The bond strength of highly filled flowable composites placed in two different configuration factors.

OBJECTIVE: The aim of this study was to evaluate the microtensile bond strength (μTBS) of different flowable composite resins placed in different configuration factors (C-factors) into Class I cavities.
MATERIALS AND METHODS: Fifty freshly extracted human molars were divided into 10 groups. Five different composite resins; a universal flowable composite (AeliteFlo, BISCO), two highly filled flowable composites (GrandioSO Flow, VOCO; GrandioSO Heavy Flow, VOCO), a bulk-fill flowable composite (smart dentin replacement [SDR], Dentsply), and a conventional paste-like composite (Filtek Supreme XT, 3M ESPE) were placed into Class I cavities (4 mm deep) with 1 mm or 2 mm layers. Restored teeth were sectioned vertically with a slow-speed diamond saw (Isomet 1000, Buehler) and four micro-specimens (1 mm × 1 mm) were obtained from each tooth (n = 20). Specimens were subjected to μTBS test. Data were recorded and statistically analyzed with two-way analysis of variance and Tukey's post-hoc test. Fractured surfaces were examined using a scanning electron microscope.
RESULTS: The μTBS in SDR-1 mm were higher than other groups, where Filtek Supreme XT-2 mm and GrandioSO Flow-2 mm were lower. No significant differences were found between C-factors for any composite resin (P > 0.05).
CONCLUSION: Bulk-fill flowable composite provided more satisfactory μTBS than others. Different C-factors did not affect mean μTBS of the materials tested.

INTRODUCTION

Flowable composites adapt well to the cavity wall and reduce the risk of gap formation due to their low viscosity. The first flowable composites were not appropriate for deep cavities due to their low mechanical properties and high polymerization shrinkage in comparison with paste-like composites.[1] In order to enhance the mechanical properties, the filler content of new generation flowable composites was increased.

In recent years, newer flowable restorative composites have been introduced that can be used as bulk-fill up to 4 mm in thickness. Bulk-fill composite resins have lower polymerization shrinkage and stress values when compared with conventional flowable paste-like microhybrid and nanohybrid composite resins.[2]

Polymerization shrinkage is a risk for long-term clinical success of composite resins. The reduction of monomer volume causes shrinkage stresses to debond the material from dentin during polymerization.[34] The amount of polymerization shrinkage is influenced by various characteristics of the composite formulation, such as polymerization kinetics, matrix type, filler content, elastic modulus, and degree of conversion.[5] Several researches have therefore been made to minimize the amount of polymerization stress.

Application of composites with different configuration factors (C-factors) is one of these research topics.[6] C-factor describes the ratio between bonded to unbonded surfaces and should be reduced with incremental layering technique to prevent gap formation caused by polymerization shrinkage. In this technique, the composite resin is placed into the cavity with different geometrical layers so that each layer contacts less cavity surface. When restoring cavities with high C-factor, the outcome stresses place tooth-resin interfaces under increased tension as there is less chance for relaxation of shrinkage stress.[7] Researchers have reported[8] that the increase in C-factor has associated with a progressive decrease in bond strength and a potentially harmful effect on marginal integrity and gap formation. Although incremental placement techniques have the advantage of maximizing polymerization of each increment and increased adaptation of the composite to cavity walls, the effect on shrinkage stress should be questioned.[9]

Armstrong et al.[10] investigated the relation between microtensile bond strength (μTBS) and different C-factor values and found that low C-factor cavity design ensure more durable bond. Lutz et al.[11] also confirmed that C-factor affects composite resin bonding to cavity floor. In high C-factor, composite placement with increments is commonly used to reduce polymerization stress.

The objective of this study was to compare the μTBS of highly filled flowable composites with bulk-fill and conventional composites placed in cavities with different C-factors. The null hypotheses were that different C-factors affect the bond strength and that using highly filled flowable composites can increase bond strength due to high filler content that may reduce polymerization shrinkage.

