metals in pros tho don tics

8/14/2019 Metals in Pros Tho Don Tics

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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 metallicelements 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.


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General characteristics of metals

A metal is an element that ionizes positively in solution

Metal have certain typical and characteristic properties thatdistinguish 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 andworkability 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.


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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 differentarrays.

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


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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 agrain.

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 increasethe 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


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formation occurs faster than the crystallization, a small grain size can be

obtained.

Consequently, a slow cooling results in large grains. In apolycrystalline 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 grainsize decrease metals have higher valves of elastic limit.


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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 :


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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


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completely recrystallized material can range from rather fine to fairly

coarse.

If now the fine grain form is further annealed, the grains begin togrow. 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


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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 :


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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 - solid solution + - 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 :


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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 +

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 :


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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 castingalloys 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 ofs75% 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 :


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1) Biocompatibility

2) Ease of melting

3) Ease of casting, brazing and polishing4) 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)


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Type III (Hard) are used for inlays subjected to high stress; onlays; thin

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 isto be adequately filled by the alloy.


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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 suitablealternatives 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 RPDS crowns, and bridges and

implants


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COMPOSITION :

Cobalt chromium alloys

These alloys generally cotain 35-65% Co, 20-35% Cr, 0-30% NiNickel 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


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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


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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 :


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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


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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 ( ) phase on heating, an allotrophicphase transformation occurs. At 8830C, a body centred cubic (BCC) phase,

which is denoted by beta ( ) phase, forms. A component with a

predominantly phase is strong but more brittle than a component with as

-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 and phase,

by changing the transformation temperature for example, in Ti-6Al-4V,

aluminium in an stabilizes, which expands the -phase field by

increasing the ( + ) to transformation temperature. The elements

oxygen, carbon and nitrogen stabilize the phase as well because of their

increased solubility in HCP structure, whereas vandalium, copper,

palladium, iron are stabilizers which expand the phase field by

decreasing the ( + ) to transformation temperature.

Ti-6Al-4V :


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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

( + ) alloy.At approx 9750C an allotrophic phase transformation takes place,

transforming the microstructure to a single phase BCC 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 lightweightprostheses 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


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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 restoration 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.


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CLASSIFICATION OF ALLOYS USED FOR METAL CERAMIC

RESTORATION1) 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.


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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 thethermal 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.


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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 and2-15% 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


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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% chromium2-5% 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, 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.


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- 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 particularlyimportant 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 m, whereas a noble metal alloy deforms 225 m.

- In general, the high hardness and high strength of base metal alloys

contribute to certain difficulties in clinical practice grinding andpolishing 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 :


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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.


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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 :


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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 :


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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.


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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 gingivaor 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


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- 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).


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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 effects 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.


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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 insome 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.


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Mutagenicity and carcinogenicity :

Mutagenecity describes an alteration of the sequence of DNA.

Carcinogenecity means alternations in the DNA have caused a call to growand 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 :


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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 beable 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.


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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 OBrien 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.


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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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COLLEGE OF DENTAL SCINECES

DEPARTMENT OF PROSTHODONTICS

INCLUDING

CROWN & BRIDGE AND IMPLANTOLOGY

SEMINAR

ON

METALSMETALS

ININ

PROSTHODONTICSPROSTHODONTICS

PRESENTED BY :

DR. SUNEEL G. PATIL

metals in pros tho don tics

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