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).

Fig. 7.4 An inverted bottle containing bis-GMA monomer. Note that the resin stays in the lower part of the bottle, defying gravity due to its high viscosity. It will take in excess of 14 days for the resin to slump to the bottom of the bottle.

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.

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.

Table 7.3 Commonly used glasses as fillers in resin-based composite materials

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).

Fig. 7.5 A block of glass (A) prior to milling and (B) the ground glass product.

The size and shape of a resin composite filler has an important influence on the properties of the material together with the amount of filler (filler loading) in the product. The particle size and shape determines the amount of filler that can be added to the resin. Furthermore, a large discrepancy between the hardness of filler and the resin will affect the surface finish and wear. Classification of resin composites by the size of their filler particles is in part historical as it chronicles the development of each material.

The macrofilled composites were the first to be developed. The large size of the filler particles (range 15–35 μm maximum and minimum, 5–100 μm) meant that although the materials displayed good mechanical properties (high strength), they were notoriously difficult to finish to an acceptable level. This was because the particles would protrude above the surface in the resin and when the surface was polished these particles were displaced and a satisfactory polish was never achieved (Figure 7.6).

Fig. 7.6A,B (A) Diagram of surface of macrofilled composite showing the particle size and distribution. (B) Photomicrograph of surface. Note the large areas of resin.

The rough surface of these composites attracted plaque and during clinical use the surface became rougher because of preferential wear of the resin matrix. The wear resistance of the material is therefore relatively poor. The only way to get a good surface finish is to cure the material using a matrix strip and leaving it unpolished. The strip forces the filler particle below the surface and the resin layer produced is smoother. This resin-rich surface, however, wears away with time. Larger particles can support higher loads as they have a lower surface area to volume ratio. These materials are on average approximately 70% filled by weight or about 55% by volume.

Reduction in the particle size leads to better packing of the filler and the reduction in the inter-particular distance which is filled with resin. This reduction in exposed resin surface will reduce wear to some extent. The reduction in size achieved by grinding means that the particles approximate more in shape to spheres. This confers a number of benefits including easier finishing and a smoother surface. The increase in filler load to between 75% and 80% by weight means that the mechanical properties are enhanced. While a filler loading of 75–80% by weight sounds very impressive, it is important to remember that glass is a much denser material than the resin. This means that the volume of the filler is always less than the weight fraction mentioned. Often the volume of filler in the restoration does not exceed 60% of its total volume.

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.

Fig. 7.7 (A) The distribution of the fine particles within the microfine resin (B) Photomicrograph showing the surface of a microfine material. The field of view is 30 μm and there is no evidence of filler at the surface. The debris observed is from the matrix strip used to make the sample.

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.

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.

Fig. 7.8 Crazing paving as an illustration of a hybrid resin-based composite. The differing sizes and shapes of the stones (filler particles) reduce the amount of cement (resin) as they are able to pack more closely together.

Fig. 7.9A,B The structure of a hybrid resin composite.

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.

Fig. 7.10 Trimodal distribution of particle sizes and the ideal packing scenario.

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).

Fig. 7.11 The effect of finishing a composite. The resin matrix is preferentially removed and excessive trimming will mean a loss of filler, as the greater diameter of the filler is exposed and the filler particles are plucked out.

Nanomers are discrete non-agglomerated and non-aggregated particles of between 20– and 70 nanometres. To put this size into perspective, one human hair is 80 μm in diameter, which is 80 000 nm, a nanomer being 1/1000 of a micrometre (micron).

The nanoparticles coalesce into nanocluster fillers, which are loosely bound agglomerates of these particles. Agglomerates act as a single unit, enabling high filler loading and high strength. These materials have the strength of a hybrid material but are easier to polish as the individual filler particles are much smaller. There are some materials which are conventional hybrid materials to which nanoparticles have been added to fill inter-particular space (see Figure 7.12).

Fig. 7.12 An example of a nanomer with agglomerated particles approximately 0.6–1.4 μm using a rice cake as an example. These are made up of much smaller particles (1–10 nm, maximum 75 nm) which are clustered together.

The aim of increasing filler loading is to make the mechanical properties of the resin composite closer to those of the filler. This means that the strength in compression goes up but that the material becomes more brittle. The resistance to wear is potentially increased but overfilling may lead to surface breakdown as there is inadequate resin to bind the filler together. The more filler that is added, the stiffer and more viscous is the paste, and manipulation can become much harder. Sometimes adaptation to the cavity margin and wall is compromised.

Chemically cured resin composites are supplied as a two-paste system with the setting reaction commencing when the two pastes are blended. The base paste contains monomer and filler together with a tertiary amine such as N,N-dimethyl-p-toluidene and the catalyst paste contains monomer and filler, but in this case there is the extra addition of benzoyl peroxide dispersed in a phthalate. The chemical reaction is sometimes referred to as a dark cure, implying that it will occur without the need for light energy unlike the 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:

These materials give the dentist the advantages of setting the cement when they are satisfied with the placed restoration as well as peace of mind should insufficient light energy reach any inaccessible regions so the cement will cure by the dark or chemical cure. To permit this to occur, these products contain both chemical accelerators and light activators.

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.

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.

Fig. 7.15 The herring bone incremental build-up for resin composite placement in a posterior cavity.

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: