21: Alloys used in dentistry

Chapter 21 Alloys used in dentistry

Introduction

There is a long history of the use of metals in the mouth. These materials have been demonstrated as being the most durable in the oral environment. One of the earliest metals used was pure gold. Its advantages are:

However, over time, pure gold has been replaced by alloys of gold. The reason for abandoning gold in its pure form is that it is too soft and flexible.

The addition of other metals to gold has produced a series of alloys whose mechanical properties are superior than that of pure gold. Further developments such as the need to have more reactive materials and the inherent cost of gold are other reasons for the production of the range of alloys that are available.

Each group of alloys has been designed for specific purposes and the composition determines the behaviour and reactivity. Dental alloys are usually moulded to specific shapes using the lost wax technique. This means that they must retain their properties despite the fact that they will be heated to a high temperature and the molten material cast into a mould before being allowed to cool. The requirements put considerable demands on the performance of the alloys.

The range of applications for alloys in dentistry is far-ranging:

This chapter describes the alloys used in dentistry together with their methods of manufacture, specifically their application and practical aspects of alloy performance.

For a detailed analysis of the metallurgical features of the dental alloys, the reader should consult a metallurgy text.

Alloys

Alloying is the addition of one or more metallic elements to the primary or matrix metal. The incorporation of these additional metals alters and frequently enhances the mechanical properties of the alloy. These properties may well vary substantially from the component metals.

Alloys may be referred to as being binary, ternary or quaternary. This means they have two, three or four metallic constituents, respectively (compare with amalgam; see Chapter 6).

Structure of alloys

Alloys are essentially crystalline in structure. The crystals that initially form then grow towards each other until they touch. This is similar to how ice crystals form. At the point where the crystals touch, the water is fully frozen. In the same way, the metallic crystals grow as the alloy cools (Figure 21.1). The arrangement of the crystals depends on the size of the atoms of the various constituent metals. If these are similar, then atoms of one constituent can replace those of another. If one metal’s atoms are much smaller, they may be trapped between the larger atoms, filling the interstitial space between the crystals.

The crystalline structure consists of crystals or grains abutting one another. The boundaries between the grains are referred to as grain boundaries (Figure 21.2). The size of the grains determines the properties of the alloy. The smaller the grains the better, as more boundaries prevent dislocations in the structure. To achieve this, some elements such as iridium or ruthenium may be added to dental alloys, particularly gold-based alloys, to reduce the grain size. These elements are called grain refiners.

General mechanical properties

Effect of heat on alloys

As alloys are composed of several individual metals, they have a melting range. When an alloy is cooled, some of it will continue to be in the liquid phase while other parts will start to solidify. The converse is also true, in that when the alloy is heated, some parts of the alloy will become molten first. The temperature at which the alloy liquefies on heating is called the liquidus, and the solidus is the temperature at which it becomes a solid again.

One of the most commonly used fabrication techniques for dental restorations is casting. This process is described later in the chapter but essentially an ingot of alloy is heated to above its liquidus and thrown into a mould of the restoration to be constructed. It is important that the dental technician knows the liquidus temperature of an alloy as it must be heated above this point to cast properly. The liquidus temperature determines both the casting temperature and choice of investment material. The dental technician must also know the solidus of the alloy. This is of particular significance when working with a ceramic bonding alloy, as it must be heated to a high temperature so that ceramic may be fired onto it. Clearly it must not be heated near to a point where it starts to become a liquid.

Heat treatments are often utilized in dental technology to enhance the alloy performance. This is described in more detail later in the chapter. However there is a potential disadvantage to this technique. Heating and reheating of the alloy may be necessary during the multiple firings required to add ceramic to the metal substructure. This may be detrimental for the properties of the alloy, particularly with base metal alloys. A good example of this is stainless steel which becomes very ductile and loses its strength when it is heated.

Biocompatibility

It is obvious that metal alloys which are used in the mouth must be resistant to corrosion and tarnish. Clinically this may manifest as an unpleasant metallic taste, irritation or allergy. Nickel is added to some base metal alloys and is responsible for a hypersensitive reaction in approximately 12% of females and 7% of males worldwide. Clearly alloys containing a known allergen should be avoided in patients sensitive to it.

Economic considerations

Inevitably cost is a consideration when the raw materials are expensive, for example precious metals such as gold. As these elements are traded in the world markets, their prices may fluctuate widely as their value mirrors financial and political global events. Gold is a very safe commodity and in times of economic hardship it is often purchased. In a world of supply and demand, such purchasing practices force the price to rise. This is also true for other commodities. Before the advert of catalytic converters, when the price of gold was high, other elements were being used in dental alloys. One such element was palladium; however, all Japanese car manufactures now require this element to make catalytic converters for engines designed for using lead-free fuel. Russia as the major producer of palladium was able to push its price up to reflect demand. The consequence for dentistry in both examples was that the price of dental alloys increased and therefore the cost of the final restoration. Many laboratories charge the dentist by the weight of the metal plus a fee for the construction of the restoration; other laboratories charge a flat fee irrespective of the metal price. The latter approach may significantly decrease the profit margin of the laboratory when metal prices rise. Dentists working outwith a third party (such as an insurance company or the National Health Service (NHS) in the UK) may be advised to charge the patient the laboratory fee plus a fee for the clinical time so that their profit margin is not affected by fluctuations in the market.

