Wear behavior of light-cured dental composites filled with porous glass–ceramic particles

J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M AT E R I A L S

3 (2010) 77–84

available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/jmbbm

Research paper

Wear behavior of light-cured dental composites filled with porous glass–ceramic particles Yanni Tan a , Yong Liu a,∗ , Liam M. Grover b , Baiyun Huang a a State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan, 410083, PR China b School of Chemical Engineering, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK

A R T I C L E

I N F O

A B S T R A C T

Article history:

Wear resistance is still perceived to be one of the most important limiting factors in the

Received 16 December 2008

long-term performance of dental restorations. Consequently, a range of different materials

Received in revised form

have been used as filler particles to reduce the rate of wear, particularly in posterior

9 April 2009

restorations. In this study, novel bioactive glass–ceramic powders exhibiting different

Accepted 10 April 2009

nominal calcium–mica to fluorapatite ratios were used as fillers for light-cured dental

Published online 3 May 2009

composites. Wear tests on the resulting samples were undertaken using a micro-tribometer with a linear reciprocating ball-on-flat geometry using lubrication from artificial saliva. The surfaces of the worn composites were then evaluated using optical microscopy. In order to enhance matrix bonding, the surfaces of the different particulates were treated using hydrofluoric acid to provide a porous surface and the resulting surface morphology was evaluated using scanning electron microscopy. Although in the case of the samples containing low fluorapatite contents (20 wt%; A2), surface etching enhanced the wear resistance of the composite, etching reduced the wear resistance of materials containing 50 wt% fluorapatite (A5). The reduction in wear resistance was attributed to the friability of the A5 particles following surface treatment. This suggests that in order to optimize wear resistance, it is important to find a critical balance between surface roughness and porosity and the strength of individual particles. c 2009 Elsevier Ltd. All rights reserved.

1.

Introduction

Since the dental composite was invented in 1962 by Bowen, considerable work has been devoted to expanding their application and clinical longevity. Although dental composites are easy to handle and are relatively costeffective, there are still problems such as high polymerization shrinkage, low wear resistance and occurrence of secondary caries, which prevent these materials from being used under ∗ Corresponding author. Tel.: +86 731 8836939; fax: +86 731 8710855. E-mail address: [email protected] (Y. Liu). c 2009 Elsevier Ltd. All rights reserved. 1751-6161/$ - see front matter doi:10.1016/j.jmbbm.2009.04.004

high majority loads (e.g., posterior teeth) and large area restorations (Sarrett, 2005). Much effort has been made to improve the wear resistance of composites, including the development of new resins to improve the degree of conversion, water absorption and polymerization shrinkage (Lai et al., 2004; Pereira et al., 2002; Wu and Nie, 2006); and refining fillers by changing the filler size, size distribution and morphology (Atai et al., 2007; Karbhari and Strassler, 2007; Turssi et al., 2005; Xia et al., 2008).

78

J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M AT E R I A L S

The bonding between filler particles and resins has a significant effect on the wear resistance of composite materials. The concept of obtaining improved bonding through micromechanical interlocking of the two phases was first proposed by Bowen and Reed in 1976, as a means of overcoming the limitations of coupling agents (Luo et al., 1998). They made two-phase etchable glasses into semiporous fillers. Other research used porous ceramic particles (containing leucite crystals) as fillers to improve the flexure strength exhibited by dental composites (Zandinejad et al., 2006; Liu et al., 2009). Porous ceramic fillers can be made by selective etching of glass–ceramics, which usually consist of a glass matrix and crystalline phases. Nanoporous silica gel powder has also been used as a filler in polymeric composites and the wear resistance has been investigated (Luo et al., 1998). Many other efforts have been focused on developing new fillers to reinforce dental composites, such as glass fibers, ceramic whiskers, or fused-fiber filler particles. Each of the methods may individually affect both polymerization shrinkage and wear resistance to a different degree (Chand et al., 2000; Ruddell et al., 2002; Xu et al., 1999). The processes involved in composite wear are complex and poorly understood. There are two methods to analyze wear behavior of dental composites: in vivo and in vitro methods. The in vitro methods are used more often as they take less time, are simpler, effective and of low-cost. But there is not a test standard for the wear process. Among the published studies, large differences can be observed on the wear simulating devices, contact conditions, counterface material and abrasive agents (Chand et al., 2000; Luo et al., 1998; Palin et al., 2005; Ramalho and Antunes, 2005; Xu et al., 2004; Yu et al., 2006; Zheng and Zhou, 2007). Concerning the type of wear test, ball-on-flat geometry and reciprocating are the most used (Ramalho and Antunes, 2005; Yu et al., 2006; Zheng and Zhou, 2007). To date no wear resistance data from composites formulated using the microporous glass–ceramic fillers have been reported. In this study, the wear behavior of dental composites filled with porous fluorapatite glass–ceramic particles was investigated. The wear test selected was of the reciprocating type, with a ball-on-flat geometry.

