CERAMICS

Question 1

What are the steps in the fabrication of a crown

Answer 1

Ceramic powder is mixed with water and applied to the die with a brush

The crown is built up using different porcelains for dentine and enamel

These are not tooth coloured

The crown is heated in a furnace to coalesce the powder into ceramic

Question 2

What does heating of the crown lead to

Answer 2

Sintering

Question 3

What is sintering

Answer 3

occurs just above the glass transition temperature

It is when the ceramic particles begin to fuse into a single mass

Question 4

What happens during sintering

Answer 4

the glass phase softens and will coalesce

the material contracts by about 20%

Over time there is controlled diffusion and a solid ceramic mass is formed

Question 5

What are the aesthetics like of conventional feldspathic ceramics

Answer 5

Ceramics have the best aesthetic properties of any dental restorative material

Colour Stable

Very smooth surface
Retain their surface better than other materials, less staining long term

Question 6

What are the optical properties of conventional feldspathic ceramics

Answer 6

Reflectance
Translucency
Opacity
Transparency
Opalescence

Question 7

What is the chemical stability like of conventional feldspathic ceramics

Answer 7

Chemically very stable

Generally unaffected by the wide pH range found in the mouth

Do not take up stain from food/drink

Good BIOCOMPATIBILITY

minimal adverse effects on biological tissues

Question 8

What are the thermal properties like of conventional feldspathic ceramics

Answer 8

Similar to tooth substance

Coefficient of thermal expansion is similar to dentine (Resulting in low stresses to the restoration in the mouth during use)

Thermal diffusivity is low which is protective of the remaining tooth.

Question 9

What is the dimensional stability like of conventional feldspathic ceramics

Answer 9

Once fully fired the material is very stable

During fabrication shrinkage is a problem and must be accommodated for by the technician

Shrinkage of 20% during firing is normal for a conventional feldspathic ceramic crown

Question 10

what are the mech. properties like of conventional feldspathic ceramics

Answer 10

High compressive strength

High hardness (can lead to abrasion of opposing teeth especially if not glazed)

Tensile strength\Flexural strength\Fracture toughness – very low (all can lead to failure during loading)

Question 11

What is static fatigue and how does it happen

Answer 11

Time dependant decrease in strength even in the absence of any applied load. Probably due to hydrolysis of Si-O groups within the material, over time in an aqueous environment

Question 12

Can conventional ceramics suffer from surface micro cracks

Answer 12

Yes,

Can occur during manufacture, finishing or due to occlusal wear. These are areas where fractures can initiate

Question 13

Why do conventional ceramics get slow crack growth

Answer 13

Cyclic fatigue under occlusal forces in a wet environment over time

Question 14

Where can conventional feldspathic be used and why

Answer 14

Only anterior crowns as too brittle for use elsewhere

Question 15

what has been used since the 1960’s to reinforce feldspathic core ceramics

Answer 15

Alumina core

Question 16

What is the core material of PJC

Answer 16

Alumina core

Question 17

How do Alumina particles work

Answer 17

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

Question 18

Aluminous porvelain is….

Answer 18

opaque and can only be used as a core material

Question 19

What are the advantages and disadvantages of alumina cored crowns

Answer 19

Problems with all alumina cored crowns remains lack of flexural strength

Aesthetics are excellent but probably not suitable for anything other than single crowns

Much more successful anteriorly

Did not replace metal ceramic for bridgework or posterior crowns

Question 20

what is the most popular ceramic core material

Answer 20

Zirconia core

Question 21

What is zirconia

Answer 21

Zirconium Dioxide, is a naturally occurring mineral

Question 22

what does zirconia need to be heated to

Answer 22

> 1600

Question 23

What is the type of zirconia used in dentsitry and why

Answer 23

Yttria-stabilised zirconia as Pure zirconia can crack on cooling

Question 24

How much Yttria is present in Yttria-stabilised zirconia and what does it do

Answer 24

3 – 5%

The more Yttria the more translucency

More Yttria reduces the physical properties

Question 25

what is the structure of Yttria and what does this structure do

Answer 25

Atetragonal crystal structure If a crack begins when the stress at the crack tip reaches a critical level the crystal structure transforms to the monoclinic structure This causes a slight expansion of the material and closes up the crack tip

Question 26

Yttria Stabilisation of Zirconia makes the material very

Answer 26

Hard Strong (1000MPA flexural strength) Tough Strong enough to be used as bridge framework

Question 27

How is a Zirconia core fabricated

Answer 27

Impression is taken of the preparation and sent to the lab A model is cast and then scanned digitally Software Unit Creates a bridge substructure on virtual preparations Minimum thicknesses of connectors are determined and fabricated Raw Zirconia block is selected for milling (A presintered block is much easier to mill) The cut framework is then heat treated at around 850oC to achieve its final physical properties This causes a 20% shrinkage but the computer softwear deals with this during the milling process. The framework is also stained to an appropriate colour The Zirconia core is then veneered with feldspathic porcelain to produce the final restoration

Question 28

What are the problmes of a Zirconia core

Answer 28

Expensive equipment required Potential for veneering porcelain to debond from core Zirconia core is opaque Inert fitting surface, cannot etch or bond

Question 29

How are cast and pressed ceramics made

Answer 29

A different technique more like casting a metal restoration The restoration is waxed-up, as you would for a metal restoration Invested Cast from a heated ingot of ceramic (1100oC) No sintering occurs the ceramic ingot is already fully condensed prior to firing. Once devested and cleaned the restoration is heated to improve its crystal structure producing crack inhibiting crystals. This process is called CERAMING

Question 30

What are the type of ceramics used in cast and presssed ceramics

Answer 30

called glass-ceramics. Lithium Disilicate Glass(LiDiSi) Leucite Reinforced Glass

Question 31

What is ceraming

Answer 31

Two stage process Stage 1 crystal formation maximum number of crystal nuclei are formed Stage 2 crystal growth to maximise the physical properties Crystal phase of the ceramic can approach 100%

Question 32

List advantages of different crown types

Answer 32

Monolithic block crowns, milled from a single block of material are strongest Zirconia based crowns are stronger than LiDiSi LiDiSi have better translucency hence better aesthetics Crowns with layered porcelain rather than just stained monolithic block have better aesthetics Layered crowns are more likely to chip

Question 33

How do you descide what to use

Answer 33

Posterior teeth-Monolithic Zirconia, can be used for single crowns and shorter span bridges Anterior teeth single crowns where aesthetics are the most important factor-LiDiSi, can probably use as far back as first premolar Anterior bridgework- LiSiDi, for short span Anterior bridgework with a longer span or heavier occlusion- Zirconia cored

Question 34

How are Zirconia and LiDiSi crowns luted

Answer 34

cemented with conventional or resin cements Both kinds of crowns have intrinsic strength and do not rely upon being bonded to tooth substance to prevent fracture

Question 35

what crowns can be etched

Answer 35

Any silica containing ceramic can be etched with hydrofluoric acid to produce a retentive surface

Question 36

How are etched crowns bonded to

Answer 36

using a silane coupling agent and in turn bonded to the tooth using an appropriate bonding agent and resin cement