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

dental materials Dental Materials 18 (2002) 72±80

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Effect of novel ®ller particles on the mechanical and wear properties of dental composites D.E. Ruddell a, M.M. Maloney b, J.Y. Thompson a,b,c,* a

Curriculum in Applied and Materials Sciences, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA b Department of Biomedical Engineering, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA c Department of Operative Dentistry, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA Received 3 May 2000; revised 8 December 2000; accepted 30 January 2001

Abstract Objectives: The purpose of this study was to investigate the method of producing pre-polymerized fused-®ber ®ller modi®ed composite (PP-FFMC) particles and the effectiveness of incorporating these novel ®ller particles into dental composites. Methods: Fused-®ber ®ller (FFF) blocks were impregnated with composite by two different methods. Three-point ¯exure 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 ¯exure 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 ¯aws observed in the novel particles are the likely explanation for the observed strength and toughness values. Signi®cance: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. q 2002 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. Keywords: Composites; Wear; Mechanical properties; Fracture toughness; Strength; Filler

1. 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 dif®cult 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 ®lling 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, forti®ed glass ionomers, and * Corresponding author. Tel.: 11-919-966-4594; fax: 11-919-966-5660. E-mail address: [email protected] (J.Y. Thompson).

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.

0109-5641/02/$22.00 + 0.00 q 2002 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. PII: S01 09- 5641(01)0002 2-7

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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 sacri®cing 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 signi®cant advances in improving the properties of polymer matrix materials, recent improvements in dental composite properties are due primarily to advances in ®ller technology [5]. Given equal levels of ®ller loading, strengthening mechanisms in composites depend primarily on the geometry and size of the dispersed ®ller phase. In general, composite materials can be either particle reinforced (random or preferred orientation) or ®ber/whisker reinforced (single or multi-layer, continuous or discontinuous ®ber, 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 ®bers 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 `scaffoldlike' 3-D ®brous 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 DeÂrand [10] and Ritsco [11], use of such ®ller particles showed notable effects in reducing polymerization shrink-

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age and improving wear resistance. The hypothesized shrinkage reduction mechanism is due to con®nement of shrinkage to the microscopic spaces between ®bers in the 3-D network [10]. Enhanced wear resistance is attributed to the porous structure's ability to anchor the ®ller particle in the matrix, resisting the `plucking out' of particles commonly observed in composite wear [8]. Although these novel fused-®ber ®llers show great promise, a good deal of further research is needed to optimize this technology. Several research projects investigating the use of fused®ber ®ller have been undertaken. Initial efforts focused on incorporation of silanated fused-®ber ®ller (FFF) particles into existing dental materials. The ®ller material that has been investigated is derived from a product developed for aerospace applications. The modi®ed fused ®ber ®ller (PRIMM, M.E.D. USA, San Antonio, TX, USA) is composed of alumina and amorphous silica (25 wt% Al2O3, 75 wt% SiO2) ®bers that are fused into a 3-D `scaffold-like' structure (Fig. 1). Jones et al. [12] measured the effects of fused-®ber ®ller (FFF) on the biaxial ¯exure strength and fracture toughness of two commercial dental composites. In this study, two hybrid composites, [Herculite XRV and Prodigy (Kerr, Orange, CA, USA)], were modi®ed (0, 5, 7, 9, 11 wt%) with additions of FFF. Filler additions actually decreased the biaxial ¯exure strength and fracture toughness values of modi®ed vs unmodi®ed 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 ®nal microstructure. Lower mechanical strength of the modi®ed composites implied that no stress transfer to the FFF had occurred. A study by Thompson and Bayne [13] investigated the

Fig. 1. Scanning electron micrograph of an un®lled slab of fused-®ber ®ller showing the open, network structure of the material.

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effect of FFF additions on the indented biaxial ¯exure strength and fracture toughness of two commercial glassionomer 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 ¯exure 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 ®ller 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 ¯exure 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 ®nding was a ,70% increase in biaxial ¯exure strength displayed by GI specimens that were modi®ed 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' ®ller is indicated. Rapp et al. [15] has reported the initial development of pre-polymerized fused-®ber ®ller modi®ed 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-®ller particles (40±125 mm diameter). The composite material used to impregnate the FFF blocks was mixed in the lab using a light-activated monomer blend and silanated quartz ®ller particles. The light-activated monomer blend and ®ller particles used were very similar to those used in commercial composite materials. Subsequent SEM analysis indicated that moderate ®ller content composite (10±30 wt% ®ller) could be drawn into the FFF blocks. Additionally, SEM analysis of the fracture surface of a commercial dental composite (Revolution, Kerr, Orange, CA, USA) modi®ed with PPFFMC 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 PPFFMC 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 success-

