Elsevier

Dental Materials

Volume 18, Issue 1, January 2002, Pages 72-80
Dental Materials

Effect of novel filler particles on the mechanical and wear properties of dental composites

https://doi.org/10.1016/S0109-5641(01)00022-7Get rights and content

Abstract

Objectives: The purpose of this study was to investigate the method of producing pre-polymerized fused-fiber filler modified composite (PP-FFMC) particles and the effectiveness of incorporating these novel filler particles into dental composites.

Methods: Fused-fiber filler (FFF) blocks were impregnated with composite by two different methods. Three-point flexure tests were utilized to determine which was more effective. In order to assess the effect of the addition of PP-FFMC particles, two Bis-GMA/TEGDMA based conventional composite compositions were utilized as baselines, to which the novel particles were added. Mechanical and wear tests were performed to determine the fracture toughness, biaxial flexure strength, and in vitro wear of the materials.

Results: Mechanical testing showed that the addition of PP-FFMC particles decreased the strength and toughness of the conventional composites. Wear tests indicated that addition of the same particles improved the wear behavior of the conventional composites. SEM analysis of the fracture surfaces indicated that the PP-FFMC particles were incorporated without creating porosity, and that fracture was transgranular through the reinforcing particles. Microscopic flaws observed in the novel particles are the likely explanation for the observed strength and toughness values.

Significance:The results indicate that PP-FFMC particles have the potential to improve the wear properties of dental composites, however, they adversely affect the fracture behavior. Existing processing techniques for these particles, which introduce imperfections, limit their current usefulness.

Introduction

The ultimate goal of advanced dental composite research is to produce a material that can be used in all circumstances as an amalgam replacement material. In addition to the obvious esthetic problems, evidence continues to accumulate that dental procedures involving amalgam placement or replacement produce waste that is difficult or impossible to capture without contamination to the environment [1]. Increasing pressures from environmental regulations and public apprehension may eventually eliminate dental amalgam as a practical and inexpensive restorative filling material [2].

In recent years several large research efforts focussed on the development of amalgam replacement materials. A variety of materials, such as gallium alloys, improved composites, CAD/CAM materials, fortified glass ionomers, and various types of strengthened ceramics have been studied. Key considerations in developing a suitable material include low costs comparable to amalgam, negligible environmental concerns, wear and fracture resistance conducive to clinical longevity, and ease of clinical use. To date, no material with all of the desired characteristics has emerged. Due to the complexity and high costs of dental ceramics and exotic metal alloys, dental composites likely offer the best possibility for developing a true amalgam replacement material in the near future.

Although current dental composites are easy to handle clinically, esthetic, and relatively low cost, there are three primary problems associated with their clinical longevity. Current dental composites undergo shrinkage during curing, and have relatively low fracture resistance (e.g. strength and toughness). In addition, they are subject to higher wear rates than ceramics, and although some composites have wear rates similar to amalgam, many have higher wear rates [3].

The low fracture resistance of currently available dental composites is a characteristic that greatly limits their use. Unlike many other properties (e.g. hardness) that become counter-productive when they greatly exceed the values for natural tooth structure, the ideal fracture toughness of all dental materials should be as high as possible without sacrificing other important properties. The low fracture toughness of many direct dental composites is one aspect that limits the use of these materials for posterior restorative applications [4].

Since there have not been significant advances in improving the properties of polymer matrix materials, recent improvements in dental composite properties are due primarily to advances in filler technology [5]. Given equal levels of filler loading, strengthening mechanisms in composites depend primarily on the geometry and size of the dispersed filler phase. In general, composite materials can be either particle reinforced (random or preferred orientation) or fiber/whisker reinforced (single or multi-layer, continuous or discontinuous fiber, random or preferred orientation) [6]. Most dental materials research has focused on the use of particulate reinforcement, which has yielded limited improvements in fracture resistance [7]. Xu et al. [8] reported an increase of between 50–100% in the fracture toughness of experimental silicon nitride whisker reinforced composites when compared to commercially available particulate reinforced dental composites. However, ceramic whiskers and fibers composed of Si3N4 or SiC have the potential to act in an abrasive manner to opposing dentition, and could give rise to biocompatibility and esthetic problems.

A reinforcing particle made in the form of a ‘scaffold-like’ 3-D fibrous network has the potential to provide great improvement in each area of concern for currently available dental composites. In studies by Ehrnford [9], Ehrnford and Dérand [10] and Ritsco [11], use of such filler particles showed notable effects in reducing polymerization shrinkage and improving wear resistance. The hypothesized shrinkage reduction mechanism is due to confinement of shrinkage to the microscopic spaces between fibers in the 3-D network [10]. Enhanced wear resistance is attributed to the porous structure's ability to anchor the filler particle in the matrix, resisting the ‘plucking out’ of particles commonly observed in composite wear [8]. Although these novel fused-fiber fillers show great promise, a good deal of further research is needed to optimize this technology.

Several research projects investigating the use of fused-fiber filler have been undertaken. Initial efforts focused on incorporation of silanated fused-fiber filler (FFF) particles into existing dental materials. The filler material that has been investigated is derived from a product developed for aerospace applications. The modified fused fiber filler (PRIMM, M.E.D. USA, San Antonio, TX, USA) is composed of alumina and amorphous silica (25 wt% Al2O3, 75 wt% SiO2) fibers that are fused into a 3-D ‘scaffold-like’ structure (Fig. 1).