2.

Materials and methods

Two glass–ceramics compositions were used, with nominal mica-to-fluorapatite ratios of 80:20 (A2) and 50:50 (A5) by weight. The chemical compositions were calculated according to the mass ratio compositions. The SiO2 , Al2 O3 , MgO, NaF, CaF2 , CaCO3 and CaHPO4 reagents were mixed, and then melted in an alumina crucible at 1500 ◦ C for 2 h and quenched at room-temperature in double distilled water. The quenched glass was annealed at 650 ◦ C for 1 h and then quickly heated to 925 ◦ C for 12 h to cause crystallization. After cooling, the glass–ceramics were ground into powders and passed through a 200 mesh (74 µm) sieve for the further study. Glass–ceramic particles were etched by using 10 vol% hydrofluoric acid for 30 min to make them porous. Both dense particles and porous particles were pretreated with the coupling agent γ-methacryloyloxypropyl-1-trimethoxysilane

3 (2010) 77–84

Table 1 – The composition of artificial saliva. NaCl

KCl

CaCl2 · 2H2 O

NaH2 PO4 · Na2 S· 2H2 O 2H2 O

Urea

Distilled H2 O

0.4 g

0.4 g

0.795 g

0.78 g

1g

1000 ml

0.005 g

(γ-MPS) to improve the bonding between fillers and resins. The particles were mixed with the solution using a magnetic stirring apparatus for 2 h. The treated fillers were then dried at 120 ◦ C. The resin system used for all samples consisted of 49.25 wt% Bis-GMA, 49.25 wt% TEGDMA, 0.25 wt% camphorquinone (CQ) and 0.25 wt% N,N0 -dimethyl aminoethyl methacrylate (DMAEMA). The latter two reagents were used as photo initiators. γ-MPS, Bis-GMA, TEGDMA, CQ and DMAEMA used in this work were produced by Sigmaaldrich Chemie Ltd, Germany. Porous glass–ceramic particles were placed in a bespoke airtight container under vacuum. The resins, which had been mixed in advance, were poured into the container and then stirred for 30 min. The mixture was filled into a steel mould with size of 13 mm × 10 mm × 3 mm, and then photopolymerized for 2 min with a dental light-curing source. After curing, the samples were dried at 37 ◦ C and held for 4 h, the samples were then polished using 600-grit, 800-grit, 1000grit, 1200-grit abrasive paper successively and then cleaned using distilled water and acetone. The specific surface area of the porous fillers was determined as an indication of the degree of porosity (Monosorb-type, USA). The crystalline phases in the fillers were determined by X-ray diffraction (D/ max -2550VB+ , Cu Kα, 50 KV, 20 mA, Japan). The microstructures of particles were observed using a scanning electron microscope (SEM, JSM-5600LV, Japan). The wear test was performed on a Micro-Tribometer (UMT-3, CETR Co., USA) by using the linear reciprocating ball-on-flat geometry (Fig. 1). A chrome steel ball with the hardness of 62HRC and diameter of 9.5 mm was used as the counter part due to its relative high hardness. Generally, during the chewing process in humans, the masticatory force in the oral cavity ranges from 3 to 36 N (Ramalho and Antunes, 2005). So in this work, a normal load of 20 N was used, at the chewing frequency f = 1.2 Hz and the reciprocating amplitude D = 2 mm. The total cycle number was 5000. In order to better simulate the oral cavity environment, samples were lubricated with artificial saliva (provided by the Department of Stomatology, Xiangya Second Hospital, China), which was injected between the surfaces using a syringe with a hypodermic needle. The composition of artificial saliva is shown in Table 1. For each test condition, three samples were used. Wear damage was represented by wear volume, wear depth or wear width (Haseeb et al., 2008). In this study, the width of the wear track was taken, which was measured on the metallograph by using the e-rule. Every wear track following 5000 cycles was measured 6 times to avoid individual differences and then averaged. The friction coefficients and the final wear width of each group were decided by averaging the original data of three samples. After wear tests, the surface of dental composites was observed by optical microscopy (OM, Leica MeF3A)

