INTRODUCTION

Resin composites are widely used due to their direct filling capability, minimally invasive nature, esthetics, and clinical performance ¹ . A significant concern when placing a resin restoration is reducing polymerization shrinkage stress. One method to do so is to layer the resin composite incrementally ² . However, since its efficiency in mitigating deleterious effects at the adhesive interface has been questioned ³ , the bulk-filling technique has become more widely used following the development of materials with less shrinkage, polymerization stress and cusp deflection ⁴ . Such benefits have been attributed mainly to the increased translucency and the modification of the resin matrix, or photo-initiator dynamics ⁵ .

Although previous meta-analyses have reported that bulk-fill restorations present survival rates and clinical performance similar to those of conventional composites ^{6, 7} in contact with erosive drinks, there is evidence showing that physical and mechanical properties of bulk-fill resin composites are more negatively influenced than a conventional counterpart ⁸ . On the other hand, when the acid is of cariogenic origin, bulk-fill and conventional resin composites do not seem differ in degradation ^{9, 10} .

One reason for the acid-dependent behavior of bulk-fill resin composites may be the lower pH of erosive acids in comparison to cariogenic acids. This explanation is supported by the fact that hydrolysis can speed up under more acidic conditions, as pH affects reaction rates through catalysis ^{11, 12} . Low pH solutions can act on the polymeric matrix of composites through catalysis of ester groups from dimethacrylate monomers present in their compositions (Bís-GMA, Bis-EMA, UDMA and TEGDMA) ¹³ . The hydrolysis of these ester groups can form alcohol and carboxylic acid molecules that may accelerate degradation of the resin composites due to a lowering of the pH within the resin matrix ¹³ . In addition, low pH solutions may also cause erosion of inorganic fillers and thereby their debonding ¹³ . Despite these possibilities, to the best of the authors’ knowledge, no previous study has compared the effect of erosive and cariogenic acids on the physical and mechanical properties of bulk-fill resin composites. Thus, the aim of this study was to compare the surface roughness and the flexural strength of various bulk-fill restorative composites under simulated carious and erosive conditions.

MATERIALS AND METHODS

Experimental design and sample size calculation

This study followed a 5x3 factorial design, using five resin composites (one conventional: Filtek Z350 XT and four bulk-fill: Filtek Bulk Fill, X-tra fil, Tetric N-Ceram Bulk Fill and Aura Bulk fill, as shown in Table 1), and three storage conditions (carious, erosive, control), comprising 15 groups.

Table 1 Oharaeterization of the resin composites tested 

Sample size was calculated (G*Power 3.1.9.4, Heinrich-Heine Düsseldorf University, Düsseldorf, Germany) based on preliminary data collected from three samples per group, from which an effect size of 0.27 was obtained. At α = 0.05 and a power of 0.80, a total sample size of 215 resin composite bars would be needed. Based on the 15 groups of this study, 15 specimens per group were required.

The dependent variables were: 1) average surface roughness (Ra, in pm), measured at baseline and after storage in the allocated conditions; and 2) three-point flexural strength (in MPa). Scanning electron microscopy (SEM) images were obtained to illustrate the changes that occurred.

RESULTS

One-way ANOVA indicated that prior to storing the bars under different conditions, there were statistically significant differences among the resin composites (p < 0.001). Tetric N-Ceram and X-tra fil were significantly smoother than Filtek Bulk Fill and Aura Bulk Fill. Except for Aura Bulk Fill, which presented the highest surface roughness, none of the bulk-fill composites differed significantly from the conventional counterpart ( Table 2 ).

Table 2 Mean (SD) of baseline and increase in surface roughness (ΔRa = final - baseline) aceording to the resin composite and storage condition 

Two-way ANOVA showed a significant interaction between the resin composites and storage conditions (p = 0.025) for surface roughness change (ΔRa). As found by Tukey’s test, there was no statistically significant difference among resin composites when they were stored in artificial saliva (control). Under either carious or erosive conditions, the composite Aura Bulk Fill showed higher ΔRa than the other resin composites, which did not differ from each other ( Table 2 ). Tukey’s test also indicated that for the composites Filtek Bulk Fill, Tetric N-Ceram Bulk Fill and Filtek Z350 XT, the carious and erosive conditions did not pose increased roughness changes in relation to those caused by artificial saliva. For Aura Bulk Fill, however, both acidic conditions (carious and erosive) resulted in higher surface roughness change.

For the flexural strength data ( Table 3 ), there was a statistically significant difference among resin composites (p < 0.001), with X-tra fil providing higher values than Filtek Bulk Fill, which presented higher flexural strength than Tetric N-Ceram Bulk Fill. The latter two bulk-fill resin composites did not differ from the conventional resin composite Filtek Z350 XT. Aura Bulk Fill had the lowest flexural strength values. The storage conditions did not significantly influence the flexural strength of the tested resin composites (p = 0.148).

