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Table of Contents
ORIGINAL ARTICLE
Year : 2020  |  Volume : 7  |  Issue : 4  |  Page : 91-95

Effect of titanium oxide and zirconium oxide nanoparticle incorporation on the flexural strength of heat-activated polymethyl methacrylate denture base resins – An in vitro experimental study


1 Associate Professor, Department of Prosthodontics, Government Dental College, Alappuzha, Kerala, India
2 Assistant Professor, Department of Prosthodontics, Government Dental College, Alappuzha, Kerala, India

Date of Submission23-Nov-2020
Date of Acceptance29-Nov-2020
Date of Web Publication29-Dec-2020

Correspondence Address:
Dr. A R Adhershitha
Department of Prosthodontics, Government Dental College, Alappuzha, Kerala
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/ijpcdr.ijpcdr_49_20

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  Abstract 


Introduction: Flexural strength is one of the most important properties which determine the clinical success of dentures. Even though titanium oxide nanoparticles (nano-TiO2) and zirconium oxide nanoparticles (nano-ZrO2) are extensively used to modify the properties of polymethyl methacrylate (PMMA) resin, there is negligible documentation in the literature comparing these two.
Aim: The aim of the study is to find out and compare the effect of incorporation of 1 weight % nano-TiO2 and nano-ZrO2 on the flexural strength of heat-activated PMMA resin.
Materials and Methods: 1 weight % of TiO2 and ZrO2 nanoparticles were weighed using a digital weighing balance and mixed with PMMA resin powder using the geometric dilution method. Sixty “35 mm × 10 mm × 3 mm” rectangular-shaped specimens fabricated were divided into three different groups, each containing twenty samples each. The specimens were loaded to failure at a crosshead speed of 1 mm/min and with 500 N load cell in a Universal Instron Testing Machine, Model 3345. Flexural strength was registered by the machine with 0.001 MPa precision.
Results: PMMA with nano-TiO2 group had the maximum flexural strength, load at maximum flexure, and load at break followed by unreinforced PMMA and PMMA with nano-ZrO2. Statistically significant difference was noted between the three groups (P < 0.001). Post hoc analysis using Tukey's post hoc test showed a statistically significant difference for all three pair-wise comparison.
Conclusions: Within the limitations of this study, it can be concluded that nano-TiO2 greatly enhanced the flexural strength, load at maximum flexure load, and load at break of PMMA resin, whereas nano-ZrO2 significantly reduced these properties of PMMA.

Keywords: Denture base, flexural strength, polymethyl methacrylate, titanium oxide nanoparticles, zirconium oxide nanoparticles


How to cite this article:
Viswambharan P, Adhershitha A R. Effect of titanium oxide and zirconium oxide nanoparticle incorporation on the flexural strength of heat-activated polymethyl methacrylate denture base resins – An in vitro experimental study. Int J Prev Clin Dent Res 2020;7:91-5

How to cite this URL:
Viswambharan P, Adhershitha A R. Effect of titanium oxide and zirconium oxide nanoparticle incorporation on the flexural strength of heat-activated polymethyl methacrylate denture base resins – An in vitro experimental study. Int J Prev Clin Dent Res [serial online] 2020 [cited 2021 Jan 24];7:91-5. Available from: https://www.ijpcdr.org/text.asp?2020/7/4/91/305293




  Introduction Top


Evolution of material sciences is scaling new heights with the invention and growth in applications of nanoparticles. In conjunction with the desirable properties such as ease of laboratory manipulation, lightweight, good esthetics, low water sorption and solubility, color-matching ability, lack of toxicity, ability to be repaired easily, and being inexpensive, polymethyl methacrylate (PMMA) remains as the most preferred denture base material in dentistry.[1] However, brittleness, inferior mechanical strength, and relatively low elastic modulus make it more prone to mechanical failure during clinical use.[2],[3] There are reports of fracture of denture base during function owing to the poor transverse, impact, and flexural strengths of PMMA.[4] Mechanical properties of the resin can be augmented by altering the composition by the incorporation of fillers.[5],[6] Surprisingly, addition of nanoparticles in small quantities has the potential to radically metamorphose the host polymer. This can be attributed to the ability of nanoparticles to alter the mobility of polymer chains near their interfaces, owing to their high surface area to volume ratios.[7] The effect of nanoparticles on the host polymer depends on the type, size, shape, concentration, and their interactions with the polymer matrix.[7],[8],[9],[10],[11]

Titanium oxide nanoparticles (nano-TiO2), despite being reasonably inexpensive, have impressive properties such as biocompatibility, chemical inactiveness, high refractive index, antibacterial effect, corrosion resistance, and high microhardness.[12] Zirconium oxide nanoparticles (nano-ZrO2) too are gaining popularity nowadays because of its excellent toughness and strength, abrasion and corrosion resistance, excellent biocompatibility, as well as white color. Being the hardest among oxides, they are able to withstand crack propagation. A meta-analysis showed that ZrO2 and TiO2 substantially increase the flexural strength of PMMA compared to other fillers.[13] Esthetics is not expected to be affected much, as they both are white in color, unlike other metal nanoparticles such as silver. Nevertheless, there is negligible documentation in the literature comparing nano-TiO2 and nano-ZrO2.

