In Vitro Assessment of Vertise [TM] Flow from Kerr

CONTENTS

IN VITRO ASSESSMENT OF VERTISE TM FLOW FROM KERR

ABSTRACT

ABREVIATIONS

LIST OF FIGURES

LIST OF TABLES

CHAPTER 1: INTRODUCTION

CHAPTER 2: LITERATURE REVIEW
2.1 INTRODUCTION
2.2 DENTAL COMPOSITES
2.2.1 COMPOSITION AND STRUCTURE
2.2.2 RESIN/ORGANIC MATRIX
2.2.3 FILLER
2.2.4 COUPLING AGENT
2.2.5 INTIATORS AND ACCELERATORS
2.2.6 CLASSIFICATION OF COMPOSITES
2.2.7 PACKABLE COMPOSITES
2.2.8 POLYMERIZATION REACTION
2.3 PHYSICAL PROPERTIES OF DENTAL COMPOSITES
2.3.1 Working and setting time:
2.3.2 Polymerization Shrinkage
2.3.3 Thermal Properties
2.3.4 Water Sorption
2.3.5 Solubility
2.3.6 Colour and Colour Stability
2.4 MECHANICAL PROPERTIES
2.4.1 Strength and Modulus
2.4.2 Hardness
2.4.3 Bond Strength and Dental Substrates (Ceramics, Alloys, etc….)
2.5 CLINICAL PROPERTIES
2.5.1 Depth of cure for light-cured composites
2.5.2 Radiopacity
2.5.3 Wear Rates
2.5.4 Biocompatibility
2.6 DENTAL ADHESIVES
2.7 FLOWABLE COMPOSITES
2.7.1 PROPERTIES OF FLOWABLE COMPOSITES
2.7.2 VERTISE FLOW
2.7.3 GRANDIO FLOW
2.7.4 PREMISE FLOWABLE
2.7.5 HYDROXYETHYL METHACRYLATE (HEMA) Figure 2.6

CHAPTER 3: AIMS AND OBJECTIVES
3.1 AIM
3.2 OBJECTIVES

CHAPTER 4: MATERIAL AND METHODS
4.1 COMPOSITE MATERIALS
4.2 IMMERSION SOLUTIONS
4.3 METHODOLOGY FOR WATER ABSORPTION AND DESORPTION
4.3.1 SAMPLE PREPARATION
4.3.2 ABSORPTION
4.3.3 LONG-TERM IMMERSION
4.3.4 DESORPTION
4.3.5 DIFFUSION THEORY AND FICK’S LAW
4.3.6 CALCULATING DIFFUSION COEEFICIENT
4.3.7 CALCULATING SOLUBILITY
4.3.8 CALCULATING REAL UPTAKE
4.4 COMPOSITE RESIN DEGREE OF CONVERSION
4.4.1 METHODOLOGY FOR COMPOSITE DEGREE OF CONVERSION
4.5 POLYMERIZATION EXOTHERM
4.5.1 METHODOLOGY FOR EXOTHERM MEAUREMENTS
4.6 DEPTH OF CURE OF LIGHT CURED COMPOSITES
4.6.1 METHODOLOGY FOR DEPTH OF CURE
4.7 STATISTICAL METHODOLOGY

CHAPTER 5: RESULTS
5.1WATER UPTAKE OF COMPOSITES
5.1.1 Vertise TM Flow
5.1.2 AVERAGE OF VERTISE TM FLOW, GRANDIO FLOW and PREMISE FLOWABLE IN DISTILLED WATER AND ARTIFICAL SALIVA
5.2 DESORPTION OF COMPOSITES
5.2.1 THREE MONTHS CONTINOUS UPTAKE OF FLOWABLE COMPOSITE (VF, GF & PF) IN ARTIFICIAL SALIVA
5.3 DIFFUSION COEEFICIENT FOR ABSORPTION AND DESORPTION AND SOLUBILTY %
5.4 DEGREE OF CONVERSION
5.5 POLYMERISATION EXOTHERM
5.6 DEPTH OF CURE

