E10. Recognize the property changes due to filler particle addition to the resin and identify the characteristics of dental fillers.

Modulus of Elasticity  

Series model/Parallel model

E_C=V_R*E_R+ V_F*E_F 

1/E_C=V_R/E_R + V_F/E_F

The modulus of elasticity can be calculated by assuming that the resin and the filler are either in series or parallel to each other. The red circle shows the approximate volume fraction of small particle filled composites, while the blue circle represents composites containing the maximal amount of pyrogenic silica.

E_C = Young's modulus, composite

E_R = Young's modulus, resin

E_F = Young's modulus, filler

V_R = Volume fraction resin

V_F = Volume fraction filler

The modulus of elasticity of a dental composite falls between the two predicted values for these two models.

Tensile strength  

The addition of filler particles does not have a dramatic tensile strength improving effect (blue horizontal line). In fact, if the filler is not bonded to the resin it weakens the composite in tension as the volume fraction filler increases (red line).

If the filler particles are spherical and equal in size, the highest possible volume fraction one can have is 74%, which is the highest possible packing density for spheres.

Compressive Strength  

Compressive strength increases with filler fraction whether or not the filler is bonded to the matrix material. Because of this behavior, compressive strength is not a particularly good predictor if one wants to know how well the filler particles are bonded to the matrix.

Resilience  

As the filler fraction of composites has increased, the resilience has decreased. We can also force that such a decrease has also happened with the toughness. This change may explain why it seems to be a small but noticeable increase in the frequency of composites. However, this increase has not exceeded the fracture frequency of amalgams.  

Summary

The modulus of elasticity of ceramic filler particles is significantly higher than the modulus of elasticity of dental resins. Thus, as the filler volume increases the modulus of the composite increases. Increased filler fraction results in decreased polymerization shrinkage and lower thermal expansion coefficient. Increased filler fraction also increases compressive strength. In order to retain or slightly improve the tensile strength of a dental composite, the filler must be well bonded to the polymer matrix. If the filler is not well bonded, tensile strength decreases as filler fraction increases. The same is also true regarding wear resistance.  

CONCLUSION: The best dental composite should contain large volume fraction well-bonded filler. At the same time, the filler size should be as small as possible to give as smooth surface as possible. However, as mentioned earlier, a decrease in particle size increases the total filler surface area, making it impossible to incorporate large volume fraction filler. This explains why an optimal filler particle size for load carrying composites (Classes I and II) is often in the range of 0.5-5 um, while restorations of smooth surface caries lesions (Classes III and V) are best treated with composites containing microfill particles. Restorations such as Class IV restorations are best rebuilt by use of a strong composite on the load carrying surface and a smooth surface composite on the labial surface.