Ceramics

Question 1

What is the composition of dental ceramics?

Answer 1

Feldspar (70-80%)
Quartz (silica- 12-25%)
Metal oxides
Glass
Kaolin (main component of decorative ceramic)

Question 2

What is the purpose of metal oxides?

Answer 2

Colouration - different oxides have different colours eg copper= green, cobalt= blue)

Question 3

What is the purpose of feldspar?

Answer 3

Lowers the fusion and softening temperature of the glass,
Flows during firing, forming a solid mass around other components

Question 4

How are conventional dental ceramics made?

Answer 4

Powder is made by heating constituents to high temperature >1000 degrees
Cooling rapidly (fritting)
Melt the frit to fine powder
Add binder (starch)
Mixed with distilled water and built up

Crown is heated in furnace into ceramic (20% shrinkage) - sintering

Question 5

What is fritting

Answer 5

Rapid cooling of powder

Question 6

What is sintering?

Answer 6

Ceramic particles begin to fuse into a single mass when heated.
Material contracts by 20%.

Question 7

What are the ideal properties of dental ceramics?

Answer 7

Aesthetics (resistance to stain)
Chemically stable (unaffected by wide range of pH)
Biocompatibility
Thermal properties (similar to tooth, low stress between restoration and tooth, low thermal diffusivity- protective to remaining tooth)
Dimensionally stable (no creep)
Mechanical properties (high compressive strength and hardness, however, low flexural strength and toughness)

Question 8

What is static fatigue?

Answer 8

This is the time dependant decrease in strength even in the absence of applied load.

Question 9

What are surface micro cracks?

Answer 9

Occur during manufacture/ finishing/ during occlusal wear- areas where fractures initiate

Question 10

What effect does cyclic fatigue have on the micro-cracks?

Answer 10

Cyclic fatigue under occlusal forces in a wet environment over time can cause slow crack propagation.

Can only be used in low stress areas

Question 11

What is used to overcome the poor mechanical properties of conventional dental ceramics?

Answer 11

Metal coping/ alumina core/ zirconia core

Question 12

What is the function of an aluminia core and what are the contraindications?

Answer 12

Aluminia particles act as crack stoppers, preventing cracks propagating through the material.
Increases fracture toughness although not strong enough for posterior teeth.

Question 13

How is a zirconia core fabricated?

Answer 13

Impression is taken and sent to labs where a model is cast and is scanned digitally.
Framework is milled, heat treated (20% shrinkage) and stained.
Core is veneered with feldspathic porcelain to produce the final restoration

Question 14

What is an advantage of cast and pressed ceramics?

Answer 14

No sintering occurs
Restoration is heated to improve its crystal structure producing crack inhibiting crystals - ceraming (lithium disilicate)

Question 15

What is the difference between sintered and milled crowns?

Answer 15

Milled crowns stronger (heat treatments maximise properties)

Question 16

How are zirconia and LiDiSi crowns cemented?

Answer 16

Conventional or resin cements
Silica containing ceramics (LiDiSi) must be etched with hydrofluoric acid to produce a retentive surface and are bonded using silane coupling agent
Zirconia does not contain silica- can be air abraded to create retentive surface