Simulated cuspal deflection and flexural properties of high viscosity bulk-fill and conventional resin composites

Journal of the Mechanical Behavior of Biomedical Materials 87 (2018) 111–118

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Simulated cuspal deflection and flexural properties of high viscosity bulk-fill and conventional resin composites

T

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Akimasa Tsujimotoa, , Yuko Naguraa, Wayne W. Barkmeierb, Hidehiko Watanabec, William W. Johnsond, Toshiki Takamizawaa, Mark A. Lattab, Masashi Miyazakia a

Department of Operative Dentistry, Nihon University School of Dentistry, Tokyo, Japan Department of General Dentistry, Creighton University School of Dentistry, Omaha, NE, USA c Department of Restorative Dentistry, Oregon Health & Science University School of Dentistry, Portland, OR, USA d Department of Adult Restorative Dentistry, University of Nebraska Medical Center College of Dentistry, Lincoln, NE, USA b

A R T I C LE I N FO

A B S T R A C T

Keywords: Simulated cuspal deflection Bulk-fill resin composite Dental bonding

Objectives: The purpose of this study was to investigate the simulated cuspal deflection and flexural properties of high viscosity bulk-fill and conventional resin composites. Methods: Seven high viscosity bulk-fill resin composites and eight conventional resin composites were used. Aluminum blocks (10 mm x 8 mm x 15 mm) with a mesio-occlusal-distal (MOD) cavity [4 (W) mm x 8 (L) mm x 4 (D) mm] were prepared and randomly divided into groups for different measurement techniques [micrometer vs CSLM] and further subdivided according to type of resin composite (high viscosity bulk-fill vs conventional resin composite). The simulated cuspal deflection resulting from the polymerization of resin composite bonded to a precisely machined MOD cavity within an aluminum block was measured with either a novel highly accurate submicron digimatic micrometer (MDH-25 M, Mitsutoyo, Tokyo, Japan) or a confocal laser scanning microscope (CLSM, VK-9710, Keyence, Tokyo, Japan) cuspal measurement method. In addition, flexural properties of tested resin composites were measured to investigate the relationship between simulated cuspal deflection and flexural properties. Scanning electron microscopy observation of tested resin composites was also conducted. Results: The simulated cuspal deflection of high viscosity bulk-fill resin composites was similar to that of conventional resin composites, regardless of measurement method. There were no statistically significant differences (p > 0.05) between the micrometer and CLSM cuspal measurement methods. There were statistically significant differences (p < 0.05) in flexural strength and elastic modulus depending on the material, regardless of the type of resin composite. Pearson correlation analysis did not show any statistically significant (p < 0.05) relationship between flexural properties and cuspal deflection. Conclusions: The results of this study indicate that high viscosity bulk-fill resin composites show similar cuspal deflection with bulk-filling techniques, to those shown by conventional resin composites with incremental filling techniques. Simulated cuspal deflection can be measured using either a micrometer or CLSM, but this experiment failed to show any relationship between the flexural properties and simulated cuspal deflection of resin composites. Significance: High viscosity bulk-fill resin composites produce the same level of cuspal deflection as a conventional incrementally filled resin composite.

1. Introduction Resin composites have come to be considered the first choice material for direct posterior restoration worldwide due to improvements in their mechanical properties (Lynch et al., 2014). Heintze et al. (2017) reported in their systematic review that, based on the quantity of restorative materials sold, as reported for corporate market insight by

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Ivoclar Vivadent, it is estimated that around 800 million resin composite restorations were placed worldwide in 2015 alone, with about 80% in posterior teeth and 20% in anterior teeth. These approximately 800 million resin composite restorations represent one of the most prevalent medical interventions in the human body. Alvanforoush et al. (2017) reported that the overall clinical failure rates of resin composite restorations in posterior teeth were similar between 1995–2005 and

Corresponding author. E-mail address: [email protected] (A. Tsujimoto).

https://doi.org/10.1016/j.jmbbm.2018.07.013 Received 7 June 2018; Received in revised form 6 July 2018; Accepted 10 July 2018 1751-6161/ © 2018 Elsevier Ltd. All rights reserved.

Journal of the Mechanical Behavior of Biomedical Materials 87 (2018) 111–118

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natural variation shown by enamel (mean elastic modulus: 84.1 GPa) and dentin (mean elastic modulus: 18.5 GPa). In addition, Park et al. (2008) reported that the cuspal deflection in the incremental filling technique was considerably lower than that in the bulk filling technique, and there was no significant difference between horizontal and oblique incremental filling techniques. Recently, high viscosity bulk-fill resin composites have been used to expedite the restoration process by enabling increments of up to 4 mm in thickness to be photo-polymerized, thereby avoiding the time consuming incremental filling process for the reduction of chair time. (Tsujimoto et al., 2017). Manufacturers claim that the polymerization shrinkage stress of high viscosity bulk-fill resin composites can be reduced with advanced technology in the filler content or monomer type, or by adding modulators to slow the polymerization rate. Therefore, it is possible that the use of high viscosity bulk-fill resin composites will lead to a reduced cuspal deflection even with the bulk-filling technique, when compared to that of conventional resin composites with the incremental filling technique. However, there has been no independent research comparing cuspal deflection between high viscosity bulk-fill and conventional resin composites with different filling techniques. The purpose of this study was to investigate measurement methods for the resultant polymerization shrinkage stress on simulated cuspal deflection of high viscosity bulk-fill and conventional resin composites, the values of that shrinkage stress, and its relationship with flexural properties. The null hypotheses tested were: (i) there would be no differences in simulated cuspal deflection between high viscosity bulk-fill and conventional resin composites; (ii) there would be no differences in the cuspal deflection of resin composites measured with different techniques; and (iii) there would be no relationship between simulated cuspal deflection and flexural properties for any measurement technique.

