Physical properties and depth of cure of a new short fiber reinforced composite

d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) 835–841

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Physical properties and depth of cure of a new short fiber reinforced composite Sufyan Garoushi a,b,∗ , Eija Säilynoja c , Pekka K. Vallittu a , Lippo Lassila a a

Department of Biomaterials Science and Turku Clinical Biomaterial Center – TCBC, Institute of Dentistry, University of Turku, Turku, Finland b Department of Restorative Dentistry and Periodontology, Institute of Dentistry, Libyan International Medical University, Benghazi, Libya c Research Development and Production Department, Stick Tech Ltd. – Member of GC Group, Turku, Finland

a r t i c l e

i n f o

a b s t r a c t

Article history:

Objectives. To determine the physical properties and curing depth of a new short fiber com-

Received 10 October 2012

posite intended for posterior large restorations (everX Posterior) in comparison to different

Received in revised form

commercial posterior composites (Alert, TetricEvoCeram Bulk Fill, Voco X-tra base, SDR,

21 February 2013

Venus Bulk Fill, SonicFill, Filtek Bulk Fill, Filtek Superme, and Filtek Z250). In addition, length

Accepted 25 April 2013

of fiber fillers of composite XENIUS base compared to the previously introduced composite Alert has been measured. Materials and methods. The following properties were examined according to ISO standard

Keywords:

4049: flexural strength, flexural modulus, fracture toughness, polymerization shrinkage and

Short fiber composite

depth of cure. The mean and standard deviation were determined and all results were

Physical properties

statistically analyzed with analysis of variance ANOVA (a = 0.05).

Depth of cure

Results. XENIUS base composite exhibited the highest fracture toughness (4.6 MPa m1/2 ) and flexural strength (124.3 MPa) values and the lower shrinkage strain (0.17%) among the materials tested. Alert composite revealed the highest flexural modulus value (9.9 GPa), which was not significantly different from XENIUS base composite (9.5 GPa). Depth of cure of XENIUS base (4.6 mm) was similar than those of bulk fill composites and higher than other hybrid composites. The length of fiber fillers in XENIUS base was longer (1.3–2 mm) than in Alert (20–60 ␮m). Conclusions. The new short fiber composite differed significantly in its physical properties compared to other materials tested. This suggests that the latter could be used in high-stress bearing areas. © 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

The use of light-cured composite resins for restoring cavities in stress-bearing posterior teeth has increased rapidly in recent years [1]. Beside the ability to bond to hard tooth tissues,

mediated by adhesive systems, they feature the advantage of good esthetics and are less expensive compared with cast gold and ceramic inlays. However, insufficient material properties limited the success of composite restorations in high stress bearing areas. Fracture within the body and margins of restorations and polymerization shrinkage have been cited

∗ Corresponding author at: Department of Biomaterials Science, Institute of Dentistry and TCBC, University of Turku, Turku, Finland. Tel.: +358 2 333 83 57; fax: +358 2 333 83 90. E-mail address: sufgar@utu.fi (S. Garoushi). 0109-5641/$ – see front matter © 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dental.2013.04.016

