Metals in
prosthodontics
Introduction :
Metals form a large part
of the earth on which we live, nearly 80% of the known elements are metals, in
the earths crust, most of the metallic elements occur in compounds and not in
the metallic state. A few of the rare and least reactive metals may be found in
the metallic state in the earths crust. These metals include gold, copper,
mercury and platinum. Scientists think the earths core in mainly made up of
nickel and iron in the metallic state.
Ancient people knew a used many native
metals. Gold was used for ornaments, plates, jewellery and utensils as early as
3500 BC, gold objects showing a high degree of culture have been excavated at
the ruins of the ancient city of ur in mesapotamia. Silver was used as early as
2400 BC. Native copper was also used at an early date for making tools and
utensils. Since about 1000 BC iron and steel have been the chief metals of
construction.
The earliest known use of dental
materials can be traced to approximately 500 BC and the Etruscans, who used
gold to make first dental bridges.
Definition :
GPT – 7 defines “metal” as any strong and
relatively ductile substance that provides electropositive ions to corrosive
environment and that can be polished to a high lusture, characterized by
metallic atomic bonding.
In dentistry, metals
present one of the four major classes of materials used for the reconstruction
of decayed, damaged or missing teeth.
General
characteristics of metals
·
A metal
is an element that ionizes positively in solution
·
Metal
have certain typical and characteristic properties that distinguish them from
non metallic elements.
The optical properties – metallic luster and high opacity
Physical properties –
high ductility and
-
high electrical and thermal conductivity.
The extensive use of metals and their
alloys in mechanical and structural applications in a result of good mechanical
properties and workability of many products.
Metallic bonding is responsible fore the
unique properties of the metals. Metals atoms have valance electrons that are
rather loosely held and these electron are free to more throughout the solid.
This diffuse nature is responsible for easy deformability of metals and their
high thermal and electrical conductivities.
They are opaque because the valance
electron absorbs the high, and they are lustrous because the electrons remit
the high.
STRUCTURE AND
PROPERTIES OF METALS
Crystal structure
:
·
Metals
usually have crystalline structure in solid state
·
The
atoms joining the crystals have a unique packing arrangement in space that is
characteristic of that metal at equilibrium. The smallest division of the
crystalline metal that defines the unique packing is called the unit cell. when
the unit cell is repeated in space, the repeating atomic position form the
crystal lattice structure of a crystalline solid.
·
Six
different crystal system have been recognized : cubic, tetragonal,
orthorhombic, monoclinic, triclinic, and hexagonal.
·
Atoms
can be arranged in the six crystal systems in only 14 different arrays.
·
The
most common arrays for metals used in dentistry are
Body – centered
cubic :
Here atoms are located
at each corner, and one atom is located at the centre – this is the unit cell of
iron and of many alloys that are used in dentistry.
Face centered
cubic :
With the face centered
cubic unit cell, atoms are located at each corner, but no atom is in the
centre, and the atoms are located in the center of each of the six faces of the
cube, this structure is found in most of the pure metals and alloys used in
dentistry including, gold, palladium, cobalt and nickel alloys.
Hexagonal close
packed :
A few metals used in
dentistry have a more complex hexagonal close packed structure ; a notable
example is titanium.
Crystallization :
When a molten metal or
alloy is cooled, the solidification process is one of crystallization and is
initiated at specific sites called nuclei. The nuclei are generally formed from
impurities within the molten mass of the metal.
Characteristically, a
metal crystallize in a 3 – dimensional tree – branch pattern from a central
nucleus. Such crystal formations are called dendrites. The growth starts from
the nuclei of crystallization and the crystals grow towards each other. Two or more crystals
collide in their growth, and the growth is stopped. Finally, the entire space
is filled with crystals. However, each crystal remains a unit in itself. The
metal is therefore made up of thousands of tiny crystals. Such a metal is said
to be polycrystalline in nature, and each crystal is known technically as a grain.
Grain size :
The size of the grain
depends upon the number and location of the nuclei at the time of
solidification. It the nuclei are equally spaced with reference to each other,
the grains will be approximately equal in size. The solidification can be
pictured as proceeding from the nuclei in all directions at the same time in
the form of a sphere that is constantly increasing in diameter when these
spheres meet, they are flattened along various surfaces. The grain tends to be
the same diameter in all dimensions such a grain is called equiaxed.
Control of grain
size :
In general, the smaller
the grain size of the metal, the better are the physical properties. The finer
grain size can raise the yield stress increase the ductility and raise the
ultimate strength. For ex : the yield strength of many types of materials has
been found to vary inversely with the square root of the grain size.
Because the grains
crystallize from nuclei of crystallization, it follows logically that the
number of grains formed is directly related to the number of nuclei of
crystallization present at the time of solidification.
This factor can be
controlled to a degree by the rate of cooling. In other words, the more rapidly
the liquid state can be changed to the solid state, the smaller or finer the
grains will be.
Another factor of equal
importance is the rate of crystallization. If the crystals form faster than do
the nuclei of crystallization, the grains will be larger than if the reverse
condition prevails. Conversely, if the nuclear formation occurs faster than the
crystallization, a small grain size can be obtained.
Consequently, a slow
cooling results in large grains. In a polycrystalline metal, the shape of the
grain may be influenced by the shape of the mold in which the metal solidifies.
Grain boundaries
:
The orientation of the
space lattice of the various grains is different. The grain boundary is assumed
to be a region of transition between the differently oriented crystal lattices
of two neighbouring grains.
DEFORMATION OF
METALS
The atoms within each
grain are arranged in a regular three-dimensional lattice. There are several
possible arrangements such as cubic, body-centred cubic and face-centred cubic
etc.
The arrangement adopted
by any one crystal depends on specific factors such as atomic radius and charge
distributions on the atoms. although there is a tendency towards a perfect
crystal structure, occasional defects occur, such defects are called
dislocations and their occurrence has an effect on the ductility of the metal
or alloy. When the material is placed under a sufficiently high stress the
dislocation is able to more through the lattice until it reaches a grain
boundary.
The plane along which
the dislocation moves is called the slip plane and the stress required to
initiate moment is called the elastic limit.
Application of a stress
greater than the elastic limit causes the material to be permanently deformed
as a result of movement of dislocations.
Grain boundaries form
natural barriers to the movement of dislocation. The concentration of grain
boundaries increases as the grain size decrease metals have higher valves of
elastic limit.
It is important to
understand that any process that impedes dislocation movement tends to harden a
metal, raise its yield stress and often lower its ductility.
COLD WORKING /
WORK HARDENING :
A process for hardening
the metal. It is the permanent deformation that takes place on the application
of sufficiently high force at room temperature, due to the movements of
dislocations along slip planes.