2006–2016 (1995–2005: 10.59%; 2006–2016: 13.13%). These researchers also noted that resin composite fracture rates showed a notable increase dependent on the increase in the size and surfaces of resin composite restorations in posterior teeth (1995–2005: 28.84%; 2006–2016: 39.07%), in that resistance to fracture is an important mechanical property, especially for large restorations. Tsujimoto et al. (2018) stated that, based on these failure rates, it can be estimated that at least 32 million resin composite restorations placed in posterior teeth in 2015 will need to be repaired or replaced due to failure by 2025. This suggests the necessity of continued improvement in the mechanical properties of resin composite materials, such as flexural properties, and thus manufacturers are attempting to develop resin composites which are superior in these respects. In order to attain superior mechanical properties in resin composites, adequate visible-light-initiated polymerization of the resin monomers to form a highly crosslinked polymer is essential (Cramer et al., 2011). However, the photopolymerization of resin composites is accompanied by volumetric shrinkage, typically in the range of 1.5–5% (Tsujimoto et al., 2016), due to the reduction in distance between monomer chains when the weak van der Waals forces are converted into covalent bonds. Volumetric shrinkage leads to the development of polymerization stresses as the resin is bonded to the tooth structures on most sides of the cavity (Kaisarly and Gezawi, 2016). Thus, polymerization shrinkage stress of resin composites can lead to internal and marginal gaps, microleakage, and micro-cracking of tooth structure due to cuspal deflection. The resultant issues associated with polymerization shrinkage are an important consideration in the failure of resin composite restorations (Ilie and Hickel, 2011). Some researchers (Condon and Ferracane, 2000; Ferracane, 2005; Pfeifer et al., 2008) have reported that resin composites with higher mechanical properties generally show higher polymerization shrinkage stress. This raises the risk that formulation modifications to avoid resin composite failure may increase the risk of problems related to this stress. While the mechanisms of polymerization shrinkage stress development within resin composite restorations are quite complex, the generation, measurement and characterization of polymerization shrinkage stress have been the subjects of much research for more than 50 years, beginning with studies by Bowen (1967) and Bowen et al. (1983), and proliferating after the appearance of work by Davidson and de Gee (1984) and Feilzer et al. (1987). Investigations of how best to measure the polymerization shrinkage stress of resin composite continue today in many research laboratories. One method for analyzing the polymerization shrinkage stress of resin composite is the measurement of simulated cuspal deflection using aluminum blocks with linear variable differential transformers (LVDT), as developed by Park et al. (2008). The advantage of measurement using LVDT is that it can measure the cuspal deflection in real time during the polymerization of resin composites. However, LVDT is not widely used, because performing these measurements requires a custom-built apparatus to hold them and the specimens. As a result, research on simulated cuspal deflection using LVDT has been conducted almost exclusively at a single research institute (Park et al., 2008; Kwon et al., 2012; Kim et al., 2016). Therefore, in order to find an alternative to the LVDT method, this study measured the simulated cuspal deflection resulting from the polymerization of resin composite bonded to a precisely prepared MOD cavity within an aluminum block with a novel highly accurate submicron digimatic micrometer (Micrometer, MDH-25M, Mitutoyo, Tokyo, Japan) or with a confocal laser scanning microscope (CLSM VK-9710, Keyence, Tokyo, Japan). Methods using a micrometer and CLSM for measurement of cuspal deflection in an aluminum block have not been previously used for this purpose and they may allow for more accessible measurement processes. Aluminum blocks (grade EN-AW 6060, elastic modulus: 68.5 GPa) have been used in previous studies (Park et al., 2008; Kwon et al., 2012; Kim et al., 2016) to simulate cuspal deflection with polymerization of resin composites as their mechanical properties are within the broad range of

