Discusión de la caracterización del material base

The range of values for the most widely recorded measurement, volumetric shrinkage, might question whether these readings give a true picture of what is happening. Previous details have described some of the factors which may cause variation between the techniques, and therefore the limitations on the methods used. The results are complicated by the fact that in a number of reports, volumetric or linear shrinkage readings appear to be calculated on a more empirical basis using a factor of 3, since one measured value is used to represent volumetric and linear shrinkage.

Many factors control the polymerisation shrinkage of composite resins, in fact, the results show that almost everything that influences polymerisation positively affects the contraction (Bausch et al 1982). In summary, the main problems and factors which may contribute to the polymerisation shrinkage measurement are given below;

(i) Dilatometric methods are more suitable for chemically cured materials, as placement into the density bottle is simpler than for the deflecting disc method. Some shrinkage values may be understated, as polymerisation is likely to have commenced following the start of mixing and before placement.

(ii) Light activated materials have been more easily measured using the bonded disc method (or measurement of deflection of a metal or glass disc), since the light source may be more easily adapted to be adjacent to the test material than in a dilatometer where it may have to pass through liquid media. This problem has been addressed to some extent by some workers (Lai and Johnson 1993).

(iii) Temperature control is more of a problem with dilatometers although some attempt to stabilise this through the use of water baths has been successful. Also, reducing the testing temperature from 37°C to 25°C made this regulation easier. While some workers suggest little difference in the polymerisation shrinkage at 37°C and 25°C, others disagree and demonstrate differences even between 30°C and 37°C (Bausch et al. 1982).

(iv) Storage of materials may be a factor whilst there are difficulties in obtaining information relating to long and uncontrolled storage history. In the short term, materials stored at 4°C had relatively more rapid contraction and high shrinkage values than those stored at room temperature, 23°C (Bausch et al. 1982). This may be because the activity of the benzoyl peroxide component in autopolymerising materials can be reduced as a result of storage at room temperature. Light activated materials are generally kept cool as recommended by the manufacturers, which implies that they are not thermally stable. This may not necessarily be for the same reason as autopolymerising materials.

(v) The oxygen inhibition layer will affect all systems but is likely to be more of a problem for those materials immersed in water. Attempts to compensate for this are reported by some studies (Lai and Johnson 1993). This may introduce further variations into the values obtained.

(vi) Differences between results have been attributed to the different material, different measurement techniques and changes in formulation o f the same material from one study to another (Rees and Jacobsen 1989).

Filler loading is known to influence the degree of conversion (Bausch et al. 1982). The more highly filled the resin, such as those for posterior use, have lower shrinkage values (Chung 1990), often with volumetric shrinkage values around 1-2% (Rees and Jacobsen 1989). However, some materials used for posterior use have apparently low filler content but still display a low shrinkage value (e.g. Heliomolar). The microfme filler particles are coated with pre-polymerised resins which contribute a fijrther 15 wt.% (in the case of Heliomolar) to a total filler loading (of 77 wt %). As a result, the total filler loading (organic and inorganic) is close to that of restorations with only inorganic filler. These restoratives containing prepolymerised resins will therefore have similar shrinkage values (Rees and Jacobsen 1989). It should be noted that it is the volumetric change which is important, and this is reflected in the density of the filler particles. The volume fraction in a filler is always lower than the weight fraction, and therefore it is the remaining unpolymerised resin volume which will contribute to the degree of the polymerisation contraction.

The configuration and molecular weight of the monomer system will also effect the shrinkage. Materials containing a high proportion of low molecular weight reacting monomers have higher degrees of conversion of un saturated methacrylate groups than materials consisting mainly o f the larger aromatic dimethacrylates. Therefore polymerisation shrinkage should be higher for those materials with higher diluent monomer content (Goldman 1983).

(vii) In some studies it has been recorded that an additional 30 second exposure to the activating light after the initial 30 second activation and a 5 minute interval, will give a further polymerisation contraction. The results were not expected by the manufacturers and workers involved (Walls et al. 1988) as it was assumed that for the thicknesses of the specimen involved (1.5 mm) the composite would reach its optimum 'cure' after 30 seconds exposure to a visible light unit. The results suggest that this is not the case, and that further polymerisation occurs with another 30 seconds exposure, although the amount in relative terms is small. It is worth recording that some light and heat treatments will produce a degree of conversion where little or no further polymerisation contraction is possible, as demonstrated by

Ferracane and Condon (1992). Here, immediate additional heat treatments of 10 minutes or 3 hours show that the shorter treatment is sufficient to enhance the properties. Further it is suggested that once the composite is hardened, and there is a delay of more than 7 days before any further heat treatment is applied, fixrther chemical reactions are unlikely (Ferracane and Condon 1992). Even before 7 days, temperatures of 120°C (near the glass transition temperature of composites) may be required to enhance the molecular mobility to further any chemical reaction (Ferracane and Condon 1992). It appears possible that in the study by Walls et al. (1988) the reason for the further polymerisation contraction was due to the fact that the polymer segments were still mobile as there was only a short delay between the initial and additional cure.

(viii) There are some questions over the nature of the adhesion of the composite sandwiched between the two glass or metal and glass plates in the so called 'deflecting disc' (Watts and Cash 1991) or bonded disc method (Watts and Marouf 2000). Some description of the volumetric/linear measurements has been given earlier, and some reports specifically relate to whether the composite is prevented from moving across a surface thereby being considered as bonded to that surface, or allowed to move across this surface in the sense of being 'free'. When the composite is prevented from moving across a surface it is held back or restrained. It would appear that in most cases where composite is placed onto a glass slide or glass coverslip or metal surface, without any preparation to that surface, some restraining force is likely to be influencing the behaviour. Some concern must therefore be expressed as to whether any shrinkage measurement using the methods to date can be considered as unrestrained or completely free.

(ix) While specimen dimension in dilatometric studies may not be critical, it appears that the dimensions or configuration of the contracting discs or cylinders between 2 plates in deflecting disc techniques may be more important. The pattern of contraction may be altered in this method so that lateral contribution to volumetric shrinkage could occur that may not be measured between the plates uniaxially (Watts and Marouf 2000). It is therefore the aspect ratio of the specimen which becomes critical.

(x) While dilatometric, bonded disc, density and pyconometric methods all give valuable data concerning the volumetric and linear shrinkage, it is essential to study

the displacement of composite resins in dental cavities. Many o f the laboratory methods of measurement appear to give a poor indication of how a composite resin behaves in a constrained, irregularly shaped environment such as a dental cavity. Some means of providing better information have been reported using microscopic evaluation techniques (Asmussen and Jorgensen 1972; Hansen 1982A). These results have demonstrated how shrinkage patterns within the cavity are variable, and how simple volumetric or linear measurements of shrinkage help to reveal the internal displacement of the composite resin within the cavity. However, there are some limitations to the amount of information which may be obtained by these more clinically based methods. Specimens for linear measurements of the gap between the cavity wall and the composite restoration are difficult to prepare, and the wall to wall measurements may be dependant on the path of the sectioning technique (Hansen 1982B). Further the measurements may be taken from the cavity margin and not in the deeper parts of the cavity (Peutzfeldt and Asmussen 1990).

This review o f some of the methods of measurement of polymerisation shrinkage has demonstrated the large number of factors which affect the dimensional changes which occur during polymerisation. Not all the studies appear to address these variables. In view of the importance of this particular property in dental composites,

some standardisation of procedures for this measurement might be helpful.