Interrelation among the handling, mechanical, and wear properties of the newly developed flowable resin composites

Author’s Accepted Manuscript Interrelation among the handling, mechanical, and wear properties of the newly developed flowable resin composites Arisa Imai, Toshiki Takamizawa, Runa Sugimura, Akimasa Tsujimoto, Ryo Ishii, Mami Kawazu, Tatsuro Saito, Masashi Miyazaki www.elsevier.com/locate/jmbbm

PII: DOI: Reference:

S1751-6161(18)31041-5 https://doi.org/10.1016/j.jmbbm.2018.09.019 JMBBM2984

To appear in: Journal of the Mechanical Behavior of Biomedical Materials Received date: 14 July 2018 Revised date: 12 September 2018 Accepted date: 17 September 2018 Cite this article as: Arisa Imai, Toshiki Takamizawa, Runa Sugimura, Akimasa Tsujimoto, Ryo Ishii, Mami Kawazu, Tatsuro Saito and Masashi Miyazaki, Interrelation among the handling, mechanical, and wear properties of the newly developed flowable resin composites, Journal of the Mechanical Behavior of Biomedical Materials, https://doi.org/10.1016/j.jmbbm.2018.09.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Interrelation among the handling, mechanical, and wear properties of the newly developed flowable resin composites Arisa Imai, Toshiki Takamizawa*, Runa Sugimura, Akimasa Tsujimoto, Ryo Ishii, Mami Kawazu, Tatsuro Saito, Masashi Miyazaki

Department of Operative Dentistry, Nihon University School of Dentistry, Tokyo, Japan

Correspondence:

Dr. Toshiki Takamizawa Department of Operative Dentistry Nihon University School of Dentistry 1-8-13, Kanda-Surugadai, Chiyoda-Ku, Tokyo 101-8310, Japan Tel: 81-3-3219-8141, fax: 81-3-3219-8347 e-mail: [email protected]

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Interrelation among the handling, mechanical, and wear properties of the newly developed flowable resin composites

ABSTRACT Objectives: This study investigates the handling, mechanical, and wear properties of the newly developed flowable resin composites and elucidate the interrelations among the tested parameters. Methods: Six flowable and two conventional resin composites are used. Five measurements are performed per resin composite to obtain the average inorganic filler content. Ten specimens per material are used to obtain the flexural strength, flexural modulus, and resilience. For sliding impact wear testing, twelve specimens are prepared. Noncontact profilometer and confocal laser scanning microscopy are used to determine the maximum facet depth and volume loss. Extrusion force and thread formation are used to measure the handling properties of the flowable resin composites. Six measurements are performed per flowable resin composite. Data evaluation is performed using analysis of variance and Tukey’s honestly significant difference test at an αlevel of 0.05. The correlation between the tested parameters is verified using the Pearson product-moment correlation coefficient. Results: A subset of flowable resin composites exhibits higher flexural properties and wear resistance as compared to the conventional resin composites. The handling properties of the flowable resin composites are material dependent. Conclusion: While the resilience parameters exhibit an extremely strong and statistically significant correlation with the wear parameters, the handling properties exhibit no interrelation with the remaining parameters.

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Significance: While the handling properties of the newly developed flowable resin composites did not correlate with the mechanical and wear properties, some new flowable resin composites have the potential for use in high-stress bearing areas, such as posterior lesions, because of the enhanced mechanical properties and wear resistance.

Keywords: Flowable resin composite Handling property Mechanical property Wear resistance

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1. Introduction Resin composites have been extensively accepted as a suitable material to ensure minimal intervention treatment due to their aesthetics and the reduced amount of drilling that is required (Lynch et al., 2014; Tyas et al., 2000). Over the decades, several types of resin composites have been developed. Further, each resin composite displayed distinctive features and improvements in terms of mechanical properties, wear resistance, polymerization shrinkage, aesthetics, and handling properties (Ferracane, 2011). Specifically, the handling properties of resin pastes for the creation of anatomical features are critical to achieve effective and precise clinical results. Additionally, the viscosity and flowability of resin pastes are important determinants of the handling properties in clinical situations (Ferracane et al., 2017). While considering these characteristics, the resin composites can be categorized into the following three different types: packable; universal; and flowable.

The objective indices of the mechanical properties of resin composites can be obtained by standardized methods such as tensile, compressive, flexural, and fracture toughness testing (Ilie et al., 2017). The data can both be used to perform comparison among materials and to illuminate the relation among the testing methods’ outcomes. Furthermore, these results are useful to predict the behavior and longevity of materials in the oral environment. Notably, only a few objective indices are available in case of the handling properties of resin composite while placing the resin paste into a cavity and creating anatomical features. Some studies have used rheological methods to compare the complex viscosity of the resin pastes. However, the handling properties have been generally evaluated by subjective methods (Beun et al., 2009; Beun et al., 2012; Ferracne et al., 2017; Lee et al., 2010; Petrovic et al., 2013; Petrovic et al., 2015).