2. Materials and methods 2.1. Study materials Seven high viscosity bulk-fill resin composites: 1) Beautifil-Bulk Restorative (BB, Shofu, Kyoto, Japan), 2) everX Posterior (EP, GC, Tokyo, Japan) and 3) Filtek One Bulk Fill Restorative (FB, 3 M Oral Care, St. Paul, MN, USA), 4) Quix Fill (QF, Dentsply Sirona, York, PA, USA), 5) Sonic Fill 2 (SF, Kerr, Orange, CA, USA), 6) Tetric N Ceram Bulk Fill (TN, Ivoclar Vivadent, Schaan, Liechtenstein) and 7) Tetric Evo Cerame Bulk Fill (TE, Ivoclar Vivadent), and eight conventional resin composites; 1) Beautifil Ⅱ (B2, Shofu), 2) Clearfil AP-X (CA, Kuraray Noritake Dental, Tokyo, Japan), 3) Clearfil Majesty ES2 (CM, Kuraray Noritake Dental), 4) Estelite Sigma Quick (EQ, Tokuyama Dental, Tokyo, Japan), 5) Filtek Supreme Ultra Restorative (FS, 3 M Oral Care), 6) G-ænial Sculpt (GS, GC), 7) Harmonize (HM, Kerr), and 8) Z100 Restorative (ZR, 3 M Oral Care) were evaluated (Tables 1 and 2). 2.2. Cuspal deflection measurement Aluminum blocks (EN-AW 6060; 10 mm x 8 mm x 15 mm) with a MOD trench [4 (W) mm x 8 (L) mm x 4 (D) mm] were fabricated using a CNC milling machine, creating two remaining cusps. These cusps were asymmetrical (widths of 4 mm and 2 mm) in order to more closely approximate the clinical situation in a tooth such as a premolar. The 10 mm width of the block had a tolerance 0.05 mm, and the 4 mm width of the trench had a tolerance of 0.03 mm. The 4 mm depth of the trench had a tolerance of 0.1 mm. The inside of the cavity was airabraded with 50 µm Al2O3 powder. Scotchbond Universal Adhesive (3 M Oral Care) was applied prior to placement of high viscosity bulkfill and conventional resin composites, following the manufacturer's instructions. The adhesive was light cured for 10 s at a standardized distance of 1 mm using a quartz-tungsten halogen (QTH) curing unit (Optilux 501, Demetron, Kerr, Danbury, CT, USA). The power density 112

Bis-GMA, PMMA, TEGDMA Bis-EMA, Bis-GMA, TEGDMA

Bis-EMA, UDMA, TEGDMA,

Bis-EMA, Bis-GMA, poly(oxy−1,2-ethanediyl), a, a′- [(1-methylethylidene)ßdi− 4, 1-phenylene]bis [x-[(2-methyl−1-oxo−2-propen−1-yl)oxy]− 2,20 -ethylenedioxydiethyl dimethacrylate, TEGDMA Bis-EMA, Bis-GMA, UDMA

Bis-EMA, Bis-GMA, UDMA

EP FB

QF

SF

TE

TN

Sonic Fill 2 (Universal)

Tetric Evo Ceram Bulk Fill (A2) Tetric N Ceram Bulk Fill (A2) Silanated barium glass filler

Silanated barium glass filler

GC, Tokyo Japan 3M Oral Care, St. Paul, MN, USA Dentsply Sirona, York, PA, USA

Short E-glass fiber filler, Barium glass Silica filler, Zirconia filler, Ziirconia/ silica cluster filler Strontium-aluminum-sodium-fluoridephosphate-silicate glass Barium glass filler, Silica glass filler

Ivoclar Vivadent, Schaan, Liechtenstein, Ivoclar Vivadent

Kerr, Orange, CA, USA

Shofu, Kyoto, Japan

Manufacturer

Fluoro-silicate glass

Inorganic filler composition

113

Poly(oxy−1,2-ethanediyl), α,α'-[(1-methylethylidene)di− 4,1-phenylene]bis[ω-[(2-methyl−1-oxo−2-propen−1-yl)oxy]− 2,2′-ethylenedioxydiethyl dimethacrylate Bis-EMA, Bis-GMA, TEGDMA

Bis-EMA, UDMA

Bis-GMA, TEGDMA

Silica filler, Zirconia filler, Ziirconia/silica cluster filler

Silica filler, Zirconia filler, Ziirconia/silica cluster filler Silica glass, strontium glass, fluoro-aluminosilicate glass Silica zirconia filler

Fluoro boro alumino silicate glass Silanated barium glass filler, Silanted silica filler, Silanated collodal filler Silanated barium glass filler Silica zirconia filler

Inorganic filler composition

Bis EMA: Ethoxylated bisphenol-A-dimethacrylate; Bis-GMA: Bisphenol-A-glycidyl dimethacrylate; TEGDMA: ; UDMA: Urethane dimethacrylate: Triethyleneglycol dimethacrylate.

ZR

HN

FS

Hermonize Universal Composite (A2) Z100 Restorative (A2)

Bis-EMA, Bis-GMA, TEGDMA, UDMA

EQ

GS

Bis-GMA, TEGDMA

CM

Clearfil Majesty ES2 (A2) Estelite Sigma Quick (A2) Filtek Supreme Ultra Restorative (A2)

Gænial Sculpt (A2)

Bis-GMA

CA

Clearfil AP-X (A2)

Bis-GMA, TEGDMA

B2

Beautifil Ⅱ(A2)

Resin Matrix Composition

Code

Resin Composite (Shade)

Table 2 High viscosity conventional resin composites used in this study.

Kerr, Orange, CA, USA 3 M Oral Care

GC, Tokyo, Japan

Kuraray Noritake Dental Tokuyama Dental, Tokyo, Japan 3 M Oral Care, St. Paul, MN, USA

Kuraray Noritake Dental, Tokyo, Japan

Shofu, Kyoto, Japan

Manufacturer

Bis EMA: Ethoxylated bisphenol-A-dimethacrylate; Bis-GMA: Bisphenol-A-glycidyl dimethacrylate; Bis-MPEPP: Bisphenol A polyethoxy methacrylate; PMMA; Poly-methyl-mehtacrylate; TEGDMA: Triethyleneglycol dimethacrylate; UDMA: Urethane dimethacrylate: .