Jones et al. [12] measured the effects of fused-fiber filler (FFF) on the biaxial flexure strength and fracture toughness of two commercial dental composites. In this study, two hybrid composites, [Herculite XRV and Prodigy (Kerr, Orange, CA, USA)], were modified (0, 5, 7, 9, 11 wt%) with additions of FFF. Filler additions actually decreased the biaxial flexure strength and fracture toughness values of modified vs unmodified composites. These results were attributed to poor incorporation of the FFF into the composite, inadequate bonding between the FFF and the resin matrix, break-up of the network structure during mixing, and the presence of porosity in the final microstructure. Lower mechanical strength of the modified composites implied that no stress transfer to the FFF had occurred.

A study by Thompson and Bayne [13] investigated the effect of FFF additions on the indented biaxial flexure strength and fracture toughness of two commercial glass-ionomer cements. In this study, the incorporation of FFF was greatly facilitated by the low viscosity (when compared to dental composites) of the glass-ionomer cements. Both mean indented biaxial flexure strength and fracture toughness were improved for each glass-ionomer (Vitremer, 3M, St. Paul, MN; Fuji Duet, GC America, Alsip, IL, USA) with additions of FFF. Improvements in these properties were close to that expected by a simple rule-of-mixtures effect due to increased filler content. However, there was still some evidence that the FFF structure was broken up during incorporation, and that porosity was produced during mixing.

Zarb et al. [14] investigated the effect of FFF additions on the biaxial flexure strength and fracture toughness of dental luting cements. That study reported that FFF did not appear to be wetted by zinc phosphate cement, but produced positive effects in polycarboxylate and glass ionomer polymeric cements for which the liquid component was very similar. The most interesting finding was a ∼70% increase in biaxial flexure strength displayed by GI specimens that were modified with FFF. Each of the initial studies indicates that incorporation of FFF directly into resin-based dental restorative materials produces uneven results at best. Therefore, another approach for incorporation of ‘scaffold-like’ filler is indicated.

Rapp et al. [15] has reported the initial development of pre-polymerized fused-fiber filler modified composites (PP-FFMC). In this study, macroscopic blocks (10×10×1.5 mm) of FFF material were impregnated under vacuum with uncured composite, pre-polymerized, and ground into macro-filler particles (40–125 μm diameter). The composite material used to impregnate the FFF blocks was mixed in the lab using a light-activated monomer blend and silanated quartz filler particles. The light-activated monomer blend and filler particles used were very similar to those used in commercial composite materials. Subsequent SEM analysis indicated that moderate filler content composite (10–30 wt% filler) could be drawn into the FFF blocks. Additionally, SEM analysis of the fracture surface of a commercial dental composite (Revolution, Kerr, Orange, CA, USA) modified with PP-FFMC particles showed transgranular fracture through the modifying particles. This indicates good coherent bonding between the PP-FFMC particles and the commercial resin.

The present study looks to further the work with PP-FFMC particles in two ways. First, the method used to impregnate the macroscopic blocks of FFF will be investigated to determine if a different method is more suitable. The method described by Rapp et al., while effective at producing low-porosity PP-FFMC particles is labor intensive and time consuming. A different method of impregnation, immersing FFF slabs in heated, uncured composite, will be considered. The heated resin could be drawn into the network structure by capillary action. If shown successful, this impregnation route would expedite PP-FFMC production.

The second goal of this study was to determine the effect of PP-FFMC particles on the wear and mechanical properties of dental composites. PP-FFMC particles were blended into custom composites that had resin and filler compositions that were designed to simulate typical commercially available dental composites. Wear and mechanical tests were performed on these materials to determine the effect of PP-FFMC additions on the conventional composites.

Section snippets

Impregnation method

FFF blocks (PRIMM, M.E.D. USA, San Antonio, TX, USA), which had been previously silane treated, were impregnated with a custom-made composite consisting of 50 wt% trimethylol propane trimethacrylate (TMTP, Polysciences, Warrington, PA, USA) 40 wt% small particle filler (0.9 μm silanated quartz, ESPE, Germany), and 10 wt% microfiller (0.04 μm silanated colloidal silica, ESPE, Germany). The FFF material has a composition that is approximately 75 wt% SiO2 fibers, 25 wt% Al2O3 fibers. Two different

Impregnation method

The results from the three-point fracture tests are shown in Table 2. The fracture test results showed that all filled sample groups showed significantly higher mean flexural strength and modulus than that of the unfilled control. The mean flexural strengths and modulii for the FFF impregnated with composite by vacuum and with unfilled resin were significantly greater than that for the FFF impregnated with composite by immersion. The group impregnated by vacuum showed a higher mean flexural

Impregnation method

The results from three-point bending tests clearly show that immersion is not the most suitable method for impregnating composite into the FFF material. A possible reason for this is seen in Fig. 2, which shows that the composite was not completely drawn into the FFF network, leaving large voids. These voids, which do not appear in the vacuum impregnated FFF, would then be responsible for the lower flexural strength. It is possible that the composite is too viscous to adequately penetrate the

Acknowledgements

This study was supported by NIH-NIDR DE12816-01, and by contributions from M.E.D USA, ESPE, and Kerr. This study is based on work presented at the 1999 annual meeting of the Academy of Dental Materials which received the Paffenbarger award.

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