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as major problems regarding the failure of posterior composites [2]. The fracture related material properties, such as fracture resistance, elasticity, and the marginal degradation of materials under stress have usually been evaluated by the determination of the material parameters flexural strength, flexural modulus and fracture toughness [3]. Fracture toughness is a mechanical property that describes the resistance of brittle materials to the catastrophic propagation of flaws under an applied load, and thus, it describes damage tolerance of the material. Fracture toughness values are dependent on the physical properties and chemical composition of the individual component of restorative material. A material which has high fracture toughness has the ability to better resist crack initiation and propagation. Consequently, the property of fracture toughness and flexural strength become important criterions in a dental materials’ longevity [4]. Depending on the studies, volumetric shrinkage of the resin based composite materials averages from 1.5% to 6% [5]. Such shrinkage induces contraction stress at interface between composite resin and walls of cavity leading to gap-formation and predisposes secondary caries. Different measurement techniques were used to follow and to understand this phenomenon, including the mercury dilatometric technique, the bonded-disk technique, strain-gage methods, and shrinkage stress tests [6–8]. Many factors affect the shrinkage of composite resins, including resin matrix composition, filler content, and the polymerization method [6,8,9]. 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 [10]. In general, composite materials can be either particle reinforced (random orientation), whisker (single or multi-layer) or fiber reinforced (long or short fibers in various orientations) [11–13]. Several manufacturers have developed posterior “bulk fill” composite resins which claimed that can be applied to the cavity in thickness of 4 mm with enhanced curing, shrinkage and physical properties [14]. Consequently, dentists can save themselves and their patients significant chairside time and make restorative process less stressful and more comfortable. A problem associated with using light cured composite resin directly in the posterior region is the decrease in curing-light intensity with depth in the material. The intensity of light at a given depth and for a given irradiance period is a critical factor in determining the extent of reaction of monomer into polymer, typically referred to as the degree of monomer conversion, and significantly associated with values of mechanical properties, biocompatibility, color stability and would therefore be expected to be associated with clinical success of the restoration [15,16]. It is thus important to achieve sufficient irradiance on the bottom surface of each of the incremental layers used in building up the restoration. The concept of the point of sufficiency in this respect is known as depth of cure. Put simply, depth of cure can be defined as the extent of quality resin polymerization depth from the surface of composite restoratives. Inadequate polymerization throughout the restoration bulk can lead to undesirable effects, e.g. gap formation, marginal leakage, recurrent caries, adverse pulpal effects and ultimate failure of the restoration [15,16].

Recently, short fiber reinforced composite (everX Posterior) was introduced as a restorative composite resin [17]. The composite resin is intended to be used as base filling material in high stress bearing areas especially in large cavities of vital and non-vital posterior teeth. It consists of a combination of a resin matrix, randomly orientated E-glass fibers and inorganic particulate fillers. The resin matrix contains bis-GMA, TEGDMA and PMMA forming a matrix called semi-Interpenetrating Polymer Network (semi-IPN) (net-poly(methyl methacrylate)inter-net-poly(bis-glycidyl-A-dimethacrylate) which provides good bonding properties and improves toughness of the polymer matrix [18,19]. Clinical results of one year trial in high stress bearing areas, showed good clinical performance, although the time frame and cases for this clinical trial were not of such duration and number as to indicate the long-term suitability of the tested restorations [17]. Thus, the aim of this study was to investigate the physical properties (i.e. flexural strength, flexural modulus, fracture toughness, and polymerization shrinkage) and depth of cure of a new short fiber reinforced composite with comparison to certain commonly used hybrid composite resins and bulk fill composite resins.

2.

Materials and methods

The composite restorative materials used in the study are listed in Table 1.

2.1.

Flexural strength and modulus

Three-point bending test specimens (2.0 mm × 2.0 mm × 25.0 mm) were made from each tested composite resin. Bar-shaped specimens were made in a half-split stainless steel mold between transparent Mylar sheets. Polymerization of the composite was made using a hand light-curing unit (TC-01, Spring Health Products, USA) according to manufacturer recommendations from one side of the metal mold. The wavelength of the light was between 380 and 520 nm with maximal intensity at 470 nm and light intensity was 1100 mW/cm2 . The specimens from each group (n = 6) were stored dry at room temperature for 48 h before testing. Three-point bending test was conducted according to the ISO 4049 (test span: 20 mm, cross-head speed: 1 mm/min, indenter: 2 mm diameter). All specimens were loaded in material testing machine (model LRX, Lloyd Instrument Ltd., Fareham, England) and the load-deflection curves were recorded with PC-computer software (Nexygen 4.0, Lloyd Instruments Ltd., Fareham, England). Flexural strength (of ) and flexural modulus (Ef ) were calculated from the following formula [20]: of =

3Fm I 2bh2

Ef =

SI3 4bh3

where Fm is the applied load (N) at the highest point of load–deflection curve, I is the span length (20 mm), b is the width of test specimens and h is the thickness of test specimens. S is the stiffness (N/m) S = F/d and d is the deflection

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Table 1 – The composite resins investigated and their composition. Brand

Manufacturer (Lot no.)

