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BDS3 CLINICAL DMS > Ceramic > Flashcards

Ceramic Flashcards

(82 cards)

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1
Q

What feature of decorative ceramic is not ideal for a dental material

A

it is opaque

we want it to be translucent

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2
Q

How do we make ceramic translucenct

A

kaolin is removed and feldspar and silica replaces

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3
Q

What is the composition of dental ceramics

A 
<5% kaolin
12-25% quartz (silica)
70-80% feldspar
1% metal oxides 
up to 15% glass
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4
Q

What is feldspar made of

A

potash feldspar (potassium alumina silicate)

soda feldspar (sodium alumina silicate)

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5
Q

What is the function of feldspar

A

it acts as a ‘flux’ which is something that promotes melting

it lowers the fusion and softening temperature of the glass

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

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6
Q

What are the components in dental ceramics

A

feldspar
borax
silica
metallic oxides

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7
Q

What are the function of metallic oxides

A

they convey color to the ceramic

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8
Q

What color does chromium give to ceramic

A

green

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9
Q

What color does cobalt give to ceramic

A

blue

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10
Q

What color does copper give to ceramic

A

green

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11
Q

What color does iron give to ceramic

A

brown

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12
Q

What color does manganese give to ceramic

A

lavender

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13
Q

What color does nickel give to ceramic

A

brown

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14
Q

How is the powder in conventional dental ceramic made

A

the constituents are heated in the oven resulting in a glassy mass which is cooled rapidly (fritting) in water and it creates cracks and crazing of the ceramic mass

it is then ground into a fine powder and a binder (often starch) is added

the powder is then given to the technician and is mixed with distilled water and built up into the restoration

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15
Q

When you make a crown from conventional ceramic what do you have to do

A

heat it again

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16
Q

What do feldspathic ceramics form when heated

A

when heated to 1150-1500 leucite is formed

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17
Q

What is leucite

A

potassium aluminum silicate

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18
Q

What does leucite form

A

it forms the glass phase of the ceramic and it gives a powder of known physical and thermal properties

the powder melts together to form the crown in what’s called ‘sintering’

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19
Q

How is the crown fabricates from conventional dental ceramic

A

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

crown is built up using different porcelains for dentine and enamel but these are not tooth colored

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

the heating leads to sintering and this occurs just above the glass transition temperature

during sintering the glass phase softens and will coalesce

overtime there is controlled diffusion and a solid ceramic mass is formed

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20
Q

What is sintering

A

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

during sintering the material contracts by about 20%

considerable skill is required by the technician to judge the contraction in 3D

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21
Q

How are the aesthetic properties of conventional dental ceramics

A

they have the best aesthetic properties
colour stable
very smooth surface and retain them better than other materials meaning less staining over time

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22
Q

What are the optical properties of conventional dental ceramics

A 
reflectance 
translucency
opacity
transparency
opalescence
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23
Q

How is the chemical stability of conventional dental ceramics

A

very stable
generally unaffected by the wide pH range found in the mouth
do not take up stain from food and drink

