Fixed Pros Flashcards
What is the effect of adding leucite crystals to leucite reinforced ceramic?
Leucite increases the strength of the ceramic
What percentage crystals is lithium disilicate?
70%
How does the amount of glass in a material affect its strength?
Increased glass content = decreased strength
How is lithium disilicate bonded?
Etch for 10-20 sec with 5-9% HF acid
Silane coupling agent
Resin bond
How does over-etching lithium disilicate affect its strength?
Overetching weakens material
How does a silane primer work?
Organic component of silane bonds to resin, inorganic component bonds to etched ceramic
What is the full name for zirconia used in dentistry?
Partially stabilised tetragonal zirconia polycrystal
What are the two phases that zirconia exists in?
Monoclinic = at room temperature, most stable phase
Tetragonal = unstable phase but stronger.
How is zirconia stabilised?
Yttrium oxide is added so that it remains in tetragonal phase
Describe transformational toughening.
When a crack propagates in zirconia, it is converted from monoclinic phase to tetragonal phase. Monoclinc phase zirconia has higher volume crystal particles. Therefore the crack is “squeezed shut” and does not propagate further.
Why is zirconia initially milled larger than required?
Shrinkage 25-30% occurs on sintering.
What is the issue with layering zirconia with porcelain?
High rate of failure at zirconia-porcelain interface due to different rates of cooling after sintering.
Fix: allow to cool very very slowly
Is zirconia bondable?
No - relies on retention and resistance forms for bonding
Ductility definition
Ability of a metal to withstand tensile stress before fracturing
Proportional limit
Maximum point at which stress is proportional to strain. After which plastic deformation occurs
Yield point
Point at which permanent deformation occurs
Describe the classification of metals used for fixed pros.
Precious: >60% precious, >40% gold
Semi precious: >25% precious, no gold requirement
Base metal alloy: <25% precious
What is the difference in resin bonding between precious metal and base metal alloy materials
Base metal alloys can bond to resin easily due to oxide layer formation. Precious metals are non-bondable
What is the difference in ceramic bonding between precious metals and base metal alloys?
Gold bonds easily to ceramic. Base metal alloys don’t bond to ceramic
What are the affects on fit of beveling?
Increased seal but worse seat
What are 3 methods of improving cement escape
- Die spacer - technician paints layer of die spacer over model everywhere except margin. When crown cast, it leaves small gap for cement
- Vent at top of crown
- Using a lower viscosity cement
2
What are the primary factors affecting retention?
- Taper - ideally 6%, but 10-20% OK. Increased taper = decreased retention
- SA - increased SA = increased retention
2
What are the secondary factors affecting retention?
Surface texture
Cement/bond used
What are the two temporary cements?
ZOE
Polycarboxylate
7
What are the characteristics of an ideal cement?
low film thickness
high compressive strenght
long working time
short setting time
low solubility
low microleakage
easy clean up
What are 3 strategies to increase micromechanical retention?
Use etchable ceramics - HF acid 5-9%
Sandblasting non-etchable materials e.g zirconia and metals
Increased surface area