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The flowable resin composites are characterized by low viscosity and high flowability. This makes them suitable for the direct filling of cavities using small gauge dispensers (Bayne et al., 1998; Takamizawa et al., 2008). Flowable resin composites exhibit low moduli of elasticity, which makes them suitable for the cervical regions (Baratieri et al., 2003; Li et al., 2006; Wood et al., 2008). During the early stages, the flowable resin composite products were not recommended for larger cavities, high-stress bearing areas, or occlusal cavities (Bayne et al., 1998). However, the recently developed products exhibit enhanced mechanical properties and wear resistance, leading to a much broader range of applicability (Irie et al., 2008; Lazaridou et al., 2015; Sumino et al., 2013). The newly developed flowable resin composites are assumed to exhibit a higher resistance to crack propagation, leading to improved wear characteristics as compared to those exhibited by the extensively used conventional prepolymerized nanohybrid resin composites (Sumino et al., 2013). However, grasping the handling properties of flowable resin composites using common physico-chemical methods (eg., compressive, flexural, or hardness tests) remains challenging (Beun et al., 2009; Beun et al., 2012; Ferracane et al., 2017; Lee et al., 2010; Petrovic et al., 2013; Petrovic et al., 2015). Importantly, in clinical situations, thread formation (stickiness) and the extrusion force of the resin paste from syringe are important factors that should be considered while choosing an optimal flowable resin composite.

In this study, we proposed a method to apply a universal testing machine for extrusion force measurement and a creep meter for the evaluation of thread formation in flowable resin composites. Specifically, the objective of this study was to evaluate the handling properties of the flowable resins. Additionally, we evaluated some of the mechanical properties and wear resistance. The null hypotheses to be verified were that i) the type of flowable resin composite

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would not affect the handling and mechanical properties or wear resistance and that ii) the properties of flowable resin composites would not differ from those of the tested conventional resin composites.

2. Materials and Methods 2.1

Study materials

Six flowable resin composites were used in this study (Table 1): Beautifil Flow Plus F00 (BF; Shofu Inc., Kyoto, Japan); Clearfil Majesty ES Flow (CE; Kuraray Noritake Dental Inc., Tokyo, Japan); Estelite Universal Flow (EU; Tokuyama Dental Corp, Tokyo, Japan); Filtek Supreme Ultra Flowable Restorative (FS; 3M ESPE, St. Paul, MN, USA); G-ænial Universal Flow (GU; GC Corp., Tokyo, Japan); Gracefil Zero Flow (GZ; GC Corp.). Two different types of conventional resin composites were compared: a micro hybrid, Clearfil AP-X (AP; Kuraray Noritake Dental Inc.) and a nano filled resin composite, Filtek Supreme Ultra (SU; 3M ESPE). These six flowable resin composites were selected because they are all recent flowable resin composites of the prevalent low flow type, but they have different filler characteristics. A halogen–quartz–tungsten curing unit (Optilux 501; sds Kerr, Danbury, CT, USA) was used to prevent any possible influence of the nonuniformity of light-emitting diode curing units (Price et al., 2010; Michaud et al., 2014). Additionally, the light irradiance (average 600 mW/cm2) of the curing unit was checked. 2.2

Inorganic filler content

The

inorganic

filler

contents

of

the

tested

materials

were

measured

using

thermogravimetry/differential thermal analysis (TG/DTA) by employing a thermogravimeter (6300; Seiko Instruments, Tokyo, Japan). Approximately 50 mg of the resin paste for each resin 6

composite was placed in a crucible (i.e., a cylindroid of pure platinum that was 7 mm in diameter and 10 mm in depth) and heated in the thermogravimeter from 25°C to 800°C at a heating rate of 10°C/min in atmospheric air until complete incineration of the organic components was obtained. The weight of the residual resin paste was automatically measured by the horizontal differential balance of the built-in high sensitivity and accuracy. Additionally, the compensated blank curve was used to estimate the inorganic filler content (wt%). The average inorganic filler content (wt%) was obtained by taking 5 measurements per resin. 2.3

Flexural strength test

We followed the ISO 4049 specifications to measure the flexural properties of the tested materials. Each resin composite paste was placed into a stainless-steel split mold measuring 25 × 2 × 2 mm. The mold was further positioned on a glass slide, and the middle third of the specimen was irradiated for 30 s. Subsequently, both the remaining thirds were irradiated for 30 s. The same technique was used to irradiate the opposite side. After eliminating the hardened specimen from the mold, all the six sides were wet polished using #1,200 silicon carbide (SiC) paper (Fuji Star Type DDC, Sankyo Rikagaku Co., Saitama, Japan). The prepared specimens were stored in distilled water at 37°C for 24 h in the dark before being subjected to testing.

