Reinforcement of dental resin composite via zirconium hydroxide coating and phosphate ester monomer conditioning of nano-zirconia fillers

Objectives: The present study aimed to evaluate effects of conditioning with the phosphate ester monomer 10-methacryloyloxydecyl dihydrogen phosphate (MDP), with and without precoating with zirconium hydroxide for nano-size zirconia fillers, on mechanical properties of dental resin composites. Materials and methods: Nano-zirconia fillers coated with or without zirconium hydroxide [Zr(OH)(4)] were prepared. Transmission electron microscopy (TEM), Fourier infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) were used to observe the coating and to characterize Zr(OH)(4) coating on the zirconia filler surface. Zirconia fillers with or without Zr(OH)(4) coating were conditioned with MDP and were subsequently used to prepare experimental resin composites. XPS was used to analyze the Zr-O-P bonds on the filler surface after MDP conditioning. Moreover, three-point bending strength and elastic modulus of prepared resin composites were measured, and Weibull analysis was performed. Resin composites without addition of zirconia fillers and the ones with addition of untreated or silane conditioned-zirconia fillers were set as controls. Cell counting kit (CCK)-8 was used to test cell cytotoxicity of these zirconia fillers-containing experimental resin composites. Results: Nano-zirconia fillers were coated with Zr(OH)(4) through chemical deposition. FTIR and XPS analysis confirmed the increase of hydroxyl groups after Zr(OH)(4) coating. XPS detected the highest contents of Zr-O-P bonds on MDP-conditioned zirconia fillers with pre-Zr(OH)(4) coating, followed by MDP-conditioned zirconia fillers. Resin composite with added MDP-conditioned zirconia fillers with and without Zr(OH)(4) coating exhibited greater three-point bending strength, elastic modulus values, and Weibull moduli. According to the cytotoxicity classification, resin composites containing experimental zirconia fillers were considered to have no significant cell cytotoxicity. Conclusion: Nano-zirconia fillers conditioned with MDP, with or without precoating with Zr(OH)(4), improve the mechanical properties of resin composites, and are potentially safe for clinical use.