difficulty in standardizing the polishing procedure²⁸ or by the prolonged bacterial incubation that was used in the study, in which new bacteria adhered to the biofilm and not to the surface of the RBC that was being tested.¹⁷ Another study showed that the surface topography might be considered more important for Streptococcus mutans biofilm formation than surface roughness, and according to the same authors, deeper and larger depressions may provide a more favorable region for bacterial colonization and biofilm formation because bacterial colonies are more difficult to remove from a rough surface.²⁸ In the present study, the topography was analyzed both by the surface roughness and by the roughness profile. Considering the surface topography of the different specimens, it is suggested that there will be increased and more mature biofilm formation around brushed Admira Fusion X-tra, Aura, Tetric EvoCeram Bulk Fill, and Tetric EvoFlow Bulk Fill RBCs. However, further investigations regarding the formation of biofilm after brushing are required as many other factors such as the chemical composition and the surface free energy of the RBC may affect biofilm formation.¹⁷

Previous studies have considered that bacterial adhesion should not occur below a profile roughness threshold of 0.2 μm.²⁹ According to this threshold, bacterial adhesion would be increased for all tested RBCs after toothbrushing (Table 3). However, this threshold was based on a profile roughness instead of the surface roughness measurement that was used in the present study. Also, a recent systematic review identified that a threshold could not reliably predict the bacterial adhesion.³⁰ This may make the comparison of the results of the present studies and this threshold unfeasible.

The roughness that patients could theoretically detect was also studied using a previously established profile roughness threshold of 0.5 μm.³¹ All RBCs tested would present a roughness increase of more than this value after the simulated 2-year toothbrushing (Table 3), which may lead to the necessity of repolishing restorations, but again, this threshold was also reported in profile roughness and not surface roughness.

Although some flowable bulk-fill composites such as Filtek Bulk Fill Flowable can be placed in small posterior occlusal restorations, other flowable bulk-fill composites, such as SDR or Tetric EvoFlow Bulk Fill, are not recommended by the manufacturers to be placed in areas of occlusal loading, where a covering layer of a more wear-resistant RBCS is required.⁴ More wear was expected for all the flowable bulk-fill composites, but this was not always the case. Instead, the flowable RBCs displayed intermediate results or even low results, such as those seen in Filtek Bulk Fill Flowable. However, this study evaluated only the wear of RBCs after toothbrushing, and the effect of the occlusal loading on these materials was not tested. Of the high-viscosity bulk-fill composites, Filtek Bulk-Fill posterior maintained the greatest gloss retention (39.7% of gloss decrease) and the least wear after toothbrushing.

Although the wear resistance may depend on the degree of conversion of the monomers,³³ these RBCs were well cured and stored for 24 hours before testing. It is expected that RBCs that have a lower filler loading may have greater wear because the resin matrix is less protected by the fillers and will be more readily removed.^32,33 However, Admira Fusion X-tra and Aura had greater wear, even though both were nanohybrid composites with 84% and 81% of filler loading, respectively. There was less wear on Filtek Bulk Fill Posterior, Filtek Supreme Ultra (nanofilled), and Mosaic Enamel (nanohybrid). Figures 1 and 2 also show a flatter surface for these RBCs compared with the other RBCs that were tested. Thus, the gloss retention and wear results reported in this study corroborate the theory that RBCs may differ after brushing due to the quality of silanization of the organic matrix,¹³ as the gloss decrease or the amount of surface loss could not be predicted from the percentage filler loading.

The fifth null hypothesis was also rejected because a significant inverse correlation between gloss and surface roughness was observed. Other authors have also observed a similar inverse linear correlation between gloss and surface roughness after toothbrushing.^19,34 This occurs because the rougher the material, the more light is scattered on its surface, leading to a decrease in the gloss.¹⁴ Also, a positive correlation between gloss decrease and surface loss and a negative correlation between surface roughness and surface loss was observed. However, these correlations were weak (R²=0.46 and 0.08). This observation may be explained by the difference in the reflective index of the exposed filler particles, reducing the gloss retention relative to the wear of the resin composites. The whitening toothpaste used in the study had an RDA of 101 and can be considered to have a medium abrasiveness. This RDA can be compared with the abrasiveness of some prophylactic polishing pastes.³⁵ Thus, after an initial surface roughness increase, the toothbrushing may have caused some polishing of the surface of the RBCs, leading to a weak positive correlation between gloss decrease and surface loss and a weak negative correlation between surface roughness and surface loss.

The excellent results for Mosaic Enamel suggest this material can be successfully used for RBC restorations, with regard to the wear expected from toothbrushing. Also, regarding anterior esthetic restorations requirements, this material retained a high gloss value. Most of the high-viscosity bulk-fill RBCs achieved properties comparable with those of conventional RBCs, except for Admira fusion X-tra, which was the most affected by toothbrushing. No difference between the sonication settings were observed for Sonic Fill 2 regarding the properties tested; however, further physical and mechanical properties should be studied to determine the best sonication setting.

CONCLUSIONS

Within the limitations of the current study, the following conclusions can be made:

1. Toothbrushing with a dentifrice reduced the gloss, increased the surface roughness, and caused surface loss for all RBCs tested.

2. There was a negative correlation between gloss and surface roughness.

3. Considering all the tested properties, Mosaic Enamel displayed excellent gloss retention, low surface profile roughness, and low wear; in contrast, Admira Fusion X-tra was the most affected by toothbrushing, exhibiting the greatest decrease in gloss, the greatest roughness, and the most wear.

4. The properties tested were product and not type dependent.