porcelain

Question 1

What is the classification of porcelain according to fusion temperature?

Answer 1

• high fusing about 1300 degrees
• medium fusing about 1200 degrees
• low fusing about 1000 degrees
• ultra low fusing < 850 degrees (used for porcelain metal restorations)

Question 2

Why are high fusing and medium fusing porcelains are contraindicated for use in porcelain metal restorations?

Answer 2

wmetal will melt/deform (sagging)

Question 3

What are the types of porcelain accordng to their dental application?

Answer 3

• core porcelain: it must have good mechanical properties
• dentine porcelain: it gives the shape and color
• enamel porcelain: it is quite translucent

Question 4

What are the types of dental porcelain restorations?

Answer 4

• porcelain metal )ceramo-metallic) restoration
• new all ceramic

Question 5

What are the properties of acrylic teeth?

Answer 5

• not as brittle as porcelain
• poor resistant to abrasion ( very soft)
• thermal expansion is the same as acrylic denture base
• density: 1.2 g/cm3 i.e. lighter two
  times than porcelain teeth
• retention to acryclic denture base: chemical bonding
• can be ground and then easily polished
• less aesthetic than porcelain
• BHN = 15 (less than natural teeth)
• transmit less forces to the underlying mucosa (because of its high resilience) –> less bone resorption
• no clicking sound on contact with the opposing teeth giving
  natural feeling to the patient

Question 6

What are the properties of porcelain teeth?

Answer 6

• very brittle
• highly resistant to abrasion (hard)
• thermal expansion is much lower than the acrylic denture base
• this causes stresses in the
  acrylic denture base –> crazing
• density: 2.4g/cm3
• retention to acrylic denture base: mechanical bonding by pins in
  anterior or undercut holes in
  posteriors
• more difficult to grind and
  difficult to polish since porcelain is very hard (loss of glaze layer)
• more aesthetic
• BHN = 470 (more than natural tooth)
• transmit more forces to the underlying mucosa (because of its low resilient) –> high bone resorption
• clicking sound occurs on contact with the opposing teeth

Question 7

What is the advantage of porcelain-metal restorations?

Answer 7

combines the advantages of both the metal (high mechanical properties) and the porcelain (good aesthetic)

Question 8

What are the requirements for porcelain metal restorations?

Answer 8

• its melting temp must be higher than the fusion temperature of porcelain
• its coefficient of thermal expansion must be the same or near that of porcelain
• must not contain copper as it causes green discoloration at the junction between alloy and porcelain
• also must not contain silver as it causes discoloration, but to a lesser effect than copper
• should contain tin and indium (in case of noble metal alloys) so that, on heating they migrate to the surface and produce oxide that is necessary for chemical bonding of
  porcelain to the alloy

Question 9

What is ceramo-metallic bonding?

Answer 9

a combination of
- chemical bonding through oxide layer: if tin or indium is present in the alloy, they will form a film of oxide which can react with porcelain (reaction between porcelain and metal oxide)
- mechanical bonding through surface irregularity: if porcelain is able to wet the metal, it will flow into surface irregularities resulting in mechanical interlocking
- compressive bonding: through the slight mismatchingin coefficient of thermal expansion between porcelain and metal (compression during cooling because metal contracts more than porcelain)