7: The tooth-coloured restorative materials I: Resin composites

Chapter 7 The tooth-coloured restorative materials I

Resin composites

Introducing the Tooth-Coloured Restorative Materials

There is a range of directly placed tooth-coloured restorative materials available to the dentist. These materials form a continuum that links resin based composites to glass ionomer cements, with resin composite at one extreme and glass ionomer cement at the other and two hybrid materials in between (Figure 7.1).

Resin composites are hydrophobic materials consisting of an inert glass and a polymerizable resin. Glass ionomer cements are hydrophilic cements which are based on the reaction between glass with an acid to form a polysalt matrix in which the unreacted glass is sheathed. They contain no polymerizable resin. Compomers (or polyacid-modified resin composites) are primarily resin based and are closer in behaviour to the resin composites than the glass ionomer cements. The reactive glasses used as the filler in these materials is similar to that used in glass ionomer cements, but the resin system uses two methacrylate based resins. Setting is achieved by a polymerization reaction, with no water being present in the material before placement. Resin-modified glass ionomer cements are glass ionomer cements to which a resin component has been added by grafting methacrylate groups onto the polyacrylic acid chain. A water soluble methacrylate resin is also added. The cement still requires water as an intrinsic part of the setting reaction. This material is closer in nature to glass ionomer cements than resin composite.

The Resin Composites

Resin composites are widely used in dentistry and their applications are increasing as technology advances. As already mentioned in Chapter 6, dental amalgam is not necessarily the automatic material of choice for restoring posterior teeth in contemporary practice. The market is patient driven as most patients wish for the excellent aesthetics afforded by tooth-coloured restorations. Resin composite is the directly placed material of choice when the best aesthetic result is required. The early composite materials did not have long-term durability as they had a tendency to wear and showed staining. However, as materials and techniques have improved the longevity of resin composite restorations has increased. This chapter describes resin composites in detail and their applications and clinical manipulation.

The original tooth-coloured restorative materials (Table 7.1), the silicate cements and the acrylics, exhibited several shortcomings. The development of resin composite materials was aimed at overcoming these factors. As their properties have improved with time, they have tended to become ubiquitous materials, being used increasingly at the expense of other traditional materials.

Table 7.1 Shortcomings of acrylics and silicate cements

Acrylics Silicates
High water sorption Low strength
High coefficient of thermal expansion Wash out
Large polymerization shrinkage Low pH
Considerable microleakage Pulpal irritation
Extrinsic and intrinsic staining  
Highly exothermic on curing  
Wear  
Poor colour stability  

Definition

A composite material is one that is composed of more than one different constituent. There are many composite materials in use in dentistry of which resin composites are one example. These materials are composed of a chemically active resin component and a filler, usually a glass or ceramic. The resin and the filler are bound together by a silane coupler. The structure of resin composite is illustrated in Figure 7.2, and the constituents of resin composite materials are shown in Table 7.2. The many component parts of a resin-based composite material are shown in Figure 7.3.

Table 7.2 Chemical constituents of resin composite materials

Ingredient Examples Reason for use
Filler (inorganic) Glass Provides strength
Ceramic Influences the optical properties of the material
Principal monomer Bis-GMA Forms polymer matrix
Bis-EMA Used as a primary monomer
UDMA
Diluent monomer TEGDMA Reduces the viscosity of the main resin so that the material can be used clinically
UDMA
Silane coupling agent γ-methacryloxypropyl-trimethoxysilane Bonds the filler to the resin
Photo-initiators Camphorquinone Initiator of polymerization reaction
PPD
Lucirin TPO
Other chemicals for the curing process Tertiary amine such as N,N-dimethyl-p-toluidene Accelerator of polymerization reaction
Ultraviolet stabilizers 2-hydroxy-4-methoxybenzophenone Prevents shade change over time due to oxidation
Polymerization inhibitors Monomethyl ether of hydroquinone Prevents premature curing of the composite prior to use
Radiopaque materials Barium, strontium and lithium salts Permits the material to be seen on radiographs (see Chapter 4)
Pigments and opacifiers Iron and titanium oxides Varies the optical properties and the colour of the final material to achieve a good shade match

UDMA, urethane dimethacrylate; TEGDMA, tri(ethylene glycol) dimethacrylate; PPD, phenylpropanedione; Lucerin TPO, ethyl 2,4,6 trimthylbenzoyldiphenyl-phosphine oxide.

