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A N EVALUATION OF COMMERCIAL A N D EXPERIMENTAL

RESIN-M O DIFIED GLASS-IONOMER CEMENTS

WIDCHAYA KANCHANAVASITA, BS, DDS, MSc

Submitted in fulfilment of the requirements for

the Degree of the Doctor of philosophy in the Faculty of Dentistry 1997

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ProQuest Number: 10106695

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ABSTRACT

Glass-ionomer cement (GIC) has become widely accepted as a restorative material due to its bonding ability and sustained release of fluoride. The cement is, however, sensitive to moisture imbalance and lacks toughness. Recently, resin-modified glass- ionomer cements (RMGIC) have been introduced. These materials contain monomeric species, such as 2-hydroxyethyl methacrylate (HEMA) in addition to the components of the conventional glass-ionomer cements. Disadvantages of RMGICs include a relatively high contraction and exotherm on polymerisation. HEMA is known to be cytotoxic, leading to problems of biocompatibility, and polyHEMA swells on exposure to water, leading to dimensional instability of the cements. Addressing these problems is im portant in the development of the RMGICs. Using alternative monomers to replace or reduce the amount of HEMA used in the current RMGIC formulations would be appropriate.

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TABLE OF CONTENT

A b stract i

L ist o f tab les vii

L ist o f figu res viii

L ist o f eq u ation s xi

C hapter 1 Introd u ction 1

C hapter 2 R eview o f the literatu re 3

2.1 Composite resins 3

2.1.1 Introduction 3

2.1.2 Composition 4

2.1.3 Chemically activated composites 8

2.1.4 Light activated composites 8

2.1.5 Degree of conversion 10

2.1.6 Properties 11

2.1.7 Advantages and disadvantages 14

2.2 Glass-ionomer cements 15

2.2.1 Introduction 15

2.2.2 Composition 16

2.2.3 Setting reaction 18

2.2.4 Monitoring of setting reaction 21

2.2.5 Properties 22

2.2.6 Advantages and disadvantages 28

2.3 Resin-modified glass-ionomer cements 30

2.3.1 Introduction 30

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2.3.3 Setting reaction 34

2.3.4 Monitoring of setting reaction 35

2.3.5 Properties 36

2.3.6 Advantages and disadvantages 44

2.4 Polyacid-modified composite resins 46

2.5 Mechanical properties and testing 47

2.5.1 Introduction 47

2.5.2 Comparisons of static strength tests 50

2.5.3 Uniaxial flexure tests of beam specimens 53

2.5.4 Biaxial flexure tests of flat plates 58

2.5.5 Sources of error in flexure tests 62

2.6 Hardness 63

2.6.1 Introduction 63

2.6.2 Microhardness tests 64

2.6.3 Vickers diamond microhardness test 65

2.6.4 The Wallace micro-indentation tester 65

2.7 Water sorption characteristics 66

C hapter 3 S tu d ies on com m ercial m aterials 73

3.1 Introduction 73

3.2 Water sorption characteristics 75

3.2.1 Methods 75

3.2.2 Results 78

3.2.3 Discussion 85

3.2.4 Conclusion 95

3.3 Microhardness 96

3.3.1 Methods 96

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3.3.3 Discussion 100

3.3.4 Conclusion 103

3.4 Static flexure tests 103

3.4.1 Methods 103

3.4.2 Results 108

3.4.3 Discussion 114

3.4.4 Conclusion 122

3.5 Temperature rise during setting 123

3.5.1 Methods 123

3.5.2 Results 125

3.5.3 Discussion 129

3.5.4 Conclusion 132

C h a p te r 4 S tu d ie s o n e x p e rim e n ta l m a te r ia ls 134

4.1 Introduction 134

4.2 Materials 140

4.3 Determination of the mixing ratio used to form cement 141

4.4 Polymerisation of THFMA monomer 145

4.5 Effect of monomer incorporation into cement formulation on working time

and setting time 149

4.6 Effect of initiator and activator inclusion on cement surface hardness 152

4.7 Temperature rise during setting 157

4.8 Determination of setting reactions of experimental cements using surface

hardness test 165

4.9 Water sorption tests 176

4.10 Effect of water storage on hardness of specimen surface hardness 187

4.11 Bi-axial flexure test 192

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A ck n ow led gem en t 203

R eferen ces 204

A p p endix A Composition o f artificial saliva 239

A p p endix B Comments on statistical analysis used in this study 240

A ppendix C Test results fo r the commercial RMGICs 243

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LIST OF TABLES

Table 2.5-1 Classification of static tests according to stress conditions in specimens 50

Table 3.1-1 Commercial RMGICs used in this study 73

Table 3.1-2 Composition of commercial RMGICs used in this study 74

Table 3.1-3 Materials selected to compare with RMGICs 75

Table 3.2-1 Equilibrium water uptake of RMGICs stored in distilled water 78 Table 3.2-2 Equilibrium solubility of RMGIC stored in distilled water 81 Table 3.2-3 Diffusion coefficient for RMGIC stored in distilled water 82 Table 3.2-4 Percentage volumetric dimensional changes at equilibrium of RMGIC

stored in distilled water 83

Table 3.2-5 Equilibrium water uptake and solubility for other materials 93 Table 3.2-6 Diffusion coefficient and percentage dimensional changes for other

materials 94

Table 3 3-1 Depth of indentation for specimens stored in distilled water 97 Table 3 3-2 Depth of indentation for specimens stored in artificial saliva 98 Table 3 5-1 Temperature rises produced by polymerisation reaction and by heat

from the light source 128

Table 3 5-2 Comparison between exposure times recommended by the manufacturers and the optimum exposures determined from this study 131 Table 4.1-1 Properties of THFMA, bis-GMA, UDMA and HEMA monomers 135

Table 4.2-1 Materials used in this study 140

Table 4.6-1 Initiator system used in commercial RMGIC formulations 155

Table 4.8-1 General experimental cement formulation 166

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LIST OF FIGURES

Figure 2.5-1 Bending moment and shear force diagrams for the flexure test 54 Figure 2.5-2 Determination of offset yield load in bending 57 Figure 2.5-3 Circular plate supported at periphery and loaded at the centre 60

Figure 2.6-1 Vickers diamond 65

Figure 2.7-1 Fickian and non-Fickian sorption curve 69

Figure 3.2-1 Percentage weight increase of RMGICs stored in distilled water during

first sorption cycle 79

Figure 3.2-2 Percentage weight increase of RMGICs stored in artificial saliva during

first sorption cycle 80

Figure 3.2-3 Percentage volumetric expansion during first sorption cycle of RMGICs

stored in distilled water 84

Figure 3.2-4 Percentage volumetric expansion during first sorption cycle of RMGICs

stored in artificial saliva 84

Figure 3.2-5 Representative plot of Mt/Mco against 91

Figure 3 3-1 Mean indentation depths for RMGICs stored in distilled water and

artificial saliva at 37°C 99

Figure 3 4-1 Four-point flexural strengths and moduli for Vitremer and Fuji IILC 109 Figure 3 4-2 Four-point flexural strengths and moduli for Vitrebond and