Bis-GMA, Bis-MPEPP, UDMA, TEGDMA

BB

Beautifil Bulk Restorative (Dentin) ever X Posterior (Universal) FiltekOne Bulk Fill Restorative (A2) Quix Fill (Universal)

Resin Matrix Composition

Code

Resin Composite (Shade)

Table 1 High viscosity bulk-fill resin composites used in this study.

A. Tsujimoto et al.

Journal of the Mechanical Behavior of Biomedical Materials 87 (2018) 111–118

Journal of the Mechanical Behavior of Biomedical Materials 87 (2018) 111–118

A. Tsujimoto et al.

Fig. 1. Schematic drawing of the experimental set-up for simulated cuspal deflection of resin composites with micrometer or CLSM.

(above 700 mW/cm2) of the QTH curing unit was checked using a dental radiometer (model 100, Demetron, Danbury, CT, USA) before preparing the specimens. The aluminum blocks were randomly divided into groups for the different measurement techniques (micrometer vs CLMS) and further subdivided according to type of resin composite (high viscosity bulk-fill vs conventional resin composite) (Fig. 1). Group 1 (micrometer x high viscosity bulk-fill resin composite): high viscosity bulk-fill resin composite was placed in bulk and photo cured from the three exposed surfaces for 40 s each. Measurement of cuspal deflection was calculated from the difference in the distance between the centers of the two remaining cusps before the placement of resin composite and 10 min after polymerization as measured by a high accuracy submicron digimatic micrometer. Group 2 (micrometer x conventional resin composite): conventional resin composite was placed in two horizontal consecutive layers. Each increment (2 mm) was photo cured from the three exposed surfaces for 40 s each to make photo curing time identical. Measurement of cuspal deflection was conducted in the same manner as Group 1. Group 3 (CLSM x high viscosity bulk-fill resin composite): high viscosity bulk-fill resin composite was placed in the same manner as Group 1. Measurement of cuspal deflection was calculated from the distance between the centers of the two remaining cusps before the placement of resin composite and 10 min after the polymerization as measured by a CLSM with built-in analysis software (VK analyzer, Keyence). Group 4 (CLSM x conventional resin composite): conventional resin composite was placed, in the same manner as Group 2. Measurement of

cuspal deflection was conducted in the same manner as Group 3.

2.3. Flexural strength measurement A Teflon split mold (2.0 mm depth, 2.0 mm width and 25 mm length) which was developed by Irie et al. (2010) was used to prepare the specimens. A Teflon split mold minimizes the stresses exerted on the specimens during their retrieval. The resin composites were placed into the mold using a condenser instrument. The top side of the mold was covered with a matrix strip and the resin composites pressed with a glass slide under a 5 N load. The exit window of the QTH curing unit was placed against the glass plate at the center of the specimen, which was light cured for 40 s. Following this, the exit window was moved to a section next to the center, overlapping the previous section by approximately one half. Photo curing was performed by sequentially curing overlapping regions until the entire sample surface had been photo cured. After photo curing, the hardened specimens were carefully removed from the mold and the specimen was polished with silicon carbide (SiC) papers (#600, Struers, Cleveland, OH, USA) to obtain smooth and flat surfaces. Twenty specimens for each resin composite were prepared under ambient laboratory conditions of 23 ± 2 °C and 50 ± 10% relative humidity. Specimen dimensions were measured using a high accuracy sub-micron digimatic micrometer. The accepted specimen size was 2.0 ± 0.020 mm in width and height and 25 ± 0.025 mm in length. The specimens were immersed in distilled water in an incubator (IC802, Yamato Scientific, Tokyo, Japan) at 37 °C for 24 h. The specimens for each resin composite were subjected to a three114

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first immersing them in ascending concentrations of aqueous tert-butanol (50% for 20 min, 75% for 20 min, 95% for 20 min, and 100% for 2 h) and then transferred from the final 100% bath to a critical-point dryer (Model ID-3, Elionix, Tokyo, Japan) for 30 min. To enhance the visibility of the fillers, the polished surfaces were etched for 30 s using an argon ion-beam (Type EIS-200ER, Elionix) directed perpendicular to the surface at an accelerating voltage of 1.0 kV and ion current density of 0.4 mA/cm2. The surfaces were then coated with a thin film of gold in a vacuum evaporator (Quick Coater Type SC-701, Sanyu Electron, Tokyo, Japan) and observed using field-emission SEM with an operating voltage of 10 kV.

