Dental Care Forum

INDEX

1. INTRODUCTION
2. HISTORICAL PERSPECTIVE ON DENTAL CASTING ALLOYS
3. DESIABLE PROPERTIES OF CASTING ALLOYS
4. CLASSIFICATION OF CASTING ALLOYS
5. ALLOYS FOR ALL METAL & RESIN VENEER RESTORATIONS
6. ALLOYS FOR METAL CERAMIC RESTORATIONS
7. BASE METAL ALLOYS
8. HANDLING HAZARDS & PRECAUTIONS
9. CONCLUSIONS













INTRODUCTION:

An alloy is defined as for dental purposes as a metal containing two (or ) more elements, at least one of which is a metal and all of which are mutually soluble in Molten State. Although there are some similarities between the characteristics of pure metals & alloys addition of other metals to a pure metal complicate the picture relative to certain fundamental aspects not yet considered.

An alloy system is an aggregate of two or more metals in all possible combinations. For example the gold - silver systems includes all possible concentrations of gold & silver.

Historical perspective on Dental Casting alloys: -
The history of dental casting alloys has been influenced by three factors:
1. The technologic changes of dental prosthesis.
2. Metallurgic advancements.
3. Price changes of noble metals.

'The lost wax technique' described by Jaggart in 1907 led to the casting of Complex inlays such as onlays, crowns, fixed partial dentures & removable partial denture frameworks.
In 1932, the dental materials group at the national bureau of standards surveyed the alloys & roughly classified them as

Type I Soft (Vickers hardness No - 50 -90)
Type II Medium (VHN - 90 - 120)
Type III Hard (VHN - 120 - 150)
Type IV Extra Hard (VHN - >150)

The base metal alloys were introduced in 1930s. Since that time both Nickel-Chromium, & Cobalt - Chromium formulation have become increasingly popular compared to conventional gold alloys, which previously were the predominant metals used for such prostheses. The advantages of base metal alloys are their lower Wt, increased mechanical properties & reduced cost.

Desirable Properties of Casting Alloys: -
The metals must exhibit biocompatibility, ease of melting, casting, brazing (or soldering) & polishing, little solidification shrinkage, minimal reactivity & the mold material, good wear resistance, high strength and sag resistance.

Classification of casting Alloys:-
In 1927, the Bureau of standards established gold Casting alloy types I through IV according to dental function, & hardness increasing from TYPE I through IV. Based on 1989 revision of specification No - 5 by the ADA, the four alloy types are classified by their properties & not by their compositions;

Type I - (Soft) - Small inlays, easily burnished & subject to very slight stress.

Type II- (Medium) - Inlays subject to moderate stress, including 3/4 crowns, abutments, pontics, & full crowns.

Type III -( Hard ) - Inlays subject to high stress, including onlays, crowns, thin cast backings, abutments, pontics,full crowns, & short span FPD'S.

Type IV - (Extra hard) - Inlays subject to high stresses, including denture base bars & clasps, long span FPD's endodontic posts & Cones, thin veneer crowns & RPDS.

In 1984 ADA proposed simple classification for dental casting alloys. Three categories, are described; High Noble (HN) Noble (N) and Predominantly base metal ( PB).

High Noble Metal - Contains >40 WT % Au, 60 WT % of the noble metal elements ( Au + Ir + Pt + Rh+Ru)

Noble Metal - Contains > 25 WT% of noble metal elements.

PB Metal - Contains < 25WT% of Noble metal Elements.



Classification of Alloys for all Metal restorations, Metal - ceramic restoration:-

Alloy Type All Metal Metal Ceramic
High noble alloys Au-Ag-Cu-Pd
Metal Ceramic Alloys Au- Pt-Pd
Au-Pd-Ag
Au- Pd. ( No Ag)
Noble metal alloys Ag-Pd-Au-Cu
Ag-Pd
Metal - Ceramic Pd- Au
Pd-Au-Ag
Pd-Ag
Pd-Cu
Pd-Co
Pd-Ga-Ag
Base metal alloys Ti - Al - V
Pure Ti
Ni - Cr - Mo- Be
Co-Cr-Mo
Co-Cr-W
Al Bronze Pure Ti
Ti - Al - V
Ni-Cr-Mo-Be
Ni-Cr-Mo
Co-Cr-Mo
Co-Cr-W




Alloys for All metal & Resin Veneer Restorations :
Traditional Type III & IV alloys are called Crown & bridge alloys, although Type IV alloys also are used occasionally for high - stress applications such as removable partial denture frameworks.

Gold Alloys can be significantly hardened if the alloy contains a significant amount of copper. The actual mechanism of hardening is probably the result of several different solid - state transformations.

Alloy that can be hardened can, of course, also be softened. In metallurgic terminology the softening heat treatment is referred to as solution heat treatment. The hardening heat treatment is termed age hardening.

