Ceramics and Porcelain Fused to Metal (PFM)	97

prefers to avoid the expense of buying the equipment, the whole process can be carried out in specially licensed laboratories.

One advantage of CAD–CAM systems is that the ceramic manufacturing processes including forming and heat treating are under the control of the manufacturer and are taken away from the dental laboratory, maximizing the physical properties of the ceramic itself. Cementation of CAD– CAM restorations normally involves the use of dual cured composite lutes. The lute is potentially the weak link in the restoration as it is much softer than either the ceramic restoration or the tooth enamel (i.e. 20 VHN compared with >300 VHN). Lute margins surrounding CAD–CAM restorations are typically 60–150 μm wide, but are sometimes reported to be even wider. The lute can undergo rapid wear to a depth of about half the value of the width. It is therefore not unusual to find a 50 μm deep defect surrounding the restoration. Improvements in marginal fit which will come as impression and milling techniques develop will help to overcome this problem.

11.10 Porcelain veneers

Porcelain veneers offer a means of improving the appearance of stained or discoloured teeth. The veneer consists of a thin shell-like structure which is ideally fabricated in such a way that it can be closely adapted to the prepared tooth. There is some controversy as to whether the veneers can be attached to unprepared teeth – a technique which would obviously conserve sound tooth substance, or whether some reduction in the tooth contour is necessary. Most authorities do advise the removal of about 0.5 mm of labial enamel.

The veneers which are normally 0.5–0.8 mm thick, may be constructed from feldspathic porcelain, glass ceramic, pressed ceramic or CAD–CAM techniques and are bonded to the tooth enamel using a composite resin luting agents. The bonding is achieved by etching the enamel with a phosphoric acid solution or gel. The fitting surface of the veneer is etched with a solution of hydrofluoric acid, then dried and treated with a silane coupling agent to aid bonding to the composite resin.

The appearance of the veneered tooth depends on the colour of the underlying tooth structure, the aesthetic qualities of the ceramic and the use of the correct shade of luting composite which may be required to mask any discoloration in the underlying tooth and give a natural appearance.

The use of a light activated luting composite is normal. These materials offer the advantage of an extended working time during which the veneer can be placed accurately.

An accurate assessment of the shade of a porcelain veneer cannot be made at trial without ‘coupling’ the veneer to the underlying tooth. This process involves optically linking the veneer to the underlying tooth to see what effect the colour of the tooth has on the finished restoration. In its simplest form this can be achieved using water, but a better alternative is a water-soluble trial paste. This commercial product has similar colour characteristics to the luting resin but can be washed off the veneer surface with water prior to luting the veneer in place.

In some respects the apparent clinical success which has been achieved with porcelain veneers is surprising. The technique involves the support of a very thin, rigid and brittle material, the veneer, by a more flexible material, the luting composite. It would be expected that cracking of the ceramic would be a frequent occurrence under such circumstances. The fact that this does not appear to be a major problem suggests that stresses generated are not great enough to cause a strain of 0.1%, the critical strain above which most ceramic materials will fracture.

Alternatives to the use of porcelain veneers involve the use of pre-formed acrylic veneers or polishable composite resin veneers. The ceramic materials have the advantage of being more durable, probably related to their greater hardness (see Table 2.2). The technique of fabrication is however more time consuming than the direct method of using a veneering composite. The preformed acrylic veneers seem to offer few advantages. They combine an involved clinical and laboratory technique with poor durability.

11.11 Porcelain fused to metal (PFM)

Porcelain fused to metal restorations involve a marrying of the good mechanical properties of cast dental alloys with the excellent aesthetic properties of porcelain. Generally, the restorations consist of an alloy substructure with bonded porcelain veneers as shown in Fig. 11.6.

A major requirement of the materials used in PFM restorations is compatibility of the metal and ceramic used. Feldspathic porcelains used for PFM work normally contain significant amounts of leucite. This increases the coefficient of thermal

98 Chapter 11

expansion of the porcelain to a value which is closer to that for the metal. This helps to prevent the development of thermal stresses during cooling from the firing temperature. The presence of leucite also helps to strengthen the ceramic. The minimum flexural strength requirement for PFM ceramics as specified in ISO Standards is 50 MPa, which is equivalent to the requirement for dentine/enamel porcelains used in all-ceramic restorations.