Any plastic deformation
of the metal by hammering, drawing, cold forging or bending processes, produce
many dislocations in the metal that cannot slip through each other as easily as
the lattice becomes more distorted.
Such cold working not
only produces a change in
microstructure, with dislocation becoming concentrated at grain
boundaries, but also a change in grain shape. The grain are no longer equiaxed
but take up a more fibrous structure.
The properties of the
metal are altered. The surface hardness, strength, and proportional limit are
increased, where as ductility and resistance to
corrosion are decreased by strain
hardening.
In dentistry, cold
working occurs when gold foil is compacted, a denture clasp is bent, an inlay
margin is burnished, or a deformed metal layer forms on a crown during
finishing and polishing.
The temperature below
which work hardening is possible is termed as recrystallizaiton temperature.
Since metals and alloys
have finite values of ductility or malleability there is a limit to the amount
of cold working which can be carried out. Attempts to carry out further cold
working beyond this limit may result in fracture.
ANNEALING :
The effects associated
with cold working such as strain hardening, lower ductility and distorted grain
can be reversed by simply heating the metal. The process is called annealing.
The more severe the cold
working, the more readily does annealing occur.
Annealing in general
comprises three stages :
Recovery,
recrystallization and grain growth :
Annealing is a relative
process ; the higher the melting point of the metal, the higher is the
temperature needed for annealing. A rule of thumb is to use a temperature
approximately one half that is necessary to melt the metal.
Recovery : It is considered the stage at which the
cold work properties begin to disappear before any significant visible changes
are observed under the microscope.
During this period there
is very slight decrease in tensile strength and no change in ductility.
Recrystallizaiton
: When a severely cold
worked metal is annealed, than recrystallization occurs after some recovery.
This involves a rather radical change in the microstructure. The old grains
disappear completely and are replaced by a new set of strain – free grains.
These recrystallization grains nucleate in the most severally cold – worked
regions in the metal, usually at grain boundaries, or where the lattice was
most severely bent on deformation.
On the completion of
recrystalization the material essentially attains its original soft and ductile
condition.
Grain growth : The recrystallized structure has a
certain grain size, depending upon the number of nuclei. The more severe the
cold working, the greater are the number of such nuclei. Thus, the grain size
for the completely recrystallized material can range from rather fine to fairly
coarse.
If now the fine grain
form is further annealed, the grains begin to grow. This grain growth process
is simply a boundary energy minimizing process. the effect, the large grains
consume the little grains. It does not progress indefinitely to a single crystal.
Rather, an ultimate coarse grain structure is reached, and then for all
practical purposes, the grain growth stops.
Excessive annealing can
lead to large grains. It should be emphasized that the phenomenon occurs only
in wrought material
ALLOYS :
An alloy is a mixture of
two or more metals mixture of two metals are called binary alloys, mixtures of
three metals ternary alloys.
The term alloy systems
refers to all possible compositions of an alloy.
To form an alloy, two or
more metals are heated to a homogenous liquid state. However, a few
combinations of metals are not miscible in the liquid state and will not form
alloys.
When a combination of
two metals is completely miscible in the liquid state, the two metals are
capable of forming an alloy. When such a combination is cooled, one of three
microstructure may form.
a) A solid solution
b) A mixture of intermetallic compound
c) An eutectic mixture s
Solid solution :
When two metals are completely miscible
in a liquid state, and they remain completely mixed on solidification,
the alloy formed is called a solid solution.
When two metals are
soluble in one another in the solid state, the solvent in that metal whose
space lattice persists, and the solute is the other metal. The solvent may be
defined as the metal whose atoms occupy more than one half the total number of
positions in the space lattice.
Eg : The copper and gold
combination crystallizes in such a manner that the atoms of copper are
scattered throughout the crystal structure (space lattice) of gold, resulting,
in a single phase system. Such a combination is called the solid solution
because it is a solid but has the properties of a solution. The configuration
of the space lattice of solid solution may be of several types.
-
Substitutional,
interstitial and ordered.
In substitution type : The atoms of the solute occupy the
space lattice positions that normally are occupied by the solvent atoms in the
pure metal.
In interstitial type : The solute atoms are present in positions between
the solvent atoms.
In ordered type : The solute atoms occupy specific sites within a common crystal lattice.
The extent of solid
solubility is determined by at least 4 factors.
1) Atomic size
: It the sizes of the
two metallic atoms differ by less than 15% they posses a favorable size factor
for solid solubility.
2) Valance : metals of the same valance and size are
more likely to form extensive solid solutions than are metals of different
valancies.
3) Chemical affinity : When two metals exhibit a high degree of chemical
affinity, they tend to form an intermettalic compound on solidification rather
than a solid solution.
4) lattice type
: Only metals with the
same type of crystal lattice can form a complete series of solid solutions
Physical
properties of solid solution :
Whenever a solute atom
displaces a solvent atom, the difference in the size of the solute atom results
in a localized distortion or strained condition of the lattice, and slip
becomes more difficult. As a consequence, the strength, proportional limit and
surface hardness are increased. Where as the ductility is usually decreased.
In other words, the
alloying of metals may be a means of strengthening the metal.
The general theory of
slip interference in alloys in same as in strain hardening, except that a
different type of lattice distortion is present initially to inhibit slip
before the structure is stressed or worked.
In general, the hardness
and strength of any metallic solvent are increased by the atoms of the solute.
Intermettalic
compounds :
If two metals show a
particular affinity for one another they may form intermettalic compounds with precise chemical formulation.
Intermettalic compounds are also formed on cooling liquid metal solution, in
the liquid state they have a tendency to unite and form definite chemical
compounds on solidifying. As far as the space lattice is concerned, the atom of
one metal, instead of appearing randomly in the space lattice of another metal,
occupy a definite position in every space lattice.
Eg : In an alloy of silver and tin containing 73.2% of Ag and 26.8% of
Sn by weight is heated above 5000C, it is a single phase liquid
system. When the alloy is cooled, it solidifies to a compound with the formula
Ag3Sn, with silver and tin atoms occupying a definite positions in
the space lattice. Such alloy is called intermetalic compound and is used in
dental amalgam alloys.
Properties of
intermetallic compounds :
The intermetallic compounds formed in some alloy systems are
usually hard and brittle. Their properties rarely resemble those of metals
making up the alloy.
Eutectic mixture
:
Eutectic
mixture occurs when the metals are miscible in the liquid state but separate
into two phases in the solid state. The two phases usually precipitate as
alternating very fine layers of one phase over the other ; such a combination
is called eutectic mixture. An example of such a combination is 72% silver and
28% copper – with this alloy the eutectic is composed of fine, alternating
layers of high silver and high copper phases.
