Chapter 21 Alloys used in dentistry
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:
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.
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.
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.
Fig. 21.2 Microstructure of (A) a solid alloy of iron, zinc and boron and (B) a titanium, aluminium, molybdenum, vanadium and chromium alloy (VT22) after quenching. Note the grains and their junctions (grain boundaries).
(A) Modern Research and Educational Topics in Microscopy. Méndez-Vilas and J. Díaz. (B) commons.wikimedia.org/w/index.php?title=User:Edward_Pleshakov&action=edit&redlink=1).
General mechanical properties
Clearly, one of the many advantages of metal alloys is that they are strong and able to withstand forces during function without permanent deformation. This allows restorations to be constructed in thin sections, which in the mouth is advantageous as tooth tissue may be conserved by minimal tooth preparation.
• Yield strength is the force per unit area (stress) required to permanently deform the alloy. Exceeding the yield strength is clearly undesirable for dental applications. Yield strength is therefore a property used to describe the behaviour of an alloy. It is measured in mega pascals.
• The yield point is defined in as the stress at which a material begins to deform plastically. Before the yield point the material will deform elastically returning to its original shape when the stress is removed. Once the yield point is passed a proportion of the deformation will be permanent and irreversible.
• Related to yield strength is hardness which increases as yield strength increased. This gives the dentist and dental technician an indication of the difficulty to grind and polish an alloy. Some metal alloys may be heat treated to increase their hardness.
• Ductility is the ability of an alloy to deform under tensile stress. This is important when clasps require to be bent and inlays burnished to enhance their fit and marginal adaptation. The stiffness of the alloy is determined by its elastic modulus and the design of the casting.
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.
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.
Tarnish: a thin layer of corrosion forming on the surface of metals such as copper, brass, silver, aluminium and other similar metals as a result of the surface undergoing a chemical reaction. Tarnish is not necessarily the sole result of contact with oxygen in air. Silver needs hydrogen sulphide in order to tarnish. Tarnish appears as a dull, grey or black film or coating over metal. It is a self-limiting surface phenomenon unlike rust. The outer layer of the metal reacts and the tarnish coating seals and protects the underlying layers from further reaction.
At least 10% of the population is sensitive to nickel and patients should be asked about it when taking the medical history. Females appear to be more prone to hypersensitivity reactions with nickel and this may be attributable to its extensive use in costume jewellery. Any patient with a history of hypersensitivity to nickel or other metallic elements should be prescribed alloys which are free of the allergen. Noble metal alloys are more likely to be biocompatible than base metal alloys because they are inert.
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).
|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).
High noble and noble alloys
The vast majority of noble alloys are based on gold (Box 21.1). As mentioned earlier, pure gold is too soft to be used alone in dentistry and to achieve adequate mechanical properties it must be alloyed with other elements (see Table 21.2). The four types of gold casting alloy used in dentistry are summarized in Table 21.3.
Box 21.1 Measuring gold content
Addition of copper: order hardening
The increase in hardness is accompanied by a decrease in ductility and corrosion resistance. The element mainly responsible for this is copper. Copper conveys order hardening to the alloy. This is where the copper atoms form ordered clusters instead of being randomly distributed within the alloy. This ordered atomic structure prevents movement or slippage of the layers of atoms. For this phenomenon to occur the alloy must contain at least 11% copper and so some effect will be seen in type III gold alloys although it is seen more so with type IV. The amount of copper added works only up to a point as the alloy will tarnish if it contains more than 16% copper. Order hardening may be achieved by heating the alloy to 400 °C and holding it in the furnace at this temperature for 30 minutes.
Platinum and palladium have similar effects on the properties of the final gold alloy. Their inclusion in the alloy leads to a higher melting point. This may be advantageous if the alloy requires to be soldered at some point, for example to join bridge components together if the technician is concerned that a large casting may not be dimensionally accurate enough if cast as one unit.
Zinc is included as a scavenger of oxygen as it will preferentially react with oxygen so preventing oxidation of the other components. It is relatively reactive and pure zinc will take up oxygen to passivate the surface. It is included in noble metal alloys for the same reason as in dental amalgam (see Chapter 6).
• Gold alloys are very strong in thin section. This means that the dentist may consider providing a gold restoration where there is little interocclusal clearance. More tooth tissue may be conserved as it need not be sacrificed in favour of accommodating the dental material. The minimum thickness of a gold alloy should be 1 mm and 1.5 mm over a functional cusp.
• Gold alloys are dimensionally very accurate as little change occurs in this respect during their construction using the lost wax technique. This minimizes chairside time as less adjustment should be required at the fit appointment.
• If any adjustment is required at the chairside, gold alloys may be relatively easily polished by the dentist prior to fitting. Unlike ceramic, the gold restoration does not need to be returned to the dental laboratory to be finished should any chairside adjustment be required.
• The patient may elect to have a gold restoration for a variety of reasons: the use of gold to restore anterior teeth is more popular in some cultures, or on the recommendation of their dentist for one or more of the reasons listed above.
• The primary dental disease should be under control and stable, that is the patient’s caries rate/risk must be low and their oral hygiene good. Therefore those patients who have a high caries rate and are unable (or unwilling) to maintain a good level of oral hygiene are unsuitable for gold alloy restorations.
• Gold alloy restorations may be contraindicated in some patients on grounds of cost. To have a gold restoration prepared, constructed and fitted requires a minimum of two surgery appointments and a laboratory bill. The price of gold, even at a low level, can be considerable.
• Many patients decline gold restorations as they do not like the appearance of gold and may prefer a tooth-coloured restoration. This problem can be overcome by sandblasting the ‘polished’ surface of the gold, which has the effect of decreasing the shine or ‘glint’ of the gold. This may be a satisfactory solution for some patients (Figure 21.4).
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).
Fig. 21.5 The fitting surface of a gold onlay which has been heat treated so that the restoration may be bonded onto the tooth surface with the use of a resin-based adhesive cement. Note the darkened surface of the gold alloy, which is now rich in copper oxide and which permits chemical bonding.
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.
When laboratory work is returned to the dental surgery, it will be contaminated with bacteria. It is therefore important that the appropriate disinfection regime is followed prior to trying in of the prosthesis in the mouth of the patient. All metal and metal-ceramic restorations may be placed in the autoclave and subjected to a normal cycle. This will have no detrimental effect on any surface oxide layer created on gold or non-precious metalwork. For wax and plastics and other low melting point materials, alternative means of disinfection such as immersion in a cold sterilization solution should be considered. However, note that pre-silanated ceramic restorations cannot be disinfected by heating as this will break down the silane layer, compromising the bond gained between the ceramic and the resin cement.
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.
Chemical constituents of alternative metal alloys and their functions
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!
If the metal surface of an indirect restoration requires adjustment, measure the thickness of the metal to be adjusted prior to making the adjustment by using an Iwannson gauge. This will prevent inadvertent perforation of the surface being adjusted (Figure 21.6).
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.