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24
Q

How is the biocompatibility of conventional dental ceramics

A

minimal adverse effects on biological tissues

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25
What are the thermal properties of conventional dental ceramics
similar to tooth coefficient of thermal expansion is similar to dentine thermal diffusivity is low meaning it can protect the pulp from hot and cold
26
Why is the coefficient of thermal expansion being similar to dentine a positive thing
results in low stresses to the restoration in the mouth during use
27
How is the dimensional stability of dental ceramics
once fully fired the material is v 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
28
How is the compressive strength of conventional ceramic
high
29
How is the hardness of conventional ceramic
high | can lead to abrasion of opposing teeth especially if not glazed
30
How is the tensile strength of conventional ceramic
very low | can lead to failure during loading
31
How is the flexural strength of conventional ceramic
very low | can lead to failure during loading
32
How is the fracture toughness of ceramic
very low | can lead to failure during loading
33
What is static fatigue
time dependent decrease in strength even in absence of any applied load
34
Why does static fatigue occur in conventional ceramic
it is probably due to the hydrolysis of Si-O groups within the material, over time in an aqueous environment
35
What are surface micro cracks and how do they occur
can occur during manufacturing, finishing or due to occlusal wear these are areas where fractures can initiate
36
What is slow crack growth
cyclic fatigue under occlusal forces in a wet environment over time smaller loads over a long period of time can cause cracks to propagate
37
What effect do the mechanical properties of conventional ceramic on its use
can only be used in low stress areas only in anterior crowns not in all patients (depends on biting forces - class II have high biting forces anteriorly) too brittle for use elsewhere
38
How are the problems with conventional ceramics overcome
want to 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 fo ceramic
39
What can the strong coping be made of
metal aluminia core zirconia core
40
What is alumina core
it is used as a core material in PJCs
41
What is the effect of using an alumina core on the flex strength
double that of feldspathic porcelain (>120Mpa) which is not high but better
42
How do the alumina particles help improve strength of the ceramic
they act as crack stoppers preventing cracks propagating through the material causing fracture
43
Why can aluminous porcelain only be used as a core material
it is opaque
44
What are the disadvantages of alumina core
not strong enough for posterior use aesthetics excellent but enough room was required for aluminous core and feldspathic layers above meaning more tooth prob possibly more palatal reduction required than a metal ceramic crown but less labial reduction is required as the prep is about a mm all the way around relatively cheap to make no specialist equipment is required, just a furnace
45
What % of alumina is in conventional aluminous cores
max of 50% | increased alumina content increases the strength resulting in new techniques called INCERAM and PROCERA
46
What is the alumina content of an inceram crown
the core material has an alumina content of around 85%
47
What is the technique used for IN-CERAM
slip casting
48
Describe the slip casting technique
the ceramic core is formed onto a refractory model a fine slurry of alumina is applied to the model and heated to 1120 degrees for 10 hrs which is below the glass transition temp for alumina partial sintering occurs and a porous core is produced the core is infiltrated with lanthanum glass at 1100 degrees this results in a high strength ceramic with >400 MPA flexural strength
49
What is in-ceram spinel
has spinel rather than alumina as its core material which provides better aesthetics as its more translucent but lower flex strength
50
What is in-ceram zirconia
has 33% zirconia replacing alumina in core higher strength, poorer aesthetics
51
What is procera
has a pure alumina core and its 99% pure it is even more complicated the core is made centrally not in every lab a fully densified alumina core is produced at around 1700 it has a high flexural strength >700 MPA has possibly better translucency than glass infiltrated core
52
What happens to the different alumina core types
both of these core types are then veneered with conventional feldspathic porcelain to produce the final crown both core types are probably suitable for single posterior but not for bridges
53
What is zirconia
zirconium dioxide naturally occurring it occurs in different forms at different temperatures very hard
54
At what temperature does zirconia powder sinter
1600
55
What is the zirconia used in dentistry
Yttria stabilized zirconia
56
What can happen to pure zirconia on cooling
it can crack
57
How much Yttria is present in the material
<1%
58
What is Yttria
tetragonal crystal structure
59
What happens if a crack begins in the yttrium stabilized zirconia
if it 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 results in a material that's strong enough to use as a bridge
60
Describe the process of fabricating a zirconia core
impression taken for the prep and sent to the lab model is cast and scanned digitally soft ware unit creates a bridge substructure on a virtual preparations minimum thicknesses of connectors are determined and fabricated raw zirconia block is elected for milling the cut framework is then heat treated at around 850 degrees to achieve its final physical properties this causes a 20% shrinkage but the computer software deals with this during the milling process its stained the zirconia core is veneered with feldspathic porcelain to produce the final restoration
61
What type of zirconia block is easier to mill
pre sintered
62
How long does milling for a 3 unit bridge take
1 hr
63
What are the problems with zirconia cored crowns
expensive equipment is required potential for veneering porcelain to debunk from the core zirconia core is opaque meaning wise aesthetics inert fitting surface so cannot etch or bond
64
Why is there potential for veneering porcelain to de-bond from the zirconia core
they have different rates of expansion and contraction so there is weakness between the core and porcelain so when subjected to forces debunking can happen
65
What are the benefits f the zirconia core
once you have the equipment they're cheaper to make as the cost of metal is increasing fit is good
66
What are the milled core crowns and bridges
``` zirconia lithium disilicate precious metal non precious metal titanium ```
67
What is the benefit of ceramics vs the milled core crowns and bridges
all have a surface sintered layer for best aesthetics
68
Compare sintered vs milled for strength
for the same material a milled crown will be stronger than a built up/pressed one the block will have been subjected to the ideal heat tx to maximize its properties and all blocks will be consistent
69
Compare sintered vs milled for aesthetics
aesthetics of hand layered crowns are better but the aesthetics of milled ones are getting better and you can stain them to improve aesthetics
70
Describe the process of fabricating a milled crown
1. cast into scanner 2. scanned image of cast 3. lower cast is scanned and articulated 4. select crown margin 5. adjust crown margin 6. select crown type and place on model 7. adjust shape and size of selected crown 8. save file and send to milling machine 9. crown requires finishing
71
Describe the process for a purely digital process
same but scan in the mouth and design on CAD machine then mill, polish, cement
72
Describe the process for cast and pressed ceramics
a different technique more like casting a metal restoration 1. restoration waxed up as you would for metal 2. invested 3, cast from a heated ingot of ceramic (1100 degrees) 4. no sintering occurs because the ceramic ingot is fully condensed prior to firing 5. once de-vested and cleaned, the restoration is heated to improve its crystal structure producing crack inhibiting crystals This process is called CERAMING 6. the cast crown can be stained more often it is cut back labially and veneered with appropriate feldspathic porcelain
73
What are the ceramics used in the cast and pressed ceramic processes called
they're glass ceramics lithium dislocate glass leucite reinforced glass
74
Describe what happens in CERAMING
stage 1 - crystal formation maximum number of crystal nuclei are formed stage 2 - crystal growth to maximize the physical properties with some materials you get 100% crystal growth
75
What is the crystal size in strong materials
small crystal size and high volume fraction of crystals makes it harder fo cracks to propagate through the material
76
What are the lithium disilicate crystals like
unique needle like crystals making crack propagation harder and giving it a good flexural strength not as strong as zirconia but better aesthetics than mono block zirconia
77
How are the silica containing ceramics cemented
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
78
How are the zirconia cored crowns cements
they do not contain silica so are not affected by acid strong enough to be self supporting and can be luted with a conventional dental cement some new zirconias contain silica to increase ability to bond but pure silica can't be bonded to tooth
79
What is translucency
Reflects light and makes it change direction but also allows transmits some of the light
80
What is reflectance
the measure of the proportion of light or other radiation striking a surface which is reflected off it.
81
What is opacity
not allowing light to pass through
82
What is transparency
transmits light

BDS3 CLINICAL DMS (9 decks)

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