Fuji Lining LC 110

Figure 3 4-1 Four-point flexural strengths and moduli for Vitremer and Fuji II LC 112 Figure 3.1-4 Biaxial flexural strengths and moduli for Vitrebond and Fuji Lining LC 113 Figure 3.5-1 Temperature rise from polymerisation reaction and from light source 124 Figure 3 5-2 Maximum temperature rise during initial polymerisation of specimens

with different thicknesses 126

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of specimens with different thicknesses 127 Figure 4.1-1 Molecular structures of bis-GMA, UDMA, THFMA and HEMA 136 Figure 4.3-1 Working time of the cements obtained from different glass/acid and

powder/water ratios 142

Figure 4 3-2 Setting time of the cements obtained from different glass/acid and

powder/water ratios 142

Figure 4 4-1 Minimum level of CQ and DMPT needed to polymerise various

THFMA/bis-GMA mixtures 146

Figure 4 4-2 Minimum level of BP and DMPT needed to polymerise various

THFMA/bis-GMA mixtures 146

Figure 4.5-1 Effect of monomer in the liquid portion on the working and

setting time 150

Figure 4.6-1 Effect of CQ and DMPT concentration and time on hardness of

specimens 153

Figure 4.6-2 Effect of BP and DMPT concentration and time on hardness of

specimens 153

Figure 4.7-1 Temperature rises of light cured specimens prepared from different

monomer systems at different concentrations 158

Figure 4.7-2 Temperature rises of chemically-cured specimens prepared from

different monomers with different concentrations 159 Figure 4 7-3 Temperature rises of specimens prepared from different monomers

and cured by both light and chemical cure methods 160 Figure 4.8-1 Effect of monomer concentration and time on the Indentation depths

of specimens prepared using 100% THFMA 168

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of specimens prepared using 70/30 HEMA/bis-GMA 171 Figure 4.8-4 Effect of monomer concentration and time on the Indentation depths

of specimens prepared using 100% HEMA 173

Figure 4.8-5 The effect of monomer types and curing modes on hardness at 60 min (specimens prepared using 30% monomer concentration) 174 Figure 4.8-6 The effect of monomer types and curing modes on hardness at 60 min

(specimens prepared using 50% monomer concentration) 175 Figure 4.9-1 Percentage equilibrium water uptake of experimental cements 179 Figure 4 9-2 Percentage solubility at equilibrium of experimental cements 181

Figure 4 9-3 Sorption diffusion coefficient 183

Figure 4.9-4 Desorption diffusion coefficient 184

Figure 4 9-5 Percentage volumetric expansion during water immersion 185 Figure 4.9-6 Percentage volumetric contraction during desorption 186 Figure 4.10-1 Changes of hardness with time of specimens stored Ih before water

immersion 189

Figure 4.10-2 Changes of hardness with time of specimens stored 24h before water

immersion 190

Figure 4.11-1 Bi-axial flexure strengths for cements prepared from different

monomers and curing methods 193

Figure 411-2 Bi-axial flexure moduli for cements prepared from different

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LIST OF EQUATIONS

Equation 2.5-1 Four-point flexural strength 56

Equation 2.5-2 Maximum deflection at the centre of beam for the four-point flexure

test 56

Equation 2.5-3 Maximum deflection at the load points for the four-point flexure test56

Equation 2.5-4 Four-point flexure modulus 57

Equation 2.5-5 Bi-axial flexural strength 61

Equation 2.5-6 Maximum deflection at the centre of plates 61

Equation 2.5-7 Bi-axial flexural modulus 61

Equation 2.6-1 Vickers Hardness Number 65

Equation 2.7-1 Diffusion flux 66

Equation 2.7-2 Fick’s first law for one-dimension diffusion 67 Equation 2.7-3 Fick’s first law with constant diffusion coefficient 67 Equation 2.7-4 Fick’s second law for one-dimension diffusion 67 Equation 2.7-5 Fick’s second law where diffusion coefficient is independent of

concentration 67

Equation 2.7-6 Diffusion in thin plane sheets 68

Equation 2.7-7 Diffusion in thin plane sheets 68

Equation 2.7-8 Diffusion in thin plane sheets 68

Equation 2.7-9 Diffusion in thin plane sheets 68

Equation 2.7-10 Equation 2.7-7 where n = 0 70

Equation 2.7-11 Equation 2.7-10 where MJM^ = ^ 70

Equation 2.7-12 Approximation of diffusion coefficient 71

Equation 2.7-13 Equation 2.7-9 where t is small 71

Equation 2.7-14 Equation 2.7-13 where r = 71

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Equation 2.7-15 Equation 2.7-13 where A/,/A/« =0.5 71

Equation 2.7-16 Slope for the MJM^ - plot 71

Equation 2.7-17 Diffusion coefficient 72

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C H A PTE R 1

IN TR O D U C T IO N

Tooth-coloured filling materials have undergone considerable evolution during the past decades. The unsatisfactory and inadequate mechanical properties of the silicate cements and unfilled resins have led to the development of composite resins and glass- ionomer cements (GICs). The GICs, commercially available since 1972, have gained acceptance from the profession for use as restorative materials due to their ability to bond directly to tooth structure and provide sustained release of fluoride. However, the disadvantages of the conventional GICs include susceptibility of the immature cement to moisture contamination, vulnerability to desiccation during use and difficulty with the production of an acceptable colour match with tooth. Their mechanical properties such as strength, toughness and abrasion resistance mean that they are inappropriate for use in high stress areas.

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contraction and exotherm during setting are likely to be their disadvantages. Large shrinkage may damage the restoration, tooth structure or the marginal seal of the tooth/restoration system. High exotherm may cause irritation to the pulp. The leaching of unconverted monomeric components into the oral environment has caused a concern over their biocompatibility. Early work on these materials has shown that there are substantial volumetric changes with respect to time due to the large quantities of water absorbed by these materials. There is also evidence that there is a fall off in mechanical properties with respect to time. This is particularly apparent when these materials are immersed in complex solutions of various solutes.

Addressing these problems is important for the development of the RMGICs. Using alternative monomers to replace or reduce the amount of HEMA used in the current RMGIC formulations would be appropriate. In the development of a new material, the aims are to lower the polymerisation contraction during setting and reduce water sorption while maintaining the advantages of the current materials. Enhancement of the mechanical properties of the materials may also help improve the clinical performance such as abrasion resistance, bond strength and tolerance to occlusal stresses.

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C H A PTE R 2

R E V IE W OF T H E LITERATUR E

2.1 COMPOSITE RESINS

2.1.1 Introd u ction

The only aesthetic tooth-coloured direct filling materials available before dental composites were introduced were silicate cements and unfilled methyl methacrylate resins. Both materials suffer from deficiencies which limited their longevity in the oral environment. Silicate cements have a low pH for a long period after placement and have been shown to be irritant to the pulp(^\ Other deficiencies include high solubility in oral fluids, sensitivity to moisture, susceptibility to dehydration and inadequate mechanical properties'^). The unfilled resins, though less prone to erosion, have relatively large polymerisation shrinkages (6% by volume). The coefficient of thermal expansion of the resin (80-90x10 6/°C) is substantially greater than that of tooth tissue

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2.1.2 C om position

As defined by Bowen et alS^\ a 'composite' is a mixture or combination of at least two different classes of materials possessing properties that could not be achieved by any of the individual components alone. Modern dental composite resin materials are composed of a number of components, the major of which are an inorganic phase, an organic polymer matrix and a coupling agent. The materials also contain an initiator- activator system necessary for the polymerisation of the materials. Additives such as inhibitors, pigments, and UV stabilisers are also included to enhance the properties'^.

