1. Precious and non precious metals metals for crown and bridges
مادة سنية
ثاني اسنان موصل
27 / 4 / 2016
Precious and non precious metals metals for crown and bridges
Dental Material
د.منية محمدنبيل &د.نشوة صبحي

2. INTRODUCTION
Metals and alloys have many uses in dentistry. Steel alloys are commonly used for the construction of instruments and of wires for orthodontics.
Crowns that have a metal component (all-metal and porcelain-fused-to-metal) are made using specific types of alloys. Alloys of precious metals are commonly used for the fabrication of metal-bonded ceramic restorations.
No pure metals are used (not even gold). This is because, that the physical properties of alloys are superior to that of pure metals, which is more beneficial for the dental applications.

3. ALLOY:
A mixture of two or more metals or metalloids that are mutually soluble in the molten state; distinguished as binary, ternary, quaternary…etc. (depending on the number of metals within the mixture). Alloying elements are added to alter the hardness, strength and toughness of a metallic element, thus obtaining properties not found in a pure metal.
Casting: Process in which: a something that has been casted in a mold; or an object formed by the solidification, from a fluid state that poured or injected into a mold.

4. Casting alloys: Those are alloys which are shaped by casting method.
DENTAL CASTED ALLOYS:
Today the dental profession has access to a wide variety of casting alloys. These alloys are designed for specific clinical purposes, like: Inlays, Onlays, Crowns, Bridges, Partial dentures and Porcelain fused to metal restorations.

5. The alloys for metal-ceramic restorations can be used for all-metal (or resin-veneer) prostheses, whereas the alloys for metal restorations should not be used for metal-ceramic restorations.
The reasons are as follows:
The alloys may not form thin, stable oxide layers to promote atomic bonding to porcelain.
Their melting range may be too low to resist sag deformation or melting at porcelain filing temperatures.
Their thermal contraction coefficients may not be close enough to those of commercial porcelains

6. Alloy classifications
In general, there are 3 basic categories of dental alloys that can be used, each of these type has its own specific advantages and disadvantages, including: cost, insurance coverage, color (yellow or white), as well as general physical properties.

7. High noble alloys (Precious metal):
The classification system that is used to categorize dental alloys included:
High noble alloys (Precious metal):
This group has a composition that is over 60% is noble metal (gold, palladium and/or platinum).
PROPERTIES OF HIGH NOBLE ALLOYS:
These alloys are the most expensive as gold, palladium and platinum.
These materials have relatively high densities that make them easier to be casted.
Due to their high melting’s point, allows they used to serve as alloys for porcelain bonded restoration.
Low corrosion properties

8. Examples of high noble dental alloys: JRVT: Alloy Color Yellow Composition 77% Gold, 13% Silver, 8.5% Copper, 1.0% Palladium, Less than 1% Indium, Iridium, Zinc. NOBLE-CAST 67: Alloy Color Yellow Composition 64.0% Gold, 23.4% Silver, 3.0% Palladium, 0.1% Platinum. ENCORE (WHITE HIGH NOBLE): Alloy Color Silver (white) Composition 48% Gold, 40% Palladium, 4.3% Zinc, 3.9% Tin, 3.75% Indium, 0.05% Rhenium.

9. Noble alloys (Semiprecious metal):
25% of noble metals does not require gold:
Silver – Palladium alloy.
Palladium – Silver alloy.
Other type:
Silver – Tin alloy.
These alloys have at least 25% of noble metal content.
PROPERTIES OF NOBLE ALLOYS:
Moderate densities 10 to 12g/cm3
Corrosion resistance slight lower than HN alloy
Cost of these alloys are less than NH alloy
Used for crown or bridges with or without porcelain covering.

10. SILVER-PALLADIUM ALLOYS (Ag-Pd)
Example of Nobel Alloys:
SILVER-PALLADIUM ALLOYS (Ag-Pd)
Developed as an alternative to those based on gold. There are two varieties:
Silver 60-70% Palladium 25-30%
Note: Some containe 10% copper.
Palladium 60% Silver 30%
Note: El 10% , other metals added to facilítate bonding with porcalain.

11. CHEMICAL PROPERTIES OF Ag-Pd:
It is possible to achieve adequate accuracy in casting and a good polishing and acceptable welds.
Pigmentation and resistance to corrosion properly handled.
CLINICAL APPLICATIONS OF Ag-Pd:
Scalling instruments
Crowns
Fixed partial
Denture
Endopostes
Note: It is possible to achieve adequate accuracy casting and a good polishing and acceptable welds.