CHAPTER 6: DISCUSSION
6.1: IMMERSION SOLUTIONS
6.2: WATER ABSORPTION AND DESORPTION PROFILE AFTER IMMERSION IN DISTILLED WATER AND ARTIFICIAL SALIVA
6.3: SOLUBILITY OF THE COMPOSITES
6.4 DIFFUSION COEEFICIENT OF THE COMPOSITES
6.5: DEGREE OF CONVERSION
6.6: POLYMERISATION EXOTHERM
6.7: DEPTH OF CURE

CHAPTER 7: CONCLUSIONS

FUTURE STUDIES

CLINICAL RELEVANCE

REFERNCES

ABSTRACT

Introduction

Flowable composites are of low viscosity and a modification of small particle-filled and hybrid composites. They have reduced filler load and modified resin monomers which provide a consistency that allows the material to flow readily. They have better adaptability to cavity walls thus preventing microleakge. However their lower filler loading results in greater polymerisation shrinkage and reduced mechanical properties compared to other hybrid composites.

Aims and Objectives

The aim of this study was to compare the physical and mechanical properties of a new low viscosity commercial flowable composite (VertiseTM Flow) with other flowable composites (Grandio Flow and Premise Flowable) currently available on the market. Water absorption, depth of cure, degree of conversion (using FTIR) and polymerisation exotherm were compared.

Material and Methods

Water absorption and desorption was measured in distilled water and artificial saliva gravimetrically, where the uptake and loss was noted at set time intervals. Degree of conversion of double bonds of uncured and cured samples of composites was measured using the FTIR. The depth of cure was measured by an adapted ISO 4049 stated method. Finally polymerisation exotherm was measured using the K-type thermocouple of samples cured for 20 seconds.

Results

The results showed increased uptake of water in distilled water and artificial saliva for VF, compared to the PF and GF. The diffusion coefficients were generally similar for desorption and absorption. The solubility % in distilled water was highest for VF in artificial saliva. All materials showed weight gain after desorption. Finally the depth of cure of VF was lower and polymerisation exotherm was higher than PF and GF. Lastly degree of conversion was found to be almost similar for all the three flowable composites.

Conclusions

The presence of HEMA in VF resulted in a higher water uptake and polymerisation exotherm and lower depth of cure than the other flowable composite tested.

ABREVIATIONS

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LIST OF EQUATIONS

Equation 4.1: Mt/M∞ which is the ratio of uptake at time (t) to the 41 uptake of equilibrium

Equation 4.2: Integration of equation 4.1

Equation 4.3: The diffusion coefficient

Equation 4.4: The percentage solubility

Equation 4.5: The real uptake

Equation 4.6: The degree of conversion of the methacrylate based composite

LIST OF FIGURES

Figure 2.1: Chemical structure of Bis-GMA

Figure 2.2: Triethyleneglycol Dimethacrylate, TEGDMA

Figure 2.3: Urethane Dimethacrylate, UDMA

Figure 2.4: Coupling agent:γ-methacryloxypropyl-triethoxysilane or γ-MPTS

Figure 2.5: Gycerophosohate dimethacrylate, GPDM bonding in VF

Figure 2.6: Hydroxyethyle Methacrylate, HEMA

Figure 4.1: Flowable composite used in the study

Figure 4.2: Flowable composite samples in the study

Figure 4.3 The light curing unit used in the study

Figure 4.4: Immersion solutions in screw tight glass jars for water absorption test

Figure 4.5: The ‘initial desorption’ and ‘final desorption’ drying oven

Figure 4.6: Fourier Transform Infrared Spectroscopy (FTIR) machine used in this study

Figure 4.7: The Golden Gate ATR system

Figure 4.8: the K-type thromocouple, chart reader and silicone sheets used in the study of polymerisation exotherm

Figure 4.9: Stainless steel mould used for the study of depth of cure

Figure 4.10: curing of the sample for the study of depth of cure

Figure 5.1: Percentage weight change of Vertise™ Flow in artificial saliva

Figure 5.2: Mean percentage weight uptake of VF, GF and PF in distilled water

Figure 5.3: Mean percentage weight uptake of VF, GF and PF in artificial saliva

Figure 5.4: Mean percentage weight loss of VF, GF and PF in distilled water

Figure 5.5: Mean percentage weight loss of VF, GF and PF in artificial saliva

Figure 5.6: Stains on the sample of PF after long term immersion in artificial saliva