ful, 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 ®ller 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. 2. Materials and methods 2.1. 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 ®ller (0.9 mm silanated quartz, ESPE, Germany), and 10 wt% micro®ller (0.04 mm silanated colloidal silica, ESPE, Germany). The FFF material has a composition that is approximately 75 wt% SiO2 ®bers, 25 wt% Al2O3 ®bers. Two different impregnation methods were used. The ®rst involved initially heating the composite to 508C then pulling the composite through the FFF slab (1 mm thick) with vacuum for 3 min. In the second method, the composite was also heated to 508C, but instead of using vacuum, the FFF slab was immersed in the heated composite for 12 h. FFF slabs were also infused with un®lled TMPT by this method. After impregnation, the slabs were covered by Mylar slips and visible light cured (Optilux 401 Curing Light, Demitron Co., Danbury, CT, USA) for 40 s on each side. Surfaces were polished through 1200 grit SiC abrasive. Bars (12 £ 2 £ 1 mm) were cut from cured samples. Bars (of the same dimensions) were also cut from unimpregnated FFF blocks to be used as controls. The bars were fractured in an Instron testing machine (Model 4411, Canton, MA, USA) using a three-point ¯exure ®xture (8 mm span, 0.5 mm/s cross-head speed). 2.2. Mechanical properties FFF blocks were impregnated by the vacuum method detailed above with one of two compositions. The ®rst consisted of 40 wt% TMPT with 60% small particle ®ller. Particles generated from this composition were designated PP-FFMC(A). The second composition was 50 wt% TMPT with 40 wt% small particle ®ller and 10 wt% micro®ll. Particles generated from this composition were designated PP-FFMC(B). After impregnation and curing, the ®lled blocks were ground using a mortar and pestle. A sonic sifter (ATM Corporation, Milwaukee, WI, USA) was used to obtain particles in the desired size range of 30±75 mm. The basic monomer mixture used for the conventional

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composites was a light activated, 50 wt% bis-GMA/50 wt% TEGDMA blend. Camphorquinone (CQ) and 2-(N,Ndimethylamnio) ethyl methacrylate (DMAEMA) were added in amounts of 0.79 and 0.50 wt%, respectively. Two conventional composites (CC) were produced through additions of small particle ®ller, or a combination of micro®ller and small particles (hybrid composite). The ®rst CC had a composition of 40 wt% Bis-GMA/TEGDMA with 60 wt% small particle ®ller, and was designated CC-A. The hybrid composite contained 50% Bis-GMA/TEGDMA with 40 wt% small particle ®ller and 10 wt% micro®ll. This was designated CC-B. These composites were modi®ed by additions of the PP-FFMC ®ller to produce the matrix of modi®ed CC (MCC) compositions shown in Table 1. Mixing of the composites was accomplished by a lowspeed, high torque mixer that was constructed by the authors. The composites were mixed at 20 rpm for 60 min each. In addition, an unmodi®ed commercially available hybrid composite (Prodigy, Kerr, Orange, CA, USA) was used as an additional control. Biaxial ¯exure strength and fracture toughness were determined using a controlled-¯aw strength method. Disk specimens (9 mm diameter £ 1 mm thick) of each composite material were formed in molds, and cured on each side for 40 s. Each disk was polished through 1200-grit SiC abrasive on the side to be subjected to tension during ¯exure testing, and through 400-grit on the opposite side. After polishing, each disk was indented on the ®nely polished side with a load of 9.8 N (1000 g). The indented disks (n ˆ 10/composition) were then fractured using a pistonon-three-ball biaxial ¯exure ®xture with the indented side subjected to tensile stresses. The radial distance from the center of the disk to the center of the balls was 4 mm. The diameter of the loading piston was 0.78 mm. Testing was done in air at a loading rate of 0.5 mm/min. The biaxial ¯exure strength (BFS) of each indented disk was determined using the failure load. The fracture toughness (KIC) of each specimen was determined from the indentation load (P) and BFS (s f), using the relationship developed by Chantikul et al. [16]: K IC ˆ 0:59…E=H†1=8 …s f P1=3 †3=4 where E/H is the ratio of elastic modulus to hardness. The E/ H ratio was determined for each material by use of a technique developed by Marshall et al. [17]. Knoop hardness indentations were used to estimate the E/H ratio of each material. The load for each indentation was 9.8 N, with a dwell time of 60 s. Previous studies by Leinfelder et al. [18] have demonstrated excellent agreement between wear data obtained from simulations on the Leinfelder-type wear device and clinical research results. In this study, a Leinfelder-type device was used to simulate contact free area wear. Wear simulation was performed by placing specimens of each different composition in a block of ceramic (Dicor MGC,

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Table 1 Composition of modi®ed conventional composites Composite material

CC-A CC-B MCC 1 MCC 2 MCC 3 MCC 4 MCC 5 MCC 6 MCC 7 MCC 8

Weight (%) CC-A

CC-B

PP-FFMC(A)

