Introduction

The objective o f this study was to evaluate a number of laboratory tests and relate these to clinical practice, determining that which has a significant effect initially. This was investigated in a quasi-clinical model and the clinical significance examined. A review of four areas of study produced a number of interesting findings.

Bausch et al. (1982), state that polymerisation shrinkage, may be one o f the main factors that determine the longevity of direct composite restoration. Polymerisation contraction forces may disrupt marginal integrity (Rees and Jacobsen 1989; Kidd 1985). This can cause cuspal flexure and fracture (Jensen and Chan 1985), which will leave residual stresses within the material (Pearson and Hegarty 1987). Composite inlays were introduced to reduce this effect, as contraction occurs in the laboratory on stone dies. However, the polymerisation shrinkage will still create a marginal discrepancy which has to be filled, and this potentially generates farther clinical problems. Some workers (Lutz et al. 1987; Schaller et al. 1988), propose that this is not significant since volumes of resin cement are small. However, there is still the potential for loss of marginal integrity.

Water sorption has been cited as of particular significance to dental composite restoratives (Braden and Clark 1984). Absorbed water can release internal strains (Sonder and Paffenbarger 1942). It can facilitate the extraction o f free monomer or polymerised residues (with biological implications) (Braden and Pearson 1981; Spahl et al. 1998), and, if excessive may promote breakdown of the resin/filler bond (Braden 1977). Over longer immersion times in distilled water and saliva, filler components have been found to leach out of dental composites (Soderholm 1996). Theoretically greater conversion of monomer which may be achieved by inlay construction can lead to a reduction in water sorption and solubility compared with more conventional materials.

Mechanical strength is of some significance although no test will be totally representative of clinical conditions (Ban and Anusavice 1990). Again the increased level of polymerisation which has been achieved with the inlays has resulted in an increase in the ultimate strength of the material. However, this is at the expense o f an increase in the modulus, enhancing the brittleness of the material.

Hardness has often been used as an index to the ability o f a material to resist abrasion (Pagniano and Johnson 1993). It has also been used as a method of measuring the degree of cure, since the extent of cure may exert an effect on nearly every physical property in a resin system. Degree of conversion may play an important role in determining the ultimate success o f the restoration (Ferracane 1984). Assmusen (1982A) and Ferracane (1984) have both reported good correlations between hardness and degree of conversion.

8.2 Polymerisation Shrinkage

The difficulties in producing meaningful measurement of this polymerisation shrinkage has resulted in the wide range of laboratory based techniques to investigate it. There are advantages and disadvantages with all the systems as described earlier, but a problem common to most techniques is that the measurements are based on systems where the principle adopted is that a mass of material is allowed to shrink freely in one or more directions. In these circumstances the contraction is theoretically relatively free of stress. There is some discussion as to when is a material considered to be shrinking freely and when is it considered to be restrained or bonded (Watts and Marouf 1999). Some surfaces are greased to allow free movement or almost free movement as in the linometer described by De Gee et al. (1993) to measure the linear shrinkage. It is doubtful if this is truly free and unconstrained movement. Although there may be other factors to consider in relation to these methods of shrinkage measurement such as the aspect ratio (diameter/height) of a ‘bonded' disc or mass (Watts and Marouf 1999)(since this is a consideration of the effect polymerisation stress by thin layers of resin cement).

These methods emphasise how the surface texture/detail may influence the flow of the composite as it contracts. ‘Free' shrinkage is therefore unlikely to happen clinically due to the cavity design or bonding to the cavity wall (Bausch et al. 1982). Since the amount of flow is highly dependant on the cavity configuration (Feilzer et al. 1987; Carvalho et al. 1998), and the flow of the curing composite is an important mechanism in the reduction of the shrinkage effect (Davidson and De Gee 1984 ).

The method of activation of the composite may be important in reducing the shrinkage forces. Since chemically cured composites polymerise more slowly than light-cured ones, the time during which the flow from the free surface will take place

is longer (Itch et al. 1986). Similarly more recent resin-composites based upon a different type of monomer system have been introduced. These 'soft start’ systems, based on multi-acrylate rather than di-methacrylate chemistry may be of benefit. Acrylates lacking the methyl group of the methacrylates are less sterically hindered. Thereby greater segmental mobility is feasible for growing free-radicals during the polymerisation process. This may permit enhanced flow during the early period of reaction (Watts and Hindi 2000). Thus, where activation and polymerisation is not so rapid, some reduction in shrinkage forces may be possible. Despite the two different modes of polymerisation in this report the rapid light curing process for Brilliant and Charisma still produce similar patterns of shrinkage to the slower polymerising process utilised by Isosit in each cavity design.