Characteristics
of eutectics:
·
The
temperature at which the eutectic occurs is lower than the fusion temperature
of either silver or copper, and is the lowest temperature at which any alloy
composition of silver and copper is entirely liquid.
·
There
is no solidification range for this composition. In other words, it solidifies
at a constant temperature, which is characteristic of the particular eutectic.
Liquid ®a - solid solution + b - solid
solution
It is referred to as an invariant
transformation, since it occurs at a single temperature and composition.
Properties of
eutectic alloys:
·
Eutectic
mixtures are usually harder and stronger than the metals used to form the alloy
and are often quite brittle.
·
Eutectic
mixtures have poor corrosion resistance. Galvanic action between the two phases
at a microscopic level can accelerate corrosion.
Peritectic alloys
:
Limited solubility of
two metals can bad to a transformation referred to as “peritectic”
·
Peritectic
systems are not common in dentistry
·
An
example being a silver – tin alloy system
·
Like
the eutectic transformation, the peritectic reaction in an invariant reaction
(ie it occurs at a particular composition and temperature) the reaction can be
written as
·
liquid
+ b ® a
METALS CAN BE
BROADLY CLASSIFIED AS:
a)
Noble metals
Noble metals are elements with a good metallic
surface that retain their surface in dry air. The term noble identifies
elements in terms of their chemical stability ie. they resist oxidation and are
impervious to acids.
Gold, platinum,
palladium, rhodium, ruthenium, iridium, osmium and silver are the eight noble
metals. In the oral cavity silver is more reactive sand therefore not
considered as a noble metal.
b) Precious
metals
The term ‘Precious’
merely indicates whether a metal has intrinsic value or in other words they are
higher – cost metals. Eight noble metals are also precious metals, and are
defined as such bymajor metallurgical societies and the federal government
agencies. All noble metals are procigus but all precious metals are not noble.
c) Semiprecious metals
There is no accepted
composition that delineates “precious” from “semiprecious”. Therefore, use of
the term semiprecious should be avoided.
d) Base metals :
Although these metals
have frequently been reffered to as non precious, the preferred designation is
base metal. These are non noble elements. base metals remain invaluable
components of dental casting alloys because of their influence on physical
properties, control of the amount and type of oxidation, or their strengthening
effects. Eg : chromium, cobalt, nickel, Iron copper etc.
DENTAL CASTING
ALLOYS
The history of dental
casting alloys has been influenced by three major factors
1) The technological changes of dental
prostheses
2) Metallurgical advancements
3) Price changes of the precious metals
since 1968 – when the U.S government lifted its support on the price of gold
before then 95% of fixed dental prostheses were made by alloys containing a
minimum of s75% by weight gold and other noble metals. However, when the price
of gold increased drastically, the
development of alternative alloys increased dramatically to reduce the cost of
cast of cast dental restorations. These alternative alloys that contained no
noble metal. Today, alternative alloys compose the majority of alloys
used.
Uses :
1) Fabrications of inlays, onlays
2) Fabrication of crowns, conventional all
metal – bridges, metal – ceramic bridges, resin – bounded bridges.
3) Endodontic posts
4) Removable partial denture frameworks
Desirable
properties :
1) Biocompatibility
2) Ease of melting
3) Ease of casting, brazing and polishing
4) Little solidification shrinkiage
5) Minimal reactivity with the mould
material
6) Good wear resistance
7) High strength and sag resistance
8) Excellent tarinsto and corrosion
resistance
NOBLE METAL
CASTING ALLOYS :
Noble metal casting
alloys contain mainly gold, palladium, and platinum and silver. They also
contain limited amounts of base metal elements such as copper, indium, iron,
tin and zinc.
High – gold
alloys :
Traditional dental casting alloys contain 70% by
weight or more of gold, palladium and platinum. ADA specification no. 5 for
dental casting gold alloy divides these alloys into four types based upon
mechanical properties.
Type I – soft (VHN 60 to 90)
Type II – Medium (VHN 90 to 120)
Type II – Hard (VHN 120 to 150 )
Type IV – Extra hard (VHN minimum 150)
Compositions Of Casting Gold Alloys
|
Type
|
Au
|
Ag %
|
Cu %
|
Pt / Pd %
|
Zn %
|
|
I
|
85
|
11
|
3
|
-
|
1
|
|
II
|
75
|
12
|
10
|
2
|
1
|
|
III
|
70
|
14
|
10
|
5
|
1
|
|
IV
|
65
|
13
|
15
|
6
|
1
|
It can be seen that the gold content or nobility
decreases on going from type 1 (soft) alloy to type IV (extra hard) alloy.
The increase in hardness
observed when nobility decreases is primarily due to the solution hardening
effect of the alloying metals which all form solid solutions with gold. Type
III and Type IV can be further hardened by heat treatments. Copper is the
principal hardener ; palladium and platinum serve to hardens the alloy but also
whitens it.
Zinc is added primarily
as a oxygen scavenger during casting.
Comparative
properties of the four types of casting gold alloys
|
Type
|
Hardness
|
Proportional limit
|
Strength
|
Ductility
|
Corrosion resistance
|
|
I
|
Increases
|
Increases
|
Increases
|
Decreases
|
Decreases
|
|
II
|
|||||
|
III
|
|||||
|
IV
|
The variation in alloy
properties with composition is reflected in the application for which the
material are choosen.
Type I (Soft) – for inlay restorations – subjected to
very slight stress and which do not have to resist high masticatory forces. The
high values of ductility of these alloys enables them to be burnished a process
which improves the marginal fit of the inlay and increases the surface
hardness.
Type II (Medium)
– are used for inlays
subjected to moderate stress and are the most widely used alloys for inlays.
They have superior mechanical properties, though at the expense of ductility.
Type III (Hard) –
are used for inlays
subjected to high stress; onlays; thin ¾ crowns, abutments, pontics, full
crowns, denture bases and short span fixed partial dentures.
Type IV (extra
hard) – are used for
extremely high stress states like endodontic posts and cores, thin veneer
crowns, long span fixed partial denture and removable partial denture.
LOW GOLD-CONTENT
ALLOYS :
Large increase in the
price of gold have led to the development and increased use of alloys with
lower gold content. Some alloys contain as little as 10% gold, but more
normally a gold content of around 45-50% is used. They have high palladium
content which imparts a characteristic whitish colour to the alloys.
The properties of
low-gold alloys are broadly similar to those of the type III and type IV
casting gold alloys, with one main exception. The ductility of these alloys may
be significantly lower than the conventional gold alloys. The casting
techniques and equipment used for low-gold alloys are similar to those used for
conventional gold alloys.