J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M AT E R I A L S

79

3 (2010) 77–84

Force

Roller

Chrome Steel Ball Resin Specimen Hold-Down

Steel Mould

Oscillating Drive

Amplitude Fig. 1 – The schematic diagram of reciprocating wear test.

and SEM (JSM-5600LV, Japan). Before SEM observation, all samples were sputter-coated with gold. According to Contact Mechanics, the Hertzian contact diameter d was calculated in accordance with the following equations (Johnson, 1987):  d = 2a = 2 With :

3FR 4E∗

1

3

1 − ν21 1 − ν22 1 = + E∗ E1 E2

where F is the load; a the contact radius; R the steel ball radius; E1 , E2 the elastic moduli (E1 = 210 GPa, E2−A2 = 6.28 GPa, E2−A5 = 12.92 GPa) and ν1 , ν2 the Poisson’s ratio associated with chrome steel ball and dental composites respectively (ν1 = 0.3, ν2 = 0.33) as found from the literatures (Papadogiannis et al., 2008; Sideridou et al., 2003; Xiang, 2007). As the elastic modulus of the porous filler is available form the literature, only the contact parameters of dense filler reinforced composites were calculated. And also the other contact parameters were also calculated as defined below according to Hertzian theory (Johnson, 1987; Silva et al., 2008). Mean contact pressure: Pm = F2 πa

Contact strain as: Ra Means and standard deviations (S.D.) of all the experimental groups data reported were calculated. The results were statistically analyzed by one-way analysis of variance (ANOVA) at a significance level of 0.05.

3.

Results

After heat treatment, the A2 particles consisted of fluorapatite (Ca5 (PO4 )3 F), calcium–mica (Ca0.5 Mg3 AlSi3 O10 F2 ) and nepheline (NaAlSiO4 ) crystals, while A5 particles consisted of only fluorapatite (Ca5 (PO4 )3 F) and nepheline (NaAlSiO4 ) crystals, as indicated in Fig. 2. After etching, the main phases do not change for both samples, but the contents of nepheline

Table 2 – Specific surface areas of porous fillers (SBET). Fillers

SBET (m2 /g)