After the storage period, ten specimens per test group underwent the three-point bending test using a universal testing machine (model 5500R; Instron Corp., Canton, MA, USA) at a crosshead speed of 1 mm/min until the specimen broke. The specimens were positioned on a three-point bending apparatus with a span length of 20.0 mm. The flexural strength (σF) and flexural modulus (E) were determined from the stress–strain curve using a computer software (Bluehill version 2.5; Instron Corp.) that was linked to the testing equipment. The modulus of

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resilience (R) was calculated using the following equation from Peutzfeldt & Asmussen (Petuzfeldt et al., 1992): R = σF2/2E 2.4

Sliding impact simulated wear test

Twelve specimens for each resin composite were examined for wear resistance using a sliding impact wear testing machine (K655-06; Tokyo Giken Inc., Tokyo, Japan). The tested materials were positioned in a polytetrafluoroethylene cylindrical mold (6 mm in diameter, 2 mm in height). The specimens were light irradiated for 30 s. One flat surface for each specimen was polished using a sequence of SiC papers having up to 2,000 grit (Fuji Star type DDC). Subsequently, they were stored under dark conditions for 24 h in distilled water at 37°C.

The specimens were attached to the center of a custom fixture manufactured from cold-cure acrylic resin (Tray Resin II; Shofu Inc., Kyoto, Japan) using a small amount of model-repair glue (Zapit; Dental Ventures of America Inc., Corona, CA, USA) prior to wear testing. A stainlesssteel ball bearing (radius = 2.4 mm) was set inside a collet assembly and was used as an antagonist for the sliding impact wear-simulation. The simulator contained a plastic water bath with a constant provision of distilled water at 37°C. Four custom fixtures were mounted inside the bath. During the wear-simulation test, the antagonists directly impacted the specimens from above with a maximum force of 50 N at a rate of 0.5 Hz, which further slid horizontally for 2 mm. Each specimen was subjected to 50,000 cycles of sliding impact motion. 2.5

Sliding impact wear measurements

After the sliding impact wear tests, the tested specimens were ultrasonically cleaned in distilled water for 1 min. The maximum depth (MD: µm) and volume loss (VL: mm3) of the wear facets

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were determined using a confocal laser scanning microscope (VK-9710; Keyence Corp, Osaka, Japan) and its built-in software (VK-Analyzer; Keyence Corp). 2.6

Extrusion force measurement

With an objective of measuring the extrusion force of the tested flowable resin composites, we developed a method using a universal testing machine. A special jig was prepared to fix a syringe of unused fresh flowable resin composite to the flange. The plunger was further subjected to perpendicular load stress at a cross-head speed of 10 mm/min. We determined the cross-head speed by imitating the extraction speed of the resin paste from a syringe in clinical situations. Load stress monitoring was performed throughout the test. The test ended when all the resin paste was discharged from the prepared syringe. The flowable resin composite’s extrusion force was measured by the peak load stress ( N) over the course of testing. Six measurements were performed for each of the flowable resin composites. However, in case of conventional resin composites, this evaluation was omitted due to the high viscosity of the paste. 2.7

Thread formation property (stickiness)

A creep meter (Rheoner II, model RE 2-3305C, Yamaden Co., Tokyo, Japan) was used to evaluate the thread formation of the tested flowable resin composites. The tested flowable resin pastes were filled into a polytetrafluoroethylene cylindrical mold (10 mm in diameter, 2 mm in height) and were left for three min to allow internal stresses to reach equilibrium. A cylindrical rod that was 5 mm in diameter was inserted 1 mm into the resin paste and was subsequently pulled up at a cross-head speed of 10 mm/s. This speed is set to imitate a clinical situation in which the dentist has finished filling the resin paste and is taking the syringe away from the cavity. Continuous monitoring of thread formation was obtained using a video camera (HDRCX680, Sony Corp., Tokyo, Japan). The video allowed to measure the vertical distance between

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the bottom of the rod and the top of the mold (mm) at the moment during which the thread broke. Six measurements were performed for each flowable resin composite. The conventional resin composites were not similarly measured, as for the extrusion force measurements. 2.8

Scanning electron microscopy observations

The cured resin composite specimens were polished to a high gloss with abrasive discs (Fuji Star Type DDC) followed by a series of diamond pastes down to a particle size of 0.25 µm (DPPaste; Struers, Ballerup, Denmark). The mirror-polished surfaces were further subjected to argon-ion beam etching (IIS-200ER; Elionix Inc., Tokyo, Japan) for 40 s, with the ion beam perpendicularly pointing to the polished surface (accelerating voltage = 1 kV; ion current density = 0.4 mA/cm2). Subsequently, the surfaces were coated with a thin gold film in a Quick Coater vacuum evaporator (Type SC-701; Sanyu Electric Inc., Tokyo, Japan). Observations were performed using a scanning electron microscope (SEM; FE-8000, Elionix Inc.) at an operating voltage of 10 kV and magnifications of 5,000× and 10,000×.

