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

Abstract

Objectives

To determine the physical properties and curing depth of a new short fiber composite intended for posterior large restorations (everX Posterior) in comparison to different commercial posterior composites (Alert, TetricEvoCeram Bulk Fill, Voco X-tra base, SDR, Venus Bulk Fill, SonicFill, Filtek Bulk Fill, Filtek Superme, and Filtek Z250). In addition, length 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 4049: flexural strength, flexural modulus, fracture toughness, polymerization shrinkage and depth of cure. The mean and standard deviation were determined and all results were statistically analyzed with analysis of variance ANOVA ( a = 0.05).

Results

XENIUS base composite exhibited the highest fracture toughness (4.6 MPa m 1/2 ) 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 μm).

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.

Introduction

The use of light-cured composite resins for restoring cavities in stress-bearing posterior teeth has increased rapidly in recent years . 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 as major problems regarding the failure of posterior composites . 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 . 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 .

Depending on the studies, volumetric shrinkage of the resin based composite materials averages from 1.5% to 6% . 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 . Many factors affect the shrinkage of composite resins, including resin matrix composition, filler content, and the polymerization method . 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 . 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) . 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 . 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 . 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 .

Recently, short fiber reinforced composite (everX Posterior) was introduced as a restorative composite resin . 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 . 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 .

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.

Materials and methods

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

Table 1
The composite resins investigated and their composition.
Brand Manufacturer (Lot no.) Type Matrix composition Inorganic filler content
X-tra base Voco, Cuxhaven, Germany (11445386) Bulk-fill Bis-EMA, MMA 75 wt%, 58 vol% silica
Venus bulk fill HerausKultzer, USA (010029) Bulk-fill UDMA, EBADMA, 65 wt%, 38 vol%, barium silicate glass and silica
Filtek Superme 3M/ESPE, St. Paul, MN, USA (8CC) Nano Bis-GMA, bis-EMA, UDMA, 78.5 wt%, 59.5 vol% silica
TetricEvoCeram Ivoclar Vivadent AG, Liechtenstein (P63316) Bulk-fill Dimethacrylate co-monomers Barium glass filler 80 wt%, 60 vol%
SDR Dentsply, USA (101006) Bulk-fill TEGDMA, EBADMA, 68 wt%, 44 vol%, barium borosilicate glass
Filtek Bulk Fill 3M/ESPE, St. Paul, MN, USA Bulk-fill bis-GMA, bis-EMA, UDMA, Zirconia, 64 wt%, 42 vol%
Alert Jeneric/Pentron, Wallingford, CT, USA (3762763) Condensable Filler (conventional and micro glass fiber) 84 wt%, 62 vol%
Filtek Z250 3M/ESPE, St. Paul, MN, USA (8RX) Hybrid Bis-GMA, bis-EMA, UDMA, zirconia 78 wt%, 60 vol%
SonicFill Kerr Corporation, CA, USA (3692963) Bulk-fill Bis-GMA, bis-EMA, TEGDMA, Filler 83 vol%
XENIUS base Stick Tech Ltd., Turku, Finland (XB1005) 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-A-dimethacrylate; wt%, weight percentage; vol%, volume percentage.

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/cm 2 . 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 ( o f ) and flexural modulus ( E f ) were calculated from the following formula :

<SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='of=3FmI2bh2′>of=3FmI2bh2of=3FmI2bh2
o f = 3 F m I 2 b h 2
<SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='Ef=SI34bh3′>Ef=SI34bh3Ef=SI34bh3
E f = S I 3 4 b h 3

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

Fracture toughness

Rectangular bar (single-edge-notched) specimens ( n = 6) to measure the fracture toughness ( K IC ) (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 K IC was calculated from the following formula :

<SPAN role=presentation tabIndex=0 id=MathJax-Element-3-Frame class=MathJax style="POSITION: relative" data-mathml='KIC=3PLBW3/2Y’>KIC=(3PLBW3/2)YKIC=3PLBW3/2Y
K I C = 3 P L B W 3 / 2 Y
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Nov 25, 2017 | Posted by in Dental Materials | Comments Off on Physical properties and depth of cure of a new short fiber reinforced composite
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