A2-Dense A2-Porous A5-Dense A5-Porous

0.25 1.28 0.09 13.26

and mica crystals seem to decrease as revealed by their corresponding X-Ray diffraction patterns. It is interesting to note that the morphology of the glass–ceramics after etching depends on their sizes. Before being ground, both A2 and A5 particles showed clear needle-like fluorapatite crystals, and the crystals seem to be more perfectly aligned in A5, as shown in Fig. 3. In etched small particles, however, needle-like fluorapatite crystals could not be clearly observed. Only at high magnification, round or short fluorapatite crystals can be seen. It means that the fluorapatite crystals also fractured during mechanical failure of the large particles. Fig. 4(a), (b) show the dense structure of A5 and A2 particles before etching. Fig. 4(c) shows the highly porous structure of A5 particles after etching. A2 particles also indicate a highly porous structure after etching (Fig. 4(d)). And the micropores do not close following silanization (Fig. 4(e), (f)). Table 2 shows the specific surface area results of the individual materials. It can be seen that the specific surface area increased significantly after etching and A5 particles exhibited a higher specific surface area than A2 particles. Thus, A5 particles are coarser than A2 before etching, and A5 particles are more highly porous than A2 particles after etching. The average friction coefficients (FC) of the samples containing different fillers, as a function of time, are shown in Fig. 5. For each group, the FC reached a steady state after 3000 cycles. From the shape of the friction curves, it can be seen that dental resins filled with dense A5 particles and porous A2 particles have a rather steady friction state. The steady friction coefficients calculated from the last

J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M AT E R I A L S

a

1500

Intensity(Counts)

80

1000

3 (2010) 77–84

Fluorapatite Nepheline Ca-mica

A2-Dense

500 A2-Porous

0 10

20

30

40

50

60

70

80

90

2-Theta(°)

b Intensity(Counts)

1500

Fluorapatite Nepheline

A5-Dense

1000

500

A5-Porous

0 10

20

30

40

50

60

70

80

2-Theta(°)

Fig. 2 – XRD patterns of A2 and A5 powder before and after etching (a) A2 powder; (b) A5 powder.

a

b

Fig. 3 – Needle-like fluorapatite crystals appear in as-quenched A2 and A5 specimens after etching (a) A2 specimen; (b) A5 specimen.

1000 s are shown in Table 3. The steady friction coefficients of composites containing A5 fillers were higher than the composites containing A2 fillers. Figs. 6 and 7 show the wear surface of four kinds of dental composites. For all samples, obvious parallel furrow-like wear scars and the polishing effect are observed (Fig. 6). But the degrees of wear damage are different from one another. It can be seen that composites containing A5 porous fillers sustained more wear damage than others. The damage of A5-porous composites was mainly due to fracture of the

reinforcing particles and their removal from the resin matrix (Fig. 7(d)). There existed obvious pores in the A5-porous composites (Fig. 6(d)), that could seldom be seen in other samples. From Fig. 7(a), it could be noted that there were gaps evident between A2 dense fillers and resins and many cracks across the particles. For the chrome steel ball used as the counter part, there was no measurable wear due to the higher hardness of chrome steel compared with the resin composites.

J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M AT E R I A L S

a

b

c

d

e

f

81

3 (2010) 77–84

Fig. 4 – Morphology of A2 and A5 powders before and after etching (a) A5 before etching; (b) A2 before etching; (c) A5 after etching; (d) A2 after etching; (e) A5 after silanization; (f) A2 after silanization.

Table 3 – Steady state friction coefficient of samples. Samples

Friction coefficient

A2-Dense A2-Porous A5-Dense A5-Porous

0.222 ± 0.002 0.207 ± 0.002 0.277 ± 0.002 0.290 ± 0.001

Table 4 shows the average wear track width following 5000 cycles and the Hertzian contact parameters of A2-dense

and A5-dense composites. The ANOVA analysis revealed that there was a significant difference (P < 0.05) in the wear track widths. For the composites containing A2 fillers, the average wear track width of A2-porous composites was significantly smaller than that of dense composites. But for the composites containing A5 fillers, the result was reversed: the average wear track width of A5-porous composites was much larger than that of the dense composites. A2-porous composites exhibit a comparable average width of wear track with the A5-dense composites. The Hertzian contact diameter is much smaller than the wear width.

Table 4 – The average width of the wear track on samples and the Hertz contact parameters of A2 and A5-Dense samples. Samples

Width of wear track (mm)

A2-Dense A2-Porous A5-Dense A5-Porous

0.781 ± 0.095 0.608 ± 0.048 0.593 ± 0.057 0.824 ± 0.075

Hertzian contact diameter (mm) 0.437 – 0.347 –

Mean contact pressure (GPa) 0.133 – 0.211 –

Contact strain 0.046 – 0.037 –

82

J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M AT E R I A L S

0. 35

0.30 Friction Coefficent

A5-Porous A5-Dense

0.25 A2-Dense A2-Porous

0.20

0.15

0.10

0

1000

2000

3000

4000

5000

Time/sec

Fig. 5 – Friction coefficient as a function of time for different dental composites.