X-tra base Venus bulk fill Filtek Superme TetricEvoCeram SDR Filtek Bulk Fill Alert Filtek Z250 SonicFill XENIUS base

Type

Matrix composition

Inorganic filler content

Voco, Cuxhaven, Germany (11445386) HerausKultzer, USA (010029)

Bulk-fill

Bis-EMA, MMA

75 wt%, 58 vol% silica

Bulk-fill

UDMA, EBADMA,

3M/ESPE, St. Paul, MN, USA (8CC) Ivoclar Vivadent AG, Liechtenstein (P63316) Dentsply, USA (101006)

Nano Bulk-fill

Bis-GMA, bis-EMA, UDMA, Dimethacrylate co-monomers

Bulk-fill

TEGDMA, EBADMA,

3M/ESPE, St. Paul, MN, USA Jeneric/Pentron, Wallingford, CT, USA (3762763) 3M/ESPE, St. Paul, MN, USA (8RX) Kerr Corporation, CA, USA (3692963) Stick Tech Ltd., Turku, Finland (XB1005)

Bulk-fill Condensable

bis-GMA, bis-EMA, UDMA,

Hybrid Bulk-fill

Bis-GMA, bis-EMA, UDMA, Bis-GMA, bis-EMA, TEGDMA,

65 wt%, 38 vol%, barium silicate glass and silica 78.5 wt%, 59.5 vol% silica Barium glass filler 80 wt%, 60 vol% 68 wt%, 44 vol%, barium borosilicate glass Zirconia, 64 wt%, 42 vol% Filler (conventional and micro glass fiber) 84 wt%, 62 vol% zirconia 78 wt%, 60 vol% Filler 83 vol%

Reinforced-base

Bis-GMA, PMMA, TEGDMA

Short E-glass fiber filler, barium glass 74.2 wt%, 53.6 vol%

PMMA, polymethylmethacrylate; MMA, methylmethacrylate; bis-GMA, bisphenol-A-glycidyl dimethacrylate; TEGDMA, triethylene glycol dimethacrylate; UDMA, urethane dimethacrylate; EBADMA, ethoxylated bisphenol-A-dimethacrylate; bis-EMA, ethoxylated bisphenol-Adimethacrylate; wt%, weight percentage; vol%, volume percentage.

corresponding to load F at a point in the straight-line portion of the trace.

2.2.

Fracture toughness

Rectangular bar (single-edge-notched) specimens to measure the fracture toughness (KIC ) (n = 6) (2.0 mm × 5.0 mm × 25.0 mm) were prepared using metal brass mold so that no force was required to remove the cured bars. A sharp central notch of specific length (a) was produced by inserting a straight edged blade into an accurately fabricated slot at mid-height in the mold; the slot extended down half the height to give a/W = 0.5. The crack plane was perpendicular to the specimen length. Fracture toughness KIC was calculated from the following formula [21]: KIC =

 3PL  BW 3/2

Y

where P is the peak load at fracture; L is the length; B is the width; W is the height; and Y is the calibration functions for given geometry (1.93[a/W]1/2 − 3.07[a/W]3/2 + 14.53[a/W]5/2 − 25.11[a/W]7/2 + 25.80[a/W]9/2 ).

2.3.

