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Dental Material II lecture # 11

Our exam 15th of November, the topics that will be included in the exam are: Amalgam, Composite, GIC, Adhesions and bonding agents. So this lecture won't be included in the Midterm exam. Now there will be a lecture today and on Wednesday, but next Monday before the exam there will be no lecture.

Metals in Dentistry (1)

we will talk today about Metals that are used in dentistry, now pure metals aren't commonly used in dentistry, for example we don’t use Copper on its own or Silver on its own… usually a group of metals are combined together; they are mixed together in certain amounts because the properties could be much better rather than using a pure metal on its own.

Alloy:

An alloy means a mixture of two or more metals. Pure metals are rarely used in dentistry because they are too weak.

Different alloys are available, can be used for restorations that are inside the crown; intra- coronal, or extra- coronal like crowns and bridges or partial dentures. Cast metal alloys are produced using the lost wax technique; that's where we carve the restoration wax, and then the wax is melted away and replaced by the metal.

A metal frame work of a partial denture made from a casting wax, you remember the casting wax, it comes into different shapes, you attach them into your cast or model, the wax is melted away and replaced by the metal to make a metal frame work, now to this metal frame work teeth are attached using wax and this wax would be replaced by acrylic resins, so we will have a Chrome- Cobalt partial denture, a partial denture that is made partially from metals.

Some partial dentures is made from acrylic, but they are weaker than these metal reinforced partial dentures because metal is stronger. And this is an example of a bridge that is made from metal (slide 4).

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Classification of dental casting alloys:

Several alloys are available; they can be classified into 3 categories, depending on the amount of gold:

1- High noble alloys:

Gold is a noble metal (نبيل) and some metals are precious metals ( like Silver; so because silver corrodes it's precious but not (ثمينnoble.

High noble metals do not corrode, so gold is corrosion resistant.

Examples of combinations:

Gold and platinum

Gold and palladium

Gold with copper and silver

Now in these high noble alloys at least 60% of the combination should be made from a noble metal; noble metals are gold, platinum and palladium, these three are high noble metals. 40% of this 60% needs to be made of gold and the rest of the alloy (40%) can be made of other types of metal (base metals) like silver, copper, nickel, titanium… it doesn't matter but 60% of the alloy should be made of noble metal.

2- Noble alloys:

Available in different combination, 25% of it at least should be made of noble metals and the rest is base metal alloys (silver, copper, nickel…). In noble alloys there is no specification for the amount of gold just to be 25% noble metals.

3- Base metal alloys:

The amount of any noble metal is less than 25% (no gold specific requirements).

Changing in the amount of noble metals in different alloys makes different properties:

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Gold Alloys:

Pure gold is 24 karat.

Gold alloys can be classified by different ways:

** Hardness: resistance to penetration

** Malleability: the ability to be shaped by tapping

** Ductility: the ability to elongate

Gold crowns used to be very common, they produce a nice surface finish and polish, they don't corrode, and they are highly biocompatible, but because of esthetic reasons (aren't very esthetic) they have been replaced by porcelain; because it's more esthetic and is available in shades like composite for example, which is a highly biocompatible material and it does not corrode.

Another high noble metal, platinum; the problem with it that it's too expensive, have a high melting temperature, and it doesn’t mix easily with gold, so it's not used commonly.

Palladium is another alternative, but nowadays it's becoming more expensive so they go back to platinum. Palladium provides a good corrosion resistance and it improves the hardness of the alloy, which is good because a hard alloy is a strong alloy.

Silver as I said, it's precious and is added to different alloys to make them harder, but the problem of silver is corrosion.

Base metal alloys:

less than 25% is made from noble metals, so mostly more than 75% of it made from a combination of these metals (silver, copper, nickel, tin, zinc…) each type of metal adds a different property to the mixture, ex; Copper and Silver improves hardness of the alloy when you add them to a mixture of metals, Zinc (reducing agent) prevents oxidation that leads to weakness of the alloy.

Usually these base metal alloys are stiffer and harder than gold alloys, and have higher resistance to stress, so any mixture with

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high percentage of gold the flexibility would be higher and its hardness is less.

Drawbacks and disadvantages of base metal alloys:

1- Less biocompatible than noble metals; some of them can cause an allergic response or irritation

2- They are difficult to finish and cut, so the surface finish is not as good as gold

3- They need more equipment to make them (special machines used for mixing).

4- Melting temperature is much higher than gold alloys.

But still they are used commonly!!

Crystal formation:

When we use these metals to form a crown or a bridge or a metal framework, we need to melt it, then it will go inside the mould (the investment material), and it will be shaped like a crown or a metal framework. It starts as a liquid metal and then it will cool down and becomes solid by forming crystals (crystallization).

If these crystals are small in size; the surface finish and the properties of the metal are better, so this is related to the internal structure of the metal.

Annealing:

Some alloys can be heated, cooled down, reheated, and cooled down again; this will cause rearrangement of the crystals and structure inside, which will produce better properties. This is used specially for gold alloys, but they don't do this process for base metal alloys because they become weaker.