4.

Discussion

Wear resistance is very important to dental resins, especially when being used in heavily-loaded areas. The literature

a

3 (2010) 77–84

suggests that failure rates are higher for larger restorations, and that wear may still be a significant mode of failure for patients with bruxism and clenching habits (Ferracane, 2006). So investigating the friction and wear behavior is of vital importance to the progress of dental material development. It is well known that wear is a complicated process, which will be affected by loads, speed, friction mode, surface characteristics, and lubrication. For dental resin composites, the wear process is controlled mainly by filler properties and the interfacial bonding strength. These two aspects are interrelated. The filler characteristics, such as particle size and shape, can not only influence the friction and wear behavior directly for they act as strengthening phases, but also can have an indirect influence on mechanical properties through influencing the interfacial bonding. So on the one hand, the composites mechanical properties can be improved by being strengthened using harder and higher-strength fillers. On the other hand, to improve chemical bonding between fillers and resin matrix, the most used method is to pre-treat the surface of the fillers with a silane coupling agent. Another concept is to improve bonding through micro mechanical interlocking of the inorganic particles and the matrix, resisting the ‘plucking out’ of the matrix (Luo et al., 1998).

b

400µm

c

400µm

d

400µm

400µm

Fig. 6 – OM images of macroscopical wear scar: (a) A2-Dense; (b) A2-Porous; (c) A5-Dense; (d) A5-Porous.

J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M AT E R I A L S

a

b

c

d

3 (2010) 77–84

83

Fig. 7 – SEM images of partial wear scar: (a) A2-Dense; (b) A2-Porous; (c) A5-Dense; (d) A5-Porous.

In this study, the A5 and A2 glass–ceramic particles were etched to make them porous, and hence improve bonding with the resin matrix. For A2 dental composites, porous particles had a positive effect on the wear behavior of dental composites. Based on the wear track width (Table 4), the composites containing A2 porous particles sustained less damage than those containing dense particles. Enhanced wear resistance is attributed to the porous structure’s ability to anchor the filler particle in the matrix (Ruddell et al., 2002). Comparing the damage area of A2-dense composites (Fig. 7(a)) to that of A2-porous composites (Fig. 7(b)), it can be observed there are more obvious gaps between A2 dense fillers and resins and many cracks across the particles, displaying clear evidence of a weaker organic-inorganic phase interface. For A5 particles, however, the dental resins filled with porous particles show a reduction in wear resistance. This may be due to the following three factors. Firstly, A5 particles exhibit a much higher porosity than A2 particles after etching. If the filler contained too much porosity it is difficult for penetration of the polymer using a vacuum. If there was some gas remaining in the porous structure, it would have prevented the resin from infiltrating. So there is some residual porosity in the porous A5 particles in the dental resin, which will lead to the degradation of wear behavior. Secondly, the strength of particles was reduced due to high porosity. So the A5 porous particles were more friable and were easily broken and then removed from the matrix under the load, as shown in Fig. 7(d). Thirdly, there are obviously many pores on the surface of composites containing A5 porous fillers, as shown in Fig. 6(d). These pores were detrimental to wear resistance. As a consequence, the composites containing A5 porous fillers were abraded more easily and so the effect of fillers porosity varies for different component particles.

The composites containing A5 dense particles showed less wear damage and could bear more contact pressure than the A2-dense composites (Table 4), which may be related to the fillers’ hardness. Usually, dental composites which exhibit a higher hardness exhibit a better wear resistance (Ramalho and Antunes, 2005). The hardness of composites is determined largely by the hardness of reinforcement fillers since the fillers make up about 70 wt% or more of the whole composites. In our previous study, the results showed that the mechanical properties and crystallinity of apatite-containing glass ceramics increase with apatite content (Liu et al., 2006). Moreover, the relative density of fluorapatite (3.16 g/cm3 ) is higher than that of nepheline (2.55 ∼ 2.66 g/cm3 ) and mica (2.6 ∼ 2.8 g/cm3 ), so the density of A5 glass ceramic is higher than that of A2 glass ceramic since A5 glass ceramic contain higher levels of fluorapatite. So A5 glass ceramic, with higher crystallinity and density, exhibits higher hardness than A2 glass ceramic. Consequently, the composites containing A5 glass ceramic fillers exhibit better wear resistance than the samples containing A2 glass ceramic fillers. Comparing the Hertzian contact diameter with the wear width, it is clear that the abrading at the tribology contact considerably expand the wear scar width after 5000 testing cycles. In this study, it seems that the friction coefficients have no direct corresponding relation with wear. The steady friction coefficients of composites containing A5 fillers are higher than the composites containing A2 fillers and the porous fillers have no significant effect on the friction coefficients of composites. It is known that friction is dominated by the encounters of asperities at the sample surfaces (Palin et al., 2005). So the surfaces of specimens containing A5 fillers are coarser than those containing A2 fillers. That is because

84

J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M AT E R I A L S

firstly, A5 particles have a much higher specific surface area and are coarser than A2 particles. Secondly, in the process of wear, some particles were scaled off from the resin matrix, which may roughen the sample surface Fig. 7(d). Accordingly, the friction coefficients of A5-Porous will become higher.

5.

Conclusions

The friction and wear behavior of dental composites containing fluorapatite glass–ceramic fillers were investigated, using a reciprocating wear model where specimens were lubricated with artificial saliva. The fillers’ surface features and porosity had a significant influence on the friction and wear of dental composites filled by fluorapatite glass–ceramic particles. Although a certain level of porosity was shown to be beneficial to the wear resistance of the composites, the incorporation of excessive porosity into the structure (A5) resulted in a reduction in wear resistance, which was attributed to an increase in the friability of the particles. In order to optimize wear resistance, it is essential to find a balance between porosity, which enhances matrix bonding and particle strength.

Acknowledgements The authors are grateful for the support by National Science Foundation of China under contract Nos. 50634060 and 50721003. And we would like to acknowledge the support of the China Scholarship Council for Y. Tan’s UK research scholarship. REFERENCES

Atai, M., Yassini, E., Amini, M., Watts, D.C., 2007. The effect of a leucite-containing ceramic filler on the abrasive wear of dental composites. Dent. Mater. 23, 1181–1187. Chand, N., Naik, A., Neogi, S., 2000. Three-body abrasive wear of short glass fibre polyester composite. Wear 242, 38–46. Ferracane, J.L., 2006. Is the wear of dental composites still a clinical concern?: Is there still a need for in vitro wear simulating devices?. Dent. Mater. 22, 689–692. Haseeb, A.S.M.A., Albers, U., Bade, K., 2008. Friction and wear characteristics of electrodeposited nanocrystalline nickeltungsten alloy films. Wear 264, 106–112. Johnson, K.L., 1987. Contact Mechanics. Cambridge University Press. Karbhari, V.M., Strassler, H., 2007. Effect of fiber architecture on flexural characteristics and fracture of fiber-reinforced dental composites. Dent. Mater. 23, 960–968. Lai, J.H., Johnson, A.E., Douglas, R.B., 2004. Organosilicon dental composite restoratives based on 1,3-bis[(p-acryloxymethyl) phenethyl] tetramethyldisiloxane. Dent. Mater. 20, 570–578. Liu, Y., Sheng, X., Dan, X., Xiang, Q., 2006. Preparation of mica/apatite glass–ceramics biomaterials. Mater. Sci. Eng., C. 26, 1390–1394. Liu, Y., Tan, Y., Lei, T., Xiang, Q., Han, Y., Huang, B., 2009. Effect of porous glass–ceramic fillers on mechanical properties of light-

