•3.2 Micro-fill

The response to these problems was the introduction of the so-called ‘micro-fill’ products. These used extremely fine filler particles, a silica ‘flour’ called ‘aerosil’ or fumed silica. This material has a typical particle size of the order of 0.05 μm, one tenth of the wavelength of yellow light, and less than one hundredth of the size of previous ‘fine’ particles, hence these are sometimes referred to as ‘micro-fine’. It therefore has no visible effect on the optical properties compared with unfilled resin. However, while this did allow products that were easy to polish (because the filler particles were much smaller than the size of any abrasive particle), the very large surface area of such powders caused unusably high viscosities when present in even a modest volume fraction (4§9.2). The only answer to this, of course, was to use substantially lower volume fractions of filler, the practical limit being about 30 %, and this had the expected effects on elastic modulus (lower), strength (lower), and wear rate (higher). This example illustrates very well the type of compromise which crops up so often in dentistry (as it does in all materials applications).

Since then there have been various other attempts at resolving the conflict between these opposing effects of filler loading. Thus, using the greater power of machines, very high volume fractions of fumed silica can be incorporated into a resin which is then polymerized. This mass is then broken up and reduced to a fine powder, which then is mixed – as if it were a filler itself – with resin containing only a moderate amount of fumed silica. This then is the product delivered to the dentist. This means that the lower viscosity of the uncured mixture allows ready handling; but when polymerized polishing is straightforward since all of the actual filler is very fine. The surface of the restoration thus presents only a varying filler loading from point to point, and the overall wear experienced is less than that of the simple ‘micro-fill’ materials, while the strength and modulus of elasticity are somewhat higher. However, the wear rate remains unacceptably high. ‘Hybrid’ materials have been created using both ‘fine’ and ‘micro’ fillers in the same matrix, and carefully graded particle size distributions used to minimize the gaps between particles supposedly by allowing them to fit together more efficiently. It should be remembered that the total filler surface area is increased by such means for a given volume fraction, and therefore so must be the viscosity. It follows that the volume fraction overall must be reduced. From time to time, minor variations on these themes have appeared. None of these designs has overcome what should be seen as the inevitable and irresolvable compromise between the demands of viscosity and service performance. The expected life of a filled resin restoration is realistically only a very few years. Regular replacement is necessary, and this means progressive loss of tooth tissue at each treatment. The consequences are obvious.

Whether or not filled resin materials have a place in dentistry is not the issue now, but a recognition of certain fundamental limitations in the nature of the material itself is required. Firstly, the carbon-carbon bond is the weak link which determines ultimately the achievable strength of any such material. Secondly, the viscosity of the uncured material controls the ease with which it may be placed in the cavity, and therefore the amount of filler which may in practice be incorporated. Allowing for the demands on the design of the resin matrix itself, cross-linking, water absorption, shrinkage, and so on, the likelihood of substantial improvement in the performance of the system as it stands must be very small. There must be a limit because the component materials involved themselves have absolute limits (see Fig. 1§14.1).