In o rg a n ic (o r d isp ersed ) p h ase

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The translucency of the fillers should be similar to that of tooth tissue in order for the restorations to achieve acceptable aesthetics. The refractive indices of most glasses and quartz used as fillers are about This value appears adequate to achieve sufficient translucency and is also close to that of the enamel (1.65), bis-GMA (1.55), TEGDMA (1.46), and UDMA (1.51)("'^^).

O rganic (o r m atrix) p h ase

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The extent of the shrinkage depends on the molecular weight of the monomers; the shrinkage increases as the molecular weight of the monomer decreases^^^^ due to a greater loss of volume. Therefore, dilution of bis-GMA increases polymerisation shrinkage. This limits the amount of the diluents which can be used in the composites.

An alternative resin system used in dental composites is based on UDMA. The most commonly used monomer of this type is the aliphatic l,6-bis(methacrylyloxy-2-ethoxy- carbonylamino)-2,4,4-trimethylhexan. This monomer has a molecular weight nearly equal to that of bis-GMA but it is less viscous, thus a diluent monomer is not required(^). The monomers are more flexible due to the urethane linkage and the absence of an aromatic ring structure, resulting in tougher polymer chains(^^\ Some commercial composite resins contain UDMA as the main ingredient in the resin phase with or without the use of diluent monomers while in other products alternative additional monomers, such as bis-GMA and TEGDMA are also included(^°\ The polymerisation shrinkage of UDMA is higher than that ofbis-GMA(^^).

In order to reduce the polymerisation shrinkage of the resin composites, several low- or non-shrinking monomer systems have been developed. These include bicyclic monomers such as spiro orthocarbonates^^^'^^) and oxybismethacrylates^^'^^\ The former undergo double ring opening with either no change in volume or an expansion. In the latter, a reduction in polymerisation shrinkage is thought to result from an orientation of carbon double bonds into a pseudo-cyclic conformation which occupies a larger free volume compared to a more linear structure. These resins are currently under evaluation with regard to mechanical properties of the polymers. None of these materials are currently used in commercial dental resins.

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Interfacial p h ase (cou p lin g agent)

Coupling agents, using chemical bonding, have been used to achieve bonding between filler and resin^^\ The most commonly used coupling agent in dental composites is 3- methacryloxy-propyl-trimethoxy silane (MPS) which is a bifunctional molecule containing methoxy (OCH3) and methacrylate (C H 2= C H -) groups. During the deposition of the silane on the fillers, the methoxy groups hydrolyse to hydroxyl groups. These can react with the hydroxyl groups on the filler surfaces or with the hydroxyl groups on the adjacent hydrolysed silane to form a homopolymer film on the filler surface(^°\ During polymerisation, the methacrylate groups on the silane react with the methacrylate groups on the resins, forming covalent bonds and binding the matrix and the fillers.

The coupling agent forms a means of continuous stress distribution between the fillers and the matrix, allowing the more plastic polymer matrix to transfer stresses to the more rigid filler particles(^^\ It may also provide hydrolytic stability of the composites by preventing water penetration along the filler-resin interface(^).

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2.1.3 C hem ically activated co m p o sites

Early composites were cured by ‘chemically activated’ polymerisation. Benzoyl peroxide (BP) is commonly used as an initiator in this system although other peroxides such as acetyl peroxide, alkyl peroxides, and hydroxyl peroxides in the amount 1-2% on the monomer have been used^^^»^^. Decomposition of benzoyl peroxide into free radicals occurs slowly at room temperature and therefore the incorporation of activator, usually an aromatic tertiary amine, is required. The initiator and the amine are contained in two separate pastes to prevent their reaction(^^\ On mixing the two components, the amine and peroxide react at room tem perature to form free radicals. These radicals initiate the polymerisation process which proceeds uniformly and rapidly throughout the mass of the material. The viscosity of the mix increases immediately due to the rapid setting reaction^^^^ If the insertion of the material into the cavity is delayed, good adaptation of the material may not be obtained.

2.1.4 Light activated com p osites

More recently, materials are supplied as a single component, containing both the light- sensitive initiator and an activator. The addition polymerisation is initiated by the generation of radicals by the initiator system after it has absorbed photons from an external light source^^^^ This facilitates a longer working time compared to that of the ‘chemically activated’ materials, together with a 'command setting'. Moreover, materials are produced as a single paste which is frequently encapsulated. No blending is required and porosity is therefore reduced.

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dimethyl-p-toluidine (DMPT) in the amount 0.1-1.4 wt% is also included in the system as a co- initiator(^°\ Upon activation by the photon energy to the carbonyl group, the diketone goes into an excited state^^^l At an appropriate excited state, camphorquinone combines with the amine to form an excited complex. The breakdown of this complex gives two types of free radicals which initiate the polymerisation, one originating from the amine and the other from the ketone^^^»^). Higher levels of the initiator are found in hybrid and conventional composites compared with microfilled composites on the basis of weight percentage^^®'^^ Since the volumetric filler content of the former is higher than the latter, the light is more scattered by the fillers, resulting in a greater reduction of the light intensity at depth. Therefore, a greater amount of camphorquinone is required. In addition, the higher filler content could reduce the yellow colour more effectively, allowing for more incorporation of camphorquinone.

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2.1.5 D egree o f co n v ersio n

The polymerisation reaction is never complete, i.e. a certain number of double bonds remain in the polymeri^^\ The di methacrylate monomers used in the dental resin- based materials polymerise to form a three-dimensional network containing unreacted pendant methacrylate groups. The lengths of these pendant groups vary depending on the structure of the monomers which may be linear, branch, or with aromatic rings in the chain. The degree of conversion (DC) is a measure of the percentage of the carbon double bonds (C=C) converted into carbon single bonds (C-C) during polymerisation. Infrared spectroscopy (IR) has been the principal method of determining the degree of conversions^). The decrease in the C=C absorption in the region 1635-1640 cm** after the addition polymerisation is determined and compared to the same absorption in the unpolymerised materiah^^). If the ratio of unreacted C=C groups after and before polymerisation is R , then the percentage degree of conversion is (1 • R ) x lOO. The degree of conversion in the range 50-70% is commonly obtained by this method when the composite resins are polymerised at room temperatures^^). Recently, Watts DC et aZ.S'^) reported that the solid-state '^c-Nuclear magnetic resonance (NMR) also provides an accurate method for determination of the degree of conversion in dental resins and adhesives if the correct technique is employed.

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linked. The lower degree of conversion for dimethacrylates is also attributed to the more restricted movement of the monomers^'^^).

Asmussen^'^^) showed that in ‘chemically cured’ systems, the degree of conversion increased with increasing concentration of amine and peroxide and decreased with increasing amount of inhibitor. In the light activated system, Yoshida and Greener<^^) showed that there were several combinations of camphorquinone and amine concentrations that produced the maximum degree of conversion. From an aesthetic consideration, the concentration of camphorquinone and the amine should be as low as possible to reduce the yellowness and discolouration of the restoration.

