Ceramics Flashcards
What are the main differences between decorative ceramic and dental ceramic?
Dental ceramic has much less Kaolin- less than 5%.
Dental ceramic has more Feldspar.
Dental ceramic has up to 15% glass, whereas decorative ceramic has 0%.
What does Feldspar do in the ceramic?
Lowers the fusion and softening temperature of the glass.
What components make up Feldspathic porcelain?
Feldspar
Borax
Silica
Metallic Oxides
What advantages does feldspathic ceramic have?
Very good aesthetics- colour stable, less staining long term
Chemically very stable- do not take up stain from the oral cavity, unaffected by the wide pH range in the oral cavity.
Very biocompatible
Thermal expansion similar to dentine
Low thermal diffusivity
Very stable
High compressive strength
High hardness
What disadvantages does Feldspathic ceramic have?
Low tensile strength
Low flexural strength
Low fracture toughness
Shrinkage occurs during firing- must be accommodated by the technician.
Static fatigue- decrease in strength even in the absence of applied load
Surface micro-cracks- due to finishing or occlusal wear
Slow crack growth
What is Feldspathic ceramic used for?
Anterior crowns- can only be used in low stress areas.
What can be added to improve the properties of ceramic?
Strong coping- metal coping, alumina core or zirconia core.
What is an advantages of Alumina reinforced feldspathic core ceramics?
Increases flexural strength
Alumina acts as crack stoppers- crack comes into contact with the alumina and needs to go around it. By doing so, it loses energy and cannot propagate throughout the whole material.
What was Alumina core typically used for?
Anterior crowns. Still not strong enough for posterior use.
What is Zirconia?
Naturally occurring mineral.
What type of zirconia is used in dentistry?
Yttria-stablised zirconia.
What happens to the structure when you add more Yttria?
More translucency
Crystals changed from small to big and this reduces crack propagation.
Pure zirconia can crack on cooling.
Uses of Yttria stabilisation of zirconia?
Bridge framework.
What are some problems with Zirconia cored crowns?
Expensive equipment
Potential for porcelain to debone from core
Inert fitting surface- cannot etch or bond.
How is a Zirconia Core fabricated?
Impression taken of the prepped tooth./teeth
Model is cast and scanned digitally
Software creates the bridge substructure
Raw zirconia block is selected for milling
Cut framework is heated to around 850 degrees
This causes 20% shrinkage but the computer software seals with this during milling
Zirconia core is then veneered with feldspathic porcelain to produce the final restoration.
What is a milled crown?
Impressions taken of the preparation and then cast and placed into a scanner.
Cast is scanned and articulated
Select the crown margin
Select crown type and place on model- adjust the shape and size of the crown
Send to milling machine
What materials can be used for a milled crown?
Zirconia
Lithium dislocate
Metal
Ceramic filled composite resin
What type of ceramic is used in cast and pressed ceramics?
Lithium Dislocate glass
Leucite reinforced glass
What is the process of cast and pressed ceramics?
Ceraming
- Stage 1- crystal formation.
- Stage 2- crystal growth to maximise the physical properties
What is an advantage of lithium dislocate core?
Unique needle-like crystals which make it difficult for cracks to propagate.
Better aesthetics- more translucent
What is the strongest core material?
Zirconia.
Where in the mouth is best to use Zirconia?
Posterior teeth- single crowns or short spanned bridges
Longer span bridges or heavier occlusion.
Where in the mouth should you use LiDi core?
Anterior teeth- incisor to premolars.
Short span bridges or single crowns.
What cement can be used for zirconia and LiDiSi crowns?
Conventional or resin cement.
Both crowns have intrinsic strength and do not reply upon being bonded to tooth substance to prevent fracture.
What must LiDiSi crowns be traded with first to create a retentive surface?
Hydrofluoric acid.
What must zirconia crowns be treated with first to create a retentive surface?
Air abraded to create a retentive surface.
