INTRODUCTION

With advances in materials science and clinical techniques, the indications of resin-based composites (RBCs) have expanded to include large posterior stress-bearing restorations that were traditionally restored using amalgam.^1,2 Nonetheless, the posterior RBC restorations are still technically challenging to perform due to the incremental layering technique and depth-of-cure issues.^3,4 The incremental layering technique is time consuming and might lead to void formation between composite layers. Bulk-fill RBCs were developed to address the previously mentioned problems associated with conventional materials. They can be placed and cured in layers of up to 4 to 5 mm in thickness.⁵ The greater depth of cure is achieved by means of novel resins, modified initiator systems, polymerization boosters, unique fillers, and filler control.^4,6,7 Moreover, special modulators and light-sensitive fillers are incorporated into some products to provide expanded working time by acting as a protective shield against operatory and ambient lights. Bulk-fill RBCs come in either restorative (sculptable/packable) or flowable forms.⁵ Typically, flowable bulk-fill RBCs, with their lower filler content, are used as liners or bases in large class I/II restorations where they are placed and bulk cured in 4-mm increments and “capped” occlusally with more highly filled RBCs.^2,8 On the other hand, restorative bulk-fill RBCs are more highly filled and can be used in stress-bearing situations without the need for another overlying final layer.⁹

The mechanical properties of bulk-fill RBCs have been the subject of some debate. While some authors have reported lower mechanical properties than conventional highly filled RBCs, others have reported values that are close to conventional materials.^5,10,11 Prior data on curing efficiency had also been ambivalent, with some reporting depths of cure of more than 4 mm and others describing insufficient curing at 4-mm layers.^12-16 The differences in mechanical properties as well as depth of cure may be attributed to variances in resin compositions, material translucency, viscosity, filler type, and content.⁸

Until now, a limited number of studies have investigated the flexural properties of bulk-fill RBCs, and few, if any, have assessed the performance of bulk-fill giomers or prereacted glass (PRG) composites. Giomers are based on PRG technology where acid-reactive fluoride containing glass is prereacted with polyacids in the presence of water, freeze-dried, milled, silanized, ground, and used as fillers.⁴ PRG technology has been incorporated into many Shofu products, ranging from restoratives to bonding agents. Besides being biocompatible and having antiplaque formation properties, giomers can also release and recharge fluoride.^17,18

Physical properties of RBCs are well known to be affected by their surrounding chemical environment. Different chemicals from food substances can cause softening and dissolution of matrices of RBCs as well as filler damage, debonding, and leaching, resulting in decreased restoration durability and longevity.^19,20 The use of dietary solvents permits the accelerated assessment of dental RBCs in short periods of time together with an appraisal of chemical affinity and the elution process.²¹ The chemical environment and food substances have been found to affect the viscoelastic properties of bulk-fill RBCs as well.²²

The objectives of the study were to compare the flexural modulus and strength of restorative and flowable bulk-fill RBCs to their conventional materials and to determine the effects of conditioning environment on the flexural properties of bulk-fill composites. The null hypotheses were as follows: 1) there are no differences in flexural modulus and strength between restorative and flowable bulk-fill RBCs as well as their conventional counterparts, and 2) the flexural properties of bulk-fill restorative and flowable RBCs are not affected by their conditioning environment.

METHODS AND MATERIALS

Materials and Specimen Preparation

Table 1 shows the materials evaluated and their technical profiles. They included three conventional RBCs (Filtek Z350 [FZ], Tetric N Ceram [TN], and Beautifil II [BT]), three restorative bulk-fill RBCs (Filtek Bulk-Fill Restorative [FB], Tetric N Ceram Bulk-Fill [TB], and Beautifil Bulk-fill Restorative [BB]), as well as three flowable bulk-fill RBCs (Filtek Bulk-Fill Flowable [FF], Tetric N Flow Bulk-Fill [TF], and Beautifil Bulk-Fill Flowable [BF]).

Table 1 Technical Profiles and Manufacturers of the Materials Evaluated

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The International Organization for Standardization (ISO) recommends the use of specimens that are 25 × 2 × 2 mm for flexural testing.²³ Such specimens are challenging to fabricate without flaws and necessitate the use of multiple overlapping irradiation because of the comparably smaller light exit windows of curing tips.²⁴ Moreover, these ISO specimens are not clinically relevant since the mesio-distal widths of molars are around 11 mm, and the cervico-incisial length of central incisors usually do not exceed 13 mm.²⁵ The miniflexural test, employing 12-mm specimens, was selected due to their significant correlation to the ISO flexural test, clinical relevance, and better efficiency.^26,27 Forty beam-shaped test specimens (12×2×2 mm) of each of the various RBCs were fabricated using customized stainless-steel molds. The conventional and bulk-fill RBCs were placed in a single increment, while the flowable materials were injected into the molds. Excess material was removed by compressing the molds between two Mylar strips with glass slides. The top surface of the specimens were light polymerized through the glass slide with two overlapping irradiations of 10 seconds each using a calibrated LED curing light (Demi Plus, Kerr, Orange, CA, USA) with an output irradiance of 1330 mW/cm² and wavelength range of 450 to 470 nm. The glass slides were removed, and the specimens were light cured for another 10 seconds. The Mylar strips were subsequently discarded, and the composite beams were removed from their molds. Any minor material excess, or “fins,” was gently removed by fine polishing discs (Sof-Lex, 3M ESPE, St Paul, MN, USA). The composite specimens were visually examined for the presence of voids, and any defective specimens were replaced. The final dimensions of the specimens and the parallelism between their opposite surfaces were verified with a digital caliper (Mitutoyo Corp, Kawasaki, Japan).

