Biaxial flexural strength of new Bis-GMA/TEGDMA based composites with different fillers for dental applications
Introduction
Nowadays composite resin materials are mainly used for dental fillings, because amalgam, the material preferred in earlier times, has many disadvantages as the possibility of toxic reactions [1], lack of adhesion to the tooth, limited mechanical strength and lack of esthetics [2]. Composite resins should provide similar structural, physical and mechanical properties as the natural tooth structures [3]. Therefore composite resin materials are mainly made of inorganic components that are embedded in an organic matrix. This is imitating the natural tooth, as the dentin and the enamel also have a large content of inorganic material [4]. However, a clinical study has shown that the survival rate of the composite fillings after seven years was lower (85.5%) than the survival rate of the amalgam (94.4%) [5].
Often zirconia fillers are used to improve the mechanical properties of composite resins materials [6]. However not only filler materials properties but also the shape of the filler particles have a considerable influence on the mechanical properties of the final composite resin. It has been shown that the mechanical properties of the composite can be improved by changing the particle shape from spherical to tetrapodal [7], [8]. While it is quite simple and cost effective to produce tetrapodal shaped particles, if the chosen material is zinc oxide [9], it is quite difficult to produce zirconia filler particles of tetrapodal shape.
The main reason for failure of the dental fillings is secondary caries caused by bacteria [10]. It has been shown that the colonization of caries-producing bacteria is enhanced by the higher surface energy and surface roughness of the composite fillings [11], [12], [13]. Several approaches to prevent the bacterial growth have been made. Antibacterial agents can be added to the filling materials, but the supply will be exhausted after a short time [10]. Additives may also change the mechanical properties of the composites [14], [15]. However, it has been shown that lauric acid is an antibacterial agent against bacteria in various regions of the human body [16], [17]. In another study it has been shown that lauric acid inside the feeding stuff of rats reduces their amount of caries [18].
Therefore, it was the purpose of this study to evaluate whether the biaxial flexural strength of composite resins can be enhanced by using tetrapodal zinc oxide filler particles instead of spherical filler particles of zirconia or zinc oxide. Another purpose of this study was to evaluate if the functionalization of the particles with lauric acid will affect the biaxial flexural strength of the composite. The first hypothesis of this study is that the functionalization with lauric acid will have no influence on the biaxial flexural strength. The second hypothesis of the study is that tetrapodal zinc oxide filler particles will lead to a higher biaxial flexural strength of the BisGMA/TEGDMA based composite resins than spherical zinc oxide and zirconia fillers.
Section snippets
Sample preparation
In this study a monomer mixture of BisGMA (Sigma–Aldrich) and TEGDMA (Sigma–Aldrich) with a weight ratio of 60:40 has been used. For the polymerization 0.5 wt-% camphorquinone (97%, Sigma–Aldrich) as photo initiator and 0.5 wt-% ethyl 4-(dimethylamino) benzoate (99%, Sigma–Aldrich) as accelerator were added. After a complete mixture of these components the filling particles were mixed mechanically into the matrix material. Besides the groups of composites with functionalized and not
Results
Means and standard deviations of the biaxial flexural strength in test groups are given in Table 2. In addition, Fig. 2a and b shows the box plots for low and high filler content, respectively. Three-way ANOVA revealed that there was no statistical significant interaction of all three factors (p = 0.936). But there is a statistical significant two factorial interaction (p ≤ 0.001) of the factors filler particles and filler content for the influence on the biaxial flexural strength, but no further
Discussion
That only one of the filled composite resins had a significant higher biaxial flexural strength than the pure matrix material seems surprising on the first sight. The reason for the reduction in biaxial flexural strength is the low particle/matrix adhesion. It is well known that a strong interfacial bonding between filler particles and matrix is crucial for an effective stress transfer leading to strong influence on mechanical strength of the composite [19]. Ikejima at al. have shown that
Conclusions
Taking the conditions and results of this study into account, the following conclusions can be drawn:
- 1.
Even without usage of a chemical coupling agent, tetrapodal zinc oxide filler particles significantly enhance the biaxial flexural strength of BisGMA/TEGDMA based composite resins. The use of proper particle functionalization requires further evaluation.
- 2.
The functionalization of filler particles with lauric acid to achieve antibacterial properties cannot be recommended as it results in a decrease
Acknowledgments
The authors wish to thank Frank Lehmann from the Department of Prosthodontics, Propaedeutics and Dental Materials of the University of Kiel and Christoph Ochmann from the chair for Functional Materials of the Institute of Materials Science at the Faculty of engineering for the support during the experiments.
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