Softening heat treatment: -
The Casting is placed in an electric furnace for 10 min at a temp of 700. C and then it is quenched in water. Such a Rx reduces the tensile strength, proportional limit & hardness but the ductility is increased

Hardening heat Rx: -
One of the most practical hardening Rx is by 'soaking' or aging the casting at a specific temperature for a definite time usually 15- 30 minutes, before it is water quenched. The temperature ranges between 200 & 450. C. Proportional limits & modulus of resilience is increased and elongation is reduced by age hardening of an alloy.

Silver - Palladium alloys: -
These alloys are white & predominantly silver in composition but have substantial amount of palladium. (Atleast 25%) that provides nobility & promotes the silver tarnish resistance. The copper free Ag- Pd alloys may contain 70- 72 % silver & 2.5 % palladium. The major limitation of Ag- Pd alloys in general & the Ag - Pd - Cu in particular is their greater potential for tarnish & corrosion.
Because of increasing interest in aesthetics by dental pts, a decreased use of all metal restorations has occurred during the past decade. The use of metal - ceramic restorations in post sites - has increased relative to use of all metal crowns & inlays.
Ni - Cr & Co - Cr alloys and titanium & titanium alloys can be used for all metal & metal ceramic restorations.

Aluminum - Bronze Alloy:-
Although bronze is traditionally defined as Copper - rich. (Cu - Sn) alloys with or without other elements such as Zn & P, there exist essentially binary, ternary & quaternary bronze alloys that contain no Tin. Such as Aluminum Bronze (Cu - Al), Si Bronze (Cu - Si) Berilium bronze (Cu - Be). Al Bronze family of alloys may contain between 81 & 88 Wt % Cu; 7 - 11% Wt % Al; 2- 4 Wt% Ni; 1- 4 wt% Fe. There is a potential for Cu alloys to react with Sulfur to form Copper Sulfide, which may tarnish the surface of this alloys in the same manner that silver sulfide darkens, the surface of gold base or silver base alloys, that contain a significant Ag content.

High Noble Alloys for metal - Ceramic Restoration:-
The chief objection to the use of dental porcelain as a restorative material is its low tensile shear strength. Although porcelain can resist compressive stresses & reasonable success, substructure design does not permit shapes in which compressive stress is the principal force. This can be minimized by bonding porcelain directly to a cast alloy substructure made to fit the prepared tooth.

The original metal ceramic alloys contained 88% gold and were much too soft for stress bearing restorations such as FPD's. Because there is no evidence of a chemical bond - between these alloys & dental porcelain, mechanical retention & undercuts were used to prevent detachment of ceramic veneer. By adding less than 1% of Oxide forming elements such as Fe, In, Sn to these high gold content alloys, the porcelain metal bond strength was improved by a factor of 3. Fe also increases the proportional limit & strength of alloys. This 1% addition of base metals Au - Pd & Pt alloy was all that was necessary to produce a slight Oxide film on the surface of substructure to achieve a porcelain - metal bond strength level that surpassed the cohesive strength porcelain itself. Gold based metal - ceramic alloys containing more than 40 wt % Au atleast 60 Wt% of noble metals are generally classified as high noble alloys.

Au - Pt - Pd Alloys:-
Gold Content is upto 88% & varying amounts of Pd, Pt & small amounts of base metals. These are yellow in color and susceptible to sag deformation, & FPD s should be restricted to 3 - unit spans, anterior cantilevers or crowns.

Au - Pd - Ag Alloys:-
These contain between 39 % & 77% Au upto 35 % Pd & Ag levels are high as 22%. Silver increases thermal contraction coefficient, but it also has tendency to discolor porcelains.

Au - Pd Alloys :-
Au content ranging from 44% to 55% Pd - 35 %. The lack of Ag results in a decreased thermal contraction coefficient and the freedom from Ag discoloration of porcelain. Alloys of this type must be used with porcelains that have low coefficients of thermal contraction to avoid the development of axial of circumferential tensile stresses in porcelain during the cooling part of the porcelain firing cycle.

Noble Alloys for metal ceramic restorations

Palladium based alloys;
Noble Pd based alloys offer a compromise between the high noble gold alloys and base metal alloys. The price of Pd alloy is 1/3 that of a gold Alloy. The density is mid - way between that of base metal & of high noble alloys. Pd based alloys after moderate price compared with Au alloys, workability similar to Au & scrap value.



Pd- Ag Alloys:-
Pd - Ag Alloys were introduced widely in late 1970's later their use has been declined because of their tendency to greenish - yellow discoloration. Popularly termed "greening”, is that the silver vapor escapes from the surface of these alloys during firing of porcelain, diffuses as ionic silver into the porcelain, and is reduced to form colloidal metallic silver in the surface layer of porcelain.