Figure 5.7: Mean percentage weight loss of VF, GF and PF in artificial saliva for 3 months

Figure 5.8: Percentage degree of conversion with the standard deviation

Figure 5.9: Mean polymerisation exotherm profile of VF, GF and PF

Figure 5.10: Difference of depth of cure in VF

Figure 5.11: Mean depth of cure of cured composites with standard deviation

LIST OF TABLES

Table 2.1: Classification of Dental Adhesives

Table 4.1: Composition of flowable composites used in the study

Table 4.2: Time intervals for weighing of weight change and weight loss of samples

Table 5.1: Diffusion coefficient of absorption/desorption in different solutions and mean solubility of composites in solvents with standard deviation in parenthesis (negative sign signifies net weight loss)

Table 6.1: Summary of diffusion coefficients for absorption and desorption.

CHAPTER 1: INTRODUCTION

Restorative dentistry is going through a dynamic transition towards adhesive dentistry. A class of resin-composite systems known as ‘flowable composites’ has become an essential part of the restorative process since their introduction in the mid ninetees (Baroudi et al, 2007). These materials were developed in response to a demand from the clinicians for easy handling. They are characterized by having less filler load and greater portion of diluent monomers (Lee et al, 2003). Designed to be less viscous, and so the flowable composites offer a better adaptation to internal walls of the cavity, easier insertion and greater elasticity. Flowability of these materials allows them to be dispensed through injectable dispensers and simplifies easy placement procedures.

Vertise ™Flow (VF) is the flowable composite used in the study that follows. This material was introduced in 1992. It incorporates the Optibond bonding mechanism to dentine by two-fold. Firstly, it binds chemically via the phosphate group of the glycerophosphate dimethacrylate (GPDM) to the calcium of the tooth and secondly micromechanical adhesion by forming a hybrid layer composed of resin impregnating with collagen fibers and dentine smear layer. The University of Leuven, in Belgium, has also proven this adhesion through SEM and TEMs studies. Several investigations have been performed on this material by the American manufacturer Kerr. VF incorporates four types of fillers; the nano-ytterbium that confirms good radiopacity, pre-polymerised fillers which reduce microleakage, improved polishability due to nano particles plus thixotropic properties.

This material was selected for the research due to; its composition which contains HEMA, a hydrogel, which has the ability of water absorption and increased polymerization exotherm compared with composite matrix monomers (Patel, 2001). As composite restorations are surrounded in an aqueous environment, they tend to absorb water and release un-reacted monomers. These further pose a potential harm in causing allergic reactions and seepage of water which may result in increased bacterial growth and eventually lead to secondary caries. The absorbed water also weakens the filler matrix bond and causes degradation of the material eventually leading to failure of the restoration (Sideridou et al, 2007). This diffusion controlled water sorption in composites can also cause several time dependent effects (Wei et al, 2011) which include; hygroscopic expansion (Ferracane, 2006), hygroscopic stress leading to cracks in the restoration (Ruttermann et al, 2007), weakened mechanical properties (Palin et al, 2005 and Musanje et al, 2001), reduction in hardness, glass transition temperature (Ferracane, 2006) and decreased wear resistance (Gohring et al, 2002).

During this investigation the VF was tested with respect to degree of conversion and depth of cure. The Light Emitting Diode (LED) light source was used in this study as it is said to be efficient at curing better dental materials Mills et al, (1999). The authors reported that an irradiance of 290mWCm-2 produced greater depths of cure as compared to halogen light. Furthermore, it is suggested that a decreased filler loading and type and size of the filler particles in the composite material affect the depth of cure. According to Qasim, (2011), the higher the filler load the lesser would be the depth of cure, and the small the particles, the higher would be the scattering of light thus less depth of cure.

Finally, the purpose of this thesis was to compare some of the physical properties of HEMA containing flowable composites to non-HEMA (GF and PF) containing flowable composites and have a look at the effect of external media (distilled water and artificial saliva) on the three composites.

