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BDS3 DMS > Dental Ceramics > Flashcards

Dental Ceramics Flashcards

1
Q

Kaolin

A

in dental ceramics

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2
Q

all porcelain is

A

ceramic

but not all ceramic is porcelain

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3
Q

Kaolin is

A

clay

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4
Q

clay a.k.a

A

kaolin

hydrated aluminium silcate

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5
Q

opacity in clay

A

important for appearance of the final product (e.g. mug)

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6
Q

opacity in dental ceramics

A

need to be translucent

so Kaolin is removed and feldspar and silica replace it

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7
Q

composition of decorative ceramic

A

Kaolin 50+%

Quatz (silica) 15-25%

feldspar 15-25%

metal oxides <1%

glass 0

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8
Q

composition of dental ceramics

A

kaolin <5%

quartz (silica) 12-25%

feldspar 70-80%

metal oxides 1%

glass up to 15%

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9
Q

detal ceramics are classed as

A

glasses

feldpathic ‘porcelain’ PJC

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10
Q

feldspar types

A

potash feldspar (pottasium alumina silicate)

soda feldspar (sodium alumium silicate)

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11
Q

feldspar

A

acts as flus

lowers the fusion and softening temperature of glass

it is the lowest fusing component and flows during firing forming a solid mass around the other components (binds)

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12
Q

4 key components of dental ceramics (glasses)

A
  • feldspar
  • borax
  • silica
  • metallic oxides
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13
Q

what do metal oxides convey to the ceramic

A

colour

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14
Q

colour chromium conveys

A

green

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15
Q

colour cobalt conveys

A

blue

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16
Q

colour copper conveys

A

green

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17
Q

colour iron conveys

A

brown

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18
Q

colour maganese conveys

A

lavendar

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19
Q

colour nickel conveys

A

brown

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20
Q

conventional dental ceramics supplied as

A

powder

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21
Q

how is dental ceramic powder made

A
  • heating constituents to a high temperature >1000oC
  • cool rapidly (fritting)
    • in water creating cracks and crazing of the ceramic mass
  • mill the Frit to a fine powder
  • add binder
    • often starch
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22
Q

dental ceramic powder is mixed with

A

distilled water and built up into the restoration (wet sand)

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23
Q

what do feldspathic ceramics form when heated to 1150oC-1500oC?

A

leucite

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24
Q

leucite

A

potassium aluminum silicate

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25
Q

leucite forms

A

around the glass phase of ceramic

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26
Q

reason for leucite in dental ceramics

A

forms around the glass phase of the ceramic

gives a powder of known physical and thermal properties

no further chemical reaction is required during fabrication of the restoration

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27
Q

how are dental ceramic powders made into restorations?

A
  • The powder is mixed with distilled water and built up into the restoration (wet sand)
  • Feldspathic ceramics form leucite when heated to 1150-1500oC
  • Leucite is potassium aluminium silicate
  • This forms around the glass phase of the ceramic.
  • Gives a powder of known physical and thermal properties.
  • No further chemical reaction is required during fabrication of the restoration
  • The powder melts together to form the crown
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28
Q

fabrication of a crown

A
  • 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
    • Heating leads to SINTERING
      • This occurs just above the glass transition temperature
      • It is when the ceramic particles begin to fuse into a single mass.
  • During sintering the glass phase softens and will coalesce
  • Over time there is controlled diffusion and a solid ceramic mass is formed
  • During sintering the material contracts by about 20%
    • Considerable skill required by technician to judge the contraction in 3D
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29
Q

sintering

A

heating leads to sintering (crown in furnance to coalesce the powder)

  • This occurs just above the glass transition temperature

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

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30
Q

6 properties of conventional dental ceramics

A
  • Aesthetics
  • Chemical Stability
  • Biocompatibility
  • Thermal Properties
  • Dimensional Stability
  • Mechanical Properties
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31
Q

aesthetics of conventional dental ceramics

A
  • 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
  • Optical properties
    • Reflectance
    • Translucency
    • Opacity
    • Transparency
    • Opalescence
32
Q

chemical stability of conventional dental ceramics

A
  • 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
33
Q

thermal properties of conventional dental ceramics

A
  • Similar to tooth substance
  • Coefficient of thermal expansion is similar to dentine
    • Results in low stresses to the restoration in the mouth during use
  • Thermal diffusivity is low
    • Protective of the remaining tooth.
34
Q

dimensional stability of conventional dental ceramics

A
  • 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
35
Q

mechanical properties of conventional dental ceramics

A
  • High compressive strength
  • High hardness
    • Can lead to abrasion of opposing teeth especially if not glazed
  • Tensile strength – very low
  • Flexural strength – very low
  • Fracture toughness – very low
    • All lead to failure during loading
  • Static Fatigue
    • 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. (small principle)
  • Surface micro-cracks
    • Can occur during manufacture, finishing or due to occlusal wear.
    • These are areas where fractures can initiate – potential to get bigger and fail
  • Slow crack growth
    • Cyclic fatigue under occlusal forces in a wet environment over time
  • All of these problems mean that conventional feldspathic ceramics can only be used in low stress areas.
    • Only anterior crowns.
    • Not in all patients
      • Too brittle for use elsewhere
36
Q

overcoming the problems with conventional ceramics

(improving the strength of the ceramic)

