Describe how a natural tooth's characteristics concerning light reflection and refraction is replicated within a PFM crown.

Describe how a natural tooth's characteristics concerning light reflection and refraction is replicated within a PFM crown.

1. In ceramics the greatest light scattering effect is achieved by an increasing difference in refractive index between particles and the main bulk of the porcelain phase.

2. Porcelain itself can be regarded as optically heterogeneous i.e. it is a transparent medium, containing small particle such as metallic oxides (opacifiers), crystals or glassy grains of dissimilar refractive indices to the porcelain.

   • When a beam of light enters such a system, a portion of beam is scattered, and the intensity of the beam is reduced.

3. The scattering is strongly dependant on particle size, so that the maximum scattering occurs at a particle size of the same magnitude as the radiation wave-length.

   • In order to increase the light scattering in dental ceramics and simulate the prismatic effect of human enamels, it is necessary to introduce opacifiers, crystals or a second phase of porcelain powder to the basic frit.
   • During the building up process of different porcelain layers, a porcelain with a better ability to reflect light is used to stimulate dentine. A porcelain with a better ability of scattering light is used to mimic enamel to allow translucency of the prosthesis around the incisal edges.

4. Refraction will occur as light passes through the internal (eg, cracks) and external surfaces of the tooth. An example of an internal surface within a porcelain-bonded restoration is the interface between a pressed coping and the superficial veneering porcelain.

5. opaque porcelains increase the brightness/value reflecting a higher degree of light. Without this layer the metal substructure will be more visible, or harder to cover.

   • An uneven surface to the opaque layer increases the degree of light scattering in the crown.