Silver-palladium
alloys :
These alloys are
white-colored and predominantly silver in composition but with substantial
amounts of palladium to provide mobility and promote the silver tarnish
resistance. There is generally a minimum of 25% of palladium along with small
quantities of copper, zinc and indium, in addition to gold which is present in
small quantities. The silver-palladium alloys have significantly lower density
than gold alloys, a factor which may affect castability. For a given volume of
casting, there is a lower force generated by the molten alloy during casting.
Attention must be paid to details such as casting temperature and mould
temperature. If the mould is to be adequately filled by the alloy.
The properties of
silver-palladium alloys are similar to those of the type III and IV gold alloys
with exeption to their lower ductility. The corrosion resistance is not as good
as gold alloys. These alloys are suitable alternatives to gold alloys. They
offer a considerable saving in cost when compared to gold alloys.
BASE METAL
CASTING ALLOYS :
According to the ADA
classification of 1984, any alloy that contains less than 250weight
% of the noble metals gold, platinum, and palladium is considered a
predominantly base metal alloy. Alloys within this category include Co-Ca,
Ni-Cr, Ni-Cr-Be, Ni-Co-Cr and Ti-Al-V.
Base metal alloys are
used extensively in dentistry and have been in used for the past 70 years. The
attractiveness of these materials stems from their corrosion resistance, high
strength, modules of elasticity (stiffness), low density and low cost.
Co-Cr and Ni-Cr have
been used for many years for fabricating partial denture frameworks and have
replaced type IV gold alloys completely for this application.
Ni-Cr alloys are used in
fabricating crowns and bridges
Ni-Cr and Co-Cr alloys
are used in PFM restorations
Titanium and titanium alloys are used for RPD’S crowns, and bridges and
implants
COMPOSITION :
Cobalt chromium
alloys
These alloys generally cotain 35-65% Co, 20-35% Cr, 0-30% Ni
Nickel chromium
alloys
Generally contain 70-80% Ni, 10-25% Cr.
Both these alloys contain minor alloying
elements such as carbon, molybdenum, beryllium, aluminium, silicon etc.
The concentration of
minor elements have a great effect on the physical properties of alloys.
Functions of
Various alloying elements :
Cobalt and Nikel are hard and strong metals.
Chromium – further hardens the alloy by solution
hardening and responsible for tarnish and corrosion resistance.
Carbon – increases the hardness of the alloy.
About 0.2% increase over the amount of the alloys becomes too hard and too
brittle. Conversely, 0.2% reduction would reduce the alloys ultimate and
tensile strength.
Molybdenum – 3% to 6% molybdenum contributes to the
strength of the alloys.
Aluminium – Increases the ultimate and tensile
strength of the nickel containing alloys.
Beryllium – Refines the grain structure and
reduces the fusion temperature of the alloys.
Silicon – Imparts good casting properties and
increases the ductility.
Microstructure :
Microstructure of any
substance is the basic parameter that controls the properties. In other words,
a change in the physical properties of a material is a strong indication that
there must have been some alteration in its microstructure. The microstructure
of Co-Cr alloys in the cast condition is inhomogeneous, consisting of a
austenitic matrix composed of a solid solution of cobalt and chromium in a
cored dendritic structure.
Many elements present in
a cast base metal alloy, such as chromium, cobalt and molybdenum are carbide
forming elements depending on the composition
of a cast base metal alloy and its manipulative condition, it may form
many types of carbides. During crystallization the carbides become precipitated
in the interdendritic regions which form the grain boundaries. If the
precipitated carbides form a continuous phase, the alloy becomes extremely hard
and a brittle, as the carbide phase acts a barrier to slip. A discontinuous
carbide phase is preferable since it allows slip and reduces the brittleness.
Whether a continuous or
discontinuous carbide phase is formed depends on the amount of carbon present
and on the casting technique.
High melting temperature
during casting favour discontinuous carbide phases but there is a limit to
which this can be used to any advantage since the use of very high casting
temperature can cause interactions between the alloy and the mould.
Manipulation of
base metal casting alloys :
The fusion temperature
of Ni/Cr and Co/Cr alloys are generally in the range of 1200-15000C.
This is considerably higher than for the casting gold alloys (9500C).
Melting of gold alloys can readily be achieved using a gas-air mixture. For
base metal alloys, however, either an acetylene-oxygen flame or an electric
induction furnace is required.
Investment moulds for
base metal alloys must be capable of maintaining their integrity at high
casting temperature used, Silica-bonded and phosphate bonded investments are
favoured.
The density values of
base metal alloys are approximately half those of the casting gold alloys,
therefore the thrust developed during casting may be somewhat lower, with the
possibility that the casting may not adequately fill the mould. Casting
machines used for the base metal alloys must therefore be capable of producing
extra thrust which overcomes this deficiency.
Base metal alloys are
very hard and consequently difficult to polish. After casting, to remove
surface roughness sandblasting and electrolytic polishing is carried out. Final
polishing is carried out using high-speed polishing buff.
Physical properties :
Melting temperature : Most base metal alloys melt
at 14000C to15000C.
Density : Average
density is between7 and 8gm/cm3 which is approximately half that of
gold alloys.
Mechanical properties :
Yield strength: They have yield strength greater than 600 Mpa. Dental alloys should
have at least 415 Mpa
to withstand permanent deformation when
used as partial denture clasps.
Modulus of elasticity : Is 220 Gpa ie.
Approximately Twice that of type
IV gold alloys. The higher the elastic modulus, the more rigid structure can be
expected.
Hardness : VHN is about 400 i.e. they have a hardness one third greater than
that a gold alloys. Although it makes the polishing of the casting a difficult
process, the final finished surface is very durable and resistant to
scratching.
Elongation : These alloys are quite brittle. Cobalt-chromium alloys exibit
elongation values of 1% to 2% whereas cobalt-chromium-nickel alloy, which
contains lesser amounts of molybdenum and carbon than other cobalt based
materials, shows an elongation of 10%.
Chemical properties:
Co-Cr / Ni-Cr
alloys have very good corrosion resistance by virtue of the passivating effect.
The alloys are covered with a tenacious layer of chromic oxide which protects
the bulk of the alloy from attack.
Chromium
containing alloys are attached vigourously by chlorine; household bleaches
should not be used for cleaning appliances made from chromium-type alloys.
Disadvantages:
Although
certain physical and mechanical features of the chromium type alloys are
superior to those of partial denture golds, clinical application of these
materials may be burdened by the following occurrences.
1.
Clasps cast from relatively
nonductile base metal alloys can break in service, some break within a short
period of time.
2.
Minor but necessary adjustments
required upon the delivery of the base metal partial denture can be made
difficult by the alloys high hardness and strength, and accompanying low
elongation.
3.
High hardness of the alloy can
cause excessive wear of restorations and natural teeth that they contact.