A
  • Aesthetics are good but they need to be stronger
  • Produce a strong coping, resistant to fracture, and cover in conventional porcelain
  • Cast or press a block of harder ceramic
  • Mill a laboratory prepared block of ceramic
37
Q

strong coping types

A
  • Metal coping
    • See DMS Porcelain-fused-alloys
  • Alumina core
  • Zirconia core
38
Q

alumina core history

A
  • Alumina reinforced feldspathic core ceramics since 1960s
  • This type of core was used as the first choice crown for anterior teeth for decades
  • It is not strong enough for posterior use
  • Aesthetics could be excellent but enough room was required for aluminous core and feldspathic layers above.
  • Possibly more palatal reduction required than in a metal ceramic crown, but less labial reduction required.
39
Q

alumina core

properties

A

Flex strength double that of feldspathic porcelain >120Mpa

  • Alumina particles act as crack stoppers preventing cracks propagating through the material and causing fracture
  • Aluminous porcelain is opaque and can only be used as a core material

1st choice for anterior teeth

not strong enough for posterior use

40
Q

alumina core

manufacturer

A

Aesthetics could be excellent but enough room was required for aluminous core and feldspathic layers above.

  • Possibly more palatal reduction required than in a metal ceramic crown, but less labial reduction required.

Relatively cheap to make

  • No specialist equipment required, just a furnace

Conventional Aluminous cores are a maximum of 50% alumina

  • Increased alumina content increases the strength leading to new techniques
    • INCERAM
    • PROCERA
41
Q

alumina content of aluminous cores

A

50%

increased alumina content increases the strength leading to new techniques

  • INCERAM
  • PROCERA
42
Q

new techniques with increased alumina content in aluminous cores

A

INCERAM

PROCERA

43
Q

uses of alumina cores

A

PJC

anterior teeth 1st choice

not strong enough for posterior

44
Q

In-ceram

A
  • Core material has an alumina content of around 85%
  • Complicated technique (Slip Casting)

In-Ceram-Spinel has Spinel (MgAl2O4) rather than alumina as its core material

  • Better aesthetics but lower flex strength

In-Ceram-Zirconia has 33% Zirconia replacing Alumina in core

  • Higher strength but poorer aesthetics
45
Q

in-ceram split casting

A

The ceramic core is formed onto a refractory model

A fine slurry of alumina is applied to the model

It is heated to 1120oC for 10 hrs

This is below the glass transition temperature for alumina

Partial sintering occurs

A porous core is produced

Core is infiltrated with lanthanum glass at 1100oC

HIGH STRENGTH CERAMIC >400 MPA flexural strength

46
Q

in-ceram-spinel

A
  • has Spinel (MgAl2O4) rather than alumina as its core material
    • Better aesthetics but lower flex strength
47
Q

in-ceram-zironica

A
  • has 33% Zirconia replacing Alumina in core
    • Higher strength but poorer aesthetics
48
Q

in-ceram core material alumina content

A

85%

49
Q

procera

A
  • Pure Alumina core >99% pure
  • Even more complicated
    • Core is made centrally not in every lab
      • A fully densified alumina core is produced at around 1700oC
  • High flexural strength > 700MPA
  • Possibly better translucency than glass infiltrated core.
50
Q

alumina core types

A

in-ceram

procera

51
Q

alumina core types veneered

A

with conventional feldspathic porcelain to produce the final crown

52
Q

alumina core (in-ceram and procera) uses

A

single posterior crowns

not used often as bridge material

53
Q

zirconica core

A
  • Probably the most popular ceramic core material.