3 (2010) 77–84

cured dental resin composites. Dent. Mater. 25, 709–715. Luo, J., Lannutti, J.J., Seghi, R.R., 1998. Effect of filler porosity on the abrasion resistance of nanoporous silica gel/polymer composites. Dent. Mater. 14, 29–36. Palin, W.M., Fleming, G.J.P., Burke, F.J.T., Marquis, P.M., Pintado, M.R., Randall, R.C., Douglas, W.H., 2005. The frictional coefficients and associated wear resistance of novel lowshrink resin-based composites. Dent. Mater. 21, 1111–1118. Papadogiannis, D.Y., Lakes, R.S., Papadogiannis, Y., Palaghias, G., Helvatjoglu-Antoniades, M., 2008. The effect of temperature on the viscoelastic properties of nano-hybrid composites. Dent. Mater. 24, 257–266. Pereira, S.G., Nunes, T.G., Kalachandra, S., 2002. Low viscosity dimethacrylate comonomer compositions [Bis-GMA and CH3Bis-GMA] for novel dental composites; analysis of the network by stray-field MRI, solid-state NMR and DSC & FTIR. Biomaterials 23, 3799–3806. Ramalho, A., Antunes, P.V., 2005. Reciprocating wear test of dental composites: Effect on the antagonist. Wear 259, 1005–1011. Ruddell, D.E., Maloney, M.M., Thompson, J.Y., 2002. Effect of novel filler particles on the mechanical and wear properties of dental composites. Dent. Mater. 18, 72–80. Sarrett, D.C., 2005. Clinical challenges and the relevance of materials testing for posterior composite restorations. Dent. Mater. 21, 9–20. Sideridou, I., Tserki, V., Papanastasiou, G., 2003. Study of water sorption, solubility and modulus of elasticity of lightcured dimethacrylate-based dental resins. Biomaterials 24, 655–665. Silva, N.R.F.A.d., Lalani, F., Coelho, P.G., Clark, E.A., Fernandes, C.A.d.O., Thompson, V.P., 2008. Hertzian contact response of dentin with loading rate and orientation. Arch. Oral Biol. 53, 729–735. Turssi, C.P., Ferracane, J.L., Vogel, K., 2005. Filler features and their effects on wear and degree of conversion of particulate dental resin composites. Biomaterials 26, 4932–4937. Wu, G., Nie, J., 2006. Synthesis and evaluation of N,N-dimethyl-N0 , N0 -di[2-(methylacryloyl)-ethoxycarbnylethyl] propyldiamine as a coinitiator. J. Photochem. Photobiol. A: Chem. 183, 154–158. Xia, Y., Zhang, F., Xie, H., Gu, N., 2008. Nanoparticle-reinforced resin-based dental composites. J. Dent. 36, 450–455. Xiang, Q., 2007. Study on the preparation and properties of fluorophlogopite/fluorapatite glass–ceramics. State Key Laboratory of Powder Metallurgy. Central South University, ChangSha. PhD. Xu, H.H., Martin, T.A., Antonucci, J.M., Eichmiller, F.C., 1999. Ceramic whisker reinforcement of dental resin composites. J. Dent. Res. 78, 706–712. Xu, H.H.K., Quinn, J.B., Giuseppetti, A.A., Eichmiller, F.C., Parry, E.E., Schumacher, G.E., 2004. Three-body wear of dental resin composites reinforced with silica-fused whiskers. Dent. Mater. 20, 220–227. Yu, H.Y., Cai, Z.B., Ren, P.D., Zhu, M.H., Zhou, Z.R., 2006. Friction and wear behavior of dental feldspathic porcelain. Wear. 261, 611–621. Zandinejad, A.A., Atai, M., Pahlevan, A., 2006. The effect of ceramic and porous fillers on the mechanical properties of experimental dental composites. Dent. Mater. 22, 382–387. Zheng, J., Zhou, Z.R., 2007. Friction and wear behavior of human teeth under various wear conditions. Tribol. Int. 40, 278–284.