CHAPTER 2: LITERATURE REVIEW

2.1 INTRODUCTION

This chapter begins with the history of composite restorative materials followed by a description on current composite restorative materials. The clinical properties, including physical and mechanical, of current composite restorative materials are also discussed, such as VertiseTM Flow (VF). Their properties together with comparing HEMA (hydroxyethyl methacrylate) containing composites with those containing no HEMA are also described.

2.2 DENTAL COMPOSITES

Dental composites, as the name implies, consist of a mixture of two or more materials. Each of these materials individually contributes to the composite’s properties and is present in a discrete form. The most commonly used types of composites are resin-based, which are used in a wide variety of clinical procedures, for example filling materials, luting agents, indirect restorations and metal facing to endodontic cores and post. (van Noort, 2007)

2.2.1 COMPOSITION AND STRUCTURE

There are four main components found in resin based restorative materials are:

1. Organic polymer matrix
2. In-organic filler particles
3. Coupling agent
4. Intiator-acclerator system(van Noort, 2007 and Powers & Sakaguchi,2006)

Each one of these is dealt with, in detail, subsequently.

2.2.2 RESIN/ORGANIC MATRIX

Primarily, in a fluid monomer form, the resin is the chemically active element, which gets converted to a rigid polymer, by a free radical polymerization reaction. Due to its ability to convert from a plastic to rigid form it is favorable to be used for restorations (van Noort, 2007). These fluid resins (monomers) are viscous liquids and their viscosity is reduced, to a useful clinical level, by adding diluent monomers (Powers & Sakaguchi, 2006)

The most common monomers used in dental composites are dimethacrylates amongst which 2, 2-bis [4(2-hydroxy-3 methacryloyloxy-propyloxy)-phenyl) 1] propane (Bis-GMA) (Figure 2.1), which is derived from reacting bis-phenol-A and glycidylmethacrylate; it is referred to as Bowens-resin, after its inventor. The other monomer which is used in a number of composites, in place of Bis-GMA, is Urethane Dimethacrylate (UDMA figure 2.2) (van Noort. 2007)

Both Bis-GMA and UDMA contain carbon-carbon double bonds at each chain of their chemical structures, which undergo addition polymerization (Powers and Sakaguchi, 2006). Due to their high molecular weight, even a small addition of filler results in a composite with a viscosity that is inappropriate for clinical use. (van Noort, 2007) Bis-GMA and UDMA (Figure 2.2) are highly viscous fluids (Patel, 2012) due to the hydrogen bonding interactions between hydroxyl group and monomer molecules,(Chen, 2010).Thus low viscosity diluents such as tri-ethylene, glycol dimethacrylate (Figure 2.3) are added by the manufacturers (Powers and Sakaguchi, 2006).

Hydroquinone acts as an inhibitor and is added to the monomers to avoid pre mature polymerization and for a sufficient prolonged shelf-life (van Noort, 2007).

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Figure 2.1: Chemical structure of Bis-GMA (Schneider et al, 2010)

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Figure 2.2 Chemical structure of UDMA (Foster and Walker, 1974; van Noort, 2007)

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Figure 2.3: Chemical structure of TEGDMA (Schneider et al, 2010)

2.2.3 FILLER

To improve the physiochemical properties of dental composites a wide range of filler types are added (Ferracane et al, 1987). In the late 50s fillers, such as quartz, were added to the metharylate based restorative materials; this is when this practice began. Addition of different types, sizes shapes, volume fractions and distribution of fillers resulted in the following:

- As the filler is not part of the polymerization process, this then leads to a decreased amount of resin in the system resulting in decreased polymerization shrinkage (Powers & Sakaguchi, 2006)
- The methacrylate monomer has a high coefficient of thermal expansion. This is reduced by the addition of ceramic fillers, which have similar coefficient of thermal expansion as that of tooth (van Noort, 2007 and Powers & Sakaguchi,2006)
- Mechanical properties are improved through the addition of fillers; these include hardness (Braem et al, 1989), radiopacity (van Dijken et al, 1989), fracture toughness, water sorption (Kim et al, 2002), gloss retention, roughness (Cavalcante et al, 2009) and compressive strength
- Aesthetic features, which include fluorescence, color plus translucency, are also controlled by the fillers (van Noort, 2007)
- Different brands of fillers are marketed these days; these can be silica, zirconium, quartz, barium, strontium and others (Schneider et al, 2010).