Most appropriate ratio reported was 1 weight percentage for nanoparticles added with mechanical methods without any surface application.[14] Studies using scanning electron microscopy have revealed that nanocomposites with high flexural strength values showed more homogeneous distribution of particles and the agglomeration tendency increased with respect to the ratio of nanoparticles.[14] In this context, the present study was conducted to evaluate and compare the flexural strength of heat-activated conventional PMMA resin, PMMA resin modified with 1% nano-TiO2 (PMMA Ti) and PMMA resin modified with 1% nano-ZrO2 (PMMA Zr).


  Materials and Methods Top


Preparation of polymethyl methacrylate resin modified with titanium oxide and zirconium oxide nanocomposites

Appropriate amounts of nano-TiO2 and nano-ZrO2 were weighed (1 weight %) using a digital weighing balance and mixed with PMMA resin powder using geometric dilution method,[15] a pharmaceutical process that thoroughly mixes a small amount of a drug with an appropriate amount of a diluent which ensures equal distribution of drug throughout the resulting compound.

Plastic die and fabrication of heat-activated acrylic resin specimens

Rectangular-shaped plastic dies of dimension 35 mm × 10 mm × 5 3 mm were fabricated in a high precision milling laboratory to ensure uniform size for all the test specimens. Sixty rectangular-shaped specimens fabricated using the plastic dies were divided into three different groups (A1, A2, and A3), each containing twenty samples each. The samples were preserved in water at room temperature and tested for their flexural strength after 14 days. Group A1 – Heat-activated PMMA Ti. Group A2 – Heat-activated PMMA Zr. Group A3 – Heat-activated PMMA resin.

Testing for flexural strength

Tests were undertaken with a Universal Instron Testing Machine, Model 3345. The specimens were loaded to failure at a crosshead speed of 1 mm/min and with 500 N load cell. The span length used was 25 mm and the apparatus exerted force on the specimens by three-point loading principle until they broke [Figure 1]. Flexural strength was registered by the machine with 0.001 MPa precision.
Figure 1: Testing for flexural strength

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  Results Top


The obtained data were analyzed using IBM SPSS Statistics for Windows, Version 20.0. Armonk, NY: IBM Corp. Descriptive statistics were expressed as mean ± standard deviation. Normality of observed values was assessed using Shapiro–Wilk test. Comparison of the study variables (flexural strength, load at maximum flexural load, and load at break) between the three study groups was performed using one-way ANOVA test. Post hoc pair-wise analysis was done using Tukey's post hoc test. A P < 0.05 was considered statistically significant. A post hoc power analysis was also performed to ensure the adequacy of sample size using G-power software Faul F, Erdfelder E,Buchner A, and Lang A (2009) (F tests-ANOVA: fixed effects, omnibus, one-way). A total of 20 samples in each group were tested for flexural strength, load at maximum flexure load, and load at break. [Table 1] shows the results of Shapiro–Wilk test (P > 0.05). [Table 2] describes the mean ± standard deviation of the study variables. The comparison of variables between study groups is presented in [Table 3]. A statistically significant difference was noted between the three groups (P < 0.001) for flexural strength, load at maximum flexure load, and load at break. Descriptive analysis of study variables are presented in [Figure 2]. A post hoc comparison revealed that the difference was significant between each pairs of the three groups [Table 3]a. Values were highest for the PMMA Ti group and least for the PMMA Zr group. As we were unable to find any previous study comparing these three groups, this exploratory study first took into consideration 20 samples in each group. A post-hoc power analysis was performed to compute the achieved power using mean, standard deviation and sample size.
Table 1: Test of normality of distribution of each variable for the three study groups

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Table 2: Mean±standard deviation of study variables in each study group

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Table 3: Comparison of study variables between study groups

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Figure 2: Descriptive analysis of the study variables

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  Discussion Top


Superior mechanical properties are vital for improved clinical performance of PMMA denture base material. According to the results of this study, nano-TiO2 could be added to PMMA resin for improving mechanical properties. Metal inserts in the form of wires, metal oxides, metallic meshes, and plates, as well as fibers such as Kevlar, glass, carbon graphite, aramid and ultra-high-molecular-weight polyethylene, were used by many researchers to improve its mechanical properties.[13] Since its introduction in 1980s, the concept of “nanomaterials” gained popularity in material sciences and they are referred to as zero-, one-, two-, and three-dimensional materials with a size of less than 100 nm.[16],[17] The added advantages of nanoparticles in material sciences may be attributed to small size effect, quantum size effect, quantum tunneling effect, and surface effect.[18],[19] The present study showed a reduction in flexural strength with the addition of Zr nano-fillers to acrylic resin. This finding was in contradiction to many published literature.[20],[21] One research confirmed maximum flexural strength when 2.5% nano-ZrO2 and 2.5% glass fibers were added together and this enhancement in flexural strength was assumed to be due to the synergistic effect of nano-ZrO2 and glass fibers.[22]

As denture has a function of improving esthetics, denture base materials should be esthetically pleasing. In recent studies, nano-TiO2 was reported to cause color changes.[23],[24],[25] Some researchers even tried color pigments along with acrylic resins after the addition of nano-TiO2.[26] The present study too demonstrated a mild opacity in PMMA Ti specimens. Use of color pigments too may be advocated in future studies to evaluate the improvement in esthetics.


  Conclusions Top


Within the limitations of this study, it can be concluded that nano-TiO2 greatly enhanced the flexural strength, load at maximum flexure load, and load at break of PMMA resin, whereas nano-ZrO2 significantly reduced these properties of PMMA. Further studies with different concentrations of nanoparticles along with coloring pigments and aging procedures in conditions simulating the oral environment are recommended.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

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    Figures

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    Tables

  [Table 1], [Table 2], [Table 3]



 

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