Influence of incorporation of ZrO2 nanoparticles on the repair strength of polymethyl methacrylate denture bases

Background: Repeated fracture of the denture base is a common problem in prosthodontics, and it represents a nuisance and a time sink for the clinician. Therefore, the possibility of increasing repair strength using new reinforcement materials is of great interest to prosthodontists. Aim of the study: This study aimed to evaluate the effects of incorporation of zirconia nanoparticles (nano-ZrO 2 ) on the flexural strength and impact strength of repaired polymethyl methacrylate (PMMA) denture bases. Materials and methods: One hundred eighty specimens of heat-polymerized acrylic resin were fabricated (90 for each test) and divided into three main groups: one control group (intact specimens) and two groups divided according to surface design (45° bevels and butt joints), in which specimens were prepared in pairs to create 2.5 mm gaps. Nano-ZrO 2 was added to repair resin in 2.5 wt%, 5 wt%, and 7.5 wt% concentrations of acrylic powder. A three-point bending test was used to measure flexural strength, and a Charpy-type test was used to measure impact strength. Scanning electron microscopy was used to analyze the fracture surfaces and nano-ZrO 2 distribution. The results were analyzed with a paired sample t -test and an unpaired t -test, with a P-value of ≤0.05 being significant. Results: Incorporation of nano-ZrO 2 into the repair resin significantly increased flexural strength ( P <0.05). The highest value was found in the bevel group reinforced with 7.5% nano-ZrO 2 , whereas the lowest value was found in the butt group reinforced with 2.5% nano-ZrO 2 . The impact strength values of all repaired groups were significantly lower than those of the control group ( P <0.05). Among repaired groups, the higher impact strength value was seen in the butt group reinforced with 2.5% nano-ZrO 2 . The bevel joint demonstrated mainly cohesive failure, whereas the butt joint demonstrated mainly adhesive failure. Conclusion: Incorporation of nano-ZrO 2 into the repair resin improved the flexural strength of repaired denture bases, whereas it decreased impact strength, especially with high nano-ZrO 2 concentrations.