E12. Even though heterogeneous microfilled composites may be wear resistant, they are not suitable for Class I and II restorations. Explain why they should be avoided for such applications.

As mentioned earlier, it is impossible to have high filler loading in a microfilled composite. In an attempt to increase the filler fraction of macrofilled composites, so called heterogeneous macrofilled composites have been made.  

Mixing the microfiller particles with a low-viscous monomer such as TEGDMA (see above) makes it possible to add a relatively large volume fraction. However, because of the low "stickiness" of TEGDMA, a high filler loading results in a material that behaves like dry clay. Such a material cannot be handled. Because of the latter, the manufacturer of the microfilled composite heat cures this resin-filler mixture. The cured composite block is then crushed into composite particles, often exceeding 100 um in diameter.  

These prepolymerized microfilled composite particles (shown above) are then mixed with a light curable (or cold curable) monomer that also contain some microfilled particles (the pyrogenic silica particles). Unfortunately, because the resinous composition of the prepolymerized particles, these particles cannot be silanized and bonded to the clinically curable matrix. The only bond is caused by diffusion of monomer into the prepolymerized composite block, a bond that is often poor. The reason is simply that during the manufacturing of the precured composite particles, the degree of conversion of the cross-linked resin becomes quite high, making it difficult for monomer molecules to diffuse into the prepolymerized composite particles. This in turn results in a material that has a tendency to chip if it is loaded, particularly if the load is of a cyclic nature.

Light microscopic picture of a heterogeneous microfilled composite that has been stored in a staining solution for 4 weeks. As seen from the slide (particularly in the upper left quadrant), some staining has been accumulated around the rather course composite filler particles. That accumulation relates to lack of silane treatment (most of the composite surface consists of resin that can not be silanated). Because of the poor matrix to composite filler bond, microfilled composites have a tendency to chip if exposed to cyclic loading.  

Fractured surface of heterogeneous microfilled composite shows protruding composite filler particles.

Microfilled composites often contain about 30 vol.% filler (around 50 wt.%). This amount of filler is too low to significantly improve the modulus of elasticity. However, despite the rather low filler fraction inorganic filler, the interparticle spacing is short, which means that the fine filler particles are rather effective in protecting the wear prone resin. In other words, microfilled composites are rather wear resistant.

Let us now imagine that a Class II restoration is going to be placed. What happens if a material with low modulus of elasticity is selected? During loading a low modulus material will deform more than a high modulus material as long as we assume that the two materials are carrying the same load. During such a deformation, what happens with the width of the material? According to Poisson's ratio, v, the increase in width is equal to the strain in the load direction divided by v. From this relationship we can conclude that the more a material is deformed in the apical direction the more it pushes the cusps apart. Such a "push" increases the risk for both cusp fractures and debonding.  

In addition to the drawback with the lower modulus, repeated load cycles will cause fatigue and dislodgment of some of the prepolymerized composite particles located at the occlusal contact points (shown above). When these particles are dislodged, the material may sometimes fail catastrophically locally.

Despite the above shortcomings, microfilled composites are ideal for Class III and Class V restorations and as a veneering material on Class IV restorations as well as on enamel surfaces.