Table 3 Simulated Cuspal Deflection of High Viscosity Bulk-fill and Conventional Resin Composites. Rank Order

Resin Composite

Type of Resin Composite

Simulated Cuspal Deflection (µm) Micrometer a,A

1 2 3 4 5 6 7 8 9 10 11 12

HN EQ BB QF AP SF TN FB CM FS TE GS

Conventional Conventional Bulk-fill Bulk-fill Conventional Bulk-fill Bulk-fill Bulk-fill Conventional Conventional Bulk-fill Conventional

11.4 11.8 13.8 14.3 14.5 14.8 15.6 16.1 16.4 16.7 17.0 17.2

(1.2) (0.8)a,A (1.2)b,A (1.1)b,A (0.6)b,A (1.0)b,A (1.4)b,c,A (1.1)c,A (0.9)c,A (0.9)c,A (1.0)c,A (1.1)c,d,A

13

B2

Conventional

17.6 (1.0)c,d,A

14 15

EP ZR

Bulk-fill Conventional

18.0 (0.9)c,d,A 19.0 (1.0)d,A

CLSM 10.7 (1.5)a,A 11.1 (1.0)a,A 13.1 (1.4)b,A 14.1 (1.0)b,A 14.5 (0.6)b,A 14.6 (0.9)b,A 15.8 (1.1)c,A 16.0 (1.0)c,A 16.3 (0.8)c,A 16.8 (0.6)c,A 16.8 (0.9)c,A 17.6 (1.0)c,d,A 18.0 (0.9)c,d,A 19.0 (1.2)d,A 20.3 (1.0)d,A

2.5. Statistical analysis The sample size for the cuspal deflection measurement tests, based on preliminary data, was examined with G*Power calculator (http:// www.gpower.hhu.de/) and a commercial statistical software package (SPSS Statistics Base, IBM, Armonk, NY, USA). The results gave an effect size of f = 2.26 (resin composite type factor) through one-way analysis of variance (ANOVA) of cuspal deflection measurements. The sample size was then checked using the effect size, α = 0.05, and power= 0.95. This gave a minimum n of 5. Cuspal deflection data were analyzed with two-way analysis of variance (ANOVA), using factors type of resin composite and measurement followed by Tukey's post-hoc honestly significant difference (HSD) test with a significance level (α) of 0.05. The flexural strength and elastic modulus data were analyzed using one-way ANOVA along with Tukey's HSD test with a significance level of 0.05. Pearson correlation analysis between cuspal deflection using the micrometer and CLSM and the flexural strength and elastic modulus was also conducted.

Values in parenthesis are standard deviations (n = 5). Same small letter in same vertical column indicates no significant difference (p > 0.05). Same capital letter within individual rows indicates no significant difference (p > 0.05).

point bending test using a universal testing machine (5500 R, Instron, Norwood, MA, USA) at a cross-head speed of 1.0 mm/min until the specimen fractured, as outlined in ISO 4049. The flexural strength in MPa and elastic modulus in GPa were determined from the stress-strain curve using a computer with custom software package (Bluehill 2 Ver. 2.5, Instron) linked directly to the testing machine. Flexural strength (σ) was calculated as follows:

σ = 3PD/2bd2

3. Results

P = maximum load at fracture point, D = distance between the supports (20 mm), b = specimen width, and d = specimen height. Elastic modulus (E) was calculated as follows:

3.1. Cuspal deflection The results for the cuspal deflection of high viscosity bulk-fill and conventional resin composites using both the micrometer and CLSM are shown in Table 3. Cuspal deflection of high viscosity bulk-fill resin composites was 13.1–18.0 µm for the micrometer and 13.8–19.0 µm for CLSM, and was material dependent. In the bulk fill resin composites, EP showed significantly higher cuspal deflection than the other bulk-fill resin composites, regardless of measurement method. On the other hand, the cuspal deflection of conventional resin composites was 10.7–19.0 µm for the micrometer and 11.4–20.3 for CLSM, and was material dependent. Pearson correlation analysis showed a statistically significant relationship between cuspal deflection measured with the micrometer and with CLSM (R=0.98, p < 0.001).

E = P1D/4bd3δ P1 = the load at an intersection point within the elastic region of stress strain curve, and δ = specimen deformation at P1 2.4. Scanning electron microscopy observation Ultrastructural observations were conducted on the polished surfaces of tested resin composites using scanning electron microscopy (SEM). A Teflon mold, 10.0 mm in diameter and 2.0 mm in height, was used to form the specimens of the resin composites. The mold was placed on a glass slide covered by a matrix strip. The resin composites were placed into the mold using a condenser instrument. The top side of the mold was covered with a matrix strip and the resin composites pressed with a glass slide under a 5 N load. The exit window of the QTH curing unit was placed against the glass slide, and the resin was photo cured for 40 s. After photo curing, the hardened specimens were carefully removed from the mold and were polished with 600 grit SiC papers to obtain smooth and flat surfaces. Three specimens for each resin composite were prepared under ambient laboratory conditions of 23 ± 2 °C and 50 ± 10% relative humidity. The specimens were immersed in distilled water in an incubator at 37 °C for 24 h. After storage in an incubator, the specimen surfaces were prepared and polished using a gradually increasing sequence (#320, #600, #1200, #2000, and #4000) of SiC papers in a grinder-polisher (Minitech 333, Presi, Eybens, France). Finally, the surfaces were polished with a soft cloth using 1.0 µm-grit diamond paste (DP-Paste, Struers, Ballerup, Denmark). SEM specimens of the resin composites were dehydrated by

