Dental ceramics Flashcards
What is Kaolin?
- Decorative ceramics contain Kaolin
- It is a clay
- Hydrated Aluminium silicate
- Opaque
- Opacity imp for appearance of final product
Why does Kaolin need to be removed from Dental ceramics?
- Ceramics need to be translucent so Kaolin removed
- Feldspar and silica replace it
What are the components of Decorative ceramic?
- Kaolin 50+%
- Quartz (silica) 15-25%
- Feldspar 15-25%
- Metal oxides <1%
- Glass 0
What are the components of Dental ceramics?
- Kaolin <5%
- Quartz (silica) 12-25%
- Feldspar 70-80%
- Metal oxides 1%
- Glass up to 15%
What are the two types of Feldspar?
- Potash Feldspar (potassium alumina silicate)
- Soda Feldspar (sodium alumina silicate)
What does Feldspar do in dental ceramics?
- Acts as a flux
- Lowers fusion and softening temp of glass
- Has lowest fusing component and flows during firing forming a solid mass around other components
What are dental ceramics considered as ?
- Glasses
What property do metal oxides gives the ceramic?
- Metallic oxide determine colour of ceramic
What colour does Chromium convey to ceramic?
- Green
What colour does Cobalt convey to ceramic?
- Blue
What colour does Copper convey to ceramic?
- Green
What colour does Iron convey to ceramic?
- Brown
What colour does Manganese convey to ceramic?
- Lavender
What colour does Nickel convey to Ceramic?
- Brown
How are conventional dental ceramics used to build up into a restoration?
- Supplied as a powder
- Powder made by heating constituents to high temp >1000oC
- Cool rapidly (Fritting) in water
- This creates cracks and crazing of ceramic mass
- Mill the fritt to fine powder
- Add binder (usually starch)
- Powder is mixed with distilled water and built up into restoration
What do feldspathic ceramics form when heated to 1150oC-1500oC?
- Form leucites around glass phase of ceramic
- Gives powder of known physical and thermal properties
- No further chemical reaction needed during fabrication of restoration
- Powder melts together to form the crown
What is Leucite?
- Potassium aluminium silicate
How are conventional dental ceramics fabricated?
- Ceramic powder mixed with water and applied to die with brush
- Crown built up using different porcelains for dentine and enamel
- They are not tooth coloured
- Crown is heated in furnace to coalesce the powder into ceramic
What is Sintering?
- Heating the crown leads to Sintering
- When the ceramic particles begin to fuse into a single mass
- Occurs just above glass transition temp
What happens during sintering of dental ceramic?
- The glass softens and will coalesce
- Over time controlled diffusion occurs and solid ceramic mass formed
- The material contracts by about 20%
What is meant by the term Coalesce?
- Come together to form one mass or whole
What are some important properties required of conventional dental ceramics?
- Aesthetics
- Chemical Stability
- Biocompatibility
- Thermal Properties
- Dimensional Stability
- Mechanical Properties
Why do ceramics have the best aesthetics properties of any dental restorative material?
- Colour stable
- Very smooth surface
- Retain their surface better than other materials leads to less staining long term
What are the optical properties of dental ceramics?
- Reflectance
- Translucency
- Opacity
- Transparency
- Opalescence
What is the chemical stability of dental ceramics?
- Chemically very stable
- Unaffected by wide pH range found in mouth
- Do not take up stain from food/drink
- Good Biocompatibility so minimal adverse effects on biological tissues
What are the thermal properties of Dental ceramics?
- Sim to tooth substance
- Coefficient of thermal expansion sim to dentine
- Results in low stresses to restorations in mouth during use
- Thermal diffusivity low which protects remaining tooth
What is the dimensional stability of dental ceramics?
- Once fully fired material is very stable
- During fabrication shrinkage occurs which is problem and must be accommodated by technician
- Shrinkage of 20% during firing normal
What are the mechanical properties of conventional dental ceramics?
- 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)
What is Static fatigue?
- Decrease in strength even in absence of any applied load, it is time dependent
Why is it thought Static fatigue occurs?
- Due to hydrolysis of Si-O groups within material, over time in aqueous environment
When can surface micro-cracks occur?
- Occur during manufacture, finishing or due to occlusal wear
- Fractures can initiate slow crack growth
What is Slow crack growth?
- Cyclic fatigue under occlusal forces in wet environment over time
Why can conventional feldspathic ceramics only be used in low stress areas?
- Due to static fatigue, surface micro cracks and slow crack growth
What low stress areas can conventional feldspathic ceramics be used in?
- Anterior crowns
- Not in all patients
- Too brittle elsewhere
How can we overcome problems with conventional ceramics?
- Aesthetics are good but need to be stronger
- Produce strong coping, resistant to fracture
- Cast or press a block of harder ceramic
- Mill a laboratory prepared block of ceramic
Why are alumina Cores used to reinforce feldspathic ceramics?
- Core material
- Flex strength double feldspathic porcelain >120Mpa
- Alumina particles act as crack stoppers so prevent cracks propagating through material and causing fracture
- Aluminous porcelain is opaque and can only be used as core material
What are the benefits of alumina core?
- Cheap to make
- No specialist equipment needed, just furnace
- Less labial reduction needed
What are the negatives of Alumina core?
- More palatal reduction required than metal crown
- Not strong enough for posterior use
- Enough room was needed for aluminous core and feldspathic layers above so aesthetics good but not amazing
- Lack of flexural strength
- Did not replace metal ceramic for bridgework or posterior crown
What are some new techniques with increased alumina content?
- INCERAM
- PROCERA
- Increases the strength
What are alumina core veneered with to produce final crown?
- Veneered with conventional feldspathic porcelain
What are some newer techniques and materials other than Alumina?
