Evaluation of Equivalent Flexural Strength for Complete Removable Dentures Made of Zirconia-Impregnated PMMA Nanocomposites

Abstract

High-impact (HI) polymethyl methacrylate (PMMA), obtained from modification of conventional PMMA, is commonly used in prosthodontics as a denture base material for improved impact resistance. However, it suffers from poor flexural strength properties. The aim of this study was to investigate the flexural strength of complete removable dentures made of HI heat-polymerised PMMA resin reinforced with zirconia nanoparticles at two different concentrations. The effect of fatigue loading on the flexural strength behaviour of the dentures was also investigated. A total of 30 denture specimens were fabricated from PMMA with different concentrations of zirconia nanoparticles: 0 (control), 3, and 5 wt.%. Ten specimens in each group were divided into two subgroups, with five specimens in each, to conduct both flexural strength and fatigue loading test of each of the subgroups. Fatigue loading was applied on the dentures using a mastication simulator and equivalent flexural strength was calculated with data from bending tests with and without fatigue cyclic loading. One-way analysis of variance (ANOVA) of the test data was conducted with the Bonferroni significant difference post-hoc test at a preset alpha value of 0.05. Paired t-test was employed to identify any difference between the specimens with and without the application of fatigue loading. The fractured surface of the denture specimens was examined with a scanning electron microscope (SEM). The bending tests demonstrated that the mean equivalent flexural strength of reinforced HI PMMA denture specimens with 5 wt.% zirconia nanoparticles increased significantly (134.9 ± 13.9 MPa) compared to the control group (0 wt.%) (106.3 ± 21.3 MPa) without any fatigue loading. The mean strength of the dentures with PMMA +3 wt.% zirconia also increased, but not significantly. Although the mean strength of all specimen groups subjected to fatigue loading slightly decreased compared to that of the specimen groups without any fatigue cyclic loading, this was not statistically significant. Denture specimens made of HI heat-polymerised PMMA reinforced with 5 wt.% zirconia nanoparticles had significantly improved equivalent flexural strength compared to that made of pure PMMA when the specimens were not subjected to any prior fatigue cyclic loading. In addition, the application of fatigue cyclic loading did not significantly improve the equivalent flexural strengths of all denture specimen groups. Within the limitations of this study, it can be concluded that the use of zirconia-impregnated PMMA in the manufacture of dentures does not result in any significant improvement for clinical application.

Prosthodontic Applications of Polymethyl Methacrylate (PMMA): An Update

“…For instance, PMMA absorbs water, which may compromise its physical and mechanical properties while in use [ 63 , 64 ] and make it vulnerable to failure under cyclic loading [ 65 ]. Currently, a number of researchers are focused on mechanical reinforcement [ 30 , 38 , 51 , 53 , 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 , 74 ] and chemical modification [ 53 , 75 , 76 ] of PMMA to overcome its drawbacks and improve its properties, which have been reviewed in this article. In order to better understand and interpret these recent modifications, it is important to understand the chemistry, types, and properties of PMMA materials.…”

“…Therefore, good mechanical properties are required for the functional performance of denture base materials (biofunctionality). Considering the significance of the mechanical properties, several modifications have been reported for various mechanical properties of PMMA, including its flexural strength [ 14 , 30 , 33 , 68 , 134 , 135 , 136 ], impact strength [ 14 , 137 , 138 , 139 , 140 ], fracture toughness [ 141 , 142 , 143 ], and surface hardness [ 67 , 98 , 141 , 144 , 145 ]. The flexural strength (also known as the modulus of rupture or transverse strength) is assessed using a 3-point bending test according to the guidelines of ISO 20795-1 [ 81 ].…”

“…Ideally, the denture base should have a high flexural strength to bear the complex forces of mastication without permanent deformation or fracture [ 9 ]. A number of studies have investigated [ 14 , 30 , 33 , 68 , 134 , 135 , 136 ] flexural strength using various methods and types of PMMA. Barbrosa et al [ 146 ] reported good flexural strength for heat-cured, cold-cured, and microwave-cured PMMA (92.84 ± 4.73, 84.40 ± 1.68, and 109.63 ± 5.31 MPa, respectively).…”

“…Various modifications to PMMA materials can improve their strength for denture repair, with promising outcomes [ 31 , 68 , 141 , 142 ]. For example, the incorporation of ZrO 2 nanoparticles improved the flexural strength of PMMA repair materials [ 29 , 30 , 31 , 142 ]. Previous studies have investigated light-cured PMMA for denture repair and reported several benefits, such as ease of manipulation, controlled polymerization time, no monomer issues, and better mechanical properties [ 93 , 183 , 184 ].…”

“…Although there are a number of concerns associated with using PMMA, such as the fracture of dentures due to water sorption and poor impact and flexural strength, the ongoing research has introduced a variety of modifications to overcome and further improve its properties (such as its conductivity, water sorption, solubility, impact and flexural strengths). For example, several studies reported the improvement of PMMA materials while reinforced using a variety of fibers [ 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 ], nanoparticles [ 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 ], and nanotubes [ 38 , 39 , 40 , 41 , 42 ]. Similarly, PMMA-based biocomposites with the addition of epoxy resins, polyamide, or butadiene styrene have been reported to improve the impact strength of PMMA [ 43 ].…”