To assess the elastic modulus (EM), volumetric shrinkage (VS), and polymerization shrinkage stress (PSS) of experimental highly filled nanohybrid composites as a function of matrix composition, filler distribution, and density.
One regular viscosity nanohybrid composite (Grandio, VOCO, Germany) and one flowable nanohybrid composite (Grandio Flow, VOCO) were tested as references along with six highly filled experimental nanohybrid composites (four Bis-GMA-based, one UDMA-based, and one Ormocer ® -based). The experimental composites varied in filler size and density. EM values were obtained from the “three-point bending” load–displacement curve. VS was calculated with Archimedes’ buoyancy principle. PSS was determined in 1-mm thick specimens placed between two (poly)methyl methacrylate rods ( Ø = 6 mm) attached to an universal testing machine. Data were analyzed using oneway ANOVA, Tukey’s test ( α = 0.05), and linear regression analyses.
The flowable composite exhibited the highest VS and PSS but lowest EM. The PSS was significantly lower with Ormocer. The EM was significantly higher among experimental composites with highest filler levels. No significant differences were found between all other experimental composites regarding VS and PSS. Filler density and size did not influence EM, VS, or PSS.
Neither the filler configuration nor matrix composition in the investigated materials significantly influenced composite shrinkage and mechanical properties. The highest filled experimental composite seemed to increase EM by keeping VS and PSS low; however, matrix composition seemed to be the determinant factor for shrinkage and stress development. The Ormocer, with reduced PSS, deserves further investigation. Filler size and density did not influence the tested parameters.
Modern direct resin composites reflect decades of filler and polymer technology development. Great improvements in mechanical properties , wear resistance , and clinical performance are acknowledged. Promising data are published on the clinical success of direct resin composites, even superior to amalgams . However, the main reason for failure is still secondary caries followed by restoration fracture . Material development should therefore intensively focus on improving composite strength to prevent restoration fracture and further reducing volumetric polymerization shrinkage (VS). However, shrinkage is not only the matter of concern, but also the resulting stress which builds up at the cavity walls. Polymerization shrinkage stress (PSS) is a main parameter influencing marginal leakage and formation of secondary caries . Recently, attempts have been made to reduce the VS by developing new low-shrinkage monomers such as siloranes, ormocers, TCD (Venus ® ), or DX 511 (Kalore™) based monomers. Those products have been introduced and exhibit an improved VS down to 1 vol.% . Since it is a monomer shrinkage concern, reducing the monomer fraction by increasing the reinforcing filler content is an efficient alternative for further reducing the VS and to additionally increasing the composite fracture strength .
During polymerization, the establishment of covalent bonds between monomers reduces their interatomic distances causing significant VS . The matrix composition is one of the determinants of VS. Bisphenylglycidyl dimethacrylate (BisGMA), which has a high molecular weight (512 g/mol) and VS of 5.2%, is the most often used base-monomer in commercial composites . However, because of its high viscosity, the addition of diluent monomers is necessary, enabling the incorporation of initiators, inhibitors, and fillers. Triethyleneglycol dimethacrylate (TEGDMA) is the most common diluent monomer used in association with BisGMA. Because of its lower molecular weight (286 g/mol) and increased flexibility, TEGDMA has a high VS . Other low viscosity and high molecular weight dimethacrylate monomers, such as urethane dimethacrylate (UDMA) and ethoxylated bisphenol-A dimethacrylate (BisEMA), are used in commercial formulations as an alternative to TEGDMA to reduce VS. Also, in an attempt to reduce shrinkage, organically modified ceramics (Ormocer ® ) were developed .
Ormocer consists of inorganic–organic copolymers. Its mechanical parameters are determined by the formation of an inorganic Si–O–Si network by hydrolysis and polycondensation reactions with the alkoxysilyl groups of the silane, as well as by the reaction of methacrylate groups . Low polymerization shrinkage values were observed in Ormocer in previous studies and short-term clinical trials show comparable performance of Ormocer and Bis-GMA-based composites .
Besides the polymeric matrix, the composite inorganic content determines the paste viscosity, VS, and EM. The increase in inorganic content reduces the amount of matrix in the composition leading to an increase in EM and decrease in VS . Thus, one could expect that composites with high filler contents would have reduced polymerization shrinkage stress (PSS). However, a study showed that packable composites are less capable of reducing PSS during the early setting stage . Another study showed that flowable resin-based composites (less packed) generate polymerization contraction stress values similar to those produced by conventional packable resin-based composites, despite their lower EM . As for nanohybrid composites, the incorporation of nanoparticles increases the filler fraction by filling the interstices of larger particles. This increases the density of the packing without compromising the paste viscosity and handling properties. Higher filler content means a lower fraction of the organic portion responsible for shrinkage, which should reduce shrinkage stress. However, an inverse relationship was shown for hybrid composites in which an increase in the filler fraction led to increased contraction stress development. Such a response has not yet been systematically determined for highly filled nanohybrid composites. Variations in filler size were also shown to have a relevant role in shrinkage-strain development as well as on the degree of conversion and wear resistance .
Based on a commercial, BisGMA-based, nanohybrid resin composite, experimental formulations were tailored regarding matrix composition and filler packing. The aim of this study was to optimize the physical performance of the experimental composites in terms of shrinkage behavior and mechanical performance.
Materials and methods
Two commercially available nanohybrid composites, one with regular and one with flowable viscosity, were tested along with six experimental materials ( Table 1 ). All experimental composites had high filler contents and presented a weight ratio of glass:nanofillers of 3:1 to guarantee a dense filler packing. Silica nanoparticles with a size distribution of 20–50 nm were used as nanofillers for all the experimental composites. Four different types of glass were used as specified in Table 1 . The matrix composition of the experimental composites was labeled M1–M5 according to the monomer composition and ratio (see Table 1 ). The photo-initiator system for all experimental composites was 0.2–0.5 wt% of camphoroquinone as used commercially.
|Resin composites||Glass type||Filler fraction||Matrix composition||Ratio||Batch|
|Label||d 50 (μm)||Density (g/cm 3 )||Wt.%||Vol.%||Label|
Elastic modulus (EM)
Specimens were manufactured according to ISO 4049 using a tungsten carbide mold (2 mm × 2 mm × 25 mm, n = 10). Photoactivation was carried out with a halogen light-curing unit (500 mW/cm 2 , Translux Energy, Heraeus Kulzer, Hanau, Germany) on five overlapping spots of 8 mm (20 s each) on both the upper and lower surfaces. Specimen edges were manually finished with 800-grit SiC-paper. The specimens were stored in distilled water at 37 °C for 24 h. To determine the EM, the stress–strain curves were obtained between 10 MPa and 20 MPa for each specimen by loading them in a 3-point bending test set-up at a loading rate of 0.75 mm/min (Zwick Z2.5, Zwick, Ulm, Germany) according to the equation:
EM = d σ d ε = const
where σ is the stress and ε is the strain. An extensometer (Zwick, Germany) was used for the strain measurements.
Volumetric shrinkage (VS)
The VS was determined through density determinations in accordance with the buoyancy method (Archimedes’ principle). Material syringes and buoyancy medium (distilled water containing 0.01% sodium laurylsulfate) were kept at room temperature (23 ± 2 °C). The density determination kit was installed on a balance (YDK01 and CP124S, Sartorius, Goettingen, Germany) with an accuracy of ±0.1 mg to measure material densities of the uncured ( ρ u ) vs. cured ( ρ c ) samples according to Archimedes’ principles of buoyancy. The VS (vol.%) was calculated by the equation:
VS = ρ c − ρ u ρ c × 100