DMS for fixed pros Flashcards
What type of gold is used for gold crowns?
Type III gold alloy
16 karat (>60% gold)
What are the key properties of precious metal alloys for crowns?
Mechanical strength
Ductility (able to be shaped without losing toughness)
Corrosion resistance
What are the main constituents of gold alloy, and what purpose do they serve?
Gold - corrosion resistance and malliability
Silver - Hardness and durability
Copper - Hardness and durability
Platinum - Corosion resistance
Tin - Improves cast ability
What is the main distinction between decroative ceramic and dental ceramic?
Low kaolin content
High feldspar content
Higher glass content
What is dental porcelain?
A type of ceramic
Feldspathic aluminium silicate
What is the purpose of metal oxides in porcelain?
Convey different colours to the ceramic
Describe the aesthetic properties of feldspathic dental porcelain.
Best of any restoriative material
Stable colour
Smooth surface
Less long term staining
Good optical properties (reflectance, opacity, translucency etc)
Describe the thermal properties of feldspathic dental porcelain.
Similar to tooth subtance
Thermal expansion similar to dentine
Low stresses to restoration during use
Low thermal diffusivity
Describe the dimensional stability of feldspathic porcelain.
Very stable once fired
Shrinkage can happen during fabriction
Describe the mechanical properties of feldspathic porcelain.
High compressive strenght
High hardness (leads to abrasion of opposing teeth)
Very low tensile and flexural strength
Fracture toughness very low
What about the mechanical properties of feldpathic porcelin indicate its use?
Low tensile/flexural strenght and low fracture toughness indicate that it can only be used in low stress areas.
Too brittle for posterior crowns and patients with heavy contacts/bruxsim.
What steps can be taken to improve the undesirable mechanical properties of porcelain?
Metal coping (PFM)
Aluminia core
Zirconia core
Advanced materials
Milled/monolithic crowns
Why are alumina core crowns no longer as desriable as they once were?
Low flexural strength
Still not used posterior crowns or bridgework
What are zirconia core crowns?
Porcelain crowns with a zirconia core
The zirconia is yttria stabilised, which gives good mechanical properties, strong enough to use as bridge framework.
Why is pure zirconia not used in for dental crowns?
It can crack on cooling if not stabilised with yttria.
What is the process of prodcuing a monolithic zirconia crown?
CAD/CAM
Milling from a single block
What are some of the drawbacks of ziroconia cored crowns?
Expesnive equipment
Cannot etch or bond fitting surface
Porcelain can debond from core
Similar aesthetics to PFM crown
What are the two main types of ceramic milled crown?
Zirconia
Lithium disilicate (E-Max)
Why do layered ceramic crowns have better aesthetics than milled crowns?
Layered porcelain rather than just a single monolithic block.
Lack of sintered layer on the core of the crown.
What are the differences between the aesthetics and strength of zirconia and lithium disilicate crowns?
Zirconia is general stronger than LiDiSi
LiDiSi have better translucency, and hence better aesthetics
Which types of porcelain crown can be used for posterior teeth and short span bridges?
Monolithic Zirconia
What type of porcelain crown can be used for anterior teeth with high aesthetic demand?
LiDiSi, can use as far back as first premolar
What type of porcelain can be used for short span anterior bridgework?
LiDiSi as long as no parafunction
What type of porcelain can be used for longer span bridgework, or those with heavier occlusions?
Zirconia cored with additional zirconia where occlusal contacts meet.
What luting agents can be used for zirconia and LiDiSi crowns?
Conventional cement (GI, RMGIC)
and
Resin cements (dual, self)
How can you make a silica containing cermaic core more retentive?
Etching using HF acid, which can then be bonded to with a silane coupling agent.
Why can zirconia cored crowns not be bonded to?
The core does not contain any silica, and are not effected by acid or bonded to.
However, they are strong enough to be self supporing and can be luted with a conventional cement.
How can you make a zirconia cored crown more retentive?
Air abrasion (sand blasting)
What layers are present in a PFM crown?
Porcelain
Metal oxide
Metal alloy
What metals can be used in a PFM crown?
Ni-Cr
Co-Cr
Gold alloys
What can you do to make a PFM crown more retentive?
Sandblasting
Acid etch with HF
What is the purpose of using a metal coping in a PFM?
Utilises the great aesthetic qualities of porcelain, with the good mechanical propeties of a metal alloy.
The alloy acts as a support and limits the strain the porcelain experiences.