Additional prolonged curing is carried out extra-orally in specially designed chambers, and results in materials which have greater levels o f conversion and shrinkage than directly placed composites. The potentially larger contraction forces are not such a problem for composite resin inlays since almost all polymerisation shrinkage of the materials occurs before bonding to the tooth structure (Kildal and Ruyter 1997). Indeed some workers have suggested an ‘almost perfect marginal adaptation for composite inlays to enamel margin can be achieved when the enamel margins are acid etched, and the in situ shrinking composite mass is reduced’. (Zuelling-Singer et al. 1991). This of course assumes dimensional stability o f the cement lute. Whilst it has the potential for greater shrinkage it is of a smaller bulk. The discrepancy therefore of the inlay is made up by the cement lute, so the problems o f the marginal gap are transferred to the shrinkage of the cement lute.

The problems of the resin cement are different to those of the bulk of the composite inlay. While the interfacial gap should be small it has already been demonstrated in this report and by others that the average interspace of composite resin is 50-200 |Lim , compared with 25-50 |im for gold inlays (Rees and Jacobsen

1990). Therefore, there is the potential for degradation of the more lightly filled resins at the cavity margins. Further it is possible that combined with the method of curing a variable thickness of cement within the constraints of the interfacial space may bring about stresses in the adjacent composite and dental structures. This is of importance depending on the cement lute thickness (Feilzer et al. 1989).

While all the inlay systems require a specialised vessel for the additional curing process, a distinct feature which is not present in directly placed composite resins is the interruption which occurs in the curing method of some inlay systems. For Charisma and Brilliant inlays, the primary cure with the hand held surgery light is followed by a short period at room temperature before being transferred to the curing vessel for the last stage of the tempering process. The composite inlay following the primary cure is of fixed dimensions before removal from the working die in order to prevent distortion. This implies that if fiirther shrinkage is to occur it will take place in the post-gel phase and is likely to set up stresses within the material, although it has been suggested that at higher temperatures near the glass transition temperature, some stress relief may be possible. In amorphous polymers these temperatures are at the point where molecular motions become such that whole chains are able to move (McCabe 1990). The temperatures recorded from the curing chambers used for Charisma and Brilliant are relatively low (in the range 49-96°C) well below the range of the glass transition temperatures for these composites noted by Ferracane and Condon (1992) which was in the region of 120°C.

Further, with less mobility o f the polymer chains, and progressive cross-linking of the unreacted methacrylate units, lower conversion rates are likely to occur. It is also interesting to note that for a secondary cure a greater amount of thermal energy is required to activate a given level o f internal molecular movement (Dionysopoulos and Watts 1989). This means that following the initial cure the constituent polymer chain segments of the resin phase are now in a stiffer, less mobile condition.

For a process where the polymerisation is continuous this may not be such a problem. In this case (Isosit) samples are subject to a gradually increasing temperature which may allow for contraction stress relief since the curing vessel temperature reaches 120°C. Here, molecular movement is easier and higher conversion rates are possible

Isosit exhibited the greatest contraction of all the readings taken. The linear shrinkage readings which were recorded ranged from 0.76% to 1.96% and calculated volumetric shrinkages were in the range 1.75% for Brilliant to 4.97% for Isosit. These measurements are of a similar order to those found by previous workers (Goldman 1983; De Gee et al. 1993). The greater conversion of the material as a result of heat and pressure is likely to have produced the higher contraction in the

Isosit material. Ruyter (1992) suggested that conversion rates for Isosit may reach 90%. Kildal and Ruyter (1994) state that the maximum possible degree of conversion for Charisma and Brilliant after secondary curing is 71%.

Even allowing for experimental variation, a greater contraction was noted in width than in length in all the samples irrespective of the composite type. The reason for the differential pattern of shrinkage in the cylindrical specimens was not clear and requires further investigation. Although it has been reported in more clinically based studies that there may be a greater shrinkage in one direction. Qualtrough et al. (1993) demonstrated an apparent tendancy for greater inlay contraction along the mesio-occlusal-distal axis as premature contacts were recorded along the axial walls. Others initially hypothesizing that this may be due to larger amounts of composite undergoing polymerisation contraction between the mesial and distal boxes did not find the contraction in this plane sufficient to prevent satisfactory removal o f the inlay from the cavity (Wassell et al. 1992). Carvalho et al. (1996) concluded after a review of polymerisation contraction in resin composites that the polymerisation contraction forces may be controlled to some extent by the cavity design and the relationship between the surface area of composite in contact with the cavity walls and the free unbonded surface area.

8.3 Water Sorption and Water Solubility