2.1.6 P rop erties

W ater so rp tio n and solu b ility

Water sorption of composites ranges from 0.5-2.0 wt% depending on the types of materials. Microfilled composites absorb more water than conventional and hybrid composites due to a greater volume of resin present in the m a t e r i a l T h i s can lead to a greater expansion. Most composites absorbed 70% of the equilibrium uptake within 5 days after which the sorption slows dramatically^'^®'^®^. Water solubility of composites ranges from Q.5-2.0 wt%(^^\ For the light activated composites, adequate light exposure is important since this results in adequate polymerisation which reduces water sorption and solubility^^^).

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Water sorption has been identified as the principal factor in the chemical and mechanical degradation of resin composite materials. Water can cause degradation of the fillers, filler/matrix interface and the matrix. A reduction in the tensile strength and wear resistance of the materials is observed. In distilled water, the inorganic species leached from composites include silicon, boron, barium, strontium and lead, as a result of hydrolytic degradation of the filler particles^^^*^^. Glass-modifiers such as barium, strontium, and sodium reduce the stability of the fillers, thus increasing the leaching rate(^^'^^\ Degradation of the fillers also results in a build-up of osmotic pressure at the filler/matrix interface which leads to separation of fillers from the matrix(^^. Composites release more fillers when stored in artificial saliva than in distilled water due to an ion exchange mechanism between the negatively- charged ions on the filler surface and the positively-charged ions in the artificial saliva(^^\ This implies that the oral environment is likely to cause more pronounced filler degradation than indicated by storage in distilled water.

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risks of pulpal toxicity of these components depend on the quantities which diffuse through dentine and accumulate in the pulp.

Su rface m icro h a rd n ess

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2.1.7 A dvantages an d disadvantages

Advantages of composite resins, particularly light activated materials, include extended working time and command setting, better colour stability, ability to obtain a polishable surface and superior physical and mechanical properties. They have many clinical applications. However, two major drawbacks of these materials are polymerisation shrinkage and exotherm.

Polymerisation shrinkage of composite resins, although significantly less than that of unfilled resins, has been reported to be in the range 1-4 vol%, depending on the products and the methods used to determine the shrinkage. The polymerisation shrinkage can potentially cause marginal discrepancies when adhesives are not used. However, when the materials are used in conjunction with adhesives, internal stresses developed between the restoration and cavity walls may exceed the cohesive forces within the materials, resulting in the cohesive failure of the materials themselves(^). Other possible consequences of the shrinkage are deformation or fracture of the tooth. The cusps of premolars and molars were reported to deflect inward for a distance of 15 to 40 pm after placement of composite resins^^'^^^). This has been thought to cause post-operative tooth sensitivity. Polymerisation exotherm of some composites has also been reported to be as high as 12°C for light activated materials^^^) and 2-3°C for chemically activated composites^^^).

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Resin-based materials also have a problem with the inhibition of polymerisation by oxygen(^^\ Oxygen has a strong potential to react with the free radicals once they are formed, thus inhibiting the ability of the radical to initiate the polymerisation process. This results in the formation of an air inhibited layer on the surface of the resin, usually when the resins are cured without a matrix band. The thickness of this layer is dependent on the composition of the resin and the initiator used(^^\ Viscous materials which contain a lower amount of the diluent had a thinner unpolymerised layer than the low viscosity materials^^^). The inhibited layer has similar composition to the uncured resin except that the initiator-activator complex has been consumed^^) and therefore it still has the potential for polymerisation if supplied with sufficient free radicals since the initiators can diffuse into this layer and induce further chemical reaction.

2 .2 GLASS-IONOMER CEMENTS (GIG)

2.2.1 Introd u ction

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This new material was given the name 'glass-ionomer cement' by Kent(^\ This has also been used as a generic name for all glass polyacid cements, including polycarboxylate and polyphosphonate-based materials^^). The word ‘ionomeri refers to a polymer containing a small proportion, usually 5-10%, of ionisable groups(^) and does not properly apply to the components of GIC since polyacrylic acid, which is one of the component of the cement, is a polyelectrolyte rather than an ionomer. The systematic name recommended by the International Organisation of Standards for GIC is ‘glass poly(alkenoate) cement’. However, this terminology does not apply to the glass polyphosphonate cement invented by Ellis and Wilson(^) and has never become as popular among clinicians as its original name. Throughout this thesis, the term 'glass- ionomer cement', abbreviated as GIC, will be used.

GICs have a variety of clinical applications, including restorations of cervical lesion and anterior teeth, lining or basing of cavities under composite or amalgam restorations, fillings of occlusal pits and fissures, cementation of crowns, bridges and inlays, and core build-up(^).

2 .2 .2 C om position

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The relative ratios of silica, alumina and fluorite present in the glass affect its properties. The range of the silica/alumina ratio which provides a glass with both translucency and cement forming capability is limited. Glasses high in alumina or fluorite are opaque. Clear glasses are obtained only when the alumina/fluorite mass ratio is approximately 1:1 but the silica/alumina mass ratio exceeds 1.3:1^“ ^ The glasses have the potential of forming a cement matrix when the silica/alumina mass ratio is 2:1(^\ Below this value, the cement will form rapidly due to the high reactivity of the glass. If the ratio is 3:1 or more, the glasses will not be susceptible to acid attack and will not release metallic ions.

The glass may be made radiopaque by incorporating heavy metal ions, such as barium, lanthanum, strontium or zinc into the formulation, although some of these elements may have adverse effects on the properties, e.g. opacity, of the set cement.

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and a rapid set. An increase in chain cross-linking may also occur in the set cement which may lead to better physical properties.

The molecular weight of the polyacids influences the viscosity of the liquid. Increasing the polyacid molecular weight not only increases the strength of the cement but also results in a higher viscosity solution, making it more difficult to mbc with the powder. This problem may be overcome by using vacuum-dried polyacids(^^\ These dried acids are incorporated into and blended with the glass powder component. The powder is then mixed with the liquid which is either distilled water or an aqueous solution of (+)- tartaric acid. In this way, high molecular weight polyacids can be used without the handling property of the cement being affected. This results in a cement with improved physical properties(^°'^^\ These materials are the so-called 'water-hardening' GID®^^.

2 .2 .3 S ettin g reaction

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As the concentration of the metal ions in the solution increases, the pH of the solution rises^^^. The polyacid chains, previously in a random coil conformation due to the hydrogen bonding between the carboxylic acid groups, extend, increasing the viscosity of the paste. This chain extension allows greater access for the combination of the metallic ions to the carboxylic acid groups. The resulting cross-linked, insoluble polyacid salts precipitate into a sol and then convert into a gel, forming the cement matrix. This occurs within 5-6 minutes after mixing and is recognised as the initial or clinical set^^^l The majority of the polyacid salts initially formed are calcium poly(acrylate)s which are more soluble than aluminium poly(acrylate)s formed at a later stage. This is clinically relevant because at this stage the soluble matrix is readily affected by water. Moreover, there are some ions which have not reacted, as well as those which are in the process of reacting, with the polyacids. These are all in a soluble state. If the initially set cement contacts with water, the soluble ions will be lost and the formation of a strong matrix is prevented. Therefore, suitable surface protection to prevent direct contact of the cement surface with water, such as varnish or light-cured unfilled resin, is required^^^"). After the initial set, the cement gradually hardens as more aluminium ions are bound to the polyacid chain(^°°). During this period, a number of changes in cement properties occur, including increased translucency, strength and hardness as well as a decrease in the sensitivity to moisture. This post-hardening process continues for months after the cements have been formed^^^.

