Elsevier

Dental Materials

Volume 37, Issue 1, January 2021, Pages 168-174
Dental Materials

Multi-scale analysis of the influence of filler shapes on the mechanical performance of resin composites using high resolution nano-CT images

https://doi.org/10.1016/j.dental.2020.10.030Get rights and content

Highlights

  • Fracture criteria of resin composites (RCs) were assessed at the micro-scale.

  • Homogenization analysis confirmed anisotropy of the RCs at the micro-scale.

  • Maximum principal strain was identified as a useful fracture criterion of the RCs.

  • Localization analysis was used to visualize RC micro-scale strain concentration.

  • Multi-scale analysis confirmed the advantage of irregular-shaped filler.

Abstract

Objective

The aim of this study was to investigate the criteria for predicting the fracture initiation of resin composites (RCs) at the micro-scale and assess the influence of filler shapes on the flexural properties of RCs by combining nano-CT imaging and in silico multi-scale analysis.

Methods

Experimental RCs composed of irregular-shaped (IS) silica filler (31.2 vol%/50.0 wt%) and Bis-GMA/TEGDMA were prepared. The RC specimens were scanned by a nano-CT with 500-nm resolution, and 10 micro-scale models (100 × 100 × 100 μm) were randomly extracted from a scanned region. In silico micro-scale models containing sphere-shaped (SS) fillers with the same volume content as the experimental RC were designed. Each RC model’s elastic modulus and Poisson’s ratio at the macro-scale were calculated using homogenization analysis. The flexural strength of the RC models were predicted by finite element analysis using the elastic moduli and Poisson’s ratio values.

Results

Significantly greater elastic modulus values were obtained in the X, Y, and Z directions for RC models containing IS fillers than SS fillers. Similarly, smaller Poisson’s ratio values were observed in the Y and Z directions for RC model containing IS fillers than SS fillers (p < 0.05). The flexural strength of RC model containing IS fillers was significantly greater than the RC model containing SS fillers (p < 0.05).

Significance

The in silico multi-scale analysis established in this study demonstrated that RC model containing irregular-shaped fillers had greater flexural strength than RC model loaded with SS fillers, suggesting that the mechanical strength of the RC can be improved by optimizing the shape of the silica fillers.

Introduction

Posterior restorations have been increasingly performed with resin composites (RCs) to satisfy the aesthetic demands of patients [1]. However, the mechanical properties of RCs have some limitations compared with metal and ceramics [2]. RCs with universal shade, consisting of a resin matrix and glass filler, can be color matched to the tooth being restored, providing an aesthetic advantage [3,4]. Newer compositions of these RC materials have been improved to withstand stress and wear [5]. RCs have been invented to closely mimic the aesthetics and function of natural tooth tissue, and their longevity in the oral environment depends largely on their fatigue or wear properties [6]. Identification of the mechanical properties of commercially-available RCs is difficult because of the variety of synthesis conditions. Clinical reviews have shown that the wear of RCs largely depend on the particle size and volume of filler or the resin matrix [7,8]. However, other factors such as filler shape, filler distribution, coupling agent between filler and monomer, and filler/monomer composition ratio have not been considered.

Although the impact of the size and content of filler on the mechanical properties of RCs has been reported [9], the relationship between the mechanical properties and the RC micro-structure has not been clarified because the above other factors are influenced by one another and are not controllable as an independent parameter in vitro. Also, distinguishing nano-hybrids from micro-hybrids is difficult and their mechanical properties, such as flexural strength and modulus, tend to be similar [10]. In this study, the mechanical properties of RCs at the micro- and macro-scale were evaluated using in silico models with precise simulation of the micro-structure reconstructed based on high-resolution nano-CT images. Multiple factors, such as the amount of unpolymerized methacrylate monomer, the degree of hydrolysis at the interface between the resin matrix and filler [11], and temperature [12], are considered in an in vitro test. However, a method identifying the fracture initiation of materials has not been established and details of the 3D shape/structure of filler has not been investigated because of their small diameter.

In silico multi-scale analysis is a method for analyzing physical properties and their behavior among different size scales, which can be used to address issues in the characterization of heterogeneous substances by considering the material properties at the micro-scale [13]. In silico multi-scale analysis has been further enhanced to solve issues with the analysis of the mechanical properties of RCs [14] and the influence of the physical properties of an implant on cortical bone resorption [15].

The finite element analysis used in the in silico multi-scale analysis is a numerical approach to investigate the mechanical properties, and the maximum principal strain is used as a fracture criterion to predict the flexural strength of RCs [16,17]. The utility of the maximum principal strain at the micro-scale has not been reported because of the limited resolution of three-dimensional imaging technologies such as micro-CT to construct numerical model reflect to real morphology of fillers [9].

The aim of this study was to investigate the fracture criteria for predicting the fracture initiation of RCs at the micro-scale and assess the influence of filler shapes on the flexural properties of RCs by combining nano-CT imaging and in silico multi-scale analysis.

Section snippets

Sample preparation

An experimental RC containing irregular-shaped (IS) silica filler and 2,2-bis[4-(2-hydroxy-3-methacrylyloxypropoxy)-phenyl]-propane (Bis-GMA)/triethylene glycol dimethacrylate (TEGDMA) was prepared. The content (monomer/filler) of the experimental RC was 68.8/31.2 vol% (50.0/50.0 wt%). The monomer compositions were Bis-GMA (75 wt%), TEGDMA (25 wt%), benzoyl peroxide (BPO) (1.4 wt%), dl-camphorquinone (CQ) (0.7 wt%), and aldehyde compound with dibutylhydroxytoluene (2.17 wt%). IS silica glass

Homogenization analysis

In the X, Y, and Z directions, significantly greater elastic modulus values were obtained for the RC models containing IS filler than with RC models containing SS filler (p < 0.05) (Fig. 3a). In the Y and Z directions, significantly smaller Poisson’s ratios were obtained for RC models containing IS filler than with RC models containing SS filler (p < 0.05) (Fig. 3b).

In silico three-point bending test

The flexural strength of the RC models containing IS filler was significantly greater than RC models containing SS filler (p <

Discussion

We established an in silico multi-scale analysis of RCs with different filler shapes (irregular/sphere). The multi-scale analysis clarifies the influence of the filler shape on the flexural properties of RCs.

Few systematic investigations have reported the effect of particle size and shape on RCs [[18], [19], [20], [21], [22]]. In the previous studies, the impact of the filler shape on the mechanical properties of RC were investigated using in vitro testing of various commercially-available

Conclusions

The in silico multi-scale analysis demonstrated that RC models containing irregular-shaped filler had greater flexural strength than RC models loaded with sphere-shaped filler, suggesting that the mechanical strength of RCs can be improved by optimizing the shape of silica filler.

Acknowledgements

This research was supported by a Grant-in-Aid for Scientific Research (No. JP 19K10244) from the Japan Society for the Promotion of Science(JSPS). The authors are grateful to Kuraray Noritake Dental for kindly providing some of the materials used. We thank Ashleigh Cooper, PhD, from Edanz Group (https://en-author-services.edanzgroup.com/) for editing a draft of this manuscript.

References (31)

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