The requirements of the alloy used to form the substructure are similar to those for non-porcelain bonding work with additional requirements as follows

(1)The alloy, having been previously cast into the desired shape, should be capable of withstanding porcelain firing without melting or suffering creep. Hence the alloy must have a high fusion temperature.

(2)The alloy should be sufficiently rigid to support a very brittle porcelain veneer otherwise fracture of the veneer is inevitable.

(3)The alloy should be capable of forming a bond with the porcelain veneer in order that the latter does not become detached.

Fig. 11.6 Photograph showing a metal-bonded porcelain restoration. Porcelain is built up on an alloy substructure.

(4)The alloy should have a value of coefficient of thermal expansion similar to that for the porcelain to which it is bonded.

There are four types of alloy currently available for porcelain bonding. These are (a) high-gold alloys, (b) low-gold-content alloys, (c) silver– palladium alloys and (d) nickel–chromium alloys. Table 11.3 gives a summary of the comparative properties of the four alloys.

High-gold alloys

The composition of a typical high-gold-content porcelain-bonding alloy is shown in Table 11.4. The major differences between these alloys and the non porcelain-bonding alloys are the high platinum/palladium content, the absence of copper and the presence of small amounts of base metals such as tin and indium.

The high platinum/palladium content raises the melting temperature of the alloy, reducing the risk of softening and creep during porcelain firing. In addition, these two metals decrease the coefficient of thermal expansion of the gold alloy to a value closer to that for porcelain. Copper is absent from porcelain-bonding gold alloys since, when present, it imparts a green hue to the porcelain veneer. The minor quantities of base metals such as tin and indium are essential in promoting bonding between the alloy and the overlying veneer. The base metals become oxidized at the surface and the oxide layer forms a chemical bond with porcelain during firing.

The high-gold alloys have two disadvantages when used for porcelain bonding. Despite the high platinum/palladium content, the melting range is still sufficiently low that there is a risk of alloy ‘sag’ during porcelain firing. Secondly, the modulus of elasticity of the high-gold alloys is less than ideal. Subsequently, copings must be produced in fairly thick section in order to prevent flexing which would result in porcelain fracture. The

Table 11.3 Properties of alloys used for porcelain bonding.

Alloy	Castability	Creep resistance during firing	Modulus	Bond strength	Biocompatibility
High-gold	+++	−	+	+	++
Low-gold	++	+	++	+	+
Silver–palladium	−	+	++	+	+
Nickel–chromium	− −	++	+++	−	− − −

Ceramics and Porcelain Fused to Metal (PFM)	99

Table 11.4 Composition of a typical high-gold-content porcelain-bonding alloy.

Metal	Percentage
Gold	85
Platinum	10
Palladium	3
Silver	1
Tin	0.5
Indium	0.5

requirement of a minimum copying thickness of around 0.5 mm results in the risk of an overcontoured restoration and gingival irritation.

Low-gold alloys

Low-gold porcelain-bonding alloys contain approximately 50% gold, 30% palladium to raise the melting temperature and lower the coefficient of thermal expansion, 10% silver and 10% indium and tin for porcelain bonding.

The mechanical properties of the low-gold alloys are similar to those for the high-gold materials. They have a slightly greater modulus of elasticity which is an advantage for porcelain bonding. The higher melting range produces better creep resistance for these materials during porcelain firing.

Good properties and a significant cost saving compared with high-gold alloys account for the widespread use of these materials for bonded porcelain work.

Silver–palladium alloys

These alloys contain about 60% palladium, 30% silver and 10% indium and/or tin to aid porcelain bonding. They have the advantages of a higher modulus value and a higher melting range than the high-gold alloys. They offer a suitable alternative to the high-gold materials for bonded porcelain work at a considerable saving in cost, providing care is taken during casting to avoid defects and gas inclusions.

The taking up of a green hue in some ceramics in contact with high silver content alloys (silver greening) has been reported, although this appears to be as much a feature of the composition of the ceramic as the composition of the alloy.