3.2. Flexural properties The results for the flexural strength and elastic modulus of high viscosity bulk-fill and conventional resin composites are shown in Table 4. The high viscosity bulk-fill resin composites showed 98.7–157.0 MPa for flexural strength and 6.6–13.5 GPa for elastic modulus. For the conventional resin composites, the flexural strength was 90.6–143.1 MPa and elastic modulus was 6.3–14.1 GPa. There were statistically significant differences in flexural strength and elastic modulus depending on the material, regardless of the type of resin composite. Pearson correlation analysis did not show any statistically significant relationship between flexural strength and cuspal deflection (R=0.32, p = 0.344 for micrometer; R=0.33, p = 0.334 for CLSM) or elastic modulus and cuspal deflection (R=0.22, p = 0.761 for micrometer; R=0.21, p = 0.755 for CLSM). 115

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3.3. SEM observation

Table 4 Flexural Properties of High Viscosity Bulk-fill and Conventional Resin Composites. Rank Order

Resin Composite

Type of Resin Composite

Flexural Strength (MPa)

Elastic Modulus (GPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

EP ZR CA TE TN FB FS QF SF EQ B2 BB CM HN GS

Bulk-fill Conventional Conventional Bulk-fill Bulk-fill Bulk-fill Conventional Bulk-fill Conventional Conventional Conventional Bulk-fill Conventional Conventional Conventional

157.0 (7.2)a 143.1 (8.1)b 139.9 (7.7)b 130.6 (7.5)c 127.2 (7.7)c 126.0 (6.6)c 118.7 (7.1)d 114.8 (6.6)d 112.5 (6.7)d 105.4 (6.2)e 103.1 (6.6)e 98.7 (6.7)e,f 94.6 (5.8)f 92.2 (5.9)f 90.6 (6.1)f

13.5 (1.2)a 14.1 (1.1)a 12.1 (1.3)b 12.3 (1.1)b 11.5 (0.9)b 11.8 (1.0)b 8.9 (0.8)c 9.8 (0.9)c 8.8 (0.8)c 7.6 (0.6)d 6.8 (0.6)e 6.6 (0.6)e 6.4 (0.5)e 6.8 (0.7)e 6.3 (0.6)e

Representative SEM micrographs of high viscosity bulk-fill and conventional resin composites are shown in Figs. 2 and 3. A wide variety of fillers in the resin composites were observed, and filler particle size and shape were material dependent. In the high viscosity bulkfill resin composites, BB, FB, SD and XB showed a wide size range (< 1–6 µm for BB and FB, < 1–10 µm for SF and < 1–15 µm for QF) of irregular shaped fillers, and TE and TN showed relatively uniform small size (< 1–2 µm) of irregular shaped fillers. Short E glass fiber and relatively uniform small size (< 1 µm) of irregular shaped filler was observed for EP. In conventional resin composites, a wide size range (< 1–4 µm for ZR, < 1–7 µm for B2, CM and FS, < 1–20 µm for CA and HN) of irregular shaped fillers were observed. Uniformly small size (< 1 µm) were observed for EQ with spherical shaped fillers. GS and ZR showed non-uniformly small sized (< 1 µm for GS, < 1–2 µm for ZR) of irregular shaped fillers.

4. Discussion Values in parenthesis are standard deviations (n = 15). Same small letter in same vertical column indicates no significant difference (p > 0.05).

The bulk-fill resin composites (13.1–19.0 µm) showed similar cuspal deflection to their conventional equivalents (10.7–20.3 µm), regardless of measurement method. Thus, the first null hypothesis, that there would be no differences in simulated cuspal deflection between high

Fig. 2. Representative SEM images of the surfaces of high viscosity bulk-fill resin composites at 5000 × (a) and 20,000 × (b) magnifications. BB: Beautifil-Bulk Restorative; EP: everX Posterior; FB: Filtek One Bulk Fill Restorative; QF: Quix Fill; SF: Sonic Fill 2; TN: Tetric N Ceram Bulk Fill; TE: Tetric Evo Cerame Bulk Fill. 116

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Fig. 3. Representative SEM images of the surfaces of high viscosity conventional resin composites at 5000 × (a) and 20,000 × (b) magnifications. B2: Beautifil Ⅱ; CA: Clearfil AP-X; CM: Clearfil Majesty ES2; EQ: Estelite Sigma Quick; FS: Filtek Supreme Ultra Restorative; GS: G-ænial Sculpt; HN: Harmonize; ZR: Z100 Restorative.

higher cuspal deflection than other bulk-fill resin composites, and, according to the manufacturers, EP contains short E glass fibers in the resin matrix to improve the toughness of resin composite. This effect was seen in the results for flexural properties, but this unique filler did not contribute to the reduction of cuspal deflection. In addition, according to the manufacturers, other bulk-fill resin composites contain high molecular weight polymerization modulators in the resin matrix. These have a lower ratio of functional groups to make double bonds through photo-polymerization to molecular weight, which is said to reduce polymerization shrinkage (Leprince et al., 2014). Thus, the results for the cuspal deflection of resin composites appear to have been mainly influenced by the modifications of the resin matrix in this study. In the present study, either a micrometer or CLSM was used to measure the cuspal deflection of resin composites, as a more accessible replacement for LVDT. There was no significant difference between the cuspal deflection measured using the micrometer and that measured using CLSM, and Pearson rank correlation analysis revealed that there was a statistically significant relationship (R=0.98, p < 0.001) between the values measured with the micrometer and with CLSM. Therefore, the second null hypothesis, that there would be no difference in the cuspal deflection of resin composites measured with different measurement techniques, was not rejected. One of the concerns about using the micrometer was the possible influence of the stress exerted on the aluminum block by the

