E13. Explain why small particle-filled composites are among those with the highest volume fraction filler.

It is a general misperception that so-called hybrid composites contain more filler than small particle filled composites. In really the opposite is true. The reason is simply that hybrid composites consist of a mixture of pyrogenic silica and ground filler particles, while the ideal small particle filled composite consists only of ground particles. Because of these differences, the filler surface area per gram filler is larger in the hybrid composites than it is for the ideal small particle filled composite. The smaller total filler surface area per gram filler means that less monomer is immobilized by the filler surfaces in the small particle filled composites. The latter means that more filler can be incorporated in the ideal small particle filled composite before it becomes saturated, explaining why the true small particle filled composites contain larger volume fraction filler than the hybrid composites.  

Having said the above, it is also important to realize that today the term "hybrid composite" has very little meaning. The reason is simply related to the evolution of dental composites. The first composites on the market (the traditional composites) consisted of rather coarse filler particles. These particles had a tendency to sediment over time. To solve this problem, manufacturers of dental composites started to add small fractions of pyrogenic silica to the monomer to slow down the sedimentation process. These materials were not called hybrid composites even though they could have been called hybrid composites.

As time progressed, the filler particles were ground smaller and smaller. The competition among manufacturers increased at the same time. Better and better "small particle filled" composites appeared on the market. In order to stand out on the market, one manufacturer of a composite started to call his product for a "hybrid composite." Consumers who still changed from the old traditional composites to small particle filled composites found that this "hybrid composite" was as good as any of the other small particle filled composites. However, the term "hybrid composite" stood out, which meant that other dentists wanted to try the "hybrid composite." Other manufacturers had to meet the composition, which they made by mainly just changing name. At the same time, manufacturers who had succeeded to grind the filler particles even smaller could not use pyrogenic silica to any greater extent. However, because the good reputation of the "hybrid composites" these products could not be marketed as "small particle filled composites" and were therefore also called "hybrid composites."

From the above we must ask how we can select a good posterior composite. The answer is simply the following. Select a composite that contains 60 vol.% or more filler that has as small filler particle size as possible and have high tensile strength values. The high volume fraction filler means a high modulus, low shrinkage, and short interparticle spacing. Small particles mean smooth surface, but also low volume fraction filler and what that mean. In other words, the 60 vol.% level forces the particles to be around 1 um in diameter. The high tensile strength is used to determine whether the filler-matrix bond is good, which is needed for a wear resistant posterior composite.  

The smaller the filler particles are, the larger the total filler surface is. As seen from this drawing, the total filler surface area starts increasing dramatically when the filler particles approach 0.1 microns and become smaller. Because of that increase, smaller particles tie up larger volume resin per gram filler.  

For a dentist, the viscosity of a composite is a very important handling characteristic. A too viscous material is difficult to adapt to cavity margins, while a too fluid material is sticky and does not retain shape. Since the volume movable resin determines the viscosity of a composite, a composite with smaller filler surface will tie up less volume resin. Consequently, more filler can be added to the courser filler if the objective is to retain the same viscosity.

From the above we can conclude that microfilled composites cannot contain as much filler as the small particle filled composites. At the same time, we can also conclude (previous slide) that an increase in filler size above a few tens of a micron will not affect the total filler surface dramatically.

Considering the above, and knowing that a smooth composite surface is preferable, we can conclude that composite optimization should occur when the filler particle sizes range from 0.1 to a few microns.  

Top left drawing simulates a composite filled with course filler. Could be a traditional composite with a filler particle size of 20-40 microns.

Top right drawing simulates a composite filled with the pyrogenic silica. Because of the large total surface area of that filler, fairly little filler can be added to the resin before most of the resin has been tied up by the filler surface. Because of that, these so-called homogeneous microfilled composites contain less than 30% by weight filler.

Lower left drawing shows a composite with finely ground filler. The filler particle size is around 1 micron. This is a so-called small particle filled composite.

Lower right drawing shows a so-called heterogeneous microfilled composite. First a heavy loaded homogeneous microfilled composite is made and cured. This composite is ground into rather course composite particles (50-100 microns). These composite particles are then mixed with a resin that also contains pyrogenicca particles.