2.2.4 COUPLING AGENT

For a composite to have superior properties it is essential that a stable bond is formed amidst the organic matrix and the filler. If there is a collapse of this interface, the stresses developed under load will not be efficiently dispersed through the material, and hence fracture will occur at this interface, resulting in consequent composite disintegration (van Noort, 2007). Manufacturers treat the filler with a coupling agent before it mixes with the monomer. The most commonly used coupling agent is called a silane (Powers & Sakaguchi, 2006) (Figure 2.4).

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Figure 2.4: Silane coupling agent (Patel, 2012)

The fundamental problem is that resins are hydrophobic and silicate based glass fillers are hydrophilic; hence the resin has no natural affinity for bonding to the glass surface. The solution to this is the use of a silane coupling agent, with a hydroxyl group at one end, which gets drawn to the hydroxyl group (or methoxy group) on the surface of the glass. Meanwhile its methacrylate group bonds to the resin with carbon-carbon double bond (methacrylate group) on the other end. Enhancement of this bond between the resin and glass results in better wear resistance for the material, thus making it able to be used in anterior and posterior restorations. (van Noort, 2007).

2.2.5 INTIATORS AND ACCELERATORS

Light or chemically activated composites are more commonly used in dentistry. Light activation is accomplished by blue-light which is absorbed by camphroquinone (photoactivator). The amine and the camphroquinione are stable in the presence of resin matrix at room temperature. But when expose to light, the camphorquinone and amine react to form free radicals with an addition polymerisation reaction taking place (Powers and Sakaguchi, 2006).

Chemical activation is accomplished at room temperature when free radicals are produced due to an organic amine reacting with an organic peroxide. This then leads to polymerization because the free radicals attack the C=C (carbon-carbon double bonds) (Powers and Sakaguchi, 2006)

2.2.6 CLASSIFICATION OF COMPOSITES

Dental composites are classified on the basis of their nature and particle size of the filler (van noort, 2007).

Traditional composites:

They contain glass filler particles with a mean size of 10-20um. These composites have a poor, rough surface finish, due to filler particles protruding from the surface.

Microfilled Resins Composites:

The first of this kind was introduced in the 1970s and contain colloidal silica with an average particle size being 0.02um. This small size of filler particles allows the composite to be polished to a smooth surface. It also delivers a larger surface area for the resin to react. These were actually called the nanocomposites due to their small particle size and as their filler level was low it could be increased by adding the pre-polymerized resin fillers (PPRF). A disadvantage these materials had, due to their low filler content, was that they were weak and a compromise was needed to increase their strength with enhancing the polishability. (Ferracane, 2011). However they give good esthetics to the material (Sideridou et al, 2011).

Hybrid Composites:

These composite contain two particle sizes, an average size of 15-20um with a small quantity of colloidal silica having a particle size of 0.01-0.05um. They give intermediate esthetics but they have good mechanical properties, due to higher filler loading than the microfilled resins, and incorporation of fillers with different particle sizes (Sideridou et al, 2011). Flexural modulus of this type of composite is approximately 10 GPa (Attar et al, 2003).

Small Particle Hybrid Composites:

Better methods and techniques of grinding glass particles have led to the introduction of composites which contain filler particles with an average particle size of 0.4- 1um. These smaller size filler particles allow composites to be polished to smoother surface finish, they are stronger, more resistant to fracture and mimic the natural enamel to a high extent (Christiansen et al, 1999). These have also been developed in shades of dentine and translucent enamel, thus helping the clinician to produce more natural dental enamel (Tay et al, 1994).

2.2.7 PACKABLE COMPOSITES

Packable composites are also known as condensable composites. They were introduced in the late nineteen-nineties to enable clinicians to apply techniques similar to those used for amalgam. Their filler content by weight is 65-81% and they show a curing shrinkage of 2-3% (Anusavice, 2003). Due to the increased filler load these materials result in decreased polymerization shrinkage and marginal leakage. However, their increased viscosity and modulus of elasticity is thought to prevent wetting of the cavity walls during placement (Majety, 2011). These materials do not have any advantage over hybrid composites, only that they can be packed in like amalgams (Anusavice, 2003).