TITANIUM AND TITANIUM ALLOYS:
Titanium
resistance to electrochemical degradation, the benign biological response that
it elicits; its relatively light weight and its low density, low modulus and
high strength make titanium based materials attractive for use in dentistry.
Ti is a very
reactive metal, it form a very stable oxide layer with a thickness of the order
of angstroms and it repassivates in a time of the order of nanoseconds. This
oxide formation in the basis for the corrosion resistance and biocompatibility
of Ti.
Commercially
pure titanium (c.p.Ti) is used for dental implants, surface coatings and more
recently for crowns, partial and complete dentures and orthodontic wires.
Several titanium alloys are also used of these alloys, Ti-6AtGv is the most
widely used.
Commercially pure titanium:
c.p.Ti is
available in four grades, which vary according to the oxygen (0.18 to 0.40 wt
%) and iron (0.20 to 0.50 wt%) content.
These apparently slight concentration differences have a substantial effect on
the physical and mechanical properties.
At room
temperature c.p. Ti has a hexagonal close packed crystal lattice, which is
denoted as alpha (a)
phase on heating, an allotrophic phase transformation occurs. At 8830C,
a body centred cubic (BCC) phase, which is denoted by beta (b) phase, forms. A component with a predominantly b phase is strong but more brittle than a component with as a-phase microstructure. As with other metals, the temperature and
time of processing and heat treatment dictate the amount, ratio and
distribution of phases, overall composition and microstructure, and resulting
properties.
Titanium alloys:
Alloying
elements are added to stabilize either the a and b phase, by changing the b transformation temperature for example, in Ti-6Al-4V, aluminium in
an a stabilizes, which expands the a-phase field by increasing the (a+b) to b transformation temperature. The elements oxygen, carbon and
nitrogen stabilize the a phase as
well because of their increased solubility in HCP structure, whereas vandalium,
copper, palladium, iron are b
stabilizers which expand the b phase
field by decreasing the (a+b) to b transformation temperature.
Ti-6Al-4V:
It
is the most widely used alloy because of its desired proportion and predictable
productivity at room temperature Ti-6Al-4V is a two phase (a+b) alloy.
At
approx 9750C an allotrophic phase transformation takes place,
transforming the microstructure to a single phase BCC b alloy.
Properties :
Titanium
has a density of 4.5 g/cm3, which is half of the weight of other non
precious metals used in dentistry and one quarter that of gold. The low density
of titanium is advantages because it allows lightweight prostheses to be
fabricated.
The
protective passive oxide film of on titanium mainly TiO2, is stable
over a wide range of pHs, potentials and temperature.
Minimum
yield strength of Ti ranges between 240 to 890 MPa. It has low modulus of
elasticity 103 to 113 MPa.
And has favorable microhardness – 210
VHN.
High melting point of 17000C
Alloys have a slightly lower melting
point
In
theory, the light weight of titanium and its strength-to-weight ratio, high
ductility and low thermal conductivity would permit design modifications in Ti
restorations and removable prosthesis.
Casting:
because of high affinity of titanium has for hydrogen, oxygen and nitrogen,
standard crucibles and investment materials cannot be used.
Dental
castings are made via pressure-vaccum or centrifugal casting methods. The metal
is melted using an electric plasma arc or inductive heating in melting chamber filled with inert gas or held
in a vacuum. The molten metal than is transferred to the refactory mould centrifngal or pressure vaccum. Filling
casting of titanium commonly are used to fabriate crowns, bridge frameworks,
and full and partial denture frameworks. The casting machines are very
expensive. Investment material such as phosphate bonded silica and phosphate
investment materials with added trace elements are used.
Other
alloys: Ti 15 V, Ti – 20 Cu, Ti 30 pd, Ti – Co, Ti – Cu.
Disadvantages:
1) High melting point 2) High reactivity
3) low roasting efficiency 4) Inadequate
expansion of investment. 5) casting porosity 6) Difficulty in finishing this
metal 7)Difficult to weld and solder 8) Expensive equipment.
Alloys for metal-ceramic restoration
All
ceramic anterior restorations can appear very natural. Unfortunately, the
ceramics used in these restorations are brittle and subject to fracture from
high tensile stresses. Conversely, all metal restoration are strong and tough
but, from an aesthetic point of view, acceptable only for posterior
restoration. Fortunately the esthetic qualities of ceramic materials can be
combined with the strength and toughness of metals to produce restorations that
have both a natural tooth like appearance and very good mechanical properties.
A
cast metal coping provides a substrate on which a ceramic coating in fused. The
ceramics used for these restorations are porcelains.
The
bond between the metal and ceramic is the result of chemisorption by diffusion
between the surface oxides on the alloy and in the ceramic. These oxides are
formed during wetting of the alloy by the ceramic and firing of the ceramic.
Noble
metals, which are resistant to oxidizing, must have other, more easily
oxidizing element added such as indium and tin to form surface oxides. The
common practice of “degassing” or preoxidizing the metal coping before ceramic
application creates surface oxides that improve bonding.
Base
metal alloys contain elements, such as nickel, chromium, and beryllium which
form oxides easily during degassing.
CLASSIFICATION OF ALLOYS USED FOR
METAL CERAMIC RESTORATION
1)
High noble - Gold – Platinum – Palladium (Au-pt-pd)
Gold – Palladium – Silver (Au-pd-Ag)
Gold –
Palladium (Au-Pd)
2)
Noble – Palladium – Gold (Pd – Au)
Palladium – Gold – Silver
(Pd-Au-Ag)
Palladium – Silver (Pd-Ag)
3)
Base metal – Pure Titanium
Titanium –
Aluminium – Vanadium (Ti-Al-V)
Nikel –
Chromium – Molybdenum (Ni-Cr-Mo)
Nikel –
Chromium – Molybdenum – Berillyum (Ni-Cr-Mo-Be)
Inspite of
vastly different chemical compositions, all alloys share at least three common
features
1)
They have potential to bond to
dental porcelain
2)
They posses co-efficient of thermal
contraction compatible with those of dental porcelain.
3)
Their solidus temperature is
sufficiently high to permit the application of low-fusing porcelains.
HIGH
NOBLE ALLOYS:
The
high noble alloys are composed principally of gold and platinum group metals
with minor additions of tin, indium, and iron added for strength and to promote
a good porcelain bond to metal oxide.
Gold-platinum
–palladium alloys:
These
have a gold content ranging upto 88% with varying amounts of Pd, Pt and small
amount of base metals alloys of this type are restricted to 3-unit spans,
anterior cantilevers, or crowns.
Gold-palldium-silver
alloys:
These
gold based alloys contain between 39% and 77% gold and upto 35% palladium, and
silver levels as high as 22%. The silver increases the thermal contraction
co-efficient, but it also has the tendency to discolor some porcelains.
