Lecture 4- Metal Ceramics Flashcards
what does a PFM crown consist of
a metal casting or coping which fits over the prepared tooth and has ceramic fused to the metal
how thick does the metal coping need to be with noble metal
0.3-0.5mm thick
how thick does the metal coping need to be with base metal
0.2mm
how thick does the metal coping need to be where porcelain is not overlayed
0.8-1.0mm thick
how thick is the opaquing porcelain
0.2-0.3mm thick
what is the order of body porcelain laid down in
dentin -> enamel -> translucent -> modifiers
how thick is the body porcelain
0.8-2mm thick
what is the goal of the high fusing shoulder porcelain
decrease shrinkage that happens when you fire
what is the metal coping designed to support
- with thickness to support the porcelain
- to support occlusal and proximal contacts
- with extensions to support porcelain
- margin design to adapt perfectly to margins and support the porcelain if porcelain margins are placed
describe the design of the coping
- convex, rounded angles
- ledge of metal to support a 90 degree metal-ceramic finish
what happens if there is less than a 90 degree angle between the metal and porcelain
cracking, fracture
assuming metal coping is at least 0.3-0.5mm thick, the metal coping should provide:
- an even porcelain thickness needed to provide optimum strength
- from 0.5-2mm of porcelain thickness
- provide support in stress bearing areas like cusp tips, incisal edges, marginal ridges
describe the thickness that is desired with porcelain
a thin uniform thickness
if porcelain is more than 2mm thick, what becomes a high risk
fracture
what is the ideal thickness of porcelain
1-1.5mm
where do we need adequate thickness for rigidity to prevent porcelain fracture while trying to optimize esthetics
in the pontic and connectors
how is a PFM made
- start with wax up
- design metal coping
- metal is cast at 2300-2600 degrees or milled
- porcelain is then stacked onto coping in layers
what does the opaque porcelain layer do
- masks the metal
- forms the oxide bond with oxide layer of metal coping
- silica oxide from ceramic to oxide layer of metal
- van der waal forces exist here
- acts as the base for the eventual shade of the crown
what colors are included in the body porcelain
- dentin
- enamel
- translucent
- shoulder porcelains
what are stains and glazes used for
final color match
when do you do color modification
once crown is complete
stains and glazes have a ____ fusing temperature than body porcelains
lower
what are the types of metal substructure
- Au-PT-Pd (gold- platinum - palladium)
- Pd-Ag (gold- palladium- silver)
- Ni-Cr- Be (nickel- chromium - beryllium)
what are the classes of the metal substructure
- high noble
- noble
- base metal
what are the desirable characteristics of metal ceramic alloys
- biocompatibility
- accuracy of fit
- high modulus of elasticity and high yield strength
- high resistance to deformation
- easily soldered
what are high noble metals made of
noble metal content greater than or equal to 60% with at least 40% of content being gold
describe the gold- platinum - palladium formulation
- this formulation is up to as much as 88% gold
- this much gold makes this formulation softer and susceptible to sag
describe the gold- palladium-silver formulation
the silver used to discolor some porcelain in a process called greening
that problem is mostly gone and overcome with new formulations
describe the gold- palladium formulations
- gold 44%-55%
- palladium 35-45%
- resistant to sag with excellent working properties
what formulation of high noble metals do we use at UMKC
gold-palladium
what is sag
resistance to deformation at high temperatures with good yield strength and a high modulus of elasticity
what are noble metals made of
at least 25% of noble metals but doesnt have to be gold
what are the formulations for noble metals
- palladium- silver
- palladium- copper
- palladium- cobalt
- palladium-gallium
noble metals are a compromise between the high noble metals and the base metals in terms of:
cost and in density
what factors of noble metals are similar to high noble metals
workability and fabrication
because of less gold, noble metals tend to have a:
higher strength and greater resistance to deformation
what are the formulations of base metal
- nickel- chromium
- nickel- chromium- beryllium
- cobalt-chromium
describe base metals
- stiffer metal
- harder metal
- greater sag resistance