Flowable composites are low viscosity and are a modification of small particle and hybrid composites. They have reduced filler load and consist of modified diluents monomers (GPDM, HEMA, etc…) which provide a consistency that allows the material to flow readily.

Details about flowable have been discussed in detail later in section 2.7

2.2.8 POLYMERIZATION REACTION

There are three methods of curing dental composites:

1. Autopolymerisation/self-cure system (room temperature, chemical)
2. Light cured
3. Dual cure system-using both self-cure and light cure system.

The light cure system is incorporated in the majority of modern dental composites for direct restorative procedures. Camphorquinone is usually employed by these composites as the photoinitiator activator system (Watts & Silikas, 2005). Conversion into an excited triplet state is achieved when the blue-light activates the camphorquinone. The activation and initiation stages of polymerization start when the excited camphorquinone reacts with an activator (a tertiary amine) to form free-radicals (Stansbury, 2000). Free-radicals are molecules with un-paired electrons. An active centre is created propagating the polymerization process when this reactive radical reacts with a monomer molecule. The polymer chains grow through quick sequential addition of monomer units to the active centers via the covalent bonds till the potential maximum degree of conversion of C=C double bonds into C-C bonds is attained. This second step of the polymerization process is called propagation. Van der Waals forces act and keep the monomers grouped before the polymerization process take place. Approximately 4Ao is the distance among the monomers at this moment. Distances of approximately 1.5Ao are achieved amongst these forces which are substituted by covalent bonds during the polymerization process (Kleverlaan & Feilzern, 2005). Volumetric shrinkage occurs as a result of this. Depending on their formulation and curing conditions, typical resin composites applied in restorative dentistry, display volumetric shrinkage values from less than 1% going up to 6% (Kleverlaan & Feilzern, 2005).

2.3 PHYSICAL PROPERTIES OF DENTAL COMPOSITES

2.3.1 Working and setting time:

“On demand” curing is considered for composites. The curing of composites start as it is exposed to the curing light. The material loses its ability to flow and is set after 40 or 20 seconds, after being exposed to the light source, but polymerization continues up till twenty-four hours. The percentage of uncured composite may increase if the surface of the restoration is left unprotected from air; this will create a layer of unpolymerised composite, (Visvanathan et al, 2007). Studies have shown, not all the carbon double bonds react and that some remain un-reacted in the bulk (Powers & Sakaguchi, 2006).

After the exposure to light curing, source “stiffening” takes place in the light cured composites (Powers & Sakaguchi, 2006). The efficiency of the light curing unit, according to Althoff and Hartung (2000), depends on the concept of total energy. According to this concept, there are two factors on which efficient polymerization depend. The first being the intensity of the curing light and the second is the duration of exposure to light source. For sufficient curing with less curing time, a higher intensity curing light is essential. This high intensity leads to increased polymerization shrinkage which eventually leads to the development of stresses at the tooth-restoration interface. (Feilzer et al, 1995 and Dennison et al, 2000). According to Dennison et al different intensities of light affected the polymerization shrinkage. Their study concluded that 100% intensity of curing light had significant reduction in the linear shrinkage of the material (Dennison et al, 2000).

The time duration for the working and setting time of chemically-activated dental composites depends upon the concentration of the initiator and the accelerator used by the manufacturers. (Powers & Sakaguchi, 2006 and Patel, 2012).