Depth of cure

The depth of cure analysis for tested materials was performed according to ISO standard 4049 with 10 mm high cylinder [22]. The mold was placed on a glass slide covered by a Mylar strip. The mold was then filled in bulk with one of tested composites. The topside of the mold was covered with second Mylar strip and the resin material made flush with the mold by use of a second glass slide. The specimens (n = 3) were polymerized from the top of the cylinder mold with a hand-light curing unit for 20 or 40 s (according to manufacturer recommendation) using a light source with an irradiance of 1100 mW/cm2 (TC-01, Spring Health Products). As soon as the curing was over, the material was pressed out from the mold and by using

plastic spatula, the part which had not been polymerized was removed. Then the remaining cured part was measured with a digital caliber with accuracy of ±0.1 mm and the given value was divided by two. This value was recorded as the depth of cure for each specimen.

2.4.

Polymerization shrinkage

The polymerization shrinkage strain was monitored using the strain gage method. This method was previously described by Sakaguchi et al. [23]. The uncured materials were placed in a silicon mold (10.0 mm × 10.0 mm × 1.5 mm) on top of two uniaxial strain gages. The strain gages (KFG-2N-120, Kyowa Ltd., gage length 2 mm) were used to measure shrinkage strains. The materials were placed on the polyimide backing of the strain gages on the opposite side of the electrical resistance foil without any adhesive. The adhesion between the resin paste and the strain gage was previously shown to be sufficient to transfer all the contraction strain from the resin to the gage [22]. Polymerization shrinkage data were acquired from two strain gages using a strain measurement module (PCD-300A, Kyowa Ltd.), which had been initially balanced at zero. The sampling rate of the module was 10 Hz. Data collection started 5 s before the start of polymerization and continued for 300 s. Both upper and lower surfaces of the specimens (n = 5) were covered with a separating sheet and a glass plate and irradiated for 40 s with a hand-held light curing unit (TC-01, Spring Health Products, USA). The light curing tip was maintained at 2 mm distance above the glass slide with the use of a reference plate.

2.5.

Fiber length measurement

The analysis of fiber length was done only for XENIUS base and Alert composites. In the beginning, a portion of the composite paste taken on a glassware. Then 2 ml of Tetra-hydrafuran

d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) 835–841

7

Flexural Strength Flexural Modulus

6 5

140

10

4

1 0 Filtek Voco X- Venus Tetric Bulk Fill tra base Bulk Fill EvoCeram Bulk Fill

SDR

Alert

SonicFill

XENIUS base

(THF, Riedel de Haen, Puriss 99.9%, Lot; 404750) was added on to the glassware with using a Pasteur-pipette. With using a spatula the paste portion was stirred in THF, so that extraction process would begin. When it was seen that the material had started to extract, the THF was removed with a Pasteur pipette. A total of six times this procedure was repeated and afterwards when the THF had vaporized and fibers were dried, they were photographed with a stereo-microscope (Heerbrugg M3Z, Switzerland) at a magnification of 10×. Photos were then processed with Image-J processing program to determine the final lengths of the fibers. The total number of fibers taken into the calculation were two hundred from both composites.

2.6.

Statistical analysis

Mean values of physical properties and depth of cure were statistically analyzed with analysis of variance (ANOVA) at the P < 0.05 significance level with SPSS version 13 (Statistical Package for Social Science, SPSS Inc., Chicago, IL, USA) to determine the differences between the groups.

Results

The mean values of fracture toughness, flexural strength, flexural modulus, shrinkage strain percentage and depth of cure for tested composite materials with standard deviations (SD) are summarized at Figs. 1–4. ANOVA revealed that XENIUS base composite had statistically significantly higher fracture toughness (4.6 MPa m1/2 ) and flexural strength (124.3 MPa) than all other tested composite materials. Filtek Bulk Fill had the lowest fracture toughness (1.7 MPa m1/2 ), flexural strength (85 MPa) and flexural modulus (4.8 GPa) values among the materials tested. Alert, had the highest flexural modulus value (9.9 GPa), which was not significantly different from XENIUS base (9.5 GPa). Curing depth of XENIUS base was found 4.6 mm which is similar to other bulk fill composites except TetricEvoCream Bulk Fill, SonicFill, and Alert which has the lowest depth of cure (2.3 mm). XEINUS base had lower percentage of shrinkage strain (0.17%) compared to other tested composites. The fiber length in XENIUS base was longer (1.3–2 mm) than in Alert (20–60 ␮m).