Flexural Testing

After the seven-day conditioning period, the composite specimens were loaded until fracture using a universal testing machine (Shimadzu Corp, Kyoto, Japan) with a load cell of 5 KN and crosshead speed of 0.5 mm/min. Flexural strength, σ, in megapascals (MPa), was calculated using the following equation:

where P is the maximum load exerted on the specimen in newtons, L is the distance between the supports in millimeters (10 mm), B is the width of the specimen in millimeters, and H is the height of the specimen in millimeters.

Flexural modulus, E′, in MPa, was calculated using the following equation:

where F/D is the slope, in newtons per millimeter, measured in the straight-line portion of the load-deflection graph; L, B, and H are defined in the flexural strength equation. Flexural modulus was subsequently converted to gigapascals (GPa).

RESULTS

Mean flexural modulus and strength for the various RBCs after conditioning in the different mediums are reflected in Table 2. Figures 1 and 2 compare the mean flexural modulus and strength between mediums for each material. Statistical comparisons of mean flexural properties between RBCs when grouped by manufacturers and material type after conditioning in the various mediums are summarized in Tables 3 and 4, respectively. Two-way ANOVA presented significant interactions (p<0.001) between materials and mediums for both flexural modulus and strength.

Table 2 Mean Flexural Modulus (GPa) and Flexural Strength (MPa) of the Various Resin-Based Composites (Standard Deviations in Parentheses)

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Table 3 Comparison of Mean Flexural Modulus and Strength Between Resin-Based Composites Grouped by Manufacturer After Conditioning in the Various Mediums^a

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Table 4 Comparison of Mean Flexural Modulus and Strength Between Resin-Based Composites Grouped by Material Type After Conditioning in the Various Mediums^a

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DISCUSSION

The current study investigated the differences in the flexural properties between bulk-fill and conventional RBCs and the effect of the conditioning environment on the different materials. As flexural properties were material and conditioning medium dependent, both the null hypotheses were rejected. The different RBCs were immersed continually in the various conditioning mediums for seven days at 37°C before flexural testing was performed. This conditioning period might appear considerably long since restorations come into contact with foods and liquids infrequently and for short durations intraorally. Continuous exposure may, however, take place when chemicals are absorbed by food particles or calculus at the margins or grooves of restorations.^29,30

Flexural testing is widely used in characterizing RBCs since it determines both flexural modulus and strength.²³ Flexural modulus describes the stiffness of RBCs, whereas flexural strength represents the maximum stress that RBCs can be subjected to prior to failure. Flexural testing yields complex tensions arising from the combination of compression, shear, and tensile stresses.²⁴ The variation between the flexural properties of various RBCs is useful in the different clinical situations.^31,32 For example, in class I, II, III, and IV cavities, RBCs with high flexural properties are usually selected to minimize fracture or deformation under the high occlusal forces, while in class V cavities, RBCs having low flexural modulus are preferred, as they can flex with the teeth during function and parafunction, which in turn reduces the stresses at the adhesive interface and decreases the chances of debonding.^31,33

Bulk-fill restorative RBCs were generally stiffer than their bulk-fill flowable and conventional counterparts. This may be attributed to the similar or higher filler content of the bulk-fill restoratives in comparison to the other RBCs. Results corroborated those of El-Safty and others,³⁴ who reported a significant positive correlation between modulus and filler loading. FZ had a higher filler content than FB, which explains the higher flexural modulus of FZ in air. Weak intraoral acids, such as citric acid, have been reported to degrade the inorganic fillers in RBCs.³⁰ This might explain why BB, with its relatively higher inorganic and prereacted glass ionomer filler content, was somewhat more susceptible to modulus degradation than BT. The flexural modulus of the bulk-fill flowable RBCs were mostly significantly lower than their bulk-fill restorative and conventional counterparts. This was consistent with the work of Jung and others.³⁵ These authors suggested that bulk-fill flowable RBCs, with their lower modulus, may not provide an effective buffer to occlusal stress and recommended that they be capped with conventional materials. In high-stress–bearing areas, RBCs of higher stiffness are required to prevent restoration deformation, which could accelerate marginal and restoration failures. In addition to modulus, other physical properties of RBCs, including strength, fracture, and wear resistance, must also be considered for stress-bearing situations. However, with their greater flexibility, bulk-fill flowable RBCs are preferred over bulk-fill restorative and conventional materials in deep class V cavities, as they appear to offer better marginal adaptation.³⁶