2.3.2 Polymerization Shrinkage

Volumetric shrinkage in composites is the direct function of the amount of monomer (resin matrix) and diluents. Stresses as high as 13MPa are created due to polymerization shrinkage at the tooth tissue material interface. The result of these stresses is a break between the inter-facial bonds amongst the material and the cavity walls, providing an area for seepage of saliva (Powers & Sakaguchi, 2006). As composites do not have any anti-carious properties, this can result in secondary or recurrent caries. (van Noort, 2007 and Powers & Sakaguchi, 2006 ). Micro filled composites have shrinkage values between 2% - 3% as they contain pre-polymerised particles (van Noort, 2007 and Powers &Sakaguchi, 2006) while on the other hand flowable composites have shrink by 3-5% (Anusavice, 2003)

The solution to this problem is placing composites in increments of 2mm. In that way, the net polymerization shrinkage would be reduced. This is due to a smaller volume of composite being placed; this allows the composite to shrink before successive addition of further material (van Noort, 2007 and Powers & Sakaguchi, 2006). According to Attar et al, (2003) there is a correlation between filler content and mechanical properties, thus composites consisting of less filler load will result in more polymerisation shrinkage and deformation (Attar et al, 2003).

2.3.3 Thermal Properties

Stresses developed due to the difference between the expansion and contraction of dental composites can be decreased if the coefficient of thermal expansion is similar to the tooth, (van Noort, 2007) The values of linear coefficient of thermal expansion for the fine particles composite is 25-38x10-6/0 C and for microfine particles it is 55-68x10-6/0 C (Braden et al, 1997). The glass fillers have low thermal coefficient of expansion while resin has a high coefficient of thermal expansion, in another words the more the inorganic filler loading the less will be coefficient of thermal expansion (van Noort, 2007). These cyclic thermal changes will be ongoing and may result in fatigue of the tooth material bond (Powers & Sakaguchi, 2006).

Due to the conductivity of the in-organic fillers being higher than that of the polymer matrix the thermal conductivity of composites with fine particle (25-30x10-4 cal/sec/cm2) is superior to that of composites with micro-fine particles (12-15x10-4cal/sec/cm2). The temperature changes in the mouth are transient and therefore composites do not pose a clinical issue (Powers and Sakaguchi, 2006)

2.3.4 Water Sorption.

Water sorption occurs in composites mainly due to the absorptive property of the resin, while the filler adsorbs water on its surface, no absorption occurs in the filler (van Noort, 2007 and Andrada et al, 2011). This water sorption results in unfavourable effects affecting the colour stability, which shows in the mouth as discoloration after it has absorbed oral fluids. Nevertheless, it is well understood that water absorption of dental composites in a wet environment can affect their mechanical properties. (Powera & Sakaguchi, 2006 and Draughn et al, 1985) In another words, water absorption relies on the type of resin matrix and the bond between the filler and resin (van Noort, 2007).

There are a number of possibilities that may indicate this behavior; first being that the material has a high-soluble fraction which may dissolve and leave spaces. Secondly, it could be that there are air voids in the resin, during mixing and lastly, it could be due to the hydrolytic breakdown of the bond between the filler and resin, which allows for water absorption (van Noort, 2007 and Wei et al, 2011). Stresses that are built during polymerization shrinkage can to some extent be relieved by the thermal expansion.

Microfilled composites absorb 2.5 times more water than the macrofilled composites (Bowen et al, 1982) and hybrid composites (Powers & Sakaguchi, 2006) owing to having an increased content of resin matrix. Water that is absorbed gets occupied in the spaces and voids in the composite resin material thus the effect of swelling are not pronounced (Bowen et al, 1982)

2.3.5 Solubility

In a set dental composite material, a considerable amount of monomer and short chain polymers remains uncured (Ferracane et al, 1994). When composite samples are immersed in solvents, some of their components, like the un-reacted monomer and filler leach out (Fan et al, 1985). This results in weight loss and can be classed as solubility or leaching. Leaching is normally thought to take place by diffusion of molecules through the resin matrix and is thus reliant upon the size and chemical properties of the eluted species (Ferracane et al, 1994). Inorganic ions like silicon, boron, barium and strontium are detected to leach at various degrees from various resin-filler systems, (Powers & Sakaguchi, 2006).

2.3.6 Colour and Colour Stability

Colour of composite restorations, being similar to natural tooth is the main feature and reason behind the common use of composites as restorative material. Composites are available in various shades to match the restoration to the surrounding tooth (Powers & Sakaguchi, 2006 and Van Noort, 2007). The shade of composite used affects the polymerization, depth of cure and many other mechanical properties of dental composites (Ferrcane et al, 1995). According to Qasim, (2011) smaller the filler particles the more would be the scattering of light and thus light has a difficulty to penetrate deeper regions of the material and require increased exposure time. On the other hand the ratio of filler to resin is also essential, higher the filler the more difficult it is for light to penetrate the composite.

