Saturday, August 3, 2013

Metals in prosthodontics

                                                       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.

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.         
          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.
          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.

          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

          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

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.

          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 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
Ag %
Cu %
Pt / Pd %
Zn %

          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
Proportional limit
Corrosion resistance





          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.

          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.
          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

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.
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 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.

          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.
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.

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.
          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.
          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
          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.
          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.  

          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 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.  

          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.   

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.

·          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