8

100 80

6

60

4

40 2

20 0

Flexural Modulus [GPa]

2

0 Filtek Bulk Tetric EvoCeram Fill Bulk Fill

SDR

Venus Bulk Voco X-tra Fill base

Alert

XENIUS base

Fig. 2 – Bar graph illustrating means flexural strength (MPa), flexural modulus (GPa) and standard deviation (SD). Groups joined by a horizontal line are not significantly difference (p > 0.05).

0.40 0.35 Linear shrinkage [%]

3

Flexural Strength [MPa]

120

Fig. 1 – Bar graph illustrating mean fracture toughness (KIC ) and standard deviation (SD). Groups joined by a horizontal line are not significantly difference (p > 0.05).

3.

12

160

0.30 0.25 0.20 0.15 0.10 0.05 0.00 Filtek Supreme XT

Filtek Z250

SDR

XENIUS base

Fig. 3 – Bar graph illustrating means linear shrinkage strain (%) and standard deviation (SD). Groups joined by a horizontal line are not significantly difference (p > 0.05).

4.

Discussion

Different commonly used commercially available posterior composite resins and bulk fill composite resins were evaluated in this study. All of them were manufactured to be used in high stress-bearing areas. A large variation in the loading

Measured 7.0

Value stated by manufacturer

6.0 depth of cure [mm]

Fracture toughness, Kic-value [MPam1/2]

838

5.0 4.0 3.0 2.0 1.0 0.0 Alert (40s) SonicFill Tetric Venus Bulk SDR (20s) XENIUS Filtek Bulk Voco X-tra base (20s) Fill (20s) base (20s) (20s) EvoCeram Fill (20s) Bulk Fill (20s)

Fig. 4 – Bar graph illustrating the measured curing depth (after recommended curing time) and the stated by manufacturer (mm).

d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) 835–841

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Fig. 5 – Microscopic image of XENIUS base section (left, 5×) and Alert section (right, 10×).

and constitution of filler particles can be seen (Table 1) in the different commercial composite resins tested. In the present study, the new short fiber reinforced composite (XENIUS base) exhibited the significantly higher fracture toughness (4.6 MPa m1/2 ) and flexural strength (124.3 MPa) than all other materials. Our finding are in agreement with a study by Kim and Watts which showed that fracture toughness of polymer-based materials was improved when they were reinforced with glass fiber-reinforced composite (FRC) [24]. In contrast, Drummond et al. have shown low values of fracture toughness of a fiber containing dental composite [25]. However, the fibers length used were in micrometerscale. A clinical study reported by Van Dijken showed that a restorative composite resin with micrometer-scale fibers suffers extensive wear and are prone for fractures [26], which can be partly explained by the fiber length used being well below the critical fiber length. It was measured by using a fiber fragmentation test that the critical fiber length of Eglass with Bis-GMA polymer matrix varies between 0.5 and 1.6 mm [27]. In order for a fiber to act as an effective reinforcement for polymers, stress transfer from the polymer matrix to the fibers is essential [13,28,29]. This is achieved by having a fiber length equal to or greater than the critical fiber length [13,28]. XENIUS base had fiber length between 1 and 2 mm, thus exceeding the critical fiber length (Fig. 5). It is therefore not surprising that short fiber inclusion with semi-IPN resin matrix revealed substantial improvements in mechanical properties. On other hand, fiber-reinforced composite Alert had fiber length in micrometer scale (20–60 ␮m) which explained the difference in fracture toughness values between the two materials (Fig. 5). Reinforcing effect of the fiber fillers is based on stress transfer from polymer matrix to fibers but also behavior of individual fiber as a crack stopper. Previous study of Garoushi et al. showed how short fiber fillers could stop the crack propagation and provided increase in fracture resistance of composite resin [13]. Random fiber orientation and the polymer matrix by the semi-IPN structure likely had a significant role in mechanical properties [13,18,19]. In addition to the toughening mechanism by fibers, the linear polymer chains of PMMA in the cross-linked matrix of BisGMA-TEGDMA plasticize the polymer matrix to some extend and increases the fracture toughness of the composite resin. Alert showed high values of mechanical parameters, which seems to be a result of high filler load level (Table 1). The most important and extensively investigated variable for physical performance in