Constituents of a Resin Composite

Resin component

Principal and diluent monomers

The main resin component is based on the chemical reaction of two resins, namely bisphenol A and glycidyl methacrylate, which form a chemical called bis-GMA (bisphenol A diglycidyl ether dimethacrylate), also known as Bowen’s resin after its inventor. It is a long chain monomer with a methacrylate group at either end of an aromatic spine. This chemical is highly viscous due to its high molecular weight and aromatic spine, which reduces mobility of the monomer so that it cannot be manipulated clinically (Figure 7.4).

It is necessary therefore to add other monomers to the bis-GMA to permit clinical handling and proper mixing with the inorganic components. These lower-molecular-weight monomers are called diluent monomers and have also been termed viscosity controllers. Examples of these chemicals are:

Chemicals such as methylmethacrylate have low molecular weights and have only one reactive group. This leads to greater shrinkage. The disadvantages of the single methacrylate group materials are as follows:

The dimethacrylates have active methacrylate groups at either end of a backbone. The longer the backbone, the smaller the shrinkage as the reaction only occurs at the active methacrylate groups. Bis-GMA is an example of this, having a long backbone made up of phenolic aromatic rings. UDMA also has a long chain backbone, but in this case it does not contain phenolic groups with the backbone being made up of aliphatic components. UDMA is now used quite frequently as an alternative to bis-GMA.

Filler component

The resin, if used alone, shows a number of shortcomings:

The incorporation of an inorganic filler into the system compensates for these shortcomings. By implication, when a filler is added, the amount of resin present decreases. The material thus created should exhibit:

One of the consequences of filler addition is that the resin composite material takes on the properties of the main constituent, and thus it becomes more brittle in nature. As its elastic modulus is increased, the material has a decreased capacity to withstand flexion encountered during function. This will occur in any dental restoration and is of particular significance with the flowable resin composite presentation as explained later.

Filler types

Resin composites have been classified by:

The next two sections deal with the composition of the filler and the influence of the size and distribution of the filler particles on the resin composite. The fillers include both glasses and ceramics.

Glasses

A glass is an amorphous (non-crystalline) solid material. While there are many different chemical compositions of glass, the formulations used in resin composites are quite limited. Those which are used in these materials are listed in Table 7.3.

The formulation of the glass is important as it has a major effect on the appearance of the final composite. It also in part influences the mechanical properties. Quartz is the hardest material but is not radiopaque. The silicate glasses produced for resin composite contain heavy metals such as barium but are slightly softer and also degrade very slowly when exposed to water. Those materials containing barium, strontium and lithium are easier to finish and exhibit an improved surface finish. Of the three, strontium-containing glasses degrade the fastest in water, which is why they are used in only a small percentage of the overall glass content. Colloidal silica is also frequently found in many of the modern composites. However, the behaviour of the product will be determined by the volume fraction of filler particles in the material. Some resin-based composites release very small amounts of fluoride. This fluoride is leached from the glass and so depends on the formulation of the filler used. The loss of fluoride will also accelerate the degradation process as the glass is weakened.

It must be borne in mind that the density of all these glasses is about four times that of the resin. Many manufacturers often quote the filler loading by weight. The more significant value is the volume of the filler because of the disparity between the densities of the resin and the glass – then the volume fraction is always less than the weight fraction of filler present.

Once the glass has been produced by firing the various components of the glass together, the resulting solid mass is ground to the desired size of particle. Figure 7.5 shows a block of glass as supplied by the glass manufacturer and after it has been milled (ground).

Microfilled

The failure to achieve an acceptable finish led the manufacturers to consider microfine fillers. The most acceptable material to use was colloidal silica. The particles of this material are sub-micron in size (0.04 μm). This type of material is most commonly used as a thickening agent. The particle has an affinity for water and if uncoated, takes up water leading to hydrolytic degradation. To overcome these problems and to produce a resin composite, the manufacturer takes the base resin and warm the mixture to below the glass transition temperature. The resin mixture becomes more fluid and at this point colloidal silica is introduced to the resin mass and a volume added until the resin is almost solid.

