Incorporation of inorganic fillers into experimental resin adhesives: Effects on physical properties and bond strength to dentin

This study investigated the effect of different monomeric systems and inorganic fillers on the physical properties of experimental resin adhesives and on the immediate and 6-month bond strength to dentin. Two, 2-step self-etch adhesive systems were prepared: two primers (constituted of GDMA-P, ethanol, water, and HEMA or PEG(400)-UDMA) and two adhesives (constituted of Bis-GMA, TEGDMA, and HEMA or PEG(400)-UDMA). Next, the adhesives were allocated into three groups according to filler incorporated: unfilled (control), silica (SiO2), or ytterbium trifluoride (YbF3). Degree of conversion (DC, after 30 and 60 s of light-activation), water sorption (WS) and solubility (SL), and flexural strength (σ) and modulus (Ef) tests were performed for all adhesives. A microtensile bond strength (µTBS) test to dentin was evaluated after 24 h (immediate) and 6 months (6-month) of water storage using a universal testing machine (DL500, EMIC). Data were analyzed using statistical tests (α=5%). The adhesives showed similar DC at a same light exposure time, although light-activation for 60 s improved conversion of monomers. The unfilled HEMA-containing adhesive showed higher WS, SL, σ and Ef compared to others. The incorporation of fillers into the adhesives did not affect negatively the immediate µTBS results; however, after 6 months the presence of SiO2 impaired in complete premature failures, and the presence of YbF3 reduced the µTBS in the PEG(400)-UDMA-based group. The unfilled HEMA-containing group also reduced bond strength after 6 months of water storage. In conclusion, depending on the resin matrix composition, YbF3 seems to be a good option for reinforcing adhesive systems.