Other Pd alloys contain 75% - 90% Pd and no Ag and were developed to eliminate the greening problem. Some of the high Pd alloys develop a layer of dark Oxide on their surface during cooling from the degassing cycle, and this Oxide layer has proven difficult to mask by the opaque porcelain. Because Pd is expensive than Ag, the elimination of Ag and its replacement by Pd results in being more expensive than Pd-Ag alloys.

The compositions of Pd - Ag alloys fall with in a narrow range; 53 - 61% Pd, 28 - 40 % Ag. Tin (or) Indium or both are usually added to increased alloy hardness and to promote oxide formation for adequate bonding of porcelain. In some of these alloys the formation of an internal oxide rather than an external oxide has been reported. Nodules are formed on the external surface as a result of internal oxidation.

The greening effect of these alloys due to Ag is minimized by gold metal conditioners or ceramic coating agents.
The low spf gravity of these alloys combined and their low intrinsic cost makes these alloys attractive as economical alternatives to the gold based alloys. Adherence of porcelain is considered to be acceptable for most of the Pd - Ag Alloys. Instead of formation of desired external oxide, Pd - Ag nodules may develop on the surface that effect retention of porcelain by mechanical rather than chemical bonding.

Pd- Cu - Alloys:-
This alloy type is comparable in cost to Pd - Ag alloys. Because this type of alloys are recent introduction to dental market, little clinical information is available on their long - term clinical success.

Because of their low melting range of approximately 11700c to 11900c. These alloys are expected to be susceptible to creep deformation at elevated firing temperatures. These alloys contain 74 - 80% Pd and 9 - 15 % Cu. Porcelain discoloration due to Cu is possible but does not appear to be a major problem. Some of these alloys are somewhat technique sensitive with respect to casting, pre-soldering and proper oxidation "Rx”. The Pd - Cu alloys have yield strengths of upto 1145 mpa elongation values of 5- 11 % and hardness values as high as some base metal alloys. Thus these alloys would appear to have a poor potential for burnishing, except when the marginal areas are relatively thin.

Pd- Cobalt Alloys:-
These are often advertised as gold free, Ni Free, Beryllium - free, & Ag Free Alloys. The reference to Ni & Be indicated that these alloys as is true with other noble metals are generally considered biocompatible. These alloys have a fine grain size to minimize not tearing during solidification process. This Pd - Co group is the most sag resistant of all noble metal alloys. The noble metal content (based on Pd) ranges from 78 - 88% .The content between - 4 & 10%. Some alloys may contain 8% Gallium.

Although these alloys are Ag free, discoloration of porcelain can still result because of presence of cobalt. Failure of technician to completely mask out the dark metal oxide color and opaque porcelain is a more common cause of unacceptable aesthetic results. No metal coating agents are required to mask the oxide color (or) to promote adherence to porcelain. Like the Pd - Ag & Pd - Cu alloys the Pd - Co alloys generally tend to have a relatively high thermal contraction coefficient and would be expected to be more compatible and higher expansion porcelains.

Pd - Ga - Ag and Pd- Ga - Ag - Au Alloys:-
These alloys are the most recent of noble metals. These alloys have a slightly lighter - colored oxide than the Pd- Cu or Pd - Co alloys and they are thermally compatible and lower expansion porcelains. The oxide that is required for bonding to porcelain is relatively dark, but is somewhat lighter than those of Pd- Cu & Pd - Co alloys. The Ag content is relatively low (5 - 8 Wt%) and is usually inadequate to cause porcelain greening. Little information is available on metal - ceramic bond strength or thermal compatibility. These Pd - Ga - Ag Alloys generally tend to have a relatively low thermal contraction co-efficient & would be expected to more compatible and lower expansion porcelains such as Vita porcelains.

Base metal Alloys for cast metal & metal ceramic Restoration:-

The use of base metal alloys has been increased recently due to high cost of gold and other noble metals .

Most Ni - Cr alloys for crowns & FPDs contains 61 - 81% Ni, 11 - 27 % Cr and 2- 5% Mo. Chromium is essential to provide passivation and corrosion resistance, other alloy formulation include Cr - Co and Fe - Cr. These alloys may also contain one or more of the following elements. Al, Be, Bo, carbon, Co, Cu, Cerium, Gallium, Fe, Mn, Niobium, Si, Sn, Ti & Zirconium.

The Co- Cr Alloys typically contain 53- 67% Co & 25 - 32% Cr & 2- 6 % Mo. Base metal alloys melt at elevated temperatures, the use of Po4 or silica bonded investments is indicated. Compensation for casting shrinkage required at these elevated temperatures if a clinically acceptable fit is to be obtained. Recently chemically pure Ti, & Ti- Al- V Alloys have been introduced for metal-ceramic restorations.