12. PALLADIUM- SILVER ALLOY (Pd-Ag):
They are an alternative to gold alloy for ceramics. These are alloys of high melting point and which support the firing of ceramics of porcelain. (1400°C)
ADVANTAGES OF Pd-Ag;
Good physical properties and bonding with ceramics.
Biocompatible
DISADVANTAGES OF Pd-Ag;
The likely change in color of porcelain baked.
Expensive

13. SILVER-TIN ALLOY (Ag-Sn):
These alloys have been used for decades in dentistry to make deposits, not being suitable for prosthetic purposes.
INDICATIONS OF Ag-Sn:
Temporary inlays.
ADVANTAGES OF Ag-Sn:
Cheap
Easy handling
Poor precision casting

14. DISADVANTAGES OF Ag-Sn:
Fragile marginal edges.
Pigmentation.
Can not be welded.
Slight corrosion resistance.
After a short clinical use and even before cemented in the mouth have lost shine and later changed to dark.

15. Non-noble (Nonprecious metal):
These alloys are also referred to as base metals. Their noble metal content is less than 25% (they may have none). They often contain large percentages of nickel, cobalt, chromium or beryllium.
PROPERTIES OF BASE METAL ALLOYS:
Extremely high yield strengths and hardness, makes difficult to polish.
Less corrosion resistance.
Less biocompatible .

16. Examples of non-precious (base METAL) dental alloys:
ARGELOY N.P. SUPREME
Alloy Color Silver (white)
Composition 61% Cobalt, 27% Chromium, 6% Molybdenum, 5% Tungsten, 1% Silicon, Less than 1% Manganese, Iron, Carbon.
ARGELOY N.P.
Composition 54.0% Nickel, 22.0% Chromium, 9.0% Molybdenum, 4.0% Iron, 4.0% Niobium, 4.0% Tantalum, 3.0 % Trace elements.

17. Other Classification by Bureau of Standard, 1927
Alloys can also classify into four types according to: their composition, their use, and the amount of stress that they will be subjected to.
Type I (soft-low strength)
Type II (medium- medium strength)
Type III (hard-high strength)
Type IV (extra hard- very high strength)
This is the most prevalent classification, the hardness increases from type I to type IV.
Types I and II alloys are often referred to as inlay alloys. Types III and IV alloys are generally called crown and bridge alloys.

18. Shaping of dental metals and alloys
Shaping of dental metals and alloys for dental use can be accomplished by one of three methods :
Casting: it involves heating the material until it becomes molten, then it can be forced into an investment mould which has been prepared eg: Co-Cr, Ni-Cr, titanium alloys…..etc.
Cold working: it involves mechanical shaping of the metal at relatively low temperature taking the benefits of the high values of ductility and malleability possessed by many metals eg: Type I gold alloys.
Amalgamation: some alloys can be mixed with mercury to form a plastic (sold) mass which gradually hardness by a chemical reaction followed by crystallization. The material is shaped by packing it into tooth cavity eg: dental amalgam.

19. Requirements of the desirable dental casting metals and alloys:
Dental casting metals and alloys must exhibit:
Biocompatibility.
Ease of melting, casting, brazing (or soldering) and finishing.
Little solidification shrinkage.
Minimal reactivity with the mold material.
Good wear resistance.
High strength and sag resistance.
Excellent tarnish and corrosion resistance.

20. Faults in casting: 1. Finning and bubbling:
Finning (formation of fins on the casting surface) is due to cracking of the investment caused by its rapid heating. This fault can be avoided by slow heating of the mold to allow escapement of gases without cracking of the mold.
Bubbling (formation of spheres on the casting surface) is due to the presence of surface porosities in the investment. This fault can be avoided by vacume investing.
These faults increase the time of finishing.

21. 2. Incomplete casting: This type of faults can be avoided by:
Convenient number and diameter of the sprues.
B. The balance between the molten alloy temperature and mold temperature is important to prevent solidification of the alloy before the mold can be properly filled.
C. Sufficient thrust should be created by the rotational speed of the centrifugal casting machines to ensure proper flow of the alloy to all parts of the mold cavity.
D. Gases and steam within the mold that cannot escape cause back pressure which leads to rounded edges of the casting and lacking in details. Therefore the metallic plate support the end of the ring must be perforated to allow escapement of gases and steam.

22. 3. Porosity: (pitting on the casting surface):
This is caused by broken pieces of investment or particles of dirt which have fallen down the sprue. This can be avoided by handling the molds with the sprue downwards.
4. Oversized or undersized castings:
These faults can be overcome by the following:
A. The casting shrinkage of the alloy should be compensated for by the setting expansion, thermal expansion and inversion of the investment.
B. The proper choice of the impression material and impression technique influences the final results.