Zirconia (Zirconium Dioxide) is a naturally occurring mineral

  • It occurs in different forms at different temperatures

Very hard

  • Zirconia powder does not sinter unless heated to over 1600oC
  • Pure zirconia can crack on cooling
54
Q

zironica core uses

A

used extensively in jewellery - imitation diamonds

Until CAD-CAM was introduced the use of Zirconia as a core material would not have been possible

  • The Zirconia used in dentistry is Yttria-stabilised zirconia
55
Q

Yttria stabilisation of Zironica

A

used in dentistry

Very small amounts of Yttria is present in the material <1%

  • Normal Zirconia is a monoclinic crystal at room temperature
  • Yttria is a tetragonal 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

This ability leads to a material which is very

  • Hard
  • Strong (1000MPA flexural strength)
  • Tough
  • Strong enough to use as a Bridge Framework
56
Q

Yttria stabilisation of zirconica

properties

A

Hard

Strong (1000MPA flexural strength)

Tough

Strong enough to use as a Bridge Framework

57
Q

Yttria

A

Yttria is a tetragonal crystal structure

  • Normal Zirconia is a monoclinic crystal at room temperature

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
58
Q

fabrication of a zirconia core

A
  • 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
    • Milling for a three unit bridge will take around an hour
  • 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
59
Q

zirconia systems

A
  • Most manufacturers have their own Zirconia system
    • LAVA from 3M is probably the best known
    • IPS e.max Zir CAD
    • Opalite
    • Zerion
60
Q

zirconica cored crowns

problems (4)

A
  • Expensive equipment required
  • Potential for veneering porcelain to debond from core
  • Zirconia core is opaque ? Are aesthetics much better than metal ceramic
  • Inert fitting surface, cannot etch or bond
61
Q

benefits of zirconia cored crowns

A

Once you have the equipment they are cheaper to make

  • Cost of metal is increasing

Fit is generally excellent

62
Q

5 milled core crowns and bridges

A
  • Zirconia
  • Lithium Disilicate
  • Precious metal
  • Non-precious metal
  • Titanium
63
Q

sintered layer on ceramics

A

for better aesthetics

64
Q

Sintered Vs Milled crowns

A

For the same material a milled crown will be stronger than a built up or pressed crown.

  • The block will have been subjected to the ideal heat treatments to maximise it’s properties and all blocks will be consistent

As aesthetics of ‘blocks’ of ceramic improve these will become the most commonly used crown. (darker cervically, more translucent at top – decide where in the block going to cut the crown; hand layered crowns better than milled but can dye milled to improve)

  • Probably already acceptable in posterior teeth
65
Q

fabrication of milled crowns

A
  • Cast goes into scanner
  • Scanned image of cast
  • Lower cast is scanned and ‘articulated’
  • Select crown margin
  • adjust crown margins
  • select crown type and place on ‘model’
  • Adjust shape and size of selected crown
  • Save file
  • Send to milling machine
    • Can be anywhere in the world
      • GDH mainly go to Spain
  • 30 – 40 minutes you have your crown In GDH return from Spain takes 48hrs
  • Still requires final finishing
    • In GDH this is still done on a plaster model
66
Q

benefits of a truly digital workflow

A

can dispense with models and impressions.

  • Scan in the mouth
  • Design on CAD machine
  • Mill
  • Polish
  • Cement
67
Q

chairside scanning

A

Trios scanner in GDH

  • Used mainly by postgraduates
  • Digital Dentistry SSM
  • Limited but increasing undergraduate use

Dentsply Sirona CAD-CAM

68
Q

cast and presented ceramics

A

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
  • The cast crown can be stained
    • More often it is cut back labially and veneered with appropriate feldspathic porcelains
69
Q

ceramics used in cast and present ceramics (2)

A

called glass-ceramics

  • Lithium Disilicate Glass
  • Leucite Reinforced Glass
70
Q

ceraming

A

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%

71
Q

strong material crystal size and volume

A

Strong materials have small crystal size and high volume fraction of crystals

72
Q

crystals in lithium disilicate

A

Lithium disilicate glasses have a unique needle-like crystals

  • This makes crack propagation through this material very difficult so good flexural strength
    • possibility to use this material for bridgework ?
      • can look better than block zirconia but may not be as strong
73
Q

luting crowns

A
  • Any silica containing ceramic can be etched with hydrofluoric acid to produce a retentive surface
    • This etched surface can be bonded to, using a silane coupling agent and in turn bonded to the tooth using an appropriate bonding agent
  • Zirconia cored crowns do not contain silica and are not affected by acid
    • They are strong enough to be self supporting and can be luted with a conventional dental cement – doesn’t need to rely on support of underlying tooth
74
Q

etching ceramics

A

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

  • This etched surface can be bonded to, using a silane coupling agent and in turn bonded to the tooth using an appropriate bonding agent

Zirconia cored crowns do not contain silica and are not affected by acid

  • doesn’t need to rely on support of underlying tooth
75
Q

sintering

A

produced by or subjected to sintering (the process of coalescing a powdered material into a solid or porous mass by means of heating without liquefaction).

76
Q

coalesce

A

come together to form one mass or whole.

77
Q

milled

A

reduced to fine particles by grinding in a mill.

BDS3 DMS (9 decks)

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