Recently, a cement-forming role of the siliceous species released from the glass has been suggested by Wasson and Nicholson^^-^’^^^). The formation of the silica gel in the matrix indicates that the secondary setting reaction, other than the primary neutralisation of the acid-base, contributes to the hardening and stability of the set cement. This theory has been confirmed by Wilson(^°^) and Matsuya et

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The structure of the set cement comprises two phases:- the cement matrix and the glass core. The matrix comprises the hydrated fluoridated calcium and aluminium poly(aciylate)s, silica gç\(93.95.ioi-io3) and/or aluminium phosphate geh^“ \ There are also some unreacted carboxylic groups as well as fluoride dispersed homogeneously throughout the matrix^^^\ The core is the partially decomposed glass particles sheathed in a layer of the siliceous hydrogeh^^'^°^\ The volume fraction of the unreacted glass may be as high as

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(+)-Tartaric acid plays an important role in improving the setting characteristics of the GIC. A GIC without this acid has a short working but a long setting time(^°^\ Tartaric acid is a stronger acid than polyaciylic acid. It forms complexes with calcium ions, preventing premature formation of calcium polyacrylates. Thus the working time is prolonged. It also accelerates the hardening process by increasing the rate of extraction of A13+ from the glass particles and the formation of aluminium polyaciylates^^\ (+)- Tartaric acid also inhibits ionisation and unwinding of the polyacid chain, hence delaying the onset of gelation.

2 .2 .4 M on itorin g o f settin g reaction

(36)

2 .2 .5 P roperties o f g la ss-io n o m er cem en t

A d h esio n an d b o n d stren gth

Adhesion of GICs to untreated enamel and dentine is established by the adhesive property of the polyacrylic acid and is probably a diffusion-based process(^^\ The carboxyl groups of the acid displace the surface calcium and phosphate ions on the hydroxyapatite in the tooth structure, forming an intermediate layer rich in calcium and aluminium phosphates and polyacrylates at the cement/hydroxyapatite interface(^®9\ This ion-exchange process is dynamic, i.e. the bond is broken and reformed when chemical and biological changes take place^^^^l This makes the interface more resistant to breakdown under oral conditions. Since enamel is richer in hydroxyapatite than dentine, the bond between the cement and enamel is stronger than that between the cement and dentine. GICs have also demonstrated in vitro bonding to type I collagen which makes up the matrix in dentine("^). However, the result is questionable since fundamental changes to the nature of the collagen may have occurred during its extraction and it may not represent the properties of the collagen in dentine in vivo.

(37)

GICs^^®^. The acid also removes the smear layer but can leave the smear plugs in the dentinal tubules intact(^^'^^^) and produces less demineralisation.

The bond strength of a material is a measure of the interfacial adhesion between the material and the substrate, with or without the use of an adhesive. This is determined by the force, usually shear or tensile, required to fracture or break the adhesive bond and is interpreted as force per unit area. In practice, the fracture usually occurs in some area other than at the interface, such as in the material itself, in the substrate, or may extend beyond the interface area. The reported shear and tensile bond strengths of conventional GICs to enamel and dentine are typically in the range 3-5 MPa and 2-4 MPa, respectively^"^'"'^). However, the failure has been found to occur within the cement structure rather than at the interfacial surface. Therefore, the bond strength values obtained from those tests represents the cohesive strength of the materials themselves. The true adhesive bond to enamel and dentine is probably greater than these values. Despite the low bond strengths of GICs in vitro, the ability of these materials to bond to dentine in Class V cavities is superior to many composite resins with dentine bonding systems("^\

S trength

(38)

and a decrease in the strength. GICs were reported to retain their strengths for a period up to 1 year when they were stored dry at room temperature and humidity, and the strength values were greater than specimens stored in water for the same periods^^^®^ However, dry storage of the cements is not clinically relevant. In general, there are no definite patterns of long-term changes in strength of GICs when stored in water. The cements may exhibit a rapid increase in strength. After this the strength may be either gradually increase or decrease, or a fluctuation of the strength values occurs over time("^'^^°-^^\ The time at which the GICs showed a rapid increase in strength generally occurred within the first 24 hours(^^^\

Compressive and diametral tensile tests which are used in most standards testing appeared to be less suitable for GICs since they are less discriminatory than the flexural tests(^^'^^^\ This was demonstrated in a study where the mean values of compressive and diametral strengths obtained from different test centres showed a wider variation than those of the flexural strengths^^^^. However, both compressive and diametral tests have been used extensively due to the simplicity of the tests and the specimen preparations. Compressive strength values of the cements have been a criterion to indicate clinical acceptability and material quality. The low values of flexural strengths of GICs, when compared with those of composite resins and amalgam, suggested that the cements may not be suitable for use in high-stress areas.

M icroh ard n ess

(39)

decrease in hardness for all specimens due to chemical dissolution; these specimens were softer than those stored in water. Yang and Chan(^^(^) found that application of varnishes on GICs reduced their hardness whereas the reverse was shown by O’Hara et

Woolford(^^^) reported that raising the temperature of the surface of the GIC to a maximum of 60°C significantly improved the early surface hardness, as well as the hardness at 24h, of the cement. This result may be due to the acceleration of the matrix-forming reaction by the heat.

W ater so rp tio n and solu b ility

Crisp et showed that GICs absorbed water rapidly during the first week (3.2 wt% in 24h and 3.8 wt% after 1 week). As the cement aged, the weight gain was relatively constant. The gain in weight has been attributed to water sorption which causes a swelling of the cross-linked polyacrylate matrix(^^^\ Recently, Forss(^^^) has shown that the weight gain of some GICs was continuous, both in distilled water and in acidic solution, up to a period of 122 days. The expanded matrix can partially counteract the setting contraction, but at the expense of the impaired physical properties.

D im en sio n a l stab ility

(40)

Among restorative materials, GICs probably match tooth tissue most closely with respect to the coefficient of thermal expansion (GIC = 8-llxlO'^/°C(^'^^); dentine =

8 .3xlO'V°C; enamel = 12xlO'^°C(^)). Hence, the materials expand and contract as much as adjacent tooth structure which reduces the risk of gap formation and microleakage between the tooth and the restoration(^'^^\

R e le a se o f ch em icals

The species leached from GICs are dependent on the composition of the cement and pH of the medium(^'^^). Since GIC is composed of inorganic and organic components, both are expected to elute from the cements. The organic component, i.e. the polyacid or polyacrylate, however, is immobile due to its high molecular weight. Therefore, negligible amounts were found to be leached out from the cements^^'^). Most of the species found in the storage medium were inorganic, i.e. the components of the glass^^^^'^^^^. These species are mobile and some are involved in the setting reaction of the cements.