For materials from manufacturer (A), the bulk-fill restorative had higher flexural strength than the other RBCs. However, for manufacturer (B) and (C) materials, bulk-fill flowable RBCs were generally stronger than their bulk-fill restorative and conventional counterparts despite their relatively lower filler loading. These results contradicted those of Tomaszewska and others³⁵ and Jung and others,³⁷ which reported that bulk-fill flowable RBCs have lower mechanical properties when compared to either highly filled nanohybrid or bulk-fill restorative RBCs. The variance in results may be attributed in part to the differences in bulk-fill flowable RBCs evaluated and experimental designs. Commercial flowable composites have a wide range of filler loading from 52% to 68% weight.³⁸ The higher flexural strength of the bulk-fill flowable RBCs evaluated in the present study when compared to bulk-fill restorative and conventional materials could be attributed to their relatively high filler loading, resiliency, and ability to withstand higher stress prior to fracture (Table 1).

When comparing different products, TN and TB from manufacturer (B) and FF from manufacturer (A) had significantly lower modulus than the other conventional, bulk-fill restorative and bulk-fill flowable RBCs regardless of conditioning mediums. The significantly lower modulus of these RBCs may be attributed to their relatively lower filler loading, disparities in fillers, and resin matrices used. As for flexural strength, no obvious trends were observed when conventional and bulk-fill flowable RBCs were compared. Differences between products varied with conditioning mediums, highlighting the importance of conducting flexural testing with different dietary solvents. Variances can again be attributed to differences in filler and resin content/type employed. For bulk-fill restorative materials, FB from manufacturer (A) was significantly stronger than both TN and BB from manufacturers (B) and (C), respectively. FB contains two novel monomers: a high-molecular-weight aromatic dimethacrylate and addition fragmentation monomers that act to decrease the polymerization shrinkage stress.³⁹ According to the manufacturer, aromatic dimethacrylate decreases the reactive groups in the resin, controlling the volumetric shrinkage and the rigidity of the final polymeric matrix, whereas addition fragmentation monomers contain a third reactive site that cleaves through a fragmentation process. This in turn helps provide a relaxation mechanism of the network being developed, leading to stress relief. The fragments can still react with each other or with other reactive sites ensuring that the physical properties of the material are preserved. This might play a role in developing shorter and stiffer polymeric chains, leading to a more rigid and stronger bulk-fill RBC.⁴⁰

When storage environments were compared, conditioning in air presented the highest flexural modulus and strength for all RBCs regardless of their type or manufacturer. Conditioning in air does not result in the leaching out of silica and filler particles that occurs with storage in aqueous mediums.²⁹ The lowest flexural modulus was observed when the RBCs were conditioned in ethanol with the exception of FZ and BB, which showed the lowest flexural modulus after storage in citric acid. Other than FB, which was weakest after storage in citric acid, the lowest flexural strength was also observed with conditioning in ethanol for all RBCs. The effect of citric acid on RBCs has already been elaborated on. Ethanol is known to soften the resin matrix of RBCs by removing unreacted monomers and linear polymers from the polymeric structure.^24,29 With resin dissolution, filler exposure and dislodgement may ensure weakening the RBCs. The dietary habits of patients should thus be considered during material selection to enhance the clinical longevity of composite restorations.

The current study can be improved in some areas. To begin with, the conditioning period could be extended to determine the longer-term effects of conditioning environment on flexural properties.^21,41 Static flexural testing that was carried out in the present study cannot provide insights into material structure, as dental RBCs are viscoelastic in nature and exhibit both viscous and elastic characteristics when undergoing deformation. Dynamic testing with dynamic mechanical analysis can be performed to better assess the viscoelastic properties of the RBCs.⁴² Dynamic mechanical analysis can be carried out using various frequencies, temperatures, and displacements that resemble the variations of forces and temperatures in the oral cavity. Moreover, unlike static testing, dynamic testing enables retesting of specimens over extended time periods, as it is nondestructive.^22,43 As flowable RBCs are not a homogeneous group of materials, appraisal of their rheological properties and correlating this to their flexural properties may be beneficial.

CONCLUSIONS

Within the limitations of this study, the following can be concluded:

• Flexural properties of bulk-fill restorative, bulk-fill flowable, and conventional RBCs were both material and conditioning medium dependent.

• Bulk-fill restorative RBCs were generally stiffer than their bulk-fill flowable and conventional counterparts.

• With the exception of FF, bulk-fill flowable RBCs were stronger than their bulk-fill restorative and conventional counterparts.

• While no patterns were observed for flexural strength, manufacturer (B) bulk-fill and conventional RBCs were less rigid than comparable products from manufacturers (A) and (C).

• Conditioning in air resulted in the highest flexural properties, while exposure to ethanol generally presented the lowest.