The standard photo initiator gives a wide range of shades of composite, but for darker shades further photo initiator is admixed. These changes not only affect the materials composition, but also affect the optical properties of the material (Sigusch et al, 2012). Discoloration in composites is seen in three areas. Either it could be marginally around the restoration, or generally on the surface in which there are bigger filler particles, or lastly in the bulk. Discoloration occurs due to chemical breaking down of resin-matrix bond and absorbed fluids (van Noort, 2007).

2.4 MECHANICAL PROPERTIES

2.4.1 Strength and Modulus

Investigations and studies have shown that the mechanical properties of composites are dependent on the structure of dimethacrylate (monomer) incorporated in their resin system (Douglas et al, 1979). The flexural strength of a Bis-GMA based composite is higher in dry condition and lower under wet conditions (Kawaguchi et al, 1989). On the whole, composites are similar in strength and toughness to amalgam but better than porcelain,(Ferracane, 2011). Composites with high filler loading are much stronger, stiffer and tough. Fracture toughness of current composites measured is below 2.0MPa m1/2 (Ferracane, 2011, Kim et al, 2002) and is said to augment by increasing the filler content (Kim.KH, 2002). The minimum flexural strength, according to ISO 4049 is set to be 80MPa for polymer based filling material. According to Attar, (2003) average values of flexural strength of composites ranges from between 95.6-117.4 MPa (Attar et al, 2003).

Composites composed of pre-polymerized filler particles showed lower flexural strength and modulus, while composites that had rounded irregular-shaped or a mixture of both, had higher flexural strength and flexural modulus (Kim et al, 2002). Filler content in composites is directly proportional to the modulus of elasticity. The higher the modulus, the higher would be its resistance to deformation (Attar et al, 2003). The modulus of elasticity under compression is 19GPa for dentine and 83 GPa for enamel on the other compressive modulus of flowable and packable composites are 2.6-5.9 GPA and 5.8-9.0 GPa respectively (Powers & Sakaguchi, 2006).

2.4.2 Hardness

Hardness of resin composites is evaluated most commonly by the Knoop hardness test. This is when a load of 0.01N is applied. Willem et al, (1993) discovered that the hardness values of dental composites were found to be 2.9-8.8 GPa. The Knoop hardness values of fine particle composite are greater than micro fine particles and this is due to the difference in the volume fraction of filler particles (Powers & Sakaguchi, 2006).

According to various studies, filler content of 75.0wt% and filler average size of 0.7um is said to be the hardest restorative material (Schulze et al, 2003). The composition and distribution of filler size is the most important part in evaluating hardness (Schulze et al, 2003). Micro hardness values of modern composites are more reliable as they contain fillers with a smaller particle size (Powers & Sakaguchi, 2006). Studies have concluded that hardness values of chemically cured composites are less than light-cured composites (Schulze et al, 2003).

2.4.3 Bond Strength and Dental Substrates (Ceramics, Alloys, etc….)

The bond strength of self adhesives composites ranges from 22-26MPa while the conventional composites have a bond strength of 12-16MPa (Walter, 2002 and Powers & Sakaguchi, 2006). In the latter this bond is achieved by micromechanical retention using the bonding agents as intermediates (Powers & Sakaguchi, 2006). Dental composites can be bonded to other substrates apart from tooth tissue for example; existing composite restorations, ceramics and alloys. These substrates are roughened and primed before composite placement. Bond strength to treated surface is usually greater than 20MPa (Powers & Sakaguchi, 2006).

Bond strength depends on the orientation of the enamel prismatic rods and array of dentinal tubules (Walter, 2002 and Schulze et al, 2003). A hybrid layer of bonding resin and collagen forms in dentine and a good bond is achieved by the penetration of adhesive in the dentinal tubules. The bond between enamel is stronger than dentine. Hence, on curing the resin pulls away from dentine more, resulting in the formation of a void at the cervical margin (Davidson, 1984).

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