Types of alloy

The metals used in dental alloys may be divided into two categories: noble and base metals. Examples of noble metals are gold, platinum, rhodium, ruthenium, iridium and osmium. Such elements are good for dental use as they are resistant to corrosion in the hostile environment of the mouth. From a chemistry perspective, silver is a noble metal but as far as dentistry is concerned it is not considered so because it corrodes in the mouth. These preceding elements are sometimes referred to as precious metals as they tend to be expensive. This term can be confusing as it does not refer solely to cost and therefore should be used carefully. Equally it does not mean noble as in noble elements, as silver and palladium are not dental precious metals. The term is more descriptive of the physical properties of the alloy. Nobility of the alloy depends on the sum of the amount of noble elements contained in it. The American Dental Association has defined alloys as high noble, noble and base metal alloys (Table 21.1).

Table 21.1 Definition of high noble, noble and base metal alloys according to percentage of noble metals present

Type of alloy Noble metal content
High noble Contains at least 40% by weight gold and at least 60% by weight of the noble metal elements (gold, iridium, osmium, platinum, rhodium)
Noble Contains more than or equivalent of 25% by weight noble metals
Base metal Contains less than 25% by weight of noble meals

Alloys may also be categorized by their major component, for example, a gold-based alloy. They may also be described by their appearance such as yellow or white. White gold alloys are not, of course, white but silver in appearance.

Base metals refer to metals which are not noble, e.g. titanium, nickel, copper, silver and zinc. These elements corrode more than noble alloys but are alloyed with noble metals as they have a significant effect on the properties of the alloy, such as increasing strength, decreasing flexibility and increasing wear resistance of the alloy.

Casting alloys for tooth restorations

Some cast restorations such as inlays, onlays, some crowns and bridges are composed solely of metal (Figure 21.3). The vast majority of these restorations are constructed out of noble alloys but in certain situations the clinician may prescribe the use of a base metal alloy. Both these types of alloy may also be used for bonding to dental ceramic to construct tooth-coloured restorations. To optimize the union between the alloy and ceramic, the constituents of these alloys may be varied (see later).

Bonding gold alloys to tooth tissue

Restorations constructed out of gold alloys are usually luted into or onto the preparation. Gold alloy itself has no inherent ability to chemically bond to tooth tissue. However, it may be treated so that it can bond to tooth tissue with the use of an adhesive resin-based cement. If the gold alloy contains more than 16% copper, it may be heat treated by putting it in the furnace at 400 °C for 9 minutes. This forms a surface oxide layer of copper oxide, to which the resin based adhesive may bond (Figure 21.5).

This phenomenon is advantageous as it allows the dentist to bond such restorations as gold veneers or onlays on to tooth tissue particularly where little or no mechanical retention exists. The dentist should specifically and clearly request this treatment on the laboratory prescription form if a bonding technique is going to being employed. Additional, albeit limited, micromechanical retention may be gained by sandblasting the fitting surface of the gold alloy. Both these techniques may be combined to provide the most secure method. In this case, the fitting surface is firstly sandblasted followed by the heat treatment prior to dispatch to the clinic.

Alternative metal alloys used for metal crowns

The more commonly used alternatives to gold alloys are the silver alloys. Cast base metal alloys are infrequently used to construct all-metal restorations unless cost is a very significant factor. Base metal alloys are more commonly used in the construction of resin-retained bridges and as bonding alloys.

Properties

Silver alloys have a major disadvantage in that they tarnish and corrode. They have variable properties and care must be taken in the selection as some are quite ductile and are unsuitable for use in load-bearing areas of the mouth.

Base metal alloys tend to have larger grain sizes and do not include grain refiners. They are stronger than the noble alloys. Additionally, they are also harder and their ductility is reduced. This means that they may be used in a thinner section and still possess sufficient strength for function. These alloys may be used in a thickness as low as 0.3 mm. The increased hardness of base metal alloys also imparts greater wear resistance, but it can lead to potential wear of opposing tooth tissue.

Base metal alloys are harder to adjust, finish and polish due to their hardness and lack of ductility. Many dental technicians sandblast the casting to remove any residual investment material and the green oxide layer. This may help to reduce the surface roughness. Electrolytic polishing may be used in preference to polishing and finishing these alloys by traditional means (see Chapter 19). However, many technicians believe that base metal alloys may be finished as well as noble alloys even though it takes longer to achieve and requires more work!

Commercially available products

Table 21.4 show some commonly used casting alloys currently available on the market. It is clear from Table 21.4 that alloys of different composition can have similar melting ranges and casting temperatures. Care needs to be exercised in their selection. It is wise to establish a dialogue between dentist and technician so that the dental team can determine which alloy should be used in any particular case.

Alloys are usually supplied to the dental technician as ingots (Figure 21.7).

Jan 31, 2015 | Posted by in Dental Materials | Comments Off on 21: Alloys used in dentistry
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