10 - Cast Metal Restorations Flashcards
What is the general process of fabricating a cast metal restoration
wax a pattern of the restoration on a die
invest the pattern to creat a mold space
cast the restoration
finish and polish the cast
cement the restoration intraorally
Becase of the complexity of this sequence, properties desriable for casting are governed as much by _____ as by _____
technique limitations, final intraoral service considerations
Cast metal alloys may be used to form either: ____ or _____
the entire restoration, or as a substructure and veneered with porcelain to create the tooth colored restoration
some of the names for cast metal alloys veneered to porcelain are
porcelain bonded to metal
ceramic bonded to metal
porcelain fused to metal
to successfully use with porcelain, metal alloy needs to have what?
relatively high melting point to tolerate the high firing temperatures of porcelain.
though, the melting temperature of resrotations that are all metal can be at any temperature that is convenient
what is a an essential characteristic of dental casting alloys
corrosion resistance
how can casting alloys be categorized in relation to corrosion resistance
1- mechanism of corrosion resistance
2- main elements in the composition that affect the corrosion resistance
corrosion resistance is achieved with either immune systems or passivating alloy systems.
Immune systems are divided into what two categories?
Passivating systems are divided into what three categories?
immune systems:
gold systems
gold substitute systems
Passivating systems: (NIC)
nickel chromium
cobalt chromium
iron chromium
_____ metal alloys are very resistant to corrosion.
these systems are based on what?
nobel metal alloys.
they are based on gold, platiunum, palladium, rhodium, iridium,ruthenium, or osmium
what are precious metal alloys
in the words of tony, alloys that contain precious metal… high economic value. traditionally include all the noble metals and silver
what are low gold alloys
3-50% weight gold or other noble metal elements
if less than ______ of the atoms in a gold alloy are corrosion resistant the overall corrosion resistance ________
half, decreases dramatically
what was the purpose of low gold alloys
how much gold is actually in these
produce lower cost alloys that still have some of the qualities of premium gold based alloys
the actual quantity of gold in these alloys is deceptively low
what are gold substitute alloys
precious metal alloys that don’t contain gold i.e. pd alloys
what are base metal alloys
based on active metallic elements that corrode but develop corrosion resistance via surface oxidation. it makes a tight adherent film that inhibits further corrsion.. like an aluminum canoe
an example of a base metal alloy is?
are the films tough?
Cr which makes a film of Cr2O3 on it’s surface.
the films are brittle and can rupture but if they do they reform immediately.
Ni and Co also oxidate, but ______ is principally responsible for the corrosion resistance
Cr2O3
Ti alloy is widely used in dentistry for what reasons?
It passivates (becomes unreacitve my forming a surface layer) by forming TiO2, it is biocompatible, it permits osseointigration with bone
cast restorations are constructed traditionally from what? why from this material?
gold alloys
becase they have excellent corrosion resistance
when were casting materials standardized?
how were dental gold casting alloys defined?
1930’s
they were defined by:
nobel metal concentration
physical properties
mechanical properties
The ADA classification system of gold alloys is what?
it was later switched to what?
A,B,C, gold alloys
later switched to I,II,III,IV
I,II,III correspond to the ABC but IV was added and is higher strength alloys
what is the nobel metal content (of which gold is the main one) of types 1,2,3,4?
I - 83%
II - 78%
III - 78%
IV - 75%
So as you go up, the nobel metal content goes down. this means higher strength but less burnishability… gold is soft
What are each of the different alloy types used for? (I,II,III,IV)
which ones can be heat treated?
I - small inlays that do not involve significant occlusal loads
II - inlays and onlays
III - onlays and crowns
IV - crowns, bridges, removable partial dentures
I and II cannot be heat treated but the other two can.
Gold is primarily responsible for producing ____ but is relatively ____ and requires other alloy elements to _____ the composition.
corrosion resistance
soft
harden
…. copper is the primary alloy element that increases the hardness of the alloy. it also makes the color less yellow and more orange.
what is each alloy element used for?
what has prompted the increased use of low gold, gold substitute, and base metal alloys?
the high cost of gold
many crowns and fixed bridges are made of what?
the mechanical properties of this matieral make them difficult to what?
palladium based gold substitute alloys
mechanical properties make them hard to make inlays on onlays. you can’t burnish them, they are too hard. But, gradual improvement in low gold alloys have made them acceptable for selective use in these applications.
many products containing about _____ % wieght of gold have acceptable tarnish resistance and adequate properties if too much burnishing is not required
50%
why is a low melting range desirable?
why is moderately high density advantageous?
for simplified heating and casting procedures
becasue most dental alloys are normally cast by centrifugal force machines and high density material then fills all the nooks and cranny’s.
why is high density alloy good
it helps the alloy quickly get into all the intricate details of the pattern within the mold before the cooling solidify’s the material. gold based alloys are good for this
why is a low coefficient of thermal expansion good
it reduces shrinkage that occurs from the melting temperature down to room temp
cooling produces____ so there must be some expansion somewhere else in the technique to compensate for it
shrinkage
what alloys can be controlled more easily
ones that have low coefficients of thermal expansion and low melting temperatures
the primary chemical property concern is what?
how is it achieved?
corrosion resistance
the entire alloy needs to be a single phase composition. two phase compositions are more prone to corrosion
Of the alloy types I,II,III,IV, which are single phase
I,II,III are single phase. Type IV is two phase. The advantage of the two phase is that it is much harder… but that is at the expense of corrosion resistance
what can produce unwanted phases and compromise mechanical properties and corrosion resistance?
contamination or improper casting of gold based alloys
primary mechanical properties of interest are what?
high modulus of elasticity (stiffness) and a high elastic limit (hardness) to resist deformation during use
it is harder to finish, polish, and burnish if what?
if the material has high resistance to plastic deformation (it is hard)… High values are not always good.
during these processes it is important for the casting metal to?
have margins closely adapted to the die by minimal mechanical deformation. marginal gaps bigger than 0.1 mm should not be burnished, should re make the casting
castings with ____ and ____ are easier to burnish
high percentage elongation and a low yield point (low hardness)
what is OLYMPIA
i don’t know if these are important but i am including them for completeness
what is williams academy gold
don’t think you have to know
what is SIGN
don’t think you have to know
what is MAESTRO
don’t think you have to know
what is firmilay
don’t think you have to know
what is sturicast
don’t think you have to know
for longerm success what are 3 principle considerations?
close fit
corrosion resistance
retention
gold based alloys are corrosion resistant for at least how long
ten years but if cemeneted restorations have a close fit and tooth prep is good they can be good up to 20-40 years
retention and service life of cast restorations are produced by a number of things. what are they?
taper of the tooth prep
stress distribution design of the prep
cement type
surface roughness on internal aspects of restoration
micromechanical and chemical bonding
under most circumstances, the restoration surface for gold based alloys is ______ to cement adhesion
not well suited. gold alloy surfaces do not wet well with cements and do not have the potential to be chemically bonded. But, if the internal surfaces are sand blasted, there are enough micromechanical irregularities to produce good luting (seal or joining)
in a maryland bridge, retention of the casting depends on?
mirco mechanical spaces along the bonded surfaces of the enamel and casting
the retentive surface of the casting is accomplished by what?
a two phase dental casting alloy. The metal surface to be bonded is relieved by chemical or electrolytic etching, then the relieved space is micro mechanically interlocked with composite cement onto etched tooth structure.