The semisolid mass is then heated to polymerize it and the result is a resin block with ‘seeds’ of colloidal silica embedded in it (Figure 7.7). This resin mass is then ground up to form ‘filler’ particles which are then mixed with unpolymerized resin to form the paste provided to the dentist. The inorganic phase, colloidal silica, in these materials does not exceed 45% by volume. However, the presence of already polymerized resin means that the polymerization shrinkage observed is not greatly above that for the other types of resin composite.

One of the consequences of retaining more resin is that the water uptake of the material is increased. This is not as bad as might be expected since the pre-polymerized blocks of resin have a very high conversion rate which reduces the water uptake. It should be stressed that the water uptake is a slow process and the amount involved is relatively small.

The type of ‘filler’ and the nature of the resin means that the inorganic filler loading in these materials is substantially lower than that found in other resin composites, in the range of 40–45% by weight. Any attempt to increase this usually results in the resin not coating all the particles and particles agglomerating. Substantial amounts of time, research and money have been spent on this during the development of these resin composite materials with relatively limited benefits.

The strength of these materials is not as high as that of conventional resin composites. Cases have been reported of marginal ridge or incisal tip fracture. However, over time observed wear with these materials is no better or worse than a conventional resin composite.

Hybrid

As the term suggests, a hybrid composite contains particles of various sizes and shapes. They were developed to try to gain all the benefits of the microfine and macrofilled resin composites. These products offer a higher filler density as the particles can get closer together and fit into each other so interlocking. This means that there is a decreased amount of resin. The structure of a hybrid resin composite can be compared with crazy paving (Figure 7.8 and Figure 7.9), with the stone paving slabs representing the particles and the cement grout the resin. It can be seen that they are remarkably similar although the hybrid has slightly more space between the particles. This is one of the limitations of the hybrid in that the resin viscosity tends to prevent the close apposition of the particles unlike the cement in the crazy paving which can be a much lower viscosity.

Under ideal conditions packing can be improved substantially with the resin acting as a binder. Figure 7.10 shows the ideal distribution of packing to achieve an optimum filler loading (a trimodal distribution). Here the large particles form the bulk of the restorative. In the spaces between them are smaller spheres of intermediate size. There still remain even smaller spaces between the larger and small spheres. If even smaller spheres are then used to fill those gaps, a trimodal distribution of particle sizes (large, medium and small) is established. The filler distribution in Figure 7.10 is 15 large (green), eight small (blue) and 32 smallest (red). These fillers form the bulk of the restorative. Manufacturers attempt to produce this type of distribution but are limited by the variable particle size within each category. It is also difficult to ensure that the distribution is uniform through the material.

The size range of particles is generally between 5– and 10 μm for the larger particles with the small particles being made up of colloidal silica. The amount of filler incorporated is a compromise between the stiffness (viscosity) of the paste and the handling characteristics required by the clinician. Maximum filler loading is in the region of 82–84% by weight of filler. This increase in the bulk of the filler has benefits in that the coefficient of thermal expansion is slightly reduced and the mechanical properties of the composite are enhanced. The materials can, however, become more brittle. The increase in filler loading may also lead to larger volumes of diluent monomer being used with the resulting material being prone to greater polymerization shrinkage.

Hybrid resin composites are often termed universal resin composites as they may be used in all sites within the mouth for all applications. Several varieties of hybrids with varying particle sizes and distributions are available. As the particle size is reduced, manufacturers have applied stylized names to these minor differences. These include ‘fine’ and ‘micro’ hybrid, which refers either to the average particle size or a blend of particles being included with an increased proportion of sub-micron sized particles.

As the particle size decreases, a further complication arises in that light is not reflected within the restorative and more shades and tinting agents are required to make the materials aesthetically pleasing. Modern resin composites are close to the maximum filler loading that is achievable, this being about 86% by weight (70–72% by volume). These heavily filled materials provide a reasonable surface and can be finished using fine diamonds burs, sanding discs and abrasive stones to produce a relatively smooth surface. The long-term performance is not ideal as the resin surface soon starts to wear away (Figure 7.11).