Gold-palladium
alloys: -
A gold content ranging from 44% to 55%
and palladium level of 35% to 45% is present in these metal-ceramic alloys,
which have remained popular despite their relatively high costs. Yield
strengths and hardness are favourable and elastic modulus is increased
significantly compared with high gold alloys. Corrosion resistance is excellent
because of high nobility. The only recognizable disadvantage is the
incompatible co-efficient of thermal contraction with some of the porcelains
with higher thermal contractions co-efficient, due to the lack of silver though
there is freedom from silver discolouration. Alloys of this type must be used
with porcelains which have lower coefficient of thermal contraction to avoid
the development of axial and circumferential tensile stresses in porcelain
during the cooling part of the porcelain firing cycle.
NOBLE
ALLOYS :
According to ADA classification of
1984, noble alloys must contain at least 25% to 40% silver. Tin and indium are
both usually added to increase the alloys hardness and to promote oxide
formation. These alloys were developed. When the cost of Pd was considerably
lower than Au ; those conditions no longer exist. Some ceramics used with these
high Ag alloys resulted in a greenish-yellow discolouration termed as
“greening”, due to the silver vapour that escapes from the surface of these
alloys during firing of the porcelain, the silver vapour diffuses is ionic
silver into the porcelain, and is reduced to form colloidal metallic silver in
the surface of porcelain.
Palladium-copper
alloys:
First
introduced to dental profession in 1982 ; they are comparable in cost to Pd-Ag
alloys. They are usually composed of 74-80% palladium and 2-15% copper. They
cause none of the porcelain colour problems associated with silver. High
hardness value in some of the alloys are offset by a relatively low elastic
modulus, resulting in better working characteristics than would be expected
with a high hardness value. Strength is good, and in some alloys extremely high
yield strengths are found. Some Pd-Cu alloys have a rather heavy oxide that is
difficult to cover with opaque porcelain. They are susceptible to creep
deformation at elevated firing temperatures, tending to contraindicate their
use in large-span fixed partial dentures.
Palladium-cobalt
alloys:
These
alloys contain around 88% palladium and 4-5% cobalt this groups is the most sag
resistant of the noble metal alloys. These alloys have good handling
characteristics. They tend to have relatively high thermal contraction
coefficient and would be expected to be more compatible with higher-expansion
porcelain. However, the main disadvantage is the formation of a dark oxide that
may be difficult to mask at thin margins.
Palladium-gallium-silver
and palladium-gallium-silver-gold alloys:
These
alloys are the most recent of the noble metals. This group of alloys was
introduced because they tend to have a slightly lighter-coloured oxide than
that of Pd-Cu or Pd-Co alloys, and they are thermally compatible with lower
expansion porcelains. The silver content is relatively low (5%) and is
inadequate to cause porcelain greening.
Physical
properties of high noble and noble metal alloys:
1)
The metal ceramic alloys must
have a high melting range so that the metal is solid well above the porcelain
sintering temperature to minimize distortion of casting during porcelain
application.
2)
Must have considerably low fusing
temperature
3)
Good corrosion resistance
4)
High modulus of elasticity
BASE
METAL ALLOYS FOR METAL CERAMIC RESTORATION:
Developed
in the 1970s, most of the base metal alloys are based on nickel and chromium,
but a few cobalt-chromium based alloys are also available.
Composition
:
Ni – Cr ® 61-81 wt / nickel
11-27% chromium
2-5% molybdenum
Co-Cr ® 53-67% cobalt
25-32% chromium
2-6% molybdenum
These alloys contain one or more of
the following elements; aluminum, beryllium, boron, carbon, cobalt, copper,
cerium, gallium, iron, manganese, niobium, silicon, tin, and zirconium.
Properties
of Ni-Cr, Ni-Cr-Be and Co-Cr alloys:
The
base metal alloys have different physical properties than the noble metal
alloys. The most significant are high hardness, high yield strength, and high
elastic modulus. Elongations is about the same as for the gold alloys but is
negated by the high yield strength which makes it difficult to work the metal.
The elastic modulus of base metal
alloys in as much as two times greater than the value of noble metal alloys
which decreases the flexibility to a significant degree. The flexibility of a
FPD framework constructed of Ni-Cr is less than half that of a framework of the
same dimensions made from a high-gold alloy. This property would enhance the
application of base metal alloys for long-span bridges. In a similar manner,
the high modulus of elasticity may be used to permit thinner castings.
-
The creep resistance of
nickel-based alloys at porcelain firing temperature is considered to be for
superior to the resistance of gold and palladium based alloys under the similar
conditions. It is particularly important in long span bridges where the
porcelain firing temperature may cause the unsupported structure to deform
permanently under controlled condition it has been found that base metal alloy
deforms less than 25 mm, whereas a noble metal alloy deforms 225 mm.
-
In general, the high hardness and
high strength of base metal alloys contribute to certain difficulties in
clinical practice grinding and polishing of fixed restorations to achieve
proper occlusion occasionally require more chair side time.
-
They have high casting temperature
and they have much lower densities (7 to 8gm /C3) thus on the basis
of the lower density and low intrinsic value of the component metals, the cost
difference between base metal and noble metal alloys can be substantial. The
disadvantage is adequate casting compensation is at a times a problem, as in
the fit of the coping.
-
The addition of beryllium to some
Ni-Cr alloys results in more favourable properties. Beryllium increases the
fluidity, and improves casting performance. Be, also controls surface oxidation
and results in more reliable, less technique sensitive porcelain metal bonds.
DENTAL
IMPLANT MATERIALS:
Most
commonly, metals and alloys are used for dental implants. Initially, surgical
grade stainless steel and Co-Cr alloys were used because of their acceptable
physical properties and relatively good corrosion resistance and
biocompatibility. However, it is currently more common to use implants made of
pure titanium or titanium alloys, because of the excellent biocompatibility of
titanium.
Stainless
steel:
Surgical
austenitic steel is an iron-carbon (0.05%) alloy with approximately 18%
chromium to impart corrosion resistance and 8% nickel to stabilize the
austenitic structure.
Because nickel is present, its use in
patients allergic to nickel is contraindicated.
The alloys is most frequently used in
a wrought and heat-treated condition. It has high strength and ductility, thus
it is resistant to brittle fracture.
Surface passivation is required to
maximize corrosion- biocorrosion resistance of all alloys, this one is the most
subject to crevice and pitting corrosion. Therefore, care must be taken to use
and retain the passivated (oxide) surface.
Cobalt-chromium-molybdenum
alloy :
These
alloys are most often used in an as cast or cast and annealed condition. This
permits the fabrication of custom designs, such as subperiosteal frames.