viscosity bulk-fill and conventional resin composites, was not rejected. Park et al. (2008) reported that the cuspal deflection of resin composites seen with the bulk filling technique was significantly higher than that with incremental filling technique regardless of type of resin composite (low or high viscosity bulk-fill or conventional resin composite). This means that in those experiments the bulk filling technique led to significantly more cuspal deflection than the incremental filling techniques. In this study, the cuspal deflection of high viscosity bulk-fill and conventional resin composite was investigated using the filling technique specified in the manufacturers’ instructions, and the cuspal deflection of high viscosity bulk-fill resin composites using the bulk filling technique was similar to that of conventional resin composites with the incremental filling technique. The cuspal deflection of the tested resin composites was material dependent, regardless of filling technique. This suggests that the cuspal deflection of resin composites may be mainly influenced by their composition, rather than the filling technique. In the SEM observations, a wide variety of fillers were seen in resin composites, but there was no clear relationship between filler particle size and shape, and cuspal deflection. Although Satterthwaite et al. (2012) reported an effect of filler particle size and shape on shrinkage stress, a systematic review of the polymerization shrinkage stress of resin composite (Meereis et al., 2018) found that modification of the resin matrix made the largest contribution to minimizing stress development. EP showed significantly

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of this study, it appears that the influence of flexural properties on polymerization shrinkage stress is falling due to the improvement in resin composite technology. This indicates that it may be possible to develop composites with higher flexural properties and lower cuspal deflection in the future, and thus continuing research on this topic should be valuable.

instrument, while in the case of the CLSM errors arising from the process of combining the scanned photos in order to perform the analysis and the measurements were a concern. Both of these factors had the potential to influence the measured values. Cuspal deflection measured using the micrometer was 11.4–19.0 µm, and that using CLSM was 10.7–20.3 µm. The equipment error for the micrometer is given by the manufacturer as 0.5 µm, and that for the confocal laser scanning microscope also as 0.5 µm. These errors are comparable in size to the differences noted in individual cases, and the results show no systematic difference between the two methods. A previous study reported that cuspal deflection measured using an aluminum block with LVDT was around 5–30 µm, although differences in the thickness of the aluminum walls and size of the trench make direct comparison difficult.18 If the micrometer did apply stress during the measurement period, that would be expected to cause a greater deformation of the block during the pre-polymerization measurement, especially when the cavity is not filled with composite. This would lead to a lower value for cuspal deflection. However, this underestimation was not observed, suggesting that stress from the micrometer is not a significant factor. On the other hand, stitching individual micrographs to form a complete three-dimensional rendering from the CLSM could bias the values in either direction, but no such deviations were observed. Thus, the original concerns do not appear to be justified. However, during the experiment one issue was noted. CLSM cannot measure the cuspal deflection precisely 10 min after polymerization because it takes time (5–8 min) to scan the necessary micrographs. In this study, the measurement was started 10 min after polymerization because a previous study using LVDT reported that values of cuspal deflection reached a plateau 10 min after polymerization. Thus, it seems that the influence of the duration of scanning should be minimal. However, previous studies (Park et al., 2008; Kwon et al., 2012; Kim et al., 2016) showed that the values of cuspal deflection slightly and gradually increased over a longer time period beyond 10 min after light polymerization. Thus there is a small possibility that the cuspal deflection measured with CLSM could be higher than that measured with the micrometer, but this was not observed. On the other hand, an advantage of CLSM is that it allows the experimenter to confirm that there has been no debonding between the resin and the aluminum block, as any debonded regions would be visible in the microscope images. Therefore, because no significant difference was seen in the cuspal deflection of resin composites measured with the different measurement methods (micrometer vs CLSM), the investigators believe that these measurement methods may be useful to measure cuspal deflection. As compared to the LVDT method, a micrometer may be more accessible and easier and CLSM may allow for more automation in the measurement process to measure the cuspal deflection. Overall, these measurement methods of cuspal deflection in an aluminum block with a micrometer or CLSM may be effective ways to evaluate the polymerization shrinkage stress of resin composite restorations. Further research is needed to determine the best experimental setup for measurement of cuspal deflection as one of the indicators of polymerization shrinkage stress. The flexural strength and elastic modulus of resin composites were material dependent, but the rank order of the results was different from that of cuspal deflection. Pearson correlation analysis did not show any statistically significant relationship between flexural properties and cuspal deflection (R=0.32, p = 0.344: flexural strength and cuspal deflection; R=0.22, p = 0.761: elastic modulus and cuspal deflection). Therefore, third null hypothesis, that there would be no relationship between cuspal deflection and flexural properties for any measurement technique, was not rejected. The results of the correlation analysis do not directly support the results of earlier studies, but they are not decisive evidence against a connection. Generally, it is thought that the volumetric shrinkage and mechanical strength of resin composites are the major factors influencing polymerization shrinkage stress (Pfeifer et al., 2008). From the results