- lowest density
- lowest cost and lowest quality
- higher likelihood of reaction if pt has metal allergy
what is the down side to lowest density in base metal
more difficult to cast and therefore more difficult to finish in preparation for the porcelain and not able to create strong solder joints
- can also lead to high oxide formation and thus problems with bonding
because of the nickel and beryllium in the base metals what two possible issues arise:
- allergic reaction
- toxicity to the lab tech
PFM crowns are layered with what type of porcelain
feldspathic
describe feldspathic porcelain
relatively weak material
- wears opposing teeth at a very high rate
what are the properties of ceramic
- excellent esthetic properties
- biocompatible
- excellent insulator and protection against heat and electricity
- shrinks as it is fired
- brittle
what are the results of ceramic being brittle
- crack propagation
- porosities in the porcelain act as crack initiators
- there is a stress dependent reaction between water vapor and crack tips in porcelain that cause growth and further fracture even with comparatively low occlusal loading over long periods
in ceramics the compressive strength is _____ than the tensile strength
better
what are the types of ceramics
- predominately glass
- particle filled glass ceramic
- polycrystalline ceramicd
describe the predominately glass form of ceramics
- etchable
-optical properties include most translucent - weakest strength
- most esthetic
- feldspathic porcelain
describe the particle filled glass ceramic
- etchable
- increased strength
- less translucent than feldspathic
- lithium disilicate
describe the polycrystalline ceramic
- not etchable
- highest strength possibilities
- lowest translucency
- zirconia
what are the components to feldspathic porcelain
- feldspar - 75-85%
- quartz
- alumina
- glass modifiers
- leucite - 10-20%
what does feldspar do
responsible for forming the glass matrix
what does quartz do
framework around which other components flow
what does alumina do
- hard strong oxide which increases strength
- increases the viscosity of the melt when firing
what do glass modifiers do
- alkali fluxes ( mostly potassium and sodium oxides)
- increases the coefficient of thermal expansion
- includes opacifiers and colorants
what does leucite do
key ingredient to increase coefficient of thermal expansion to bring ceramic closer to the coefficient of thermal expansion in the metal
where does the strength of the PFM come from
- the bond between the ceramic and the. metal substructure
- the compatability of the metal and the porcelain
- design and rigidity of the metal coping or framework
the metal- porcelain bond is accomplished with:
- mechanical interlocking of materials
- vander waals forces
- chemical bonding
- compressive forces
describe the mechanical interlocking in the metal- porcelain bond
- microabrasions from metal finishing or from air abrasion create a surface that allows for the interlocking of porcelain and metal
- these microabrasions also clean the surface
- these microabrasions also increase the surface area of the metal
- the microabrasions increase the wettability of the metal meaning the porcelain will flow over the metal material better and therefore flow into the abraded surface
describe the mechanism of the metal-porcelain bond
- an oxide layer is formed as the metal coping is fired in the heating treatment
- traces of gallium, tin, indium and iron move to the surface
- base metal alloys readily oxidize
- gases are released from the metal
- this oxidation stage after firing is critical and requires no contamination
- the oxide layer of the metal bonds to similar oxides in the opaque layer of the porcelain
when done correctly this oxide bond is so strong that the _____ will break before the bond gives away
porcelain
after the opaque layer is placed what layers are placed next
shoulder porcelain -> body dentin porcelain -> enamel porcelain
why is the coefficient of thermal expansion matched with dental porcelain
to reduce ceramic fracture in response to heating and cooling
is porcelain CTE higher or metal
porcelain
is the melting temperature of metal higher or lower than the fusing range of the veneering porcelain and what temp is it
higher at 1600-1900 degrees
the melting temperature of metal must be high enough so as to __________. usually needs a difference of at least ______ degrees F.
