Dental Alloys. Definition of a Dental Alloy. Properties needed in a Dental Alloy. Why Alloys are used in Dentistry

Transcription

1 Dental Alloys

2 Dental Alloys Definition of a Dental Alloy Properties needed in a Dental Alloy Why Alloys are used in Dentistry Properties provided by the metal elements in Dental Alloys Classifications of Dental Alloys

3 Definition of a Dental Alloy

4 Definition of a Dental Alloy Dental alloys are made from a mixture of metals. There may be up to 10 different metals in the alloy. These metal components and their % composition determine the mechanical properties of the alloy. Alloys are designed to have specific properties to suit their special purpose. The alloys can be divided into categories depending on their composition, colour, hardness and purpose.

5 Definition of a Dental Alloy Alloys are composed of Noble metals and Base metals. Noble metals are defined as metals which are resistant to corrosion under extreme conditions. They are also expensive. The Base metals are not resistant to corrosion under extreme conditions and are usually inexpensive.

6 Definition of a Dental Alloy The Noble metals can also be called Precious. Silver, although precious, has a tendency to corrode under extreme conditions. Therefore, in dentistry Silver is not classed as a NOBLE metal or a precious metal. The Base metals can also be called Non-Precious.

7 Definition of a Dental Alloy The Noble metals are Gold Palladium Platinum

8 Definition of a Dental Alloy The Base metals are Nickel Copper Silver Zinc Iron Tin Ni Cu Ag Zn Fe Sn

9 Definition of a Dental Alloy In dentistry, the Noble metals are Precious and the Base metals are called Non-Precious.

10 Definition of a Dental Alloy Composition The main metal elements used in dental alloys are: Gold Palladium Platinum Au Pd Pt

11 Definition of a Dental Alloy Silver Iridium Iron Tin Zinc Ag Ir Fe Sn Zn

12 Definition of a Dental Alloy Colour The colour of dental alloys can be white, yellow or deep yellow.

13 Definition of a Dental Alloy Hardness and Purpose Dental alloys can be soft or hard depending on their purpose. The purpose of an alloy could be to make a FGC or to make a PFM bridge. Many types of Dental alloys have been developed to provide different properties and strengths. Eg. A gold inlay needs less strength than a 3 unit gold bridge. A PFM alloy needs to form an oxide layer where as a FGC does not need this ability.

14 Properties needed in a Dental Alloy

15 Properties needed in a Dental Alloy Biocompatible - non allergic Corrosion Resistance Tarnish Resistance Non toxic - when grinding and finishing Suitable mechanical properties High yield stress - withstand forces of mastication Ductility able to be burnished without fracturing Hardness able to grind and polish easily

16 Properties needed in a Dental Alloy Good castability - flow well when molten Easy to solder Colour Cost economical/inexpensive

17 Why Alloys are used in Dentistry

18 Why Alloys are used in Dentistry Alloys are used in dentistry because Pure Gold is too soft and ductile. Pure Gold seems the perfect metal as it is biocompatible, corrosion and tarnish resistant, ductile, easy to cast, grind, polish and solder.

19 Why an Alloy is used in Dentistry However, Gold does not have the necessary property of high yield strength and hardness so as to withstand the forces of mastication.

20 Why an Alloy is used in Dentistry Pure Gold metal can distort, which makes it useless as a dental appliance. When Gold is mixed or alloyed with other metals, the Gold alloy becomes harder and stronger.

21 Why an Alloy is used in Dentistry Gold-based alloys are the perfect material for dental crown & bridge restorations as they provide sufficient strength and corrosion resistance. All the Noble metals, when alloyed together with small amounts of base metals, such as copper, tin, iridium, iron and zinc produce alloys with favourable chemical and mechanical properties for their specific dental application. In other words, Base metals are added to improve strength, flexibility and wear properties in dental alloys.

22 Properties provided by Different Elements

23 Properties provided by Different Noble Metals Elements Gold provides good tarnish resistance and ductility. Platinum hardens and strengthens gold the most. Increases tarnish and corrosion resistance. Melting temperature is dramatically increased therefore content is only 3-4%. Palladium - similar effects as platinum but less expensive. Often added instead of platinum.

24 Properties provided by Different Elements Base Metals Silver a neutral effect. Tends to whiten alloy, may enrich the yellow colour by neutralizing the red colour from added copper. A high silver content in PFM alloys although may cause porcelain discolouration due to the silver oxide formation during porcelain firing. Copper increases strength and hardens the alloy. 8% or more enables alloy to be heat-treated. 12% or greater reduces tarnish resistance.