(41)

Ideally, the release of ions, particularly F , in the aqueous environment should be controlled by a diffusion process and not by the dissolution or erosion of the material. The release of F has been shown to occur in two phases, i.e. an initial surface washout followed by a long-term bulk diffusion(^^°\ The set GIC released a large amount of F initially but the rate declined to a lower level. This rate of release was sustained for a long period of time(^^^\ GICs which release a greater amount of F also release more A13+Ü49), The presence of Al^+ has been reported to affect the determination of F release from GICs if the appropriate method is not carried out since this ion is readily form stable complexes with GICs release less fluoride in artificial saliva than in distilled water, probably due to the presence of various cations in artificial saliva(^%^^^\

B iocom p atib ility

GICs are biocompatible with tooth structure and have low irritancy to pulp tissue^^'^^^ Irritation to the pulp when the cement is placed in the cavity is minimaF^^). This is because polyacrylic acid is a weak acid and its high molecular weight and chain entanglement make it difficult for the acid to enter the dentinal tubules(^^^\ However, an initial mild inflammatory response has been found in some cases^^^^) but this resolves rapidly within 30 days provided that there is no bacterial involvemenb^^. In deep cavities where less than 1 mm of sound dentine remains, sublining with calcium hydroxide before placement of GICs has been recommended. However, the amount of the lining material present should be minimal to avoid the reduction of the adhesion.

(42)

Most of the ions leached from GICs are biocompatible and positively beneficial to the body. For example, Ca^+ is the main constituent of the hydroxyapatite and F has a cariostatic effecb^^^^. SM+ is benign to the body and Al^+ showed no evidence of cytotoxicity to the cell culture(^^\ possibly as a result of the formation of non-toxic complexes with the Since these ions are released from GIC at the very low leveF^'^^-^'^^, they are likely to be harmless to the body. In addition, the toxicity of these ions may be reduced by the buffering capacity of dentine.

A GIC commonly used for bone reconstruction (lonocem) has been reported to cause aluminium encephalopathy in two cases(^^^\ It is important to stress that this was the result of an incorrect use of the material. The release of Al^+ from this material when in contact with cerebrospinal fluid was substantial. The high concentration of this ion in blood and cerebrospinal fluid was responsible for the disease.

T em perature rise d u rin g settin g

The acid-base setting reaction of the conventional GICs is exothermic. However, the maximum temperature rise during the setting reaction for these materials is small; the reported values were in the range 1-4°C for the hand-mixed GICs(^^'^^'^^\ The values were lower than those for the zinc phosphate cements and composite resins(^^'^^\ The tem perature increased slowly, the maximum temperature rise occurring several minutes after mixing(^^^\ In the encapsulated materials, the heat attributable to mechanical mixing caused a relatively high initial rise in temperature (5-7°C)(^(^).

2 .2 .6 A dvantages and disadvantages

(43)

expansion of these cements matches that of the tooth structure^^^^^. The exotherm on setting is minimah^^X They bond chemically to both enamel and dentine^^^'^^^. The materials for use in non-loaded cavities, such as cervical lesions, are superior to composites used in conjunction with the second- and third-generation dentine bonding agents(^^^\ Fluoride has been reported to be released from GICs at a constant rate for more than five years(^^^\ The fluoride is believed to inhibit certain bacterial growth, thus providing an antimicrobial effect in the caries('^°\ However, Deschepper et indicated that the fluoride release may not be as im portant to the antibacterial mechanism as the initial low pH caused by the cement.

Several clinical investigations have shown that secondary caries rarely developed adjacent to GIC restorations(^^'^^). There is also evidence from in vitro studies that GICs not only protect the cavity wall from artificial caries formation but also reduce the extent of the lesion adjacent to the restoration(^^^»^^^\ However, a recent report based on a survey of the reasons for replacement of the restorations from general dental practitioners(^^^) showed that secondary caries have been diagnosed on marginal areas of 50% of the GIC restorations. The mean age of the GIC restorations was 5 years while those for composites and amalgam were 8 and 10 years, respectively. The inherent weakness of this survey included the possibility of false-positive diagnosis of secondary caries due to non-calibrated examiners and the skill of the examiners.

(44)

the cement surface may craze and contraction occurs if the cement is left dry<^7-i39). Finally they are brittle and their mechanical properties, such as strength, toughness and abrasion resistance, are inferior to posterior composite resins and amalgam. Therefore, they may be not suitable for use in high-stress areas^^^l Qvist et

reported that more failure (37%) of conventional GIC restorations in primary teeth were observed after 3 years of service compared to amalgam restorations (18%). They concluded that conventional GIC is not an appropriate alternative to amalgam for all types of restorations in primary teeth, especially in class II restorations, due to the short longevity despite the reduced caries progression and the need for restorative therapy of adjacent tooth surfaces.

Since their invention, GICs have undergone continuous improvements and modifications both in the formulations and in the placement techniques. These included the addition of inorganic or organic species into, or modification of the component of, the powder or liquid portions of the conventional materials. One of the major modifications has been the reinforcement of the conventional GIC with metals or other materials in order to improve the strength and toughness so that this material m aybe used in the posterior region of the mouth^^®^'^“ \ Another modification has been the incorporation of polymerisable resins in the formulation so that the material will set by a polymerisation reaction and have improved mechanical properties(^^'^^\

2 .3 RESIN-MODIFIED GLASS-IONOMER CEMENTS (RMGIC)

2.3.1 In trod u ction

(45)

particles to those similar to conventional GICs with some incorporated monomers. This has led to a diverse group of materials with widely different properties appearing on the market. These new materials have caused considerable confusion to dentists. Several terms have been used by the manufacturers to describe their products, for example, light-cured or light-curable, dual-cure, tri-cure or triple-cure, resin- reinforced, resin-ionomer, and resin-modified. Some of these terms {e.g. dual-cure, resin-ionomer) may give an impression to the dentists that the materials are in the same class as the 'true' glass-ionomer cement and the acid-base reaction contribute to the setting processes. Others {e.g. light-cured or light-curable) improperly implies that the acid-base reaction can be photo-initiated.

According to McLean et typical resin-modified glass-ionomer cements are glass- ionomer materials where the acid-base reaction occurring during the setting reaction is complemented by a polymerisation reaction, which may be chemically or light induced. These materials are capable of setting without polymerisation reaction, but the process is slower than the conventional GIC and an inferior material will be formed. Throughout this thesis, the term 'resin-modified glass-ionomer cement', abbreviated as

RMGIC, will be used.

2 .3 .2 C om p osition

(46)

the resin matrix to the GIC matrix so that the two different setting mechanisms lead to an interpenetrating network.