Silane coupler

When the filler is added to the resin, the resultant product actually becomes weaker than the original resin component alone, which clearly negates the objective of adding the filler. This is because no bonding occurs between the filler and the resin. Manufacturers therefore have to chemically coat the surface of the filler particles to facilitate their bonding with the chemically active resin. This chemical is called a silane coupler. The silane molecule facilitates the formation of a chemical bond between the resin and the filler. The principle is similar to silanation treatment, when ceramics are to be bonded to the tooth using a resin composite bonding system, for example in the bonding of ceramic restorations such as veneers (see Chapters 11 and 22). One example of a silane coupler in glass-filled resin composite is γ-methacryloxypropyltriethoxysilane (γ-MPTS), a vinyl silane compound. The silane molecule is bifunctional with groups that react with the inorganic filler and others that react with the organic resin, hydrophilic and hydrophobic groups, respectively. The γ-MPTS is acid activated and when reacted with the filler, the methoxy- groups hydrolyse to hydroxyl groups, which in turn react with hydroxyl groups on the filler. There is also a condensation reaction with the hydroxyl group of the hydrolysed silane. The silane coupler then bonds via its carbon double bond to the resin. The bond can be degraded by water absorbed by the material during clinical function.

The bond between the filler and the resin needs to be durable and strong. If this is not the case, stresses applied to the set material will not be equally distributed within the material and the resin will have to absorb more. This will lead to creep and wear of the restoration, further leading to fracture. Stress concentrations will also occur at the interfaces between the filler and resin, so leading to the formation of crack initiation sites. Fatigue fracture will occur as the resin does not have a high resistance to crack propagation. Stress can therefore be transferred from the strong filler particles to the next through the lower strength resin.

However, there is a major problem in that the resin is hydrophobic while the modern glasses used in composite will take up water and can undergo slow hydrolytic degradation. Once water ingress occurs, these materials start a very long and slow process of degradation, which leads to surface disruption with time.

Chemicals required for the curing process

Historically, resin composites set by chemical means and it was only in the late 1970s that light curing became more prominent. This led to a step change in performance of these materials. The resin composites can therefore be divided conveniently into:

The chemicals necessary to initiate and then complete the setting reaction are contained in the unset material. The reaction in both chemical and light curing materials is a free radical polymerization.

Light cured resin composites

The majority of light cured resin composite restorative materials are cured only when the material is exposed to light energy, which initiates a chemical reaction within the material to form the set material. The chemicals required for the curing process are contained in the unset material and can be divided into:

Mechanism of cure

On exposure to a sufficient amount of light energy at the correct wavelength of light, the monomers will polymerize to form a rigid cross-linked polymer, the various polymer chains linking via the methacrylate groups. The process of light curing is explained in detail in Chapter 2.

It is possible to accelerate the polymerization reaction by increasing the concentration of the camphorquinone. While the material sets quickly, the penalty for this is that the propagation phase of the polymerization process is shortened such that there are shorter chain lengths formed and the material is not as strong. Additionally, the material may be more susceptible to ambient light and can start to set if exposed to the dental operating light.

Once the activating light is switched on, the reactive groups on each resin chain start to polymerize. The reaction goes through two stages:

It should be remembered that the polymerization reaction is anaerobic, that is, in the presence of air the material will only be partially cured. This explains why if no matrix is used to cover the surface of the restoration the surface will remain tacky and is then called the oxygen inhibition layer. This partly cured layer should be removed and the restorative trimmed back to the fully set material.

Radiopaque materials

The need for restorative materials to be radiopaque was discussed in Chapter 4. In many cases the filler particles contain heavy metal derivatives which are inherently radiopaque, such as barium, zinc, strontium and ytterbium. In those materials containing radiolucent filler, particles such as quartz or lithium aluminium silicate, and barium salts, can be added to convey radiopacity to the material. There is an ISO requirement (see ‘The regulation of dental materials and the International Standards Organization’ on the inside back cover of this book) that the radiopacity of these materials is equivalent to 2 mm of aluminium. However, there is variation in the radiodensity of materials above this which depends on the volume of radiopaque filler used.

Properties

Polymerization shrinkage

Polymerization shrinkage of resin composites has been cited as one of the most important shortcomings of this family of materials.