Properties of base metal alloys

Compared to other alloys for metal ceramic restoration, base metal alloys generally have higher hardness and elastic modulus (stiffness) values and are more sag resistant at elevated temperatures, but they may be more difficult to cast and pre-solder than Au - Pd or Pd - Ag Alloys. These alloys are more technique sensitive than well - established noble metal alloys. The ability to obtain acceptable fitting base metal castings represents a challenge to technicians and may require special procedure to adequately compensate for their higher solidification shrinkage. Another potential disadvantage of these alloys is their potential for porcelain adherent oxide layer from the metal substrate. In addition relatively small differences in composition may produce wide variations in metal ceramic bond strength.
Ni alloys without Beryllium demonstrate poorer castability than those that contain upto 1.8 % Be. The creep resistance of nickel based alloys at firing temperatures is considered to be far superior to the resistance of gold based and Pt based alloys under the same conditions. The tarnish & corrosion resistance of base metal alloys containing nickel is of principal concern because of allergic potential of Ni & Nickel compounds.

Studies have been conducted on the potential of galvanic corrosion induced by base metal alloys with other metallic restorations. Amalgams are expected to sustain less galvanic attack when they are placed in contact with Ni - Cr ( or ) Co- Cr alloys than with gold alloys . When in contact to type III gold alloys, however the base metal alloys are more susceptible to corrosive degradation.

In general the high hardness & high strength of base metal alloys contribute to certain difficulties in clinical practice. Grinding & Polishing of fixed restoration to achieve occlusion occasionally require more chairside time.

Handling Hazards & precautions :-
High concentrations of Be & Ni dust & Be Vapor are hazardous to laboratory technicians. Although Be concentration in dental alloys rarely exceeds 2% the amount of Be vapor released into the breathing space during melting of Ni - Cr- Be Alloys may be significant over an extended period. The Occupational Health & Safety Administration (OSHA) specified that exposure to Be dust in air should be limited to a particulate Be Concentration of 2 Mg/ m3 of air (both respirable & Non respirable particles) determined from an 8 hr time-weighted coverage. The allowable ceiling concentration is 5mg / m3. Exposure to Be may result in acute & chronic forms of Be Decease. Physiologic responses vary from contact dermatitis to severe chemical pneumonitis, which can be fatal.

Of greater concern to dental patients is the intra-oral exposure to Ni especially for pts with a known allergy to this elements. Inhalation ingestion and thermal contact of Nickel or Nickel containing alloys are common, because Nickel is found in environmental sources such as air, soil, and food as well as in synthetic objects such as coins , kitchen utensils & jewelry. Females are 5 times more allergic to Nickel than males. To minimize exposure of metallic dust to pts and dentists during metal grinding operations; a high speed evacuation system should be employed when such procedures are performed intra-orally. Patients should be informed of the potential allergic effects of Nickel exposure and a thorough medical history should be taken to determine if the pt is at risk of exhibiting an allergic reaction to nickel.

Commercially pure Titanium:-
The element Titanium is a light Wt metal with a density of 4.51 gm/ cm3 compared to a density of 7.6 8/ cm3 for Ni - Cr . It has a relatively high melting point of 16680 c and a thermal expansion coefficient of 8.4  106 / 0 c . This co efficient is far below the values (12.7 - 14.2  10 6 / 0c) for the porcelains that are typically used for metal ceramic restoration. Thus a special low-fusing porcelain is required to minimize the development of thermal tensile stresses in the porcelain veneers. For general dental applications Titanium has the ability to passivate, i.e. to change its surface from a chemically active state to much less reactive state by the formation of an extremely thin Oxide layer, even when the surface is scratched or abraded it can reform this protective Oxide layer instantaneously. For Rx of Pts with known hypersensitivity to Ni; Pure Ti represents an excellent alternative to base metal alloys that contain nickel.

Titanium Alloys :-
The most common alloy for dental & medical purposes is the Ti - Al 6 V4 composition. The main benefits of alloying are significant strengthening & stabilization of the alloy against the formation of either the ALPHA phase (through Al addition) or the B-phase through addition of Cu, Pd (or) Vanadium. The ALPHA phase alloys are more resistant to high temperature creep, a most imp property for metal-ceramic applications, but these alloys are more amendable to brazing (or) soldering. B alloys are less resistant to creep deformation at elevated temp, but they can be hardened & strengthened significantly.


Conclusions: -

There have been major advances in the development of dental casting alloys during past 20 years. Because of large Number & types of casting metals available for use. One of the most complex challenges of dentists is the proper selection of casting alloys for resin veneer bridges metal-ceramic restorations and all metal cast restoration.


References :-

1. Philips Science of D M - 10th Edition.

2. CRAIG -Dental Materials - 8th edition

3. DCNA - Dental Materials

4. JPD - 1991 - 65 (1); 97