Nickel–chromium alloys

Nickel–chromium casting alloys typically contain 70–80% nickel and 10–25% chromium with small quantities of other metals such as molybdenum, tungsten and beryllium. Porcelain bonding is to the layer of ceramic oxide which forms on the surface of the alloy.

These alloys have the advantages of a very high modulus and high melting temperature. Their disadvantages are as follows.

(1)A high casting shrinkage which may affect accuracy of fit if not fully compensated by the investment.

(2)A tendency for poor castability, with voids in the castings.

(3)A bond strength with porcelain which does not compare with that achieved with the other alloys.

Indeed, fractures in Ni/Cr–porcelain systems invariably occur through the oxide layer whereas fractures in the other systems generally occur cohesively in the porcelain. In addition, these alloys are suspect from the biocompatibility point of view, as discussed on p. 76.

Tooth preparation for PFM restorations

During tooth preparation it is necessary to allow about 1.5 mm in thickness for a metal coping (0.3–0.5 mm) and the porcelain veneer (1.0 mm) to achieve optimal aesthetics. If this space is not available then the technician will either produce an overly bulky restoration with reasonable appearance or the opaque layer of porcelain used to mask the metal coping will ‘shine through’ the surface layers of porcelain producing an opaque white or cream spot. For this reason preparations for PFM crowns need to be designed to give adequate space for the technician to produce an appropriate restoration. The margin configuration for a PFM crown is a flat shoulder where there is porcelain and a chamfer or bevel where this is a metal finishing line. The appearance of the margins of PFM crowns has been revolutionized by the development of shoulder porcelains. These porcelains have adequate substantivity so that they do not flow significantly during firing. This allows the technician to cut the metal coping back from the edge of the tooth, leaving an adequate bulk of porcelain to give a reasonable marginal fit with much improved aesthetics.

100 Chapter 11

11.12 Capillary technology

The capillary technology (or CaptekTM) system is an alternative means of producing porcelain–metal restorations. The metal substructure is produced in two stages. Firstly, a wax strip loaded with powdered, high palladium content metal is adapted to the cast. This is fired in order to burn off the wax and sinter the metal, forming a threedimensional capillary network. A second wax strip, heavily loaded with almost pure gold (97% pure), is applied to the surface of the sintered layer and, during a second firing, the wax is again burnt off and the molten gold infiltrates the capillary network to form a metal substructure with a composite structure. A thin layer of veneer porcelain is finally baked onto the surface. It is claimed that this layer need only be about 35 μm thick as there is no dark oxide layer of material to cover. The bonding between the metal and porcelain is achieved through mechanical attachment.

11.13 Bonded platinum foil

A problem with the metal-bonded porcelain restoration is that a considerable thickness of tooth substance must be removed to allow space for the metal coping and the porcelain veneer. An alternative approach, which does not produce such a robust result but which may be adequate in some circumstances, is to make a porcelain crown which is bonded to a platinum foil.

The technique involves laying down two platinum foils on the working die as opposed to the normal single foil. The surface of the outer foil is then tin plated and the porcelain crown constructed and fired on top of the tin-plated surface. Porcelain bonds to the layer of tin oxide on the tin-plated surface. The inner platinum foil is removed prior to cementation of the crown whilst one platinum foil remains bonded to the inner surface of the crown. This foil helps to prevent crack formation on the inner surface.

11.14 Suggested further reading

Fan, P.L. & Stanford, J.W. (1987) Ceramics: their place in dentistry. Int. Dent. J. 37, 197.

Kelly, J.R., Nishimura, I. & Campbell, S.D. (1996) Ceramics in dentistry: Historical roots and current perspectives. J. Pros. Dent. 75, 18.

Preston, J.D. (ed.) (1988) Perspectives in Dental Ceramics. Quintessence Publishing Co., New Malden.

Raigrodski, A.J. (2004) Contemporary materials and technologies for all-ceramic fixed partial dentures: A review of the literature. J. Prosth. Dent. 92, 557.

Wassell, R.W., Walls, A.W.G. & Steele, J.G. (2002) Crowns and other extra-coronal restorations: Materials selection. In A Clinical Guide to Crowns and other Extra-coronal Restorations (R.W. Wassell, A.W.G. Walls, J.G. Steele & F.S. Nohl, eds), pp. 9–17. BDJ Books: London.