17955907
Description
Mind Map by Alice Dolman, updated more than 1 year ago
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Created by Alice Dolman
about 5 years ago
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Describe how a natural tooth's characteristics concerning
light reflection and refraction is replicated within a PFM
crown.
- 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.
- 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.
- When a beam of light enters such a system, a portion of beam is
scattered, and the intensity of the beam is reduced.
- 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.
- 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.
- 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.
- 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.
- An uneven surface to the opaque layer increases
the degree of light scattering in the crown.
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