-NOT sag or melt when porcelain is applied
- 300
higher amount of gold in the allow the _____ the melting temperature
lower
what does adding platinum and palladium do to the melting temperature
raises it
what does adding base metals do to the alloy melting temperature
raises it
because some porcelains fuse at a higher temperature than the melting temperature of the metal, what type of ceramic is used
low fusing
metal flexure can cause porcleain to:
fracture
where should occlusion be in anteriors
- not on the finish line
- preferable to be at least 1.5mm away
why should you not use lingual porcelain on a maxillary anterior
- will cause wear on opposing teeth
- contraindicated in deep vertical overlap
- lingual metal collar to support porcelain
- more tooth reduction would be required
what are the advantages of a metal lingual on maxillary anterior
- easier to form anatomy
- easily polished
- less wear on opposing teeth
- less tooth reduction is requried
should there be metal or porcelain on contacts and why
porcelain for esthetics
if there is no contact can the lab tech add on porcelain
yes
what are the possible occlusal surface designs on posterior teeth
- full porcelain
- full metal
- combination
when would you do a metal occlusal surface on a posterior tooth
if the oppposing tooth is either:
- enamel
- gold
- resin
when would you do a porcelain occlusal on a posterior tooth
if the opposing tooth is porcelain
a metal finish line on posterior teeth is best placed at ______ away from occlusal contacts
1.5mm
for mandibular posteriors what occlusal design is recommended
full porcelain or full metal
second molars almost always get _____ occlusal design
full metal
what is the necessary reduction is porcelain is on the occlusal
2mm
ideal to have ____ metal collar on the lingual of maxillary teeth to support occlusal forces
3mm
why is it ideal to make occlusal metal on maxillary posterior teeth
- prevent fracture
- less reduction on an already short tooth
describe the design on mandibular posterior teeth
- full coverage porcelain often used for esthetics
- 2.0mm reduction
- metal support under lingual cusps and marginal ridges of 3mm
how do you decide on your porcelain design
- patients concern for esthetics
- clinical crown height
- opposing occlusal surfaces
- bruxism
- possibly add a night guard at the end of tx
what is the ideal margin type and why
porcelain shoulder because strength
when would you use the metal collar
only used when you can only have a small margin
what is the disadvantage of disappearing metal
can chip and break because of occlusion and flexing
where is the best placement of porcelain shoulder margins
slight subG
what are the special considerations when using shoulder porcelain
- uses high fusing aluminous porcelain
- need a good ceramist
- need 2 additional firings at a higher temperature
- 90 degree angle for strength and sealed finish line
describe the metal collar margin
- design is a more conservative tooth preparation in cervical area
- restoration design is easier for the lab to fabricate
- great for molars that have margins deep subG
- allows for multiple types of preparation finish lines
when is the disappearing margin indicated
for areas that the margin cannot be hidden under gingiva
what is the common design flaw in disappearing margins
metal copings are over contoured and thick at gingival area
what is bond failure due to
trauma or heavy occlusion
what is adhesive fracture and what causes it
- the de-lamination of the porcelain from the metal
- due to a failure of the manufacture process where there occurs an incorrect oxidation process or contamination of oxide surface
what happens if the cooling rate after firing is too rapid
the porcelain will compress too rapidly and fracture
when will fracture occur in an issue with firing
- if the porcelain is too thick or thin
- if the metal has deformed during the firing process
where would be poor placements of metal ceramic finish line
- lingual for mandibular finish line too close to centric occlusion points or incisal edge
- too close to incisal edge
how is porcelain repairs in fractured PFMs
- can be repaired with composite or amalgam
- if porcelain is intact it is bonded back together
are porcelain repairs strong
no
what is the #1 reason for PFM fracture and #2
- # 1: poor design
- # 2: occlusion
how can you be successful with PFM crowns
- strength comes from bond between metal and ceramic
- the type of porcelain and metal enhance or detract from the bond
- design of the metal coping for proper support of the more fragile porcelain
- a preparation design that allows for good resistance and retention
- know what kind of margin you desire
what porcelain is used in PFZ crowns
3Y on the lingual and feldspathic on the buccal
