Effect of untreated zirconium oxide nanofiller on the flexural strength and surface hardness of autopolymerized interim fixed restoration resins.

OBJECTIVE: Autopolymerized poly methyl methacrylate (PMMA) resin is commonly used for the construction of interim restorations; however, it has less than optimal mechanical properties. In this article, we evaluated the reinforcing effect of adding untreated zirconia nanoparticles on the flexural strength and surface hardness of this resin.

METHODS: A total of 80 specimens were fabricated. Forty each were used for the flexural strength test and for the surface hardness test. The specimens were categorized into four groups of 10 specimens each as follows: pure PMMA, PMMA with 1%, PMMA with 2.5%, and PMMA with 5% weight of untreated zirconia nanofillers. The flexural strength of the specimens was evaluated by the three-point bending test, and the surface hardness was assessed by micro Vickers hardness test. The data obtained from these tests were statistically analyzed using one-way ANOVA and Tukey tests. In addition, the fracture surface characteristics were assessed using scanning electron microscopy.

RESULTS: Flexural strength testing showed a significant increase in the group with 2.5% zirconia nanofillers, but not in the groups with 1% and 5% nanofillers. Surface hardness was also significantly increased in the groups with 2.5% and 5% nanofillers, but not in the 1% group. The SEM images showed a highly brittle fracture in the pure PMMA group and noticeably less brittle fracture in the group with PMMA with 2.5% weight of zirconia nanofillers. Several cracks and void were also observed in the group with 5% weight of nanofillers.

CONCLUSION: Reinforcement of the autopolymerized acrylic resin with 2.5% weight of untreated zirconia nanofillers significantly increased its flexural strength and surface hardness.

CLINICAL SIGNIFICANCE: The interim restorations play an important role in protection of hard and soft oral tissue and providing the critical function and esthetics before the final restoration replacing. Temporary restorations must have sufficient flexural strength to resist deformation during mastication force. Moreover, sufficient surface hardness is also necessary to resist abrasion. The color stability of materials is considered as an important clinical criterion, specifically in esthetics zone. Several materials have been applied to improve ‏the flexural strength and surface hardness for representing clinical success. Zirconia nanoparticles show desirable features, such as high hardness, biocompatibility, and favorable color because of its white color. ‏It seems that the addition of the nano zirconia to acrylic resins can be the appropriate method for improving interim restoration.