MATERIALS AND METHODS

Table 1 lists the restorative materials used. Fifty intact, caries-free, and nonrestored human molar teeth were stored in 0.1% thymol solution at 4°C <1 month after extraction. Class I occlusal cavity preparation (3.5 mm long × 3.5 mm wide × 4 mm deep) with inner rectangular angles was made in each tooth mounted in a Micro-Specimen Former using a diamond bur (#2068 – KG Sorensen, Barueri, SP, Brazil). The cusps were flattened 2 mm, and the cavity floor was prepared parallel to the flattened cusps. After preparation, a self-etch adhesive (SE bond, Kuraray Dental, Tokyo, Japan) was applied to the cavities according to the manufacturer's instructions using a light-emitting diode (LED) curing device (Elipar FreeLight 2, 3M ESPE, Seefeld, Germany).

Table 1

Type, composition and filler load of materials used in this study

Fifty teeth with Class I cavities were randomly assigned to one of five composite resins and further subdivided into two different C-factor groups. A bulk-fill composite (SureFil smart dentin replacement [SDR] Flow, Dentsply, Konstanz, Germany); two flowable composites with high filler content (GrandioSO Flow, VOCO, Cuxhaven, Germany), (GrandioSO Heavy Flow, VOCO, Cuxhaven, Germany), “GRH;” a conventional flowable composite (Ælite Flo, Bisco, IL, USA); and a conventional paste-like composite (Filtek Supreme XT, 3M ESPE, St. Paul, MN, USA) were used. Each composite resin was light-cured with 1 mm or 2 mm layer thicknesses using a LED curing device (Elipar FreeLight 2, 3M ESPE, Seefeld, Germany) in standard mode (20 s for each layer). The output energy of the LED curing device was measured periodically with a radiometer (Demetron/Kerr, Danbury, CT, USA) to ensure that 1545 mW/cm2 were exceeded. C-factor was calculated as the ratio between bonded and unbonded surface areas as described by Feilzer et al.[7] The C-factor for 1-mm layer thickness (C1) was therefore 2.14, and 3.29 for 2 mm (C2).

After 1 week storage in 37°C water, teeth were embedded into plastic molds using self-curing acrylic resin and mounted on a low-speed diamond saw (Isomet 1000, Buehler, Coventry, UK). Four micro-specimens were obtained from each tooth with 1 mm2 × 1 mm2 surface areas (n = 20). The microtensile test was to be performed between cavity bottom dentin and composite resins, so the teeth were sectioned longitudinally. Each micro-specimen was attached to a microtensile tester (Micro Tensile Tester, T-61010 K, Bisco, USA) using cyanoacrylate adhesive (Zap-It, Dental Ventures of America Inc., Corona, CA, USA) and subjected to microtensile testing at a crosshead speed of 5.4-kg force per minute. μTBS (MPa) was calculated by dividing the failure load (N) by the cross-sectional area (mm2) of the test bar.

Specimen surfaces were examined under a stereomicroscope at ×100 magnification, and failure modes were recorded.

μTBS-fractured surfaces were mounted on aluminum stubs using a conductive silver agent (Silver Print; GC, Tokyo, Japan), sputter-coated with gold in a 20 nm layer (SCD 040; Balzers, Wiesbaden, Germany) for scanning electron microscope (SEM) evaluation. Photomicrographs were taken with a SEM (JSM-5600 Scanning Microscope; JeOL Ltd., Tokyo, Japan) at ×150 and ×2000 magnifications [Figure 1].

Figure 1

Scanning electron microscope images of three different failure modes. (a1) shows a mixed failure, white arrow shows a cohesive fractured site in dentin side; (a2) higher magnification of a1, white arrow shows resin composite remnants; (b1) shows a completely adhesive failure (b2) higher magnification of b1, it can be seen adhesive resin remnants as well as smear residues; (c1 and c2) show a completely cohesive failure in composite side

Data were analyzed with SPSS 20 software (IBM Corp.) using analysis of variance, Tukey's post-hoc test, and Chi-square test at the 95% significance level.

RESULTS

μTBS values ranged from 3.2 MPa to 50 MPa, as shown in Figure 2 and Table 2. No pretesting failure was recorded.

Figure 2

Boxplot of the microtensile bond strength results. The box represents the spreading of the data between the first and third quartile. The whiskers extend to the minimum and maximum value measured

Table 2

Mean bond strength, SD, minimum and maximum values of composite resins in different layer thickness

There were no significant differences between C1 and C2 for any composite.

