Effect of filler type on microhardness and flexural properties of flowable resins

INTRODUCTION

Methacrylate-based resin composites are widely in use as a dental material in dentistry. The advantages of resin composites as a restorative material are their excellent mechanical, physical, and aesthetic properties, easy handling and caries prevention features (Wiegand et al., 2007; Ferracane, 2011). Also, with minimum expense, the damaged part in tooth can be swiftly and timely restored. Resin composites are a commixture of different monomers and inorganic fillers. Among the monomers, Bis-GMA is of high viscosity due to high molecular weight, and it works as a backbone monomer in most resin composites. To reduce viscosity for easy handling, diluents of low molecular weight (MW) such as TEGDMA (triethyleneglycol dimethacrylate) and UDMA (urethane dimethacrylate) are mixed (Geurtsen & Leyhausen, 2001; Floyd & Dickens, 2006). Since resin products have different combinations and ratios of monomers and inorganic fillers depending on the purpose of usage, each resin products have slightly different viscosity (Ilie & Hickel, 2009; Papadogiannis et al., 2011).

Inorganic fillers are important in that they improve strength, hardness, and wear resistance of resin composites. Polymerization shrinkage that causes many unwanted problems, such as restoration fractures, marginal leakage, and recurrence of caries after restoration, can be reduced by increasing filler content (Kemp-Scholte & Davidson, 1998; Lai & Johnson, 1993). In many resin composites, microhybrid-type composites have achieved excellent roles. However, recently, with an aid of nanotechnology to further reduce polymerization shrinkage and improve esthetics of resin composites, containing of nanofillers in resin products becomes general (Chen, 2010; Sideridou et al., 2011).

For lining or basement procedure, flowable resin became the first choice (Ivanovas et al., 2011; Beun et al., 2012). The remarkable advantage of flowable resins is their easy-to-handle feature compared to conventional resin composites. High flowability was achieved by reducing the filler content and increasing diluents in the composition of flowable resins. Flowable nature due to the low viscosity is good in placing the resins into the cavity with excellent wetting and marginal adaptation (Abedian & Millstein, 2006; Lee et al., 2010). Reduction of restoration procedure and treatment time can be expected. However, the decrease of filler content had significantly reduced hardness and also much increased polymerization shrinkage compared to many other resin composites (Kleverlaan & Feilzer, 2005; Pick & Pelka et al., 2011). To improve these shortcomings, many recent flowable resins contain nanofillers in their resin matrix to increase filler weight and volume. Under these circumstances, evaluation of composite resins which have different filler types may provide valuable results such as mechanical properties.

The purpose of the present study was to test how the fillers of different types affect the mechanical properties of flowable resins.

MATERIALS AND METHODS

Flowable resins

For the study, nine flowable resins in three different filler types (all with A3 shade) were selected as outlined in Table1: nanohybrid [Premise flow (PF), Synergy D6 flow (SF), Tetric N flow (TF), Filtek Z350 flow (ZF)], microhybrid [AeliteFlo (AF), Esthet X flow (EF), Palfique Estelite LV (PL), Revolution Formula 2 (R2)], and minifil [Heliomolar flow (HF)]. For light curing, a quartz-tungsten-halogen (QTH) light-curing unit (LCU) [Optilux 501, Kerr, Orange, CA, USA] was used.

Table 1.

Materials tested in the present study

RESULTS

The microhardness of the specimens both on the top and bottom surfaces is shown in Table 2. Among the specimens, ZF and PL showed the highest and lowest microhardness, respectively. Also, interestingly, these two products showed slightly higher microhardness on their bottom surface compared to their top surface (34.2, 35.4 Hv; 8.7, 9.8 Hv). A significantly different microhardness was observed between the nanohybrid and microhybrid products (α <0.05).

Table 2.

Microhardness (Hv) of the tested specimens

The flexural strength (FS) and modulus (FM) are shown in Table 3. FS of the tested specimens ranged from 114.4 to 162.1 MPa. Among the specimens, ZF and PL showed the highest (162.1 MPa) and lowest (114.4 MPa) FS, respectively. For FS, the filler type had no statistically significant influence. However, the FS of nanohybrid products was slightly greater (122.8-162.1 MPa) than that of microhybrid products (114.4-152.1 MPa). FM ranged from 7.33 to 8.82 GPa for nanohybrid products and 5.39 to 8.40 GPa for microhydrid products. The FM of nanohybrid products was significantly different to that of microhybrid products (α <0.05).

Table 3.

Flexural properties of the tested specimens

DISCUSSION

Microhardness is the measure of how material resists to an external force that causes permanent deformation on the material. Unlike resin composites of high fillerloading, most flowable resins contain less filler than that of most other resin composites to increase flowability (Ilie & Hickel, 2009; Hadis et al., 2011). Both on the top and bottom surfaces, specimens of nanohybrid showed slightly higher microhardness than that of microhybrid and minifil specimens, it is due to the slightly greater filler content. The microhardness of the top and bottom surfaces was linearly correlated with the filler content (wt%) at the range of 0.84-0.93 (according to the curve fit) depending on the surface and if TF and PL are excluded. In the case of TF and PL, they showed exceptionally low microhardness even though their filler content was not low. Slightly higher microhardness on the bottom surface of ZF and PL compared to their top surface can be deduced from the condensation of the bottom surface. The condensed surface becomes more dense and stiff due to the shrinkage and subsequently may achieve increased hardness. Nevertheless, such microhardness increase in the bottom surface was not general in all products, but was a product-dependent outcome.

Flowable resins are usually used as a liner or base material beneath the resin composites. For the optimal and durable restoration, consistent mechanical properties among tooth, resin composite, and flowable resin are important. In the present study, flexural and compressive properties were tested. The flexural properties [strength (FS) and modulus (FM)] are the measure of material resistance against transverse stress. The FS of the tested specimens was approximately 114-162 MPa. The range of these values is similar to the most methacrylate-based proprietary products (63-161 MPa) (Ilie & Hickel, 2009). The correlation with the filler weight was negligibly low (R<0.2) and the FS difference for filler type was statistically insignificant (α >0.05). Low correlation between FS and filler content is probably due to the possibility that FS depends more on the internal defects which were formed during manufacturing process (Zeng & Odén, 1996, Della Bona & Anusavice, 2003). FM was approximately 5.6-8.8 GPa, lower than that of dentin (17-25 GPa) (Xu, 1998; Kinney & Marshall, 2003). In the load-displacement curve of the bending test, FM is the slope of the linear portion (elastic range) and is the measure of material’s stiffness, so high FM implies a great stiffness (Ferracane, 2001). Since hardness of the material in the elastic range is the level of stiffness in that range, a high correlation between microhardness and FM for the top and bottom surfaces (R=0.83) seems natural. Unlike the case of FS, FM showed a high correlation with filler content (wt%) (R=0.61 for all products; 0.82 if PL is excluded).

CONCLUSION

Within the limitations of the present study, the following conclusions could be reached:

1. Flowable resins can be diversely classified according to the filler type, however, the differences in microhardness and flexural properties for nanohybrid, microhybrid, and minifil products were minor.

2. Except one or two products, filler weight was highly correlated with microhardness and flexural modulus.

3. Additionally, microhardness and flexural modulus were strongly correlated to each other.

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