9 - Dental ceramics Flashcards
What is kaolin?
- opaque clay
- hydrated aluminium silicate
- removed from dental ceramics to create translucency
What replaces kaolin in dental ceramics?
- feldspar
- silica
What are the constituents of dental ceramic?
- kaolin <5%
- silica 12-25%
- feldspar 70-80%
- metal oxides 1%
- glass up to 15%
What is the function of feldspar in dental ceramic?
- lowers fusion and softening temperature
- flows during firing to form solid mass around other components
What colour does chromium convey to the ceramic?
Green
What colour does cobalt convey to the ceramic?
Blue
What colour does copper convey to the ceramic?
Green
What colour does iron convey to the ceramic?
Brown
What colour does manganese convey to the ceramic?
Lavender
What colour does nickel convey to the ceramic?
Brown
Describe conventional dental ceramics.
- supplied as powder which melts together to form crown
- crown is fabricated using different porcelains for dentine and enamel
- made 20% too big due to contraction on firing
Define sintering.
- when ceramic particles fuse to form a single mass
- material will contract around 20%
Describe the aesthetic properties of dental ceramic.
- colour stable
- smooth surface
- retain surface properties better than most materials and resist surface staining
- have optical properties of reflectance, translucency, transparency, opacity and opalescence
Describe the chemical stability of dental ceramic.
- chemical very stable
- unaffected by wide range of pH in mouth
- does not take up stain
- good biocompatibility
Describe the thermal properties of dental ceramic.
- similar to tooth substance
- coefficient of thermal expansion is similar to dentine (low stresses to restoration during use)
- thermal diffusivity is low which protects remaining tooth tissue
Describe the dimensional stability of dental ceramic.
- once fired material is very stable
- shrinkage during firing must be accommodated for by technician
Describe the mechanical properties of dental ceramic.
- high compressive strength
- high hardness (can cause abrasion of opposing teeth )
- low tensile and flexural strength
- low fracture toughness
- static fatigue can occur
What is static fatigue?
Decrease in strength over time even in the absence of applied load
Where can feldspathic ceramics be used in the mouth?
- low stress areas
- anterior crowns
What are alumina cores used for?
- used as core in porcelain jacket crowns (anterior)
- act as crack stoppers preventing cracks from propagating
- can only be used as core due to opacity
How are alumina cores incorporated into crowns?
- the cores are veneered to allow for conventional porcelain to be placed over top
- only used in single crowns
- more successful anteriorly
What is incorporated into zirconia to stabilise it for dental use?
Yttria (3-5%)
How does yttria improve the zirconia?
If a crack begins, the crystal structure will reach a critical level and transform to a monoclinic structure which expands and close up the crack
How is a zirconia core fabricated?
- impression of prep
- model cast and scanned digitally
- software created bridge substructure which minimum thickness connectors determined
- raw zirconia milled
- framework is heat treated and then stained
- core is then veneered with feldspathic porcelain for final restoration
What problems can arise with zirconia cored crowns?
- expensive equipment initially
- veneered porcelain could de-bond from core
- core is opaque although better than metal
- inert fitting surface cannot be etched or bonded
What are the different types of milled cores for crowns?
- zirconia
- lithium disilicate
- precious metal
- non-precious metal
- titanium
- composite
Describe the process of ceraming.
Two stages:
1 - crystal formation for maximum number of crystals
2 - crystal growth to maximise properties
What should you use for posterior teeth?
Monolithic zirconia
What should you use for anterior single crowns?
LiDiSi
What should you use for anterior bridgework (short)?
LiDiSi
What should you use for posterior teeth (long)?
Zirconia cored with zirconia where occlusal contacts meet
What type of crown do you cement with conventional cement?
- zirconia
- LiDiSi
What type of crown do you etch to create a retentive surface?
- silica containing ceramic
- can be bonded to using a silane coupling agent