Their
composition is approximately 63% cobalt, 30% chromium and 5% molybdenum and
they contain small concentrations of carbon, manganese and nickel.
Molybdenum – stabilizes the
structure
Carbon – acts as a hardener
These alloys posses outstanding
resistance to corrosion and they have a high modulus.
However they are the least ductile of
all the alloys systems and bending must be avoided.
When proper quality control is
ensured, this alloys group exists excellent biocompatibility.
Because of the requirement of low cost
and long-term clinical success, but stainless steel and Co-Cr alloys have been
used extensively in many areas of surgery and dentistry.
Titanium
and titanium-aluminium-vandalium (Ti-6A-4V) alloy :
Commercially
pure titanium (Cp Ti) has become one of the materials of choice because of its
predictable interaction with the biologic environment.
Titanium is a highly reactive metal it
oxidizes (passivates) on contact with air or normal tissue fluids. This
reactivity is favourable for implant devices because it minimizes biocorrosion.
An oxide layer 10 A0 thick forms on the cut surface of pure titanium
within a millisecond. Thus any scratch or nick in the oxide coating is
essentially self healing.
Ti
6Al 4V alloy :
In
its most common alloyed form it contains 90 wt % titanium, 9 wt % aluminium and
4 wt % vanadium.
-
Density : 4.5g/cm3,
making it 40% lighter than steel.
-
The metal posses a high strength :
weight ratio
-
Ti has modulus of elasticity
approx. one half that of stainless steel or Co-Cr alloys. However it is still
5-10 times higher than that of bone.
-
Few titanium substructures are
plasma sprayed or coated with a thin layer of calcium phosphate ceramic.
The rationale for coating the implant with tricalcium
phosphate or hydroxyapatite, both rich in calcium and phosphorous into produce
a bioactive surface that promotes bone growth and induces a direct bond between
the implant and hard tissue.
The rationale of a plasma sprayed surface is to provide a
roughened, though biologically acceptable, surface for bone in growth to ensure
anchorage in the jaw.
Other
metals and alloys:
Many
other metals and alloys have been used for dental implant device fabrication.
Early implants extra made of gold, palladium, tantalum, platinum, iridium and
alloys of these metals.
More recently, devices made from
zirconium, hafnium and tungsten have been evaluated.
BIOCOMPATIBILITY
OF DENTAL CASTING METALS:
Dental
casting alloys are widely used in applications that place them into contact
with the oral epithelium, connective tissue or bone for many years. Given these
long-term roles, it is paramount that the biocompatibility of the casting
alloys be measured and understood.
Biologically
relevant properties of casting alloys:
-
Dental alloys are complex
metallurgically, in dentistry alloys usually contain at least 4 and after 6 or
more metals.
-
Dental alloys are commonly
described by their composition. Compositions are expressed in wt % or at %.
Atomic percentage better predicts the number of atoms available to be released
and affect the body.
-
Another way of describing the
alloys is by its phase structure. Single phase alloys have similar composition
throughout the structure. Elements in multiple phase alloys combine in such a
way that some areas differ in composition than the other areas.
-
The phase structure of an alloy is
critical to its corrosion properties and its biocompatibility. The interaction
between the biologic environment and the phase structure is what determines
which elements will be released and therefore how the body will respond to the
alloy.
Corrosion:
Corrosion
of alloys occurs when elements in the alloy ionize corrosion of an alloys
indicate that some of the elements are available to affect the tissues around
it.
Corrosion
is measured by – Observing the alloy surface
– Electrochemical test
– Spectroscopic
methods
Corrosion of an alloy is of
fundamental importance to its biocompatibility because the release of elements
from the alloys is necessary for adverse biological effects such as toxicity,
allergy, or mutagenecity.
The biological response to the elements depends upon
–
Which elements is released
–
Quantity released
–
Duration of exposure to tissues
-
An alloy does not necessarily
release elements in proportion to its composition.
-
Multiple phases will often
increase the elemental release from alloys.
-
Certain elements have a higher
tendency to be released from dental alloys, regardless of alloy composition.
This tendency is called liability.
Cu, Ni, Ga are
liable elements
Ca, Zn are
relatively liable
Au, Pd, Pt
have low liability
-
Reduction in pH will increase
elemental release from dental alloys.
Geis –gerstofer (1991) measured
the substance release from NI-Cr-Mo and Co-Cr-Mo alloys using a solution of lactic
acid and NaCl. Results reveals a considerable more rate of corrosion in
NI-Ci-Mo alloy than Co-Cr-Mo alloy and
alloys with Be contents, showed extremely high ion release under the corrosive
conditions.
Yang Tai et al (1992) in a simulated 1 yr period of mastication, the results showed that
nickel and berythium metals were release both by dissolution and occlusal wear.
J. C. Wataha et al (1998) subjected high noble, noble, base metal alloys for 30min to a
solution with pH ranging from 1 to 7 and concluded saying that the transient
exposure of casting alloys to an acidic oral environment is likely to
significantly increase elemental release from nickel based alloys, but not from
high noble and noble alloys.
F. Oscar et al (2000) evaluated corrosion of Ni-Cr and Cu-Al alloys by in vitro and
invitro tests and found almost no corrosion with Ni-Cr alloys but high
corrosion of Cu-Al alloys was observed.
Systemic toxicity of casting alloys:
Elements
that are released from alloys into the oral cavity may gain access to the
inside of the body through the epithelium in the gut, through the gingiva or
other oral tissue. In contrast, elements that are released from dental implants
into the bony tissues around the implant.
The
route by which an element gain access inside the body is critical to its
biological effects. It is for this reason that elemental release from implants
in thought to be more critical biologically than elemental release from dental
alloys used for prosthetic restorations.
Once
inside the body metal ions can be distributed to many tissue, each harbouring a
characteristic amount they are distributed by
-
Diffusion through the tissues
-
Lymphatic system
-
Blood stream
Ultimately the
body eliminates metals through the urine, feces or lungs
-
There in little evidence that
elements released from casting alloys contribute significantly to the systemic
presence of elements in the body.
-
In most situations, the amounts
of elements that are released from the dental alloys are far below those taken
in as a part of the diet.
Furthermore,
no studies with dental casting alloys and implants have shown that systemic
metal levels are elevated from the use of dental crowns.
In summary,
systemic toxicity from dental casting alloys has not been demonstrated.
Local toxicity:
A
second major concern about the safety of dental casting alloys is whether
elements released can cause toxicity locally that is adjacent to the
restoration.
The
concentration that is required to have a local adverse effect may be much lower
than concentration necessary to cause systemic effects through oral route.
Dental
crown often extends below the level of the gingiva. If the elements from the
alloy are released into the sulcus they may reach high concentration as they
are not diluted by saliva.