Table 3 Acidity, Fluoride and Silver concentrations in the different groups and times 

The photomicrographs in Fig. 1 show a smooth surface and similar micromorphology for Filtek Z350 XT and Filtek Bulk Fill when comparing the bars stored in artificial saliva (1A and ID), carious (1B and 1E) and erosive conditions (1C and 1F). X-tra fil (1G, 1H and II) presented detachment of some filler particles (line arrows), and bars stored under carious and erosive conditions (1H and 1I) had a rougher surface than the control group (artificial saliva). The Tetric N-Ceram Bulk Fill composite resin subjected to erosive condition (1L) presented filler particles exposed on the surface with irregularities (right-pointing double arrow). The Aura Bulk Fill composite resin displayed marked surface damage as a consequence of carious and erosive conditions (right-pointing thick arrow, 1N and 1O).

Fig. 1 Photomicrographs of restorative bulk-fill and conventional composites after storage in conditions of carious or erosive challenges or in artificial saliva. A, B, and C) Filtek Z350 XT; D, E, and F) Filtek Bulk Fill; G, H, and I) X-tra fil; J, K, and L) Tetric N-Ceram Bulk Fill; M, N, and O) Aura Bulk Fill. The first column presents resin composites stored in artificial saliva (A, D, G, J, and M). The second column presents resin composites submitted to the in vitro caries model (B, E, H, K, and N). The third column presents resin composites submitted to the in vitro erosion model (C, F, I, L, and O). Note: The arrows indicate surface alterations: the line arrows (→) indicate a detachment of filler particles (H and I); the right-pointing double arrow (➾) represents an exposure of filler particles (L); and the right-pointing thick arrow (➯) indicates degradation associated with damage and irregular surface (N and O).  

DISCUSSION

The main clinical benefits associated with bulk-fill resin composites are reduetion in polymerization stress ¹⁸ , shorter working time, and polymerization of increments up to 4-5-mm thick ¹⁹ . Notwithstanding these advantages over conventional resin composites, some bulk-fill composites degrade in the oral cavity as much as ^{9, 10} or more ⁸ than conventional composites. This performance of bulk-fill composites seems to depend on the eonditions to which they are exposed and/or the composition/brand of these restorative materials. Such speculations are supported by 1) studies that used acids associated with caries episodes, which did not affect physical and mechanical properties of some bulk-fill composites ^{9, 10} and 2) a study that found that erosive beverages harmed the surface of bulk-fill composites ⁸ . However, to date, no research has been published on whether erosive and cariogenic acids, whose strengths are different, would affeet bulk-fill resin composites differently. Thus, the current paper compares the effect of erosive and cariogenic acids on physical and mechanical properties of varying bulk-fill composites.

The findings of our study showed that both aeidie solutions similarly increased the surface roughness of all composites, but the damage caused by cariogenic and erosive acids surpassed that caused by artificial saliva only for Aura Bulk Fill. This suggests that the behavior of bulk-fill composites may be more likely attributable to the differing chemistry of the monomeric resin formulations and filler eharaeteristies (type, volume fraction, density and particle size and distribution) ²⁰ as well as the filler-resin interface of the bulk-fill resin composite itself, than to the pH of the storage solution. Thus, even though the erosive solution had a substantially lower pH (2.3) than the cariogenic solution (4.3) and could potentially speed up hydrolysis and degradation of resin-based materials, such pH discrepancy did not cause detectable differences in the composite’s surface roughness measured through the Ra parameter. However, as can be seem in Fig. 1 F, I, L and O, there was a trend toward the presence of increased irregularities, voids and cracks when the bulk-fill composites were under erosive conditions. Such qualitative evidence may be due not only to potentially accelerated hydrolysis by the low pH erosive solution, but also by erosion and debonding of inorganie fillers ¹³ .

It is worth mentioningthat among the resin composites tested in this study, Aura Bulk Fill had the lowest filler eontent by weight and thereby the highest resin matrix content. This may have accounted not only for Aura Bulk Fill’s higher susceptibility to the acidic Solutions but also for its higher baseline surface roughness and lower flexural strength in comparison to the other materials tested. Indeed, when weight percentage of fillers decrease, the sorption increases ^{21, 22} , causing swelling and thereby softening and plasticization of composites ²³ as well as compromising their mechanical properties ²⁰ . This statement is supported by previous observations that low inorganic filler contents (<75 wt%) have been associated with low flexural strength ²⁴ and Aura Bulk Fill was the only composite having low filler loading (65.0%) by weight. This is probably a result of the inclusión of pre-polymerized fillers ²⁵ .

In contrast to Aura Bulk Fill, the other resin composites (Filtek Z350 XT, Filtek Bulk Fill, X-tra fil and Tetric N-Ceram Bulk Fill) did not differ from each other regarding surface roughness changes, and were not affected by the storage eonditions. This finding corroborates a previous paper that found that under biofilm aeeumulation and cariogenic challenges, a resin composite did not degrade faster ¹⁵ , but disagrees with another study in which the lowest pH erosive solution caused the highest increase in surface roughness ²⁶ . This may be ascribed to the fact that our solution had 10% of citric acid (0.05 M), whereas the other study used plain passion fruit juice containing 55% citric acid, in addition to other acids.

It is worth noting that although there is no clear information on the surface roughness threshold and the ideal morphological features of a resin composite to reduce biofilm aeeumulation, surface staining and wear, and to increase gloss retention, the smoother the surface, the better. In this regard, although Aura Bulk Fill became only 7% and 8% rougher after immersion in the carious and erosive acids, respectively, the roughness attained was the highest among the materials tested. Considering that the restoration surface should have a máximum roughness 0.50 pm if it is not to be detected by