The representative wear facets of the tested resin composites were chosen to perform SEM examinations. The specimens were randomly selected after performing the wear measurements, and the samples were evaporation coated using the same procedure as that in case of the polished specimens. The coated surfaces were visualized using SEM with an operating voltage of 10 kV and magnifications of 50× and 2,500×. 2.9

Statistical analysis

A statistical power analysis indicated that at least four specimens were required to effectively measure the inorganic filler content and that six to eight specimens were required for flexural properties and wear testing. Additionally, five specimens were required to effectively measure

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the extrusion force and thread formation. Thus, we initially performed the experiment using the following sample sizes: 5 specimens for inorganic filler content measurement; 10 specimens for flexural strength tests; 12 specimens for simulated wear measurements; 6 specimens for extrusion force and thread formation measurements. Post hoc power tests were performed on the gathered

data

using

two

statistical

software

systems

(G

Power

calculator;

http://www.gpower.hhu.de/, and Sigma Plot version 13.0; Systat Software Inc., Chicago, IL, USA). These exhibited an f value of 0.75, α value of 0.05, and power of 0.95. These tests indicated that the sample size was adequate.

Due to their variance homogeneity (Bartlett’s test) and normal distribution (Kolmogorov– Smirnov test), the data for each material were subjected to both analysis of variance (One-way ANOVA) and Tukey’s honestly significant difference (HSD) test at a significance level of 0.05. The Pearson product-moment correlation coefficient was used to perform pairwise comparisons, with an objective to understand the interrelations between the tested parameters of the flowable resin composites. We omitted the two conventional resin composites from this analysis. Statistical analyses were performed using a software system (Sigma Plot version 13.0).

3. Results 3.1 Inorganic filler content The average inorganic filler contents are presented in Table 1. We observed a significantly lower inorganic filler content in the flowable resin composites than that observed in the conventional resin composites AP and SU. A wide range of inorganic filler content was observed in both types of resin composites. 11

3.2

Flexural properties

The flexural properties (flexural strength (σF), flexural modulus (E), and resilience (R)) of the resin composites are presented in Table 2.

The σF value was significantly higher for the

conventional resin composite AP as compared to that of the other resin composites. The flowable resin composites, GU and GZ, demonstrated higher σF values than those exhibited by the conventional resin composite SU and significantly higher σF values than those exhibited by the other flowable resin composites.

For all the flowable resin composites, we observed

significantly lower E values as compared to those exhibited by the conventional resin composites AP and SU. Among the flowable resin composites, CE and EU demonstrated significantly lower E values vs. the other composites. For the conventional resin composites AP and SU, we observed a significantly lower resilience than that exhibited by the tested flowable resin composites, with the exception of BF. GU and GZ exhibited significantly higher resilience than that exhibited by the remaining resin composites. No statistical significance was observed while comparing the difference between GU and EU. 3.3

Sliding impact simulated wear test

The wear behaviors (maximum facet depth and volume loss) of the resin composites following the sliding impact wear test are presented in Table 3. BF and AP exhibited a significantly higher maximum facet depth than that exhibited by the other resin composites. In case of wear volume loss, GZ and SU demonstrated a significantly lower volume loss as compared to that exhibited by the other resin composites. 3.4

Handling properties

The handling properties (extrusion force and thread formation) of the flowable resin composites are presented in Table 4. GZ and FS exhibited significantly lower extrusion forces as compared

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to those exhibited by the other resin composites. However, a significantly higher extrusion force was observed for GU as compared to those observed for the other resin composites. GZ and CE exhibited significantly lower thread formation as compared to those exhibited by the other resin composites. We observed a significantly higher thread formation for GU as compared to those observed in case of the other resin composites3.5

Scanning electron microscopy observations

Fig. 1 depicts the representative SEM images of the highly polished samples of the eight resin composites after argon-ion etching. In case of flowable resin composites, GU and GZ exhibited similar morphological features even though each resin composite depicted differences in terms of filler shape, size, and distribution. GU and GZ employed densely packed nanosized irregular filler particles (Figs. 1E and 1F). Although EU and FS both employed nanosized spherical particles, FS also exhibited 0.5 to 5 μm of aggregated filler particles (Fig. 1D), and EU exhibited pre-polymerized fillers that employed the same nanosized spherical fillers (Fig. 1C). However, 0.5 to 5 μm irregular glass filler particles were observed in BF (Fig. 1A), and CE exhibited densely packed nanosized irregular glass filler particles with aggregated nanofillers (Fig. 1B). The conventional resin composite SU contained nanosized spherical fillers (Fig. 1H); additionally, irregular 0.5 to 5μm aggregates of filler particles were also employed. AP exhibited a wide size range of irregular filler, ranging from 0.5 to 10 μm (Fig. 1G).