PP-FFMC(B)

100 ± 95 90 95 90 ± ± ± ±

± 100 ± ± ± ± 95 90 95 90

± ± 5 10 ± ± 5 10 ± ±

± ± ± ± 5 10 ± ± 5 10

Caulk/Dentsply, Milford, DE, USA). A simulated cavity preparation 4.0 mm in diameter and 3.0 mm in depth was used. The ceramic was treated with a primer (Scotchbond, 3M, St Paul, MN, USA) and uncured material of each composition were packed into the simulated cavity preparation and cured for 60 s. The surface was ground through 1200-grit SiC abrasive so that a continuous smooth surface existed across the ®lled simulated cavity preparation. The entire assembly that holds the ceramic blocks was encased in a water bath, and ®lled with a slurry of deionized H2O and unplasticized PMMA beads (44 mm diameter) that simulated a food bolus. An 8.0 mm diameter polyacetal stylus was attached to each of the pistons. Each stylus was then loaded vertically, at a load of 75 N, onto the surface of the composite specimen at a rate of 1.2 cycles/s. Four samples of each composition were subjected to wear testing. Quantitative wear measurements were made using a surface pro®lometer (Surfanalyzer 5000, Federal Products, Providence, RI, USA). Four scans were performed (at 458 angles) on each wear specimen, resulting in a sample size of 16 scans for each composition. The travel length of the stylus was 5 mm. Fracture specimens and PP-FFMC particles were examined under SEM (JEOL, JSM-6300FV, Mahwah, NJ). Results from wear and mechanical testing were analyzed by ANOVA and Duncan multiple range comparison tests (P # 0.05) to determine statistical groupings.

3. Results 3.1. Impregnation method The results from the three-point fracture tests are shown in Table 2. The fracture test results showed that all ®lled sample groups showed signi®cantly higher mean ¯exural strength and modulus than that of the un®lled control. The mean ¯exural strengths and modulii for the FFF impregnated with composite by vacuum and with un®lled resin were signi®cantly greater than that for the FFF impregnated

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with composite by immersion. The group impregnated by vacuum showed a higher mean ¯exural strength than the un®lled resin group, but the difference was not signi®cant at the level chosen for the statistical analysis. SEM analysis indicated that all three impregnation methods ®lled the FFF with no macroscopic porosity, however there was some evidence that penetration of composite into the FFF block was incomplete with immersion (Fig. 2). 3.2. Mechanical properties Tables 3 and 4 contain the wear test results. In all cases, the amount of wear is signi®cantly higher for the unmodi®ed composites than the composites modi®ed with PP-FFMC particles. With the exception of MCC-2 and MCC-8, the modi®ed composite compositions did not show wear signi®cantly different from that of Prodigy. Within each group of materials (based on the two CC compositions) there are some signi®cant differences in wear based on the level and/or type of PP-FFMC particles, but these differences were not consistent across the groups. The biaxial ¯exure strength (BFS) and fracture toughness data, also contained in Tables 3 and 4, show that the mechanical properties of the two conventional composites

were better than those of the modi®ed composites. Those compositions blended with 10 wt% PP-FFMC (B) particles (MCC-4, MCC-8) showed signi®cantly lower mechanical properties than the other modi®ed composites within their respective groups. SEM showed that the PP-FMC particles were homogenously mixed into the conventional composites. Additionally, SEM showed that fracture was transgranular [Fig. 3(a)], and that the interface between the conventional composite and PP-FFMC particles was continuous [Fig. 3(b)]. Isolated PP-FFMC particles showed the formation of microcracks in some of the PP-FFMC particles (Fig. 4), as well as some evidence that bonding between the FFF and the impregnating composite was not complete (Fig. 5). 4. Discussion 4.1. 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.

Table 2 Flexural strength and ¯exural modulus of FFF blocks impregnated by various methods (superscripts (a,b,c) indicate specimens belonging to statistically equivalent groupings) Filler (method)

Mean ¯exural strength (MPa)

Mean ¯exural modulus (GPa)

Un®lled control Composite (immersion) Un®lled TMPT (immersion) Composite (vacuum)

4.22 (0.4) a 20.26 (2.1) b 40.22 (6.7) c 46.34 (3.2) c

0.32 (.03) a 2.62 (.30) b 4.47 (.49) c 4.25 (.55) c

Table 3 Wear of and mechanical properties of composites based on CC-A (superscript letters (a,b,c,d) within each property represent statistically equivalent means) Material

BFS (MPa)

Fracture toughness (MPa z m 1/2)

Wear 400 K cycles (mm)

CC-A MCC-1 MCC-2 MCC-3 MCC-4 Prodigy

155 (9) b 119 (4) c 115 (9) c 113 (5) c 103 (8) d 194 (7) a

2.02 (0.11) b 1.65 (0.05) c 1.61 (0.10) c 1.59 (0.06) c 1.48 (0.09) d 2.22 (0.08) a