When the cavities were filled according to C1, μTBS of SDR was significantly higher than Grandio Flow (P < 0.01) and Supreme XT (P < 0.05). Grandio Heavy Flow was not different from the other groups (P > 0.05).

When the cavities were filled according to C2, μTBS of “SDR” was significantly higher than of other composite resins (P < 0.001) except for “Ælite” (P > 0.05). The μTBS of “Ælite” was significantly higher than “Supreme XT” and “Grandio Flow” (P < 0.05). There was no significant difference among “Supreme XT,” “Grandio Flow” and “Grandio Heavy Flow” (P > 0.05).

Failure modes of each specimen, as listed in Table 3, were recorded as an adhesive failure (A), cohesive failure at dentin side (CD), cohesive failure at the composite side (CC) or mixed failure (M).

Table 3

Percentage of failing modes after microtensile bond test

DISCUSSION

The incremental technique has recently been questioned by some researchers,[212] who recommend bulk-filling of the entire cavity. Low-shrinking composites have been developed in order to eliminate the unfavorable effects of high polymerization shrinkage. This study compared the μTBS of the newly introduced GrandioSO Flow and GrandioSO Heavy Flow with SDR and conventional composites placed in cavities with different C-factors. The null hypotheses that different C-factors affect the bond strength and that GrandioSO Flow and GrandioSO Heavy Flow can increase the bond strength due to high filler content were rejected. No significant differences were determined between C-factors.

Bulk-fill flowable composites are highly desirable for routine dentistry, but concerns about shrinkage stress cause an important reluctance of their application. However, bulk-fill composites have also not presented a huge breakthrough, because of the difficulty of application in posterior teeth. Moreover, due to reduced polymerization efficiency, the incremental filling is most often still needed, especially with deep cavities.[1314]

In our study, we opted to employ Class I cavities to obtain high C-factors. In this situation polymerization shrinkage may be an important factor in bond strength. In a cavity with high C-factor, shrinkage stress increases due to high polymerization shrinkage.[715] In C1 and C2, the highest bond strength was measured in SDR. At the same time, Van Ende et al.[16] reported higher bond strength values of SDR compared with other composites (G-ζnial Universal Flo, GC, and Z100, 3M ESPE) using the bulk-fill technique. However, there were significantly no bond strength differences in the incremental technique.

The flowable composites GrandioSO Flow and GrandioSO Heavy Flow are composites with very high filler content (80-83 w/w%, respectively). According to the manufacturer, these flowable composites are subject to lower shrinkage than conventional flowable materials during polymerization (2.96%). Lower shrinkage could ensure less debonding of the material during polymerization and higher bond strength. However, there was no significant difference between Grandio composites and conventional composites. This may be attributed to the adhesive used. Likewise, Nikolaenko et al.[17] reported significant differences when composites were placed using layering techniques with OptiBond FL and single bond whereas there was no significant difference with One-Up Bond F.

Nikolaenko et al.[17] reported that bond strength increased when a vertical layering technique was used. Bulk application of low-shrinkage composites may not allow sufficient light polymerization of the materials. Use of the layering technique has therefore been described as essential. On the other hand, in our study cavities were filled with horizontal layers, and thickness was limited according to the manufacturers’ recommendations. SDR was not bulk-filled in order to standardize the study concept and compare the materials used.

When the five composites were used to fill cavities (4 mm deep) in four increments compared to two, both C-factor and composite volume decreased. Niu et al.[18] supported this approach as they have been determined different bond strengths with different C-factors. In our study, however, there was no significant difference between C1 and C2. This may be due to the close values of C1 and C2. Nikolaenko et al.[17] suggested that the first increment was important for acceptable bond strengths. Curing depths of C1 and C2 were within the manufacturers’ limit. Hence, the same characteristics in different layer thicknesses may lead to similar results. Varied adhesives and different C-factors could be considered to measure the bond strength in further studies.

CONCLUSION

Within the limitations of this in vitro study, it may be concluded that;

Bulk-fill composite has exhibited more reliable performance with higher bond strength in Class I cavities.

The bond strength values of highly filled flowable composites (GrandioSO Heavy Flow and GrandioSO Flow) were not superior to other composites. Thus, higher filler content is not an indicator of higher bond strength.

The bond strength of conventional paste-like composites was not different from flowable composites.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.