dental ceramics Flashcards
(31 cards)
porcelain
all porcelain is ceramic, but not all ceramic is porcelain
kaolin in decorative
Kaolin is a clay
Hydrated Aluminium Silicate
Opaque
Opacity is important for the appearance of the final product
Dental Ceramics need to be translucent so Kaolin is removed and feldspar and silica replace it
so glass is up to 15%
dental ceramics are glasses
Feldspar
Potash feldspar (potassium alumina silicate)
Soda feldspar (sodium alumina silicate)
Acts as a flux
Lowers the fusion and softening temperature of the glass
It is the lowest fusing component and flows during firing forming a solid mass around the other components
Borax
Silica
Metallic Oxides
metal oxides convey colour
Chromium GREEN
Cobalt BLUE
Copper GREEN
Iron BROWN
Manganese LAVENDER
Nickel BROWN
conventional dental ceramics
Conventional dental ceramics are supplied as powder
The powder is made by heating the 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
The powder is mixed with distilled water and built up into the restoration
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
ceramic powder is blue or pink
fabrication of the crown
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
pink dentine, blue layered on top
sintering
powder comes together and solidifies, 25% reduction in size
properties of conventional dental ceramics
Aesthetics
Chemical Stability
Biocompatibility
Thermal Properties
Dimensional Stability
Mechanical Properties
aesthetics
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
chemical stability
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
thermal properties
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.
dimensional stability
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
mechanical properties
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.
Surface micro-cracks
Can occur during manufacture, finishing or due to occlusal wear. These are areas where fractures can initiate
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
overcoming the problems with conventional ceramics
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
strong coping
metal coping
alumina core
zirconia core
allumina core
Alumina reinforced feldspathic core ceramics since 1960s
Used as core material in PJCs
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
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.
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 new techniques
INCERAM
PROCERA
Both complicated fabrication techniques
First used in the 1990s
allumina cores
core types are veneered with conventional feldspathic porcelain
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
newer techniques and materials
Zirconia
Lithium Disilicate
Cores
Pressed crowns
Monolithic/Milled crowns
Advantages/Disadvantages of different types
Luting
zirconia core
most popular ceramic core material.
Zirconia (Zirconium Dioxide) is a naturally occurring mineral
It occurs in different forms at different temperatures
Very hard
Used extensively in Jewellery industry as imitation diamonds
Until CAD-CAM was introduced the use of Zirconia as a core material would not have been possible
Zirconia powder does not sinter unless heated to over 1600oC
The Zirconia used in dentistry is Yttria-stabilised zirconia
Pure zirconia can crack on cooling
yttrium stabilisation
SMALL CRYSTAL TO BIG CRYSTAL
Very small amounts of Yttria is present in the material
3 – 5%
The more Yttria the more translucency
More Yttria reduces the physical properties
Normal Zirconia is a monoclinic crystal at room temperature
Yttria 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 a material which is very
Hard
Strong (1000MPA flexural strength)
Tough
Strong enough to use as a Bridge Framework
fabrication of a zirconia core
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
Some modern zirconias are available in different shades
The Zirconia core is then veneered with feldspathic porcelain to produce the final restoration
problems with zirconia cored crowns
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
BUT
Once you have the equipment, they are cheaper to make
Cost of metal is increasing
Fit is generally excellent
milled core crowns and bridges
Zirconia
Lithium Disilicate (E-Max)
Precious metal
Non-precious metal
Titanium
Composite
ceramics all have a surface sintered layer for best aesthetics
fabrication of a milled crown
cast goes into a scanner
same methods irrespective of material being used
scanned image of cast
lower cast is scanned is articulated
select crown margin
adjust crown margin
select crown type and place on ‘model’
adjust shape and size of selected crown
