E04. Explain why certain ceramic filler particles cannot be used in light curable composites.

To produce an esthetic composite, the optical properties of the composite should be similar to the optical properties of enamel and dentin. If not, refraction will occur at the tooth/composite interface.

Regarding the resins used in dental composites, the refractive index is around 1.50. If filler is mixed with such a resin, the filler must have a similar refractive index as the resin matrix. If not, light will be refracted and reflected at the resin-filler interface. Such a miss-match means that more light will deviate and come back to the eyes of the viewer, which results in a restoration with high value (looks white) and low translucency.

Besides the poor effect on the esthetic properties of a composite with poor resin-filler refractive index match, such a composite will also prevent light from penetrating deep into the material. The latter is detrimental during cure, because photons cannot reach too deeply and trigger the light-induced setting reaction.  

Light Curing

Be aware that light curable materials have different depth of cure. Factors affecting the depth include filler composition, color and light intensity. By making a specimen and checking the surface hardness opposite to where the light was placed, one can get a feeling for the difficulty of curing a particular material and color. As a general rule, darker and more yellowish colors require longer cure times.

In the figure below, a commercial cure mold is shown. This mold is made by Ivoclar and consists of different cavities with different depths.