Porcelain bonded alloys:

One of the applications of metals in dentistry is making porcelain fused to metal crowns, so we'll have crown and bridges that are made from both metal and on top of it we have porcelain. Metal provides strength, and porcelain provides esthetics. These metals need to have certain properties; they need to have a

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surface layer that is able to bond with porcelain which is called oxide layer, and to have this oxide layer they need to add small amount of indium and tin. التاج معادن إلى المعادن هذه اضافة مع اذن

البورسلين بين رابطا تشكل التي السطح على مؤكسدة طبقة لدينا تتشكل.والمعدن

Porcelain and metal are two different materials, so in the oral cavity they will be subjected to changes in temperature; so they need to be able to contract and expand at similar rates to prevent stress and incidence to fracture by having similar coefficient of thermal expansion.

Metals under porcelain need to withstand high temperatures, because when you make the metal crown and you want to start adding porcelain which is added layer by layer, each layer you add you need to place your crown in an oven at very high temperature (up to 1350˚C) and then you take it out and add another layer… so the metal should be able to handle very high temperature without being deformed.

This is an example of a porcelain fused to metal bridge (slide 12), you can see the inside of the crown is made from metal and the outer layer about 1mm thick is porcelain, usually these restorations have 1.5-2 mm in thickness specially labially; to accommodate the thickness of the metal and on top of it thickness of porcelain. Metal can be 0.5 mm thick it will be strong enough, but porcelain because it's brittle it needs to be at least 1 mm thick to be strong enough and also supported by metal (no metal .. thicker porcelain).

Usually when these restorations are made, you got metal underneath and you need to cover the color of this metal, you don’t want it to show through porcelain; it won't be esthetic. So when they add porcelain, they add an opaque layer of porcelain to mask the color of metal, then another layer of porcelain that shades like dentine, then a layer that is colored like enamel, and then they can add stains that it will be similar to natural teeth( like building up a tooth from zero).

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Metals that are used for removal partial dentures (metal frame works):

Can be available in different combinations, usually the most common are Cobalt-Chromium combinations and they also add Nickel.

This is a presentation of metal frame work (slide 15), how is it retained?

Some of them are retained by clasps (مشابك), and some of them are retained by these pieces that are called precision attachment; they are like a key and a lock, so part of it is attached to the tooth and part of it is attached to the partial denture, and when you put it in the patient mouth it will click and cause retention (not easily displaced).

Biocompatibility:

Most biocompatible metal is gold.

If the metal starts to cause irritation or allergy the biocompatibility drops. So if the metal starts to react; by releasing components, by corrosion, by breaking down it will become less biocompatible. Ex. Nickel; some people (9-12%) especially women suffer from allergy to nickel (any surface that comes in contact with it like skin or oral mucosa...), so we have to use something else.

Also Beryllium can cause allergy, Beryllium is usually added into Nickel-Chromium alloys to improve their properties. Also if you inhale droplets during finishing and polishing it can cause lung disease called Berylliosis.

Solders:

This is a child who had one of his primary teeth extracted (slide 19), to maintain the space for the permanent teeth to come out, we need what is called a space maintainer; this maintainer is made by covering one of the teeth with a crown and attaching a wire that will be in touch with the tooth in front, this space will be maintained until a permanent tooth appears.

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Now to attach the crown with the wire we need to fuse them together using an alloy in-between; this alloy can be made based on silver or gold, this is called a solder alloy.

Solders that are based on gold have different applications; usually used to join bridge components together, or to add contact area to the bridge, or if there is any deficiency on the margin of the crown it can be corrected by gold solder because they do not corrode.

Silver solders are used for space maintainer for Pediatrics and in Orthodontics also to fuse components together.

Welding:

Some alloys are used in similar applications which are called Welding (اللحام), in solders we are fixing two components by adding a third alloy in the middle, but in welding it's not necessarily so, we don't always need to place a third alloy in the middle, we can melt both metals and join them together, this joint or welded point is susceptible to corrosion.

In dentistry we use soldering.

Now solder alloys need to have a certain melting temperature that is not too high, it shouldn't be higher than the crown or the wire, otherwise when you apply it you might melt the wire or the crown and change their shape.

Question: why do we use different alloy components for the solders?

Because the wire and the crown have different components; that means if we use a solder with components similar to the crown it might damage the wire during melting and vice versa.

Wrought metal alloys:

Wrought means something that has been shaped differently. Ex. Metal alloys that have been shaped in forms of wires like orthodontics wires (slide 23). Another way of shaping metals is to form files (slide 25) that are used in root canal treatment, whether they are engine driven or hand instruments.

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So wrought metal alloys have mechanically changed into different forms and shapes (wires, files, plates…). So this alloy once the shape has been changed, usually it's harder and has higher yield strength; it is the point after which the metal will permanently deform. So these wrought metal alloys are harder because they have been melted and then mechanically changed into different shapes, and have higher degrees of resistance, so it takes a lot of force to become permanently deformed.

Orthodontics wire can be made from stainless steel, or noble metal alloys, used in orthodontics appliances and used to make of space maintainers

Stainless steel made up from chromium, nickel, iron, carbon in different combinations, it will resist tarnish and rust.

Endodontic files:

can be made from stainless steel, combination of different metal or from nickel- titanium which makes them more flexible, so nickel titanium files are much more flexible than the stainless steel files that’s why they don't break as much.

Reamers:

Are similar to files, but the number of twists are less, so they don't cut as effectively as files, so files and reamers whether they are used by hand or placed in a flow hand piece, they are examples of metal alloys that have been changed mechanically into different shapes.

Good luck

Sorry for any mistakes, I did it very quickly!

Written and summarized by: Ward Abu Nassar

31.10.2011

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