DENTAL CERAMICS

Question 1

What do decorative ceramics contain?

Answer 1

Kaolin

Question 2

What is Kaolin?

Answer 2

An opaque clay composed of hydrated aluminium silicate

Question 3

What are the constituents of dental ceramic?

Answer 3

• Kaolin <5%
• Quartz (silica) 12-25%
• Feldspar 70-80%
• Metal oxides 1%
• Glass (up to) 15%

Question 4

What is Feldspar?

Answer 4

Potash feldspar = potassium alumina silicate
Soda feldspar = sodium alumina silicate

Question 5

what is the function of Feldspar?

Answer 5

• Acts as a flux
• Lowers the fusion and softening temperature of the glass

Question 6

How does feldspar act during firing of ceramics?

Answer 6

flows during firing forming a solid mass around the other components

Question 7

What is the function of metallic oxides in ceramics?

Answer 7

They convey colour to the ceramic

Question 8

How are conventional dental ceramics supplied?

Answer 8

Supplied as a powder

Question 9

How is conventional dental ceramic powder made?

Answer 9

• heating the constituents to a high temperature (>1000 degrees)
• cool rapidly
• mill the frit to a fine power
• add binder
• power is mixed with distilled water & built up into the restoration

Question 10

What is the rapid cooling of conventional dental ceramic power known as?

Answer 10

Fritting
- creates cracks and crazing of the ceramic mass

Question 11

What is the binder commonly added to conventional dental ceramics?

Answer 11

often starch

Question 12

What do Feldspathic ceramics form when heated to 1150-1500 degrees celsius?

Answer 12

Leucite (potassium aluminium silicate)
- forms around the glass phase of the ceramic
- gives a powder
- powder melts together to form crown

Question 13

What are the properties of conventional dental ceramics?

Answer 13

• brilliant aesthetics
• chemically stable
• thermally similar to tooth substance
• dimensionally stable
• good mechanical properties

Question 14

Why are dental ceramics very aesthetic?

Answer 14

• colour stable
• very smooth surface
• less staining long term
• good optical properties

Question 15

The coefficient of thermal expansion of dental ceramics is similar to dentine, why is this good?

Answer 15

results in low stresses to the restoration in the mouth during use

Question 16

The thermal diffusivity of dental ceramics is low, why is this good?

Answer 16

protective of the remaining tooth

Question 17

What occurs during firing of a conventional feldspathic ceramic crown?

Answer 17

shrinkage of about 20%

Question 18

What are the mechanical properties of dental ceramics?

Answer 18

• high compressive strength
• high hardness
• very low tensile strength
• very low flexural strength
• very low fracture toughness

Question 19

The mechanical properties of dental ceramics mean that conventional feldspathic ceramics can only be used in low stress areas, why?

Answer 19

• Stress fatigue
• Surface micro-cracks
• Slow crack growth

Question 20

What can feldspathic ceramic crowns be used for?

Answer 20

Anterior crowns

Question 21

How are dental ceramics made stronger?

Answer 21

By metal coping with
- alumina
- zirconia

Question 22

What is the flex strength of alumina core ceramic crowns?

Answer 22

> 120Mpa

Question 23

What is the function of the Alumina in dental ceramic crowns?

Answer 23

Alumina particles act as crack stoppers preventing cracks propagating through the material and causing fracture

Question 24

What are the downsides of alumina core dental ceramic crowns?

Answer 24

• lack of flexural strength
• probably not suitable for anything other than single crowns
• cannot be used posteriorly

Question 25

What is the most popular ceramic core material?

Answer 25

Zirconia core

Question 26

At what temperature does Zirconia powder sinter?

Answer 26

over 1600 degrees celsius

Question 27

What zirconia is used in dentistry? Why?

Answer 27

Yttria-stabilised zirconia - pure zirconia can crack on cooling

Question 28

How much Yttria is present in yttria stabilised zirconia?

Answer 28

3-5%

Question 29

Higher levels of Yttria in zirconia crowns lead to what?

Answer 29

More translucency & reduced physical properties

Question 30

What is the flexural strength of Yttria Stabilised Zirconia?

Answer 30

1000MPA

Question 31

What are the problems associated with zirconia cored crowns?

Answer 31

- expensive equipment required - potential for veneering porcelain to deboned from core - inert fitting surface (cannot etch or bond)

Question 32

How are Cast & Pressed Ceramics made?

Answer 32

- restoration is waxed up - invested - cast from a heated ingot of ceramic - once divested and cleaned the restoration is heated to improve its crystal structure - this is called CERAMING

Question 33

What ceramic is used in cast and pressed ceramics?

Answer 33

Glass ceramics - lithium disilicate glass - leucite reinforced glass

Question 34

What is CERAMING?

Answer 34

Stage one = crystal formation maximum number of crystal nuclei are formed Stage two = crystal growth to maximise the physical properties

Question 35

What ceramic crowns should be used on posterior teeth?

Answer 35

Monolithic Zirconia (can be used for single crowns and shorter span bridges)

Question 36

What ceramic crowns should be used on anterior teeth where aesthetics are the most important factor?

Answer 36

LiDiSi

Question 37

Explain what translucency means with regard to optic properties of materials:

Answer 37

Translucency refers to the ability of a material to allow light to pass through but be scattered at one of the surfaces or internally leading to a blurring of the transmitted light

Question 38

Explain what opalescence means with regard to optic properties of materials:

Answer 38

The ability of a material to appear - blue in reflected light - orange/yellow in transmitted light

Question 39

Chemically how does a silane coupling agent work when bonding composite to ceramic?

Answer 39

- Creates a strong bond between oxide groups on the ceramic surface and the silane - Other end of the silane molecule has a C=C bond which reacts with the composite resin luting agent