Some permanent soft linings are adversely affected by denture cleansers. Oxygenating cleansers may cause surface pitting in acrylic linings whilst brushing accelerates the rate at which silicone soft linings become detached. Soaking in a very dilute solution of hypochlorite is an acceptable way of achieving denture hygiene without damaging the soft lining.

None of the soft lining materials can be considered truly permanent in nature since none could be expected to last the full lifetime of a denture. Therefore, regular reviews of patients with soft linings are essential.

14.6 Self-administered relining materials

Several types of lining materials are available which enable the patient to attempt to improve the fit of ill-fitting dentures or to provide a soft cushioning effect to the fitting surface. Such products are generally available for purchase at many retail outlets such as supermarkets or chemists shops. The products normally contain methacry-

late or vinyl polymers such as polymethyl-, poly- ethyl-, or polybutylmethacrylate or vinyl acetate along with a plasticizer such as butyl phthalate and a solvent such as acetone, ethanol or toluene. The claim for such products is that they improve the fit or comfort of a denture without having to visit the dentist. Most authorities agree however that the use of these products should be firmly discouraged, for all but short term emergency use. Long term use of these products can lead to harmful effects on the hard and soft oral tissues. Cases of irritation, severe bone loss and tumors, related to the use of self-administered denture lining materials, have been documented.

14.7 Suggested further reading

Braden, M., Wright, P.S. & Parker, S. (1995) Soft lining materials – a review. Eur. J. Prosthodont. Restor. Dent. 3, 163.

Jagger, D.C. & Harrison, A. (1997) Complete dentures

– the soft option. An update for general dental practice. Br. Dent. J. 182, 313.

15.1 Introduction

Chapters 13 and 14 have dealt with materials which are used to form the denture base and to line the fitting surface of the denture base. The other main components of a denture are the artificial teeth themselves. The materials most widely used for manufacturing artificial teeth are acrylic resin and porcelain.

15.2 Requirements

The most important requirement of artificial teeth is good appearance. They should, ideally, be indistinguishable from natural teeth in shape, colour and translucency. Good matching often requires that the shade and translucency of the artificial tooth should vary from the tip of the crown to the gingival area.

There should be good attachment between the artificial teeth and the denture base. The introduction of artificial teeth into the base should not adversely affect the base material. That is, the artificial tooth and base materials should be compatible.

It is an advantage for the artificial teeth to be of low density in order that they do not increase the weight of the denture unduly. The artificial teeth should be strong and tough in order to resist fracture. They should be hard enough to resist abrasive forces in the mouth and during cleaning, but should allow grinding with a dental bur so that adjustments to the occlusion can be made by the dentist at the chairside.

15.3 Available materials

The two materials which are commonly used for the production of artificial teeth are acrylic resins and porcelain.

Acrylic resins

Acrylic resin artificial teeth are produced in reusable metal moulds using either the dough moulding technique, described for denture base construction (Chapter 13), or by injection moulding in which the acrylic powder is softened by heating and forced into the mould under pressure.

The resins used are highly cross-linked in order to produce artificial teeth which are resistant to crazing. The main difference between the materials and those used for denture base construction is the incorporation of tooth-coloured pigments rather than pink ones.

Porcelain

The composition and manipulation of porcelain are dealt with in Chapter 11. Artificial porcelain teeth are produced to standard shapes and sizes by using moulds which are approximately 30% larger than required, in order to allow for shrinkage during firing. Small holes or metal pins are incorporated in the base of the porcelain teeth during their production. These are used to give mechanical attachment to the denture base.

15.4 Properties

Both acrylic and porcelain teeth can be made to give a realistic appearance. The slightly greater translucency and depth of colour achieved with porcelain possibly gives this material a slight advantage in terms of aesthetics. Both materials are produced to a variety of shapes, sizes, colours and shades which enable selection of teeth to suit most individuals.

One aspect of porcelain teeth which is sometimes unpopular with patients is the ‘clicking’

sound which is made when two porcelain teeth come into contact.

Attachment of the teeth to the base is through a chemical union in the case of acrylic teeth and by mechanical retention for porcelain teeth. For both materials, adequate bonding is achieved only if all traces of wax from the trial denture are removed from the teeth during the ‘boiling out’ stage.

Chemical bonding of the acrylic teeth depends on the softening of the resin at the base of the teeth with monomer from the ‘dough’ of denture base material. Some manufacturers encourage this process by constructing the base and core of the artificial teeth from uncross-linked or only lightly cross-linked resin which is more readily softened. The outer ‘enamel’ layers of the tooth are constructed from highly cross-linked resin to prevent crazing. Unfortunately it may be necessary to remove these lightly cross-linked areas when trimming denture teeth to fit into the space available within the dentures. It can then be very difficult to achieve an adequate attachment to the remaining highly cross-linked material.

Despite efforts to ensure bonding between the artificial teeth and the denture base, it has been reported that 33% of all denture repairs are required due to debonding of teeth. For acrylic teeth, bonding to heat-cured resins is more effective than bonding to autopolymerizing resins. The bond to autopolymerizing resin can be improved by grinding grooves into the teeth or through grinding followed by the use of a solvent/monomer mixture to soften the tooth base and impregnate it with methylmethacrylate. ISO specifications for ceramic (ISO 4824) and acrylic (ISO 3336) teeth adopt different approaches towards ensuring a reasonable bond. ISO 4824 requires only that there is some means of mechanical attachment in the ceramic tooth base, i.e. diatoric holes provided for attachment must be patent. ISO 3336 for synthetic polymer teeth requires a bond test to be performed. When a denture tooth is fractured away from a sample of denture base the fracture path must not occur along the interface between the tooth and denture base, i.e. the fracture must be cohesive.

Table 15.1 lists some of the physical and mechanical properties of artificial teeth.

Acrylic resin teeth are, naturally, more compatible with the denture base than porcelain teeth. There is a serious mismatch in coefficient

Table 15.1 Some properties of artificial teeth.

of thermal expansion and modulus of elasticity between porcelain and acrylic resin. This may lead to crazing and cracking of the denture base in the region around the base of the porcelain teeth. Porcelain has a density value about twice that of acrylic resin and dentures constructed with porcelain