dental composite resins is filler loading [30,31]. Previous studies found a positive correlation between filler loading and mechanical performance [32]. Kim and Watts reported that the threshold of filler loading for the highest fracture toughness values in resin composites was 55% by volume [24]. This percent of filler loading is more important than weight percent. In this study, composite Venus bulk fill had the lowest filler loading that is 38% by volume showed better mechanical values than composite Filtek Bulk Fill which has filler loading of 42% by volume. Composite TetricEvoCeram Bulk Fill, containing filler load of 60% by volume demonstrated the significantly lower fracture toughness and flexural strength values. In other words, this study demonstrated the absence of a direct relationship between volumetric content of inorganic particles and fracture resistance parameters (fracture toughness and flexural strength). The difference in fracture toughness and flexural properties values among the tested composite resins may be due to other factors than filler loading. Stress transfer from the polymer matrix to filler particles is one of the important factors effects on fracture toughness and flexural strength values. There may be difference in the adhesion between filler particles and matrix among these resin composites. Beside the filler system, monomer structures of the resin matrix also influence the mechanical properties. The magnitude of the polymerization shrinkage and the accompanying stress generated by the polymerization reaction of the resin composite material are the main factors for in vivo problems like poor marginal adaptation, post operative pain, and recurrent carries. Previously, short randomly oriented fiber-reinforced composite (FRC) reported low polymerization shrinkage compared to particulate filler composite [7,8]. In accordance, this study showed that fiber reinforced composite (XENIUS base) had lower percentage of shrinkage strain (0.17%) compared to other tested composites (Fig. 3). The overall volumetric shrinkage during polymerization can be measured by dilatometer. This provides average shrinkage figures and gives reliable results for isotropic materials that have same material properties in all orientations, such as conventional dental composites. For anisotropic materials where properties vary according to the orientation of reinforcing fibers, like in the composite XENIUS base where fibers are orientated on plane, the shrinkage is not equal to all directions. The polymerization shrinkage is controlled in direction of fibers [7,33]. Accordingly, during polymerization the material is not able to shrink along the length of the fibers. It retains

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its original dimensions horizontally but the polymer matrix between the fibers can shrink and the vertical shrinkage. The shrinkage of the anisotropic materials, like XENIUS base, can be measured by using the strain gage method that provides spatially controlled linear shrinkage data. In some clinical situations the light guide tip cannot be placed in close contact with the restoration surface. Therefore, any increase in the depth of cure obtained by curing should be considered important for daily clinical practice. Interestingly, the depth of cure of the composite XENIUS base evaluated in this study was double higher (4.6 mm) than the other reinforced fiber composite (Alert, 2.3 mm). This could be explained by the difference in filler loading and contents between the two materials. It has been demonstrated that refraction indices and extinction coefficients change during polymerization of BisGMA-TEGDMA monomer systems of fiber reinforced composites which enhance light-induced polymerization to occur [34]. Other factors that may influence depth of cure are shade of composite resin, type of curing unit and method of curing, all are widely discussed in the literature [35,36]. Le Bell et al. have shown that fiber-reinforced composites conduct and scatter the light better than conventional composite resin [37]. Moreover, the light scattering and absorption coefficients of composite resins, which affect the light distribution, should also be taken into consideration. Our study has demonstrated that a new short fiber reinforced composite resin (XENIUS base) had high mechanical parameters values and low polymerization shrinkage which is related to the orientation of fibers. This could suggest better performance and durability of the material in high stressbearing application areas.