Representative SEM images of the facets post-wear testing are presented in Fig. 2. The morphologic appearance of the wear facets was material and location dependent. A higher number of cracks were observed at the edge of the facets rather than that observed at the center in all the tested resin composites. In terms of material differences, the wear facets of BF and AP (Figs. 2A and 2E) exhibited rougher surfaces and larger facets as compared to those exhibited by

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the other resin composites. Furthermore, some plucking of the irregularly shaped filler particles was evident. GU, GZ, and ES exhibited a similar wear pattern with a somewhat smooth surface. In the latter, while some cracks were observed, it was difficult to observe evidence of the plucking of the fillers (Figs. 2B, 2C, and 2D). Although we observed a smooth SU surface, there were some cracks and evidence of the plucking of the aggregate nanofillers (Fig. 2F). 3.6

Interrelations between the tested parameters of the flowable resin composites

The Pearson product-moment correlation coefficient (r) and p values for the interrelations between the tested parameters of flowable resin composites are presented in Table 5. We observed extremely strong positive and negative correlations for σF vs R, MD vs VL, EF vs TF, R vs MD, and R vs VL. Those correlations were each observed to be individually statistically significant. We observed strong negative correlations for σF vs MD, σF vs VL, FC vs E, FC vs MD, and FC vs VL, whereas a moderate positive correlation was observed between σF and E. However, all these correlations exhibited a p value of greater than 0.05 and showed no statistical significance. No consistent or easily interpretable relationship between the inorganic filler loading of flowable resin composites and the other tested parameters was apparent. In particular, increasing the inorganic filler content did not appear to enhance the physical properties.

4. Discussion Laboratory data from standardized testing methods is among the grounds used to select optimal resin composites. Although information about physical and chemical properties in vitro is easily accessible (Ilie et al., 2017), it may be difficult to acquire information based on objective laboratory data about handling properties in clinical situations. The purpose this study was to

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determine the handling properties of recent flowable resin composites through the methods described here and compare them to other properties of the composites.

The properties of the fillers in a resin composite play an important role in its mechanical and handling characteristics (Ferracane, 2011; Ferracane et al., 2017; Ilie et al., 2017). . Ideally, the volumetric percentage of FC would be used to understand the interaction between FC and the other tested parameters, because different type of inorganic filler have different densities and the presence of organic filler may influence the surface ratio of filler and resin matrix. However, measuring the volumetric percentage of resin composite is difficult and complicated. Therefore, we used the well-established incineration method and obtained FC as weight percentage. Generally, increased filler loading of the conventional resin composites enhances both the material strength and elastic modulus. However, no straightforward relationships were observed between the filler loading of the tested flowable resin composites and the other tested parameters. In particular, although CE showed the highest FC, its E value was significantly lower than those of most flowable resin composites. On the other hand, although GZ and GU showed relatively low FC, their σF values were similar to the conventional resin composite AP and their R values were significantly higher than those of most resin composites. This may be because each of the tested resin composites has a different type of resin matrix. Different resin matrices have different molecular weights, viscosities, and backbones. Bis-GMA monomer is known as a high viscosity monomer with a stiff backbone (Gonҫalves et al., 2011). There was a need to enhance flowability, and low viscosity, flowable resin composites have to contain much lower molecular weight resin monomers such as TEGDMA to dilute the matrix (Ellakwa et al., 2007). Therefore, it seems likely that the composition of the resin matrix is the main influence on the flexural

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properties of flowable resin composites, rather than FC. A set of newly developed products, characterized by superior filler technologies, have improved their resin matrix. Therefore, it is difficult to provide an explanation based exclusively on the filler loading of the flowable resin composites’ mechanical properties.

There has been extensive investigation of the interrelation between the mechanical properties and wear resistance of resin-based materials (Tsujimoto et al., 2018). Tamura et al., (Tamura et al., 2013) indicated that occlusal wear was not directly influenced by mechanical properties when using experimental resin composites in vitro. However, a systematic review of clinical trials that explored the laboratory-measured mechanical properties of posterior resin composites and their relation with wear concluded that σF was moderately correlated with clinical wear (Heintze et al., 2017). Furthermore, a previous observation of resin composites’ clinical wear discovered that σF and R were correlated with the quantitative wear values (Petuzfeldt et al., 1992). The data presented by these reports are in line with the results that are obtained in the present study. The σF of the flowable resin composites was not correlated with the wear parameters in a statistically significant manner; however, the measured correlations were strong. On the contrary, the resilience and wear parameters exhibited an extremely strong and statistically significant correlation. Resilience is defined as a material’s ability to absorb energy without failing when it is elastically deformed by an external stress (Irie et al., 2008; Shibasaki et al., 2017). Furthermore, it has been shown that resistance to plastic deformation may reduce the marginal gap formation, restoration fractures, and wear (Li et al., 1999). In this study, most of the flowable resin composites exhibited significantly higher R-values and lower E values as compared to those exhibited by the conventional resin composites.

