Effect of zirconium oxide nanoparticles addition on the optical and tensile properties of polymethyl methacrylate denture base material

Background: Polymethyl methacrylate (PMMA) is widely used for the fabrication of removable prostheses. Recently, zirconium oxide nanoparticles (nano-ZrO2) have been added to improve some properties of PMMA, but their effect on the optical properties and tensile strength are neglected. Objective: The aim of this study was to investigate the effect of nano-ZrO2 addition on the translucency and tensile strength of the PMMA denture base material. Materials and methods: Eighty specimens (40 dumbbell-shaped and 40 discs) were prepared out of heat-polymerized acrylic resin and divided into four groups per test (n= 10). The control group for each test included unreinforced acrylic, while the test groups were reinforced with 2.5, 5, and 7.5 wt% nano-ZrO2. Acrylic resin was mixed according to manufacturer's instructions, packed, and processed by conventional method. After polymerization, all specimens were finished, polished, and stored in distilled water at 37 degrees C for 48 +/- 2 hours. Tensile strength (MPa) was evaluated using the universal testing machine while the specimens' translucency was examined using a spectrophotometer. Statistical analysis was carried out by SPSS using the paired sample t-test (p <= 0.05). A scanning electron microscope was used to analyze the morphological changes and topography of the fractured surfaces. Results: This study showed that the mean tensile strength of the PMMA in the test groups of 2.5% NZ, 5% NZ, and 7.5% NZ was significantly higher than the control group. The tensile strength increased significantly after nano-ZrO2 addition, and the maximum increase seen was in the 7.5% NZ group. The translucency values of the experimental groups were significantly lower than those of the control group. Within the reinforced groups, the 2.5% NZ group had significantly higher translucency values when compared to the 5% NZ and 7.5% NZ groups. Conclusion: The addition of nano-ZrO2 increased the tensile strength of the denture base acrylic. The increase was directly proportional to the nano-ZrO2 concentration. The translucency of the PMMA was reduced as the nano-ZrO2 increased. Clinical significance: Based on the results of the current study, the tensile strength was improved with different percentages of nano-ZrO2 additions. However, translucency was adversely affected. Therefore, it is important to determine the appropriate amount of reinforcing nano-ZrO2 that will create a balance between achieved properties -mechanical and optical.