5.

Conclusions

Short glass fiber reinforced semi-IPN composite resin (everX Posterior) revealed improvements in physical properties compared with the commercial restorative composites. This could suggest better performance of the new fiber reinforced composite in high stress-bearing application areas.

Acknowledgements Composite material development has started as the PhD research of the first author and completition of the present results were made in the REPOCOM project in the FRC Research Group of the BioCity Turku Biomaterials Research Program (www.biomaterials.utu.fi). The authors gratefully acknowledge Genevieve Alfornt for kind help in this study.

references

[1] Manhart J, Kunzelmann KH, Chen HY, Hickel R. Mechanical properties of new composite restorative materials. Journal of Biomedical Materials Research 2000;53:353–61. [2] Roulet JF. The problems associated with substituting composite resins for amalgam: a status report on posterior composites. Journal of Dentistry 1988;16:101–3.

[3] Craig RG, editor. Restorative dental materials 10. St. Louis, MO: Mosby Publishing Co; 1997. [4] Kim KH, Okuno O. Micro fracture behavior of composite resins containing irregular-shaped fillers. Journal of Oral Rehabilitation 2002;29:1153–9. [5] Ferracane JL. Placing dental composites—a stressful experience. Operative Dentistry 2008;33:247–57. [6] Davison CL, Feilzer AJ. Polymerization shrinkage and polymerization shrinkage stress in polymer-based restoratives. Journal of Dentistry 1997;25:435–40. [7] Tezvergil A, Lassila LV, Vallittu PK. The effect of fiber orientation on the polymerization shrinkage strain of fiber–reinforced composites. Dental Materials 2006;22: 610–6. [8] Garoushi S, Vallittu PK, Watts DC, Lassila LVJ. Polymerization shrinkage of experimental short glass fiber reinforced composite with semi-inter penetrating polymer network matrix. Dental Materials 2008;24:211–5. [9] Watts DC. Reaction kinetics and mechanics in photo-polymerised networks. Dental Materials 2005;21:27–35. [10] Ruddell DE, Maloney MM, Thompson JY. Effect of novel filler particles on the mechanical and wear properties of dental composites. Dental Materials 2002;18:72–80. [11] Xu HHK, Quinn JB, Smith DT, Giuseppetti AA, Eichmiller FC. Effect of different whiskers on the reinforcement of dental resin composites. Dental Materials 2003;19:359–67. [12] Zandinejad AA, Atai M, Pahlevan A. The effect of ceramic and porous fillers on the mechanical properties of experimental dental composites. Dental Materials 2006;22:382–7. [13] Garoushi S, Vallittu PK, Lassila LVJ. Short glass fiber reinforced restorative composite resin with semi-interpenetrating polymer network matrix. Dental Materials 2007;23:1356–62. [14] Ilie N, Hickel R. Investigations on a methacrylate-based flowable composite based on the SDR technology. Dental Materials 2011;27:348–55. [15] Musanje L, Darvell BW. Curing-light attenuation in filled-resin restorative materials. Dental Materials 2006;22:804–17. [16] Ferracane JL, Greener EH. The effect of resin formulation on the degree of conversion and mechanical properties of dental restorative resin. Journal of Biomedical Materials Research 1986;20:121–31. [17] Garoushi S, Tanner J, Vallittu PK, Lassila LVJ. Preliminary clinical evaluation of short fiber-reinforced composite resin in posterior teeth: 12-months report. The Open Dentistry Journal 2012;6:41–5. [18] Lastumäki TM, Lassila LV, Vallittu PK. The semi-interpenetrating polymer network matrix of fiber-reinforced composite and its effect on the surface adhesive properties. Journal of Materials Science: Materials in Medicine 2003;14:803–9. [19] Vallittu PK. Interpenetrating polymer networks (IPNs) in dental polymers and composites. In: Matinlinna JP, Mittal KL, editors. Adhesion aspects in dentistry. Leiden, The Netherlands: VSP; 2009. p. 6–74. [20] International Standardization Organization, ISO 4049-1992(E). Polymer-based crown and bridge material. Geneva: ISO; 1992. [21] ASTM E1820-05. Standard test method for measurement for fracture toughness. Philadelphia: American Society of Testing and Materials; 2005. [22] International Standardization Organization, ISO 4049-2000(3). Dentistry-Polymer-based filling restorative and luting materials; 7.10 Depth of cure, Class 2 materials; 2013.