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The two-body wear method exhibits several benefits. Specifically, it involves a simplified model and eliminates variability through the absence of a medium. Furthermore, the two-body wear method may have a role in specifically clarifying the interrelations between the observed mechanical property data and wear outcomes (Furuichi et al., 2016; Heintze et al., 2006; Koottathape et al., 2012). Using the two-body wear method, Shinkai et al., (Shinkai et al., 2014) determined the effect of cyclic impact loading on the surface properties of four flowable resin composites. Importantly, the authors obtained results similar to those of the present study. Here, we observed that most of the flowable resin composites exhibited a significantly higher wear resistance as compared to that of the conventional hybrid type resin composite AP. These results were similar to those obtained with the nanosized filler contained resin composite SU. As compared to the other tested materials, AP showed significantly higher σF and E values. However, regardless of this, AP demonstrated significantly higher maximum depth and wear volume values as compared to those exhibited by the other tested materials, with the exception of BF. When comparing SEM images between Fig. 1 and Fig. 2, resin composites with similar morphological features tend to show similar wear patterns. In particular, AP and BF exhibited similar morphological features in Fig. 1A and 1G, with larger glass fillers and a somewhat greater interparticle space. Further, AP and BF showed similar wear patterns with rougher surfaces and some plucking of filler particles (Fig. 2A and 2E). On the other hand, flowable resin composites with nanosized spherical fillers or fine irregular fillers also showed similar wear patterns. GU and GZ showed similar morphological features (Fig. 1E and 1F) and a similar wear pattern in a somewhat smooth surface with some cracks (Fig. 2B and 2C). Therefore, it can be concluded that characteristics such as a somewhat lower flexural modulus, higher resilience, and

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highly dense small fillers are more important than the FC. The lower flexural modulus and higher resilience may contribute to the absorption and endurance of direct impact forces, and highly dense small fillers may enhance a pinning effect in crack propagation (Ferracane et al., 1987), resulting in increased wear resistance.

Although handling properties are very important characteristics for the selection of a resin composite, they cover over a wide range and are dependent on the practitioner. To our knowledge, the current study is the first study to both depict the objective parameters for the handling properties of flowable resin composites and elucidate their relation with other properties. In the past, the handling properties of flowable resin composites have been assessed though in vitro rheological measurements (Beun et al., 2009; Beun et al., 2012; Ferracane et al., 2017; Lee et al., 2010; Petrovic et al., 2013; Petrovic et al., 2015). Methods, such as a simple press method, a penetrating method, a slumping tendency measurement, and a dynamic oscillatory shear test, have been applied. Such methods focused on the rheological characteristics of the resin paste itself (Bayne et al., 1998; Beun et al., 2008; Beun et al., 2009; Beun et al., 2012; Lee et al., 2010; Lee et al., 2003; Lee et al., 2006; Petrovic et al., 2013; Petrovic et al., 2015). However, there was an indirect connection between the obtained data and the practitioner’s sense of ease of handling. To overcome this limitation, we developed new methods that would allow us to take a step forward toward the clinical situation. While the tested flowable resin composites have different types of syringes, we measured the extrusion force using the syringe supplied for each resin composite, to simulate clinical situations. On the other hand, thread formation measurements were directly evaluated from the resin paste and monitored by

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video camera until the thread broke. These methods can determine the handling properties of flowable resin composite from different perspectives through simple and reproducible methods. The extraction force of flowable resin composites will not only be influenced by the viscosity of the resin paste but will also be influenced by the configuration, tip diameter, and plunger material of the syringe. If the resin paste requires a stronger force to be extruded, it may be difficult to have a precise control of the application speed and the tip’s position. On the contrary, when the extrusion force was smaller, an excessive amount of resin paste may be applied and the extract of resin paste will be obtained from the syringe plunger. If the resin paste forms threads, it will be difficult to create optimal anatomical forms when the syringe is withdrawn. These properties considerably influence the filling procedures, length of the procedure, and finishing and polishing times.

While the parameters of extrusion force and thread formation depicted an extremely strong and statistically significant correlation, we have not observed a correlation between these parameters and the other parameters of mechanical properties and wear resistance. In a previous study, the slumping tendency of the five flowable resin composites was measured. The results indicated that the variations in slumping tendency among materials could be caused due to the differences in monomer formulation, filler size, filler load, surface morphology, and the treatment method of the fillers (Lee et al., 2012). In the present study, the mechanical properties and wear behavior of GU and GZ were similar. However, we observed different trends in the extrusion force and thread formation data. The same manufacturer makes both the products. These products have similar inorganic filler loading, size, shape, and distribution and different types of resin monomers and filler surface treatments. Therefore, it can be inferred that the type of resin matrix

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and filler surface treatment methods are important factors to determine the handling properties of the flowable resin composites.