19 (9) a 12 (4) b,c 13 (4) b 10 (1) b,c 12 (1) b,c 8 (3) c

Table 4 Wear of and mechanical properties of composites based on CC-B (superscript letters (a,b,c,d) within each property represent statistically equivalent means) Material CC-B MCC-5 MCC-6 MCC-7 MCC-8 Prodigy

BFS (MPa) b

146 (9) 112 (8) c 111 (8) c 113 (5) c 102 (5) d 194 (7) a

Fracture toughness (MPa z m 1/2) b

1.94 (0.10) 1.59 (0.10) c 1.58 (0.09) c 1.60 (0.05) c 1.48 (0.05) d 2.22 (0.08) a

Wear 400 K cycles (mm) 14 (3) a 7 (4) c 9 (2) b,c 6 (3) c 11 (1) b 8 (3) c

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Fig. 2. Scanning electron micrograph of a FFF slab impregnated with composite by immersion. The lack of complete ®lling is evident.

These voids, which do not appear in the vacuum impregnated FFF, would then be responsible for the lower ¯exural strength. It is possible that the composite is too viscous to adequately penetrate the FFF network without the aid of vacuum. Using thinner FFF blocks or a longer immersion time may remedy this. The un®lled TMPT did not have this problem, and showed similar ¯exural strength to the vacuum impregnated FFF. The vacuum impregnated blocks showed a ¯exural strength that was at least as high as those immersed in un®lled resin. The numerical values suggested that there may be some advantage to utilizing the vacuum method. For this reason, the vacuum impregnation method was chosen to produce the PP-FFMC particles used in the second part of this study. 4.2. Mechanical properties The results from the fracture and wear tests appear to be contradictory at ®rst glance. It is expected that a material with better fracture properties will also possess better wear properties. That is not the case in this study; all the conventional composites have better biaxial ¯exure strength and fracture toughness, but exhibit higher wear rates. The improved wear resistance exhibited by the modi®ed composites is most likely due to two factors. The relatively large PP-FFMC particles, which are well incorporated into the composite matrix, resist plucking-out better than conventional ®ller. In addition, the presence of Al2O3 ®bers in the FFF material increases the wear resistance compared to having only SiO2 ®ller. Previous investigators [19] were not able to mix PPFFMC particles into a commercial composite without the

concurrent introduction of large pores, which were blamed for the subsequent decrease in mechanical properties. Those pores are absent from the modi®ed conventional composites used in this study. This study used a monomer mixture with a lower viscosity, as well as lower ®ller levels. The combination of a less viscous composite with blending at an elevated temperature resulted in the elimination of mixing pores. Figs. 3 and 4 show that the fracture of the modi®ed composites is transgranular and that the interface between the composite matrix and the PP-FFMC particles is continuous and coherent. This indicates that the drop in mechanical properties is not likely attributable to poor bonding between the reinforcing particles and the composite matrix, otherwise one would expect to see the fracture go around the PP-FFMC particles, with a fracture surface showing pullout. The most likely cause of the reduced mechanical properties of the modi®ed composites in comparison to the conventional composites lies in the particles themselves. Fig. 4 shows microcracking that is present in some particles. These cracks are likely to have been introduced when the impregnated FFF slabs were ground into particles. Fig. 5 shows incomplete wetting of the FFF material by the impregnating composite. Both of these phenomena would lower the ¯exural strength and fracture toughness of any modi®ed composite by providing a lower energy path for crack propagation. Because of the ¯aws present in the PP-FFMC particles, the conventional composites are, in effect, being `reinforced' with a weaker material, at least in respect to toughness and ¯exural strength. Further work is required to devise a method of reducing impregnated FFF blocks to smaller particles that does not introduce microcracks.

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Fig. 3. Scanning electron micrographs of the fracture surface of a modi®ed conventional composite. (a) A PP-FFMC particle incorporated into the conventional composite matrix. The fracture has proceeded through the PP-FFMC particle. (b) A higher magni®cation micrograph of the same surface, showing the coherent, continuous interface between the PP-FFMC particle and the conventional composite matrix.

The improved wear resistance exhibited by the modi®ed composites is most likely due to two factors. The relatively large PP-FFMC particles, which are well incorporated into the composite matrix, resist plucking-out better than conventional ®ller. In addition, the presence of Al2O3 ®bers in the FFF material increases the wear resistance compared to having only SiO2 ®ller.

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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Fig. 4. Scanning electron micrograph of an isolated PP-FFMC(A) particle. Microcracking is evident within the particle.

Fig. 5. Scanning electron micrograph of a single PP-FFMC(A) particle. An incoherent interface between the fused-®ber ®ller and the resin used to impregnate the material is indicated.

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