d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) 835–841

[23] Sakaguchi RL, Sasik CT, Bunczak MA, Douglas WH. Strain gauge method for measuring polymerization contraction of composite restoratives. Journal of Dentistry 1991;19:312–6. [24] Kim SH, Watts DC. Effect of glass-fiber reinforcement and water storage on fracture toughness (KIC ) of polymer-based provisional crown and FPD materials. International Journal of Prosthodontics 2004;17:318–22. [25] Drummond JL, Lin L, Miescke KJ. Evaluation of fracture toughness of a fiber containing dental composite after flexural fatigue. Dental Materials 2004;20:591–9. [26] Van Dijken JWV. Direct resin composite inlays/onlays: an 11-year follow-up. Journal of Dentistry 2000;28:299–306. [27] Cheng TH, Jones FR, Wang D. Effect of fiber conditioning on the interfacial shear strength of glass-fiber composite. Composites Science and Technology 1993;48:89–96. [28] Garoushi S, Lassila LVJ, Vallittu P. The effect of span length of flexural testing on properties of short fiber reinforced composite. Journal of Materials Science: Materials in Medicine 2012;23:325–8. [29] Garoushi S, Vallittu PK, Lassila LVJ. Use of isotropic short fiber reinforced composite with semi-interpenterating polymer network matrix in fixed partial dentures. Journal of Dentistry 2007;35:403–8. [30] Ersoy M, Civelek A, L’Hotelier E, Say EC, Soyman M. Physical properties of different composites. Dental Materials Journal 2004;23:278–83.

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[31] Garoushi S, Lassila LVJ, Vallittu P. Influence of nanometer scale particulate fillers on some properties of microfilled composite resin. Journal of Materials Science Materials in Medicine 2011;22:1645–51. [32] Kim KH, Ong JL, Okuno O. The effect of filler loading and morophology on the mechanical properties of contemporary composites. Journal of Prosthetic Dentistry 2002;87:642–94. [33] El-Mowafy O. Polymerization shrinkage of restorative composite resins. Practical Procedures and Aesthetic Dentistry 2004;16:452–5. [34] Lehtinen J, Laurila T, Lassila LVJ, Tuusa S, Kienanen P, Vallittu PK, et al. Optical characterization of bisphenol-Aglycidyldimethacrylate-triethyleneglycoldimethacryalate monomers and copolymers. Dental Materials 2008;24:1324–8. [35] Rueggeberg FA, Caughman WF, Curtis JW, Davis HC. Factors affecting cure at depths within light-activated resin composites. American Journal of Dentistry 1993;6:91–5. [36] Garoushi S, Vallittu PK, Lassila LVJ. Depth of cure and surface microhardness of experimental short fiber-reinforced composite. Acta Odontologica Scandinavica 2008;66:38–42. [37] Le Bell AM, Tanner J, Lassila LV, Kangsniemi I, Vallittu PK. Depth of light-initiated polymerization of glass fiber-reinforced composite in a simulated root canal. International Journal of Prosthodontics 2003;4:403–8.