The results of this study exhibited that the handling, mechanical, and wear properties of the tested flowable resin composites are material dependent. Additionally, some flowable resin composites exhibited higher mechanical and wear properties as compared to those exhibited by the conventional resin composites. Therefore, the first hypothesis, which suggested that the type of flowable resin composite would not affect the handling and mechanical properties as well as wear resistance, was rejected. Additionally, the second hypothesis, which suggested that the properties of the flowable resin composites would not differ from those of the tested conventional resin composites, was rejected. Results of the present study strongly indicate that some newly developed flowable resin composites have overcome the concerns linked to early products over their low mechanical properties and wear resistance. Additionally, there is a possibility that these new composites can be used in high-stress bearing areas. However, further research is needed from different perspectives on whether these new flowable resin composites will be long lasting in vivo or not. We believe that the selection of flowable resin composites for applications and for the assessment of newly developed materials may be supported by the extrusion force and thread formation measurements.

5. Conclusions Among the tested flowable resin composites, correlations between flexural modulus and wear parameters were observed to be strongly negative despite the absence of correlation between the inorganic filler content of the tested composites and any of the tested properties. The resilience

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and wear parameters of the flowable resin composites exhibited extremely strong and statistically significant correlations. The handling properties of the flowable resin composites were material dependent. Further, we observed no interrelation with the other parameters. The extrusion force and thread formation measurements may be used as indicators of optimal flowable resin composites for specific applications.

Conflicts of interest The authors declare no conflicts of interest.

Acknowledgments This work was partially supported by a Grant-in-Aid for Scientific Research (C) (16K11565 and 17K11716) and a Grant-in-Aid for Young Scientists (B) (17K17141, and 17K17142) from the Japan Society for the Promotion of Science. This project was also partially supported by the Sato Fund and by a grant from the Dental Research Center of the Nihon University School of Dentistry, Japan.

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Bis-GMA/TEGDMA

flowable

resin

composites

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various

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Figure Legends Fig. 1. Scanning electron micrographs of the argon-ion-etched surfaces of the resin composites. A. BF at a magnification of 5,000× and 20,000× B. CE at a magnification of 5,000× and 20,000× C. EU at a magnification of 5,000× and 20,000× D. FS at a magnification of 5,000× and 20,000× Nanosized spherical particles and aggregated nanofillers were observed. E. GU at a magnification of 5,000× and 20,000× F. GZ at a magnification of 5,000× and 20,000× G. AP at a magnification of 5,000× and 20,000× H. SU at a magnification of 5,000× and 20,000× A and G (BF and AP): different sizes of irregular filler particles, B (CE): densely packed fine irregular filler particles and sparse spherical fillers, C (EU): nanosized spherical particles and pre-polymerized fillers, D and H (FS and SU): Nanosized spherical particles and aggregated nanofillers, E and F (GU and GZ): densely packed nanosized irregular filler particles

Fig. 2. Representative facets of flowable resin composites after the sliding impact wear test. A. BF at magnifications of (a) 50× and (b) 2,500×. B. GU at magnifications of (a) 50× and (b) 2,500 ×. C. GZ at magnifications of (a) 50× and (b) 2,500×. D. ES at magnifications of (a) 50× and (b) 2,500×.

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E. AP at magnifications of (a) 50× and (b) 2,500×. F. SU at magnifications of (a) 50× and (b) 2,500×. Arrows indicate evidence of the plucking of inorganic filler particles. A and E (BF and AP): rougher surfaces and some plucking of the irregularly shaped filler particles were obvious, B, C and C (GU, GZ, and ES): a somewhat smooth surface with some cracks, F (SU): a smooth surface with some cracks and evidence of the plucking of the aggregate nanofillers

Table 1 Materials used in this study Code

Resin composite (Shade; Lot No.)

Main Components

Inorganic filler content (wt%)

Manufacturer

Flowable resin composite BF

CE

Beautifil Flow Plus

bis-GMA, TEGDMA, Al2O3,

(F00)

aluminofluoro-borosilicate glass,

(Shade; A2; 031755)

dl-camphorquinone, others

Clearfil Majesty ES Flow

TEGDMA, silanated silica filler,

(Super Low flow)

hydrophobic aromatic dimethacrylate,

(Shade; A2; BN0242)

silanated barium glass filler, accelerators,

61.2 (0.2)f

Shofu Inc., Kyoto, Japan

71.1 (0.8)c

Kuraray Noritake Dental Tokyo, Japan

dl-camphorquinone, initiators, pigments EU

FS

Estelite Universal Flow

bis-GMA, bis-MEEP, TEGDMA, UDMA,

(Super Low flow)

dl-camphorquinone, initiators,

(Shade; A2; 006)

silica-zirconia filler, pre-polymerized filler

Filtek Supreme Ultra

bis-GMA, substituted dimethacrylate,

Flowable restorative (Shade; A2; N839817)

TEGDMA, silane treated ceramic, silane treated silica, ytterbium fluoride, reacted polycaprolactone polymer, ethyl 4-dimethyl aminobenzoate, benzotriazol,

67.0 (0.4)d

Tokuyama Dental Tokyo, Japan

62.0 (0.6)f

3M ESPE St. Paul, MN, USA

diphenyliodonium hexafluorophosphate GU

G-ænial Universal Flow (Shade; A2; 1704174)