• A material which shrinks markedly will leave a gap at the tooth–restoration interface, thus allowing the passage of bacteria and oral fluids. This is termed microleakage (Figure 7.13).

• As is discussed in greater detail in Chapter 11, the bond achieved between the material and tooth is very good indeed. During curing, a large amount of shrinkage will cause high levels of stress to be built up in the remaining tooth structure, the tooth–restoration interface or the restorative material itself.

• It is also more difficult to manipulate the material to form good contact areas and stable occlusal relationships when the material will shrink (Figure 7.14).

The advantages of decreased shrinkage are summarized below:

Some manufacturers have changed the resin chemistry by changing the constituents of the resin. By attempting to eliminate the lower molecular weight monomers that exhibit greatest shrinkage on polymerization, the overall shrinkage of the resin can be reduced. Many products have as their resin component a mix of bis-GMA with bisphenol A polyethylene glycol diether dimethacrylate (bis-EMA) and UDMA, which shrink much less on curing that the original bis-GMA/lower molecular weight systems. By using this system, there is virtual elimination of TEGDMA and the higher polymerization shrinkage values seen with it. This resin is more hydrophobic and a slightly softer resin matrix is created, which may influence wear resistance. The resin base colour is lighter and the higher molecular weight leads to less shrinkage as there is greater distance between the methacrylate groups on the monomer chain.

Of the methacrylate-based resin composites currently available, the lowest shrinkage is of the order of 2.2% by volume while the average value is between 2.5% and 3%. This is seen in products using the bis-EMA/UDMA resin combination and not bis-GMA resins alone and occurs when the monomer matrix converts to the polymer state. This figure while low, still represents a significant problem clinically and so manufacturers have attempted to further reduce it in a number of ways to reduce the effect of this shrinkage.

Strategies to overcome polymerization shrinkage

In general when a polymer sets it shrinks toward the centre of its mass. If it is in the form of a sphere, the diameter of the sphere is reduced. In a cavity the problem is more complex but the basic principle is the same. When placing these materials the dentist can reduce the effect of shrinkage of the overall restoration in a number of ways. Firstly, if the material is placed in increments and each increment is cured prior to the placement of the next increment, the shrinkage is minimized and compensated for to some extent. This technique is termed incremental build-up.

The magnitude of stress depends on the composition of the composite and its ability to flow before gelation occurs. This is influenced by the shape of the cavity and can be overcome by the way the dentist places the material into the cavity. This has been termed the configuration factor (C factor) and is the ratio of bonded to unbonded surfaces. The higher the ratio the more stress is potentially incorporated into the situation. A Class I cavity has the highest C factor as there are five bonded surfaces (four axial walls and the cavity floor) and only one unbonded surface where stress relief can occur (the occlusal surface). Compare this with a Class IV cavity or a cuspal replacement where the converse occurs. Techniques have been advocated for the placement of resin composite so that only one or two surfaces are contacted by the material at any one time, which decreases the C factor to eliminate stress. This will also allow for compensation of the polymerization shrinkage, with each subsequent increment offsetting the effects of this shrinkage. Unfortunately, the recommendation to keep cavities small tends towards having cavities with high C factors, which means that greater stress is set up with the inherent risk of interface failure.

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Incremental build up allows for more effective and uniform polymerization and reduces total polymerization shrinkage (Figure 7.15), which may decrease stress generated at cavity walls and so reducing potential for debond gaps and cuspal deflection. It has, however, been shown that even with incremental build-up, once the final increment is placed the risk of cuspal flexure still exists. Proper placement of the first increment is the most important step and the dentist should ensure that it is fully polymerized as this increment is the furthest from the light.

Stresses within teeth will also depend on the compliance of the tooth. This is the ability of the tooth to withstand flexure and is dependent on the amount and quality of the remaining tooth tissue and its position. Furthermore, how each increment is placed in the cavity and polymerized can influence the stresses built up within the tooth, at the tooth–restoration interface and within the resin composite material itself. When the composite shrinks, various effects can occur in the cavity:

Jan 31, 2015 | Posted by in Dental Materials | Comments Off on 7: The tooth-coloured restorative materials I: Resin composites
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