Elements
released towards the tissue side of the RPD framework may not be diluted by
oral fluids to the same extent as elements that are released from the opposite
side of the framework consequently, the metal ion concentration may be higher
next to the tissue than in the saliva.
It
is clear that if metal ions are present at high enough concentrations, they
will other or totally disable the cellular metabolism.
Toxicity
of these metal ions is reported on the concentration to depress cellular activity
by 50% or total toxic concentration 50% (TC 50 value).
If
the exposure time of a metal ion to cell is increased, the TC50 value will
decrease. Thus alloys that release elements over longer periods are more likely
to cause local toxic effects.
Although
the release of elements from dental casting alloys is well established, the
local biologic effect of these released elements is still a topic of debate.
Studies
have clearly established that release of metallic ions is necessary for
cellular damage but does not guarantee that cellular damage will occur. Whether
damage will occur depends on the elemental species, the concentration released
and the duration of exposure to the cells.
Lamster et al (1987) reviewed 2 cases who demonstrated significant loses of alveolar bone
about the nickel rich non precious alloy and porcelain crown. The loss of alv.
bone occurred within 18 months after placement of the restorations reason for
this was thought that the electrolysis of metal leading to corrosion and
bioavailability of nickel.
John C. Wataha et al (2002) assessed the toxicity of 5 types of casting alloys commonly used
after, stimulated tooth brushing, in acidic environment and a toothpaste.
Au-Pt, Au-Pd and Ni-Cr (without Be) exhibited mitoxicity. A large increase in
the toxicity was noted for Pd-Cu-Ga and Ni-Ca-Be alloys.
We know there
is significant tolerance in vivo to low levels of elements released from dental
alloys over the short term questions of long-term responses to these low level
of elements remain unanswered.
Allergy: An
element must be released from an alloy to cause allergy. Allergy and toxic
reaction are often difficult to difficult to distinguish. Classically, allergic
responses are characterized by dose independence. In reality the boundary
between toxicity and allergy are not clear and the relationship is still an
active area of research.
Patch
tests for metal hypersensitivity are controversial allergy to metal is assessed
by either applying the metal ion to the skin in a patch or injecting a small
amount of ion below the skin, but the metal salts are in some liquid vehicle,
and the vehicle will affect the results whether it is water, oil or petrolatum.
Even the type of patch can influence the results.
The
incidence of hypersensitivity to dental alloys appears to quiet low.
Studies
indicate that about 15% of the general population is sensitive to nickel, 8% is
sensitive to cobalt, and 8% to chromium. Documented allergies have also been
reported for mercury, copper, gold, platinum, palladium, tin and zinc.
Timothy K. James (1986) stated that incidence to Ni hypersensitivity was more in women (10
times more than men) the reason was attributed to high frequency of exposure to
nickel jewellery, nickel plated objects at work and at home.
There
is probably a genetic component to the frequency of metal allergy as well.
It
is possible for metals to have cross reactive allergy some studies have
reported that patients who are sensitive to palladium are nearly always also
sensitive to nickel.
Mutagenicity and carcinogenicity:
Mutagenecity describes an alteration of
the sequence of DNA.
Carcinogenecity means alternations in the
DNA have caused a call to grow and divide inappropriately carcinogenecity
results from several mutations.
An
alloys ability to cause mutagenesis of carcinogenesis is directly related to
its corrosion.
There
is little or no evidence from the dental literature that indicates the dental
alloys are carcinogenic. It is also imperative to realize that the form of the
metal is critical to understanding its mutagenic potential.
For
example, the oxidation state of chromium is critical to understanding its
mutagenic potential Ca3+ is not a mutagen but Cr6+ is.
The
molecular form of the metal is also important Nickel ions are weak mutagens but
nickel subsulfide (Ni2S3) is highly mutagenic.
Therefore,
it is improper to state that a metal is mutagenic or carcinogenic per Se.
In
dental laboratories, the vapour forms of elements such as beryllium are the
most common mutagenic threat. The vapours are created during the casting and
finishing of the prosthesis. Exposure to beryllium may result in acute and
chronic forms of beryllium disease – beryllosis. Symptoms range from coughing,
chest pain and general weakness to pulmonary dysfunction.
Overall,
there is no evidence that dental alloys cause or contribute to neoplasia in the
body. However it may be prudent for the practitioner to avoid alloys containing
elements such as cadmium, cobalt and beryllium which are known carcinogen.
To
minimize biological risks, dentists should select alloys that have the lowest
release of elements selection of an alloy should be made using corrosion and
biological data from dental manufacturers.
CONCLUSION :
As
a wide range of metals and alloys combination are now available, it is
necessary for us to have the knowledge about the composition, properties and
biocompatibility of the constituent metals of the alloys, to be able to choose
them for the required applications. The decision is not an easy one, as it will
have financial, technical and patient satisfaction consequences. In may ways
the decision is philosophical, based on the drawbacks of using a particular
alloy versus its known clinical benefits.
REFERENCES :
REFERENCES :
1)
Science of Dental Materials –
Anusavice, 10th Edn.
2)
Restorative Dental Materials –
Craig, 11th Edn.
3)
Applied Dental Materials – Mccabe,
7th Edn.
4)
Dental Materials and their
selection – O’Brien 2nd Edn.
5)
JPD 2000; 83; 223-234
6)
Quint. Int. 1996 ; 27 : 401 – 408
7)
JADA ; 128 : 37 – 45
8)
Dent. Metr 2001 ; 17 : 7 – 13
9)
Dent. Metr. 2003 ; 19 : 174 – 181
10)
JPD 2000; 84 : 575 – 82
11)
JPD 2002 ; 87 : 94 – 98
12)
J. Periodontal. 1987 ; 58 : 486 –
492
13)
JPD 1998 ; 80 : 691 – 698
14)
JADA 2003 ; 134 : 347 – 349
15)
IJP 1991 ; 4 : 152 – 158
16)
IJP 1995 ; 11 : 432 – 437
17)
JPD 1992 ; 68 : 692 – 697
18)
JPD 1983 ; 49 : 363 – 370.
METALS
IN PROSTHODONTICS
·
Introduction
·
History of metals
·
Definition
·
General characteristics of metals
·
Structure and properties of metals
·
Deformation of metals
·
Cold working
·
Annealing
·
Alloys
o
Solid solutions
o
Intermetallic compound
o
Eutectic formation
o
Perictectic formation
·
Classification of metals
·
Dental casting alloys
o
Uses
o
Desirable properties
·
Noble metal casting alloys
·
Base metal casting alloys
·
Alloys for metal – ceramic
restoration
·
Implant materials
·
Biocompatibility of metals
·
Conclusion
·
References
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