- Zirconia
- Lithium Disilicate
- Cores
- Pressed crowns
- Monolithic/Milled crowns
- Advantages/Disadvantages of different types
- Luting
What is a Zirconia?
- Most popular ceramic core
- Zirconium dioxide naturally occurring material
- Occurs in different forms at diff temp
- Very hard
- Used in jewellery industry
What Zirconia is used in dentistry?
- Yttria-stabilsed zirconia
Why is pure zirconia not used in dentsitry?
- Pure can crack on cooling
- Zirconia powder does not sinter unless heated to over 1600oC
What is the process of Yttria stabilisation of Zirconia?
- 3-5% Yttria present in material
- The more Yttria the more translucency but it reduces physical properties
- Normal zirconia is monoclinic crystal at room temp
- Yttria is a tetragonal crystal structure
- If crack begins when stress at crack tip reaches critical level level the crystal structure transforms to monoclinic structure
- Causes slight expansion of material and closes crack tip
What properties does Yttria stabilisation give to Zirconia?
- Hard
- Strong (1000MPA flexural strength)
- Tough
Strong enough to use as bridge framework
What is the process for fabrication of Zirconia Cone?
- Impression taken of preparation snd sent to lab
- Model is cast and scanned digitally
- Software unit creates bridge substructure on virtual preparations
- Minimum thickness of connectors determined and fabricated
- Raw Zirconia block selected for milling
- Presintered block easier to mill
- Cut framework then heated to 850oC to achieve final properties
- Framework stained appropriate colour
- Then veneered with feldspathic porcelain to produce final restoration
How long does milling take for three unit bridge?
- Approx an hour
What occurs during heating stage of Zirconia cone?
- Causes 20% shrinkage
- Computer software deals with this during milling process
What are some Zirconia systems?
- Zerion
- Opalite
- Everest ZH
- LAVA from 3M first mainstream material
What are some problems with Zirconia cored crowns?
- Expensive equipment needed
- Potential for veneering porcelain to debond from core
- It is opaque so questions aesthetics
- Inert fitting surface, can’t etch or bond
What are some positives with Zirconia Cored crowns?
- Once equipment is bough they are cheaper to make
- As cost of metal is increasing
- Fit is excellent
What can be used to make Milled core crowns and bridges?
Zirconia
Lithium Disilicate (E-Max)
Precious metal
Non-precious metal
Titanium
Composite
What do all ceramics have in common to get better aesthetics?
- All have surface sintered layer
What is the process of fabrication of a milled crown?
- Cast goes into scanner
- Scanned image of cast
- Lower cast scanned and articulated
- Select crown margin
- Adjust crown margin
- Select crown type and place on model for upper and lower
- Adjust shape and size of selected crown on uppers
- Save file and send to mill (GDH go to spain)
- Requires final finishing (GDH do this on plaster model)
How long does milling take for Milled crown?
- 30-40 mins and crown is made
- If return from Spain it is 48hrs
Is the process the same for all materials used in milled crowns?
- Yes
- Zirconia
- LiDiSi
- Metal
- Ceramic filled composite resin
What is the method of fabrication of a Milled crown if you have fully digital workflow?
- Don’t need models and impressions
- Scan in mouth
- Design on CAD machine
- Mill
- Polish
- Cement
What is the method for cast and pressed ceramics and what is it called?
- Restoration waxed up as you would metal restoration
- Invested
- Cast from heated ingot of ceramic 1100oC
- Once devested and cleaned the restoration heated
- Called Ceraming
Why does sintering not occur in Ceraming?
- Ceramic ingot already fully condensed prior to filling
Why is the ceramic ingot heated in Ceraming?
- Heated to improve crystal structure producing crack inhibiting crystals
Why are cast crowns veneered?
- Can be stained
- Most often cut back labially and veneered with appropriate felspathic porcelains
What ceramics are used in cast and pressed ceramics?
- Called glass ceramics
- Lithium Disilicate Glass
- Leucite Reinforced Glass
What are the two stages of Ceraming?
- 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%
Why are lithium disilicate glasses used in cast and pressed ceramics?
- Strong material which has small crystal size and high vol fraction of crystals
- Have unique needle-like crystals
- Makes crack propagation very difficult = good fracture toughness
- Good flexural strength 350MPa
What are some advantages of different crowns?
- 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 due to stresses between core and
What process creates a stronger crown - Sintered or Milled?
- Milled crown of same material gives stronger crown
- Block is subjected to ideal heat treatments to maximise properties
- Blocks will be consistent
- As aesthetics of these blocks improve they will become most commonly used crown for all mouth not just posterior
What material should you choose for posterior teeth?
- Monolithic Zirconia
- Can be used for single crowns and shorter span bridges
What material should you choose for anterior teeth single crowns where aesthetics most important factor?
- LiDiSi
- Can probably use as far back as first premolar
What material should you choose for anterior bridgework?
- Short span with no parafunction use LiDiSi
- Longer span or heavier occlusion use Zirconia cored with zirconia where occlusal contacts meet
What can Zirconia and LiDiSi crowns be cemented onto tooth with?
- Can use conventional or resin cements
- Don’t require to be bonded to tooth substance to prevent fracture as they have own intrinsic strength
What can Silica containing crowns be cemented to tooth with?
- can be etched ,with hydrofluoric acid to produce retentive surface
- etched surface can be bonded using silane coupling agent, to tooth using appropriate bonding agent and resin cement
What are Zirconia cored crowns not affected by?
- don’t contain silica and not affected by acid
- can be abraded by air to create retentive surface but are strong enough to be self supporting
What is the definition of Translucency?
- Ratio of intensity of transmitted light to that of incident light