25 Properties provided by Different Elements Base Metals Zinc acts as a scavenger combining with excess oxygen during melting of the alloy to form zinc oxide. Found in most alloys. Iridium small amounts reduce the grain size of the alloy making it more ductile, stronger and easier to polish. Iron and Tin small amounts are used for hardening in PFM alloys. They promote ceramic bonding by forming stable oxides on the alloy surface.

26 Types of Dental Alloys

27 Types of Dental Alloys The classification called Types of Dental alloys is based on the mechanical property of hardness. High content Gold alloys are the softest and the high content base metal alloys are the hardest. Type I Soft (VHN 60 to 90) Type II Medium (VHN 90 to 120) Type III Hard (VHN 120 to 150) Type IV Extra hard (VHN 220 +) also when quenched min. VHN 150

28 Types of Dental Alloys Type I Soft Used in situations where alloy is not subjected to great stress. Class III and Class V cavities. Type II Medium Used for most types of inlays where burnishability is more important than strength. Type III Hard Has largely replaced type I and II in dentistry. For inlays, crowns and bridges where great stress is involved Type IV Extra hard Used for long span bridgework, post cores cast partial dentures, clasps and attachments.

29 Classifications of Dental Alloys

30 Classifications of Dental Alloys There are two main classification systems to categorise dental alloys. One is based on the Noble metal content and the second system is based on the Gold content within the alloys.

31 Classifications of Dental Alloys 1. Classification based on the Noble alloy content The American Dental Association (ADA) currently classifies dental alloys into three groups depending on their noble metal content. The three groups are: High Noble Alloys Noble Alloys Base Metal Alloys

32 Classifications of Dental Alloys This newer classification is more inclusive than the older system which categorises on gold content only. Alloys now can be based on Palladium, Silver or Nickel.

33 Classifications of Dental Alloys ADA System for Classification of Dental Alloys Alloy Classification Gold Content (wgt%) Noble Metal Content (wgt%) High Noble Noble Base Metal No Requirement No Requirement 25 <25

34 Classification of Dental Alloys High Noble Alloys These alloys contain 40% gold content or They can contain 60% combined content of gold, palladium and platinum They have the highest density of all the alloys, are the most expensive and the most biocompatible. Also they are the softest alloys due to the high gold content.

35 Classification of Dental Alloys Noble Alloys These alloys have a noble metal content 25% and this does not have to include Gold. The main incentive for their use is they are less expensive and more economical.

36 Classification of Dental Alloys Noble Alloys cont d These alloys form a diverse group and can be Gold-based Palladium-based or Silver-based (This group contains just enough palladium(25%) to be included as a noble alloy)

37 Classification of Dental Alloys Noble Alloys cont d This group have moderate densities making them lighter than the high-noble alloys which is an advantage when making bridgework. They have a higher or equal yield strength and hardness to high-noble alloys. This is attributed to the inclusion of palladium in the alloy.

38 Classification of Dental Alloys Noble Alloys cont d The silver content in an alloy increases the ductility but lowers the hardness and corrosion resistance. Now noble alloys are classed on the total content of the noble alloys present.

39 Classification of Dental Alloys Base Metal Alloys Base metal alloys consist mainly of base metals. They are based on Nickel, Cobalt or Titanium and have a noble metal content of <25%. Base metal alloys have the lowest density and are the hardest.

40 Classification of Dental Alloys Base Metal Alloys The term base metal alloy is different to the term a metal-based alloy. A base metal alloy is made of base metals whereas a metal based alloy describes the metal which makes up the main % composition of the alloy. eg. A gold-based alloy, a nickel-based alloy. Gold is not a base metal but you can have a gold-based alloy. Nickel is a base metal and in this case it is the main metal in a nickel-based alloy.

41 2. Classification based on the Gold Content The three groups are: High Gold Alloys Low Gold Alloys High Palladium Alloys High Gold Alloys 70% of mainly gold with palladium and platinum content. These are similar to the High Noble alloys. Due to their high gold content these alloys can range from soft (type I) to hard (type IV)

42 Low Gold Alloys Composed mainly of gold, silver and copper with very little palladium or platinum. Gold content usually 45 60%. Main incentive for use is they are more economical. Mechanical properties are similar to Type III alloys. Suitable for FGC-full gold crown but rarely for inlay due to low ductility.

43 High Palladium Alloys Composed mainly of palladium. Mechanical properties similar to Type III alloys. However the silver content increases ductility and lowers hardness and corrosion resistance. Can be used as PFM alloy. A high silver content may cause porcelain discolouration due to silver oxide formation during porcelain firing.

44 PFM Alloys Used as the substructure for porcelain restorations. Special properties are required. They must be able to : 1.Maintain strength at the high temperatures used in porcelain application 2.Provide an oxide layer that will form a strong bond with porcelain. 3.Cast into thin sections. 4.Provide high yield strength. 5.Have a similar coefficient of thermal expansion to that of the porcelain being applied.

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