UDMA, bis-EMA, silane, dimethacrylate component,

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65.5 (0.5)e

GC Corp., Tokyo, Japan

strontium glass filler, photoinitiator UV-light absorber, stabilizers, pigments GZ

Gracefil Flow (Zero Flow) 1705151G)

66.1 (0.4)e Tokyo, Japan

bis-MEEP, barium glass filler, silane treated silica, UV-light absorber, photoinitiator, stabilizers, pigments

GC Corp., (Shade; A2;

Conventional resin composite AP

SU

Clearfil AP-X (Shade; A2: 370069)

bis-GMA, TEGDMA, silane barium glass filler, silane silica filler, silanated colloidal silica, catalysts, accelerators, CQ, pigments, others

Filtek Supreme Ultra (Shade; BA2: N339152)

bis-GMA, bis-EMA, UDMA, TEGDMA, PEGDMA, silica nanofiller, zirconia/silica

84.2 (0.5)a

Kuraray Noritake Dental

74.5 (0.5)b

3M ESPE

Aggregated zirconia/silica clusters bis-GMA: 2, 2-bis[4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl]propane, TEGDMA: triethylene glycol dimethacrylate, bis-MEEP: bisphenol A ethoxylate dimethacrylate, UDMA: urethane dimethacrylate, bis-EMA: bisphenol A polyethethylene glycol diether dimethacrylate, PEGDMA: Polyethylene glycol dimethacrylate Values in parentheses indicate standard deviation. Same lower case letter indicates no difference at 5% significance level.

Table 2 Flexural strength, flexural modulus, and resilience Flexural strength (MPa)

Tukey Group

Flexural modulus Tukey Group (GPa)

Resilience Tukey Group (MJ/mm3)

BF

116.2 (7.3)

f

7.1 (0.1)

c

0.95 (0.08)

e

CE

123.1 (5.8)

f

5.7 (0.4)

d

1.33 (0.09)

d

EU

134.5 (5.6)

e

5.5 (0.5)

d

1.64 (0.12)

b,c

FS

137.9 (5.3)

d,e

6.8 (0.5)

c

1.51 (0.13)

c

GU

154.9 (8.9)

b,c

7.3 (0.8)

c

1.72 (0.14)

a,b

GZ

160.1 (6.5)

b

7.3 (0.8)

c

1.82 (0.12)

a

AP

180.1 (9.3)

a

18.5 (0.8)

a

0.89 (0.12)

e

SU

145.5 (7.7)

c,d

13.5 (0.4)

b

0.72 (0.14)

f

Values in parentheses indicate standard deviation. Same lower case letter in vertical columns indicates no difference at 5% significance level.

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Table 3 Maximum facet depth (μm) and volume loss (mm3) after sliding-impact wear test Volume loss (mm3)

Maximum depth (μm)

BF

110.7 (11.6)a

0.109 (0.013)a

CE

49.4 (10.8)b

0.032 (0.007)bc

EU

59.9 (14.8)b

0.037 (0.014)bc

FS

56.1 (12.2)b

0.041 (0.017)b

GU

49.5 (10.8)b

0.025 (0.004)bc

GZ

49.4 (8.1)b

0.021 (0.005)c

AP

105.6 (12.6)a

0.107 (0.028)a

SU

43.3 (7.9)b

0.021 (0.005)c

Values in parentheses indicate standard deviation. Same small case letter in vertical columns indicates no difference at 5% significance level.

Table 4 Handling properties of flowable resin composites Extrusion force

Tukey Group

(N)

Thread formation

Tukey Group

(mm)

BF

24.0 (1.3)

b

20.4 (1.7)

c

CE

24.0 (4.9)

b

14.3 (3.0)

d

EU

26.9 (3.8)

b

26.4 (2.6)

b

FS

13.8 (1.5)

c

24.8 (1.6)

b

GU

47.6 (8.4)

a

53.9 (3.8)

a

GZ

12.2 (2.6)

c

11.8 (0.6)

d

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Table 5 Interrelationships between the tested parameters in flowable resin composites σF FC

r P value

σF

r P value

E

r P value

R

r P value

MD

r P value

VL

r P value

EF

r P value

0.047 0.930

E

R

MD

VL

EF

TF

-0.627 0.183

0.297 0.568

-0.611 0.198

-0.600 0.208

0.163 0.757

-0.172 0.745

0.482 0.333

0.910 0.012

-0.655 0.158

-0.727 0.101

0.096 0.856

0.346 0.501

0.081 0.878

0.197 0.708

0.159 0.764

-0.020 0.970

0.272 0.602

-0.835 0.039

-0.896 0.016

0.100 0.851

0.279 0.593

-0.062 0.908

-0.158 0.765

-0.084 0.874

-0.172 0.744

0.990 0.0002

0.868 0.025

FC: Inorganic filler content, σF: Flexural strength, E: Flexural modulus, R: Resilience, MD: Maximum depth, VL: Volume loss, EF: Extrusion force, TF: Thread formation, r: correlation coefficient

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