5. Conclusions The results of this study indicate that the simulated cuspal deflection of high viscosity bulk-fill and conventional resin composites can be measured using a micrometer or CLSM. These measurements revealedthat high viscosity bulk-fill resin composites showed similar simulated cuspal deflection with bulk-filling techniques when compared to conventional resin composites with incremental filling techniques. However, this experiment failed to show a relationship between the flexural properties of resin composites and simulated cuspal deflection. References Alvanforoush, N., Palamara, J., Wong, R.H., Burrow, M.F., 2017. Comparison between published clinical success of direct resin composite restorations in vital posterior teeth in 1995–2005 and 2006–2016 periods. Aust. Dent. J. 62, 132–145. Bowen, R.L., 1967. Adhesive bonding of various materials to hard tooth tissues. VI. forces developing in direct-filling materials during hardening. J. Am. Dent. Assoc. 74, 439–445. Bowen, R.L., Nemoto, K., Rapson, J.E., 1983. Adhesive bonding of various materials to hard tooth tissues: forces developing in composite materials during hardening. J. Am. Dent. Assoc. 106, 475–477. Condon, J.R., Ferracane, J.L., 2000. Assessing the effect of composite formulation on polymerization stress. J. Am. Dent. Assoc. 131, 497–503. Cramer, N.B., Stansbury, J.W., Bowman, C.N., 2011. Recent advances and developments in composite dental restorative materials. J. Dent. Res. 90, 402–416. Davidson, C.L., de Gee, A.J., 1984. Relaxation of polymerization contraction stresses by flow in dental composites. J. Dent. Res. 63, 146–148. Feilzer, A.J., de Gee, A.J., Davidson, C.L., 1987. Setting stress in composite resin in relation to configuration of the restoration. J. Dent. Res. 66, 1636–1639. Ferracane, J.L., 2005. Developing a more complete understanding of stresses produced in dental composites during polymerization. Dent. Mater. 21, 36–42. Heintze, S.D., Ilie, N., Hickel, R., Reis, A., Loguercio, A., Rousson, V., 2017. Laboratory mechanical parameters of composite resins and their relation to fractures and wear in clinical trials-A systematic review. Dent. Mater. 33, e101–e114. Ilie, N., Hickel, R., 2011. Resin composite restorative materials. Aust. Dent. J. 56 (Supplement 1), 59–66. Irie, M., Maruo, Y., Nishigawa, G., Suzuki, K., Watts, D.C., 2010. Physical properties of dual-cured luting-agents correlated to early no interfacial-gap incidence with composite inlay restorations. Dent. Mater. 26, 608–615. Kaisarly, D., Gezawi, M.E., 2016. Polymerization shrinkage assessment of dental resin composites: a literature review. Odontology 104, 257–270. Kim, Y.J., Kim, R., Ferracane, J.L., Lee, I.B., 2016. Influence of the compliance and layering method on the wall deflection of simulated cavities in bulk-fill composite restoration. Oper. Dent. 41, e183–e194. Kwon, Y., Ferracane, J., Lee, I.B., 2012. Effect of layering methods, composite type, and flowable liner on the polymerization shrinkage stress of light cured composites. Dent. Mater. 28, 801–809. Meereis, C.T.W., Münchow, E.A., de Oliveira da Rosa, W.L., da Silva, A.F., Piva, E., 2018. Polymerization shrinkage stress of resin-based dental materials: a systematic review and meta-analyses of composition strategies. J. Mech. Behav. Biomed. Mater. 82, 268–281. Leprince, J.G., Palin, W.M., Vanacker, J., Sabbagh, J., Devaux, J., Leloup, G., 2014. Physico-mechanical characteristics of commercially available bulk-fill composites. J. Dent. 42, 993–1000. Lynch, C.D., Opdam, N.J., Hickel, R., Brunton, P.A., Gurgan, S., Kakaboura, A., Shearer, A.C., Vanherle, G., Wilson, N.H., Academy of operative dentistry european section, 2014. Guidance on posterior resin composites: Academy of operative dentistry European section. J. Dent. 42, 377–383. Park, J., Chang, J., Ferracane, J., Lee, I.B., 2008. How should composite be layered to reduce shrinkage stress: incremental or bulk filling? Dent. Mater. 24, 1501–1505. Pfeifer, C.S., Ferracane, J.L., Sakaguchi, R.L., Braga, R.R., 2008. Factors affecting photopolymerization stress in dental composites. J. Dent. Res. 87, 1043–1047. Satterthwaite, J.D., Maisuria, A., Vogel, K., Watts, D.C., 2012. Effect of resin-composite filler particle size and shape on shrinkage-stress. Dent. Mater. 28, 609–614. Tsujimoto, A., Barkmeier, W.W., Takamizawa, T., Latta, M.A., Miyazaki, M., 2016. Mechanical properties, volumetric shrinkage and depth of cure of short fiber-reinforced resin composite Dent. Mater. J. 35, 418–424. Tsujimoto, A., Barkmeier, W.W., Takamizawa, T., Latta, M.A., Miyazaki, M., 2017. Depth of cure, flexural properties and volumetric shrinkage of low and high viscosity bulkfill giomers and resin composites. Dent. Mater. J. 36, 205–213. Tsujimoto, A., Barkmeier, W.W., Fischer, N.G., Nojiri, K., Nagura, Y., Takamizawa, T., Latta, M.A., Miazaki, M., 2018. Wear of resin composites: current insights into underlying mechanisms, evaluation methods and influential factors. Jpn. Dent. Sci. Rev. 54, 76–87.

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