The first method was introduced by Mathis and Ferracane^^^^ with the ideas of reducing the brittleness of GICs by adding organic additives and a free radical initiator system into the GIC liquid. They combined the liquid component for the commercial dental composites with that used in a commercial glass-ionomer cement (13/87 wt% ). The mixture was added to the glass to produce a hybrid glass-ionomer/composite^^^. Although physical and mechanical properties of the cement were improved, this simple combination of the two systems was unlikely to function as the primary monomeric components used in composite resins are hydrophobic in nature, thus being incompatible with the aqueous environment present in the conventional GICs(^^\

The system using water-soluble or water-compatible vinyl monomers, such as 2- hydroxyethyl methacrylate (HEMA) was introduced by McKinney and Antonucci^^^^) and Antonucci and Stansbury<^^°). The formulation was found to have improved wear resistance, higher diametral tensile and compressive strength, reduced moisture sensitivity during setting and degradation after exposure to aqueous acids, and showed higher measured bond strength to dentine and composites than conventional

GICsC^'^90),

(47)

The polyacrylate matrix is formed at a later stage as a result of the acid-base reaction and may serve to harden and strengthen the already set cement(^^\ Thus, the set cement consists of glass particles and two matrices which are different in nature, i.e. an ionomer salt hydrogel which is hydrophilic and polyHEMA which is hydrophobic. For thermodynamic reasons, the two matrices may not interpenetrate, resulting in phase- separation as suggested by Wilson^^^) and Nicholson and Anstice^^^^\

Another version of RMGIC has been developed by Mitra^^^) to prevent phase- separation in the set cement. In this system, the polyacid co-polymer is modified by substituting some carboxylic acid groups by methacrylate groups capable of free radical polymerisation. Other polymerisable monomers such as HEMA are also incorporated into the liquid component to serve as a co-monomeri^^'^^^). This system has been shown to have high early strength, improved water resistance, and higher bond strength to tooth structure compared to conventional GICs^^’^J. The amount of fluoride release is similar to the conventional GICs(^^^\

(48)

2 .3 .3 S ettin g reaction

The setting of RMGICs involves two different reactions, i.e. an acid-base reaction of the conventional GIC and a free-radical polymerisation reaction typical of the composite material. The latter reaction is designed to supplement the reaction of the former and can be chemically and/or light initiated. This setting mechanism was experimentally confirmed by Mitra et using Fourier Transform Infrared Spectroscopy (FTIR) and Bourke et using Differential Thermal Analysis (DTA).

Recently, products have appeared which have been claimed to utilise a 'tri cure' or a triple-curing mechanism. The manufacturers incorporated an initiator for a self-cure, free radical polymerisation into the formulations in addition to that for light-initiation. However, this term is misleading since the materials only have a two-mechanism cure where one of the process, i.e. the polymerisation reaction, has two possible modes of initiation - chemical and light initiations^).

In practice, the acid-base reaction and the polymerisation cannot take place without reference to each other. Eliades and PalaghiasS^^^) and Kakaboura et

(49)

that the specimens had not set after 1 houH^^\ As expected from these studies, if there is too little water or no water in the RMGIC system, the acid-base reaction cannot occur and only the polymerisation reaction will take place when properly initiated.

A possible clinical implication of the reduced rate of the acid-base reaction might be a prolonged acidic behaviour of the cements to the surrounding tissues. This can be determined by measuring the pH of the setting cements. The initial pH of RMGIC liner/bases ranged from 2 to 6 during the first 90 minutes followed by a slow increase over 24 hours(^°^'^°^\ These values were comparable to those of conventional GIC

liners.(204.205).

The polymerisation reaction in some RMGICs where the modified polyacid is used, has been shown to be enhanced by the acid-base reaction through the steric orientation effecb^^^^. The suggested mechanism was based on the preferential orientation of the modified polyacid chains, the carboxylic acid groups being towards the glass particles. This brought the pendant methacrylate groups into a position favourable for a cross- linking reaction.

Another feature of the setting reaction of RMGICs is a tendency for the reaction mixture to phase separate as the reaction proceeds(^^^\ This may be explained as follows. When HEMA undergoes polymerisation, its solubility in water decreases. In addition, HEMA becomes less soluble when the pH of the mixture increases as a result of neutralisation of the acid-base reaction^^^^l This also leads to the product containing domains of different phases, i.e. the ionic discrete assemblies in the organic matrix.

2 .3 .4 M on itorin g o f settin g reaction o f RMGICs

(50)

photo-polymerisation reduced the acid-base reaction during the early setting stages. Kakaboura et also found that the degree of conversion in these materials, measured immediately after irradiation, ranged from 33-50%. Irradiation after 20 min of dark/dry storage significantly reduced the degree of conversion. Although IR spectroscopy can provide the evidence of whether a glass-ionomer reaction takes place in the cements, the method is not suitable for reliable quantification since the typical absorption pattern of the acid-base reaction are obscured by the presence of water and

HEMA(207).

Most dental materials undergo setting reactions which involve heat generation. The changes in tem perature can be monitored using thermocouples. Differential Scanning Calorimetry (DSC) or Differential Thermal Analysis (DTA). These methods are useful when the materials under investigation have a marked exothermic setting reaction. The calorimetric studies showed that the temperature rises during setting of some RMGICs, particularly the liner/bases, were greater than those of the composites^^’^'^®^ The presence of the photo-polymerisation and the acid-base reactions were clearly shown by this method(^°^'^°^\

2 .3 .5 P roperties o f resin -m o d ified g la ss-io n o m er cem en ts

A d h esio n and b o n d stren gth

(51)

'^^2iyi67;iii-2i5)^ Failures occurred cohesively within the materials which indicated that the interfacial bond strengths may be higher than the inherent strengths of the materials. Higher bond strengths of RMGICs compared to conventional materials may relate to their higher cohesive strengths. Generally, the bond strengths of RMGICs to those conditioned enamel were higher than to conditioned dentine(^"\ This is probably due to the micromechanical interlocking with the etched enamel.

RMGICs also showed significantly higher bond strengths to conditioned than unconditioned dentine^^^^'^^^). Pretreatment with polyacrylic acid improves adaptation(^°), removes the smear layer and partially déminéralisés the dentine which helps HEMA present in the materials penetrate the exposed collagen fibres^^^). HEMA has been reported to react with the collagen in dentine both mechanically by entangling the demineralised dentine matrix and chemically via hydrogen bonding^^»^\ Prado et alS?24) found no difference in shear bond strength of RMGICs to dentine when dentine was conditioned with either 10% polyacrylic acid or 10% phosphoric acid. Shear bond strengths of RMGICs to conditioned enamel and dentine were not adversely affected when the materials were stored in water for a period up to 6 months(^"\

(52)

One study^^^ has shown that the thickness of some lining materials has an effect on the bond strength of RMGICs to dentine; thin layers of materials resulted in higher bond strength. This may be due to a greater cure in the thin specimens compared to the thicker ones.

S tren gth p ro p erties

The initial compressive, tensile^^^'^*^^ and flexural strengths^^^»^^) of RMGICs have been shown to be greater than those of conventional GICs. The toughness and fracture toughness of RMGICs were also higheri^^-^), implying less brittle behaviour of these materials. The monomeric components present in the materials lower their elastic moduli and increase their resistance to crack propagation.

(53)

M icro h a rd n ess

Surface hardness has been used as a measure of the extent of the setting reaction of RMGICs. Bourke et found that for one RMGIC the ultimate hardness was significantly higher than that obtained at the termination of light activation, indicating th at the post-hardening reaction occurred in that material. In the other cement, no evidence of the post-hardening was observed. The hardness of RMGICs was significantly greater when light-cured than when allowed to set without irradiation^^). The hardness of some RMGICs was similar to that of composites(^). The surface hardness tests have also been used to evaluate the depth of cure of the materials^^»^^. Immediately after light activation, the upper surfaces of RMGICs stored in distilled water at 25°C were harder than the deeper layers, but the hardness in the deeper layers increased to that of the superficial layer within 7 days^^^). RMGICs generally attained maximum surface hardness Id after light irradiation^^^»^®). As in the case of composite resins, specimen thickness, exposure times, and distance from the light source affected the hardness of RMGICs^^^).

The effects of storage medium on the hardness of RMGICs have been investigated by Mante et RMGICs were stored in distilled water, ethanol, heptane, and O.IN NaOH solution. After 30d, the surface hardness of all RMGICs decreased.

(54)

W ater so rp tio n

When they first appeared on the market, RMGICs were thought to be less water sensitive than conventional GICs due to the formation of the polymerised network and did not require surface p r o t e c t i o R e c e n t l y , RMGICs have been shown to absorb water and swell in an aqueous environmenb^^^'^'^^^. This finding can be explained in term s of the underlying chemistry of the materials. The matrix of the set cement contains a polymerised network of HEMA as well as a polyacrylate network. Poly(HEMA) with its high proportion of hydrophilic hydroxyl groups has a strong affinity for water. This results in the cement behaving as a hydrogel, i.e. a water- swollen polymer. Synthesised poly-HEMA hydrogel has been shown to absorb a large amount of water (typically 40 wt% for the pure poly(HEMA) and 12.5 wt% for the co­ polymer) and expand(^^\ Thus, the mechanical properties of RMGICs may depend on the amount of water taken up. Nicholson et reported a reduction in compressive strengths and plastic behaviour of RMGICs aged in water.

The water uptake and expansion of the hydrogel is dependent on the osmotic pressure surrounding it. Anstice and Nicholson(^^^) found that when the cements were stored in a saline solution, the increase in mass and volume is smaller compared to the cements stored in distilled water. The alteration of the osmotic pressure as a result of the dissolved solutes, such as sodium, in the saline reduces the degree of swelling of the cements. This result implies that when the cements are exposed to the oral environment, the swelling may be less than that observed in distilled water.

(55)

indicated that dissolution of the cement component was greater than the water uptake of the cements.

D im en sio n a l stab ility

Shrinkage occurs during the setting of the resin-based materials. Several approaches have been proposed to determine the shrinkage of these materials, for example, density change determination, dilatometry, and linometiy. Polymerisation contraction occurs as a result of the conversion of the monomers into a polymer and therefore correlates with the degree of conversion(^^^l Polymerisation contraction of RMGICs has been determined in several studies^^^^'^"^^'^^^). The volumetric contraction ranged from 2.7-4.7 %. The plot of contraction with respect to time can be used to monitor the contraction behaviour, and thus the pattern of the setting reaction of the materials^^^*^\

Shrinkage may also result from desiccation of the cements. On desiccation, RMGICs undergo a reduction in both mass and volume^^^^J and exhibit a considerable shrinkage^^^’^ A calculated water loss of up to 66% of the original water content indicated that a large amount of water is present in the set RMGICs^^^^\ Severe loss of water may affect the cement integrity and results in inferior properties. Studies have shown that prolonged protection is needed when RMGICs are exposed to either air or water by glazing the surface with a light-cured resin(^°).

(56)

R elea se o f ch em icals

RMGICs showed a considerable variation in the amount and pattern of the release of ionic species, including fluoride. Similar ionic species eluted from conventional GICs, excluding the matrix-forming ions Al^+ and Ca^+, were found when RMGICs were stored in distilled wateri^^^). However, in an acidic medium^^^^) and an unbuffered physiological saline(^^^, both Ca^+ and Al^+ were released, with the amount of the former ion being significantly less than the latter. Although the leached amounts of Ca^+ and Al^+ were less than those found for conventional GICs^^*^^, this suggested that RMGICs are also susceptible to erosion. Similar to conventional GICs, the amount of Na+ released was greater than that of F despite the lower content of Na+ in the glass componenb^-^^'^^^J. In contrast to the conventional GICs, strontium has also been found(^^).

(57)

The release of fluoride from RMGICs is affected by the changes in pH as well as by the presence of saliva proteins and buffer systems. More fluoride is released into artificial saliva and acidic solution^^^^'^^^'^^^ than into distilled water. Reduced fluoride release was found in the presence of albumin and in phosphate buffer system(^^\

In addition to the inorganic species, some organic materials have been found to be released from RMGICs. HEMA was eluted from RMGICs in the range 0.5-22 ppm, depending on the duration before which the specimens were immersed in wateri^^\ The specimens stored 24h before immersion released a significantly smaller amount of HEMA than those stored for only 10 min^^^-^^^. This implies that polymerisation of HEMA continues after irradiation for at least 24h. The release of formaldehyde, the degradation product of resins when polymerised in contact with air, has been reported in 2 studies^^^-^^^, the range of which was 0.1-5 pg/cm^ when the specimens were stored in water for 24-72h at 37°C. This level was relatively low and comparable to that found in the composite resins(^^\ When the samples were cured in the absence of air, there was no release of this materiah^^). This indicates the importance of the use of a

matrix band when curing these materials.

B iocom patibility

(58)

value and produce a high polymerisation exotherm during setting, may suggest that RMGICs should not be placed in deep, unlined cavities without using a subliner.

T em perature rise d u rin g settin g

Kanchanavasita et and Bourke et reported that the temperature rises of RMGIC were higher than those for composites and compomers. The temperature rises of the restorative RMGICs were lower than those for the liner/base materials, some of which exhibited rises of as much as 20°C. This may be attributable to the greater resin content in the liner/base cements. High polymerisation exotherm may cause irreversible pulpal necrosis. An increase in temperature of around 5°C in the pulp has been shown to damage the pulp(^^^\ Therefore, pulpal protection should be considered when the materials are used in deep cavities.

In the light-cured materials, the temperature increased rapidly upon activation by the light and reached the maximum value within 15-20 seconds. The rate of temperature rise during polymerisation has been shown to relate to the rate of shrinkage, i.e. the rate of shrinkage increased immediately as the temperature rose and decreased as the temperature felh^^^).

2 .3 .6 A dvantages and disadvantages

(59)

conventional GICs and is sustained over a long period of tim e^^\ It has been questioned whether fluoride could diffuse into the tooth tissue if the tooth surface is coated with the adhesives since this may seal most of the tooth surface with which the restoration will interface. One study has shown an inhibition of artificial enamel demineralisation up to 7 mm from the marginal of the RMGIC restorations^^^^. In fact, a more pronounced effect was observed within 1 Some in vivo data are available for the 2-year clinical performance of RMGICs. Wilder et and Robbins et reported that there was 76-100% retention of class V RMGIC restorations after 2 years with no occurrence of secondary caries despite the 23% loss of anatomical form.

Figure

Figure 2.5-1 Bending m om ent and shear force diagram s for the
Figure 2.5-2 Determination of offset yield load in bending
Figure 2.5-3 Circular plate supported at periphery and loaded at the centre
Figure 3.3-1 Mean indentation depths for RMGICs stored in distilled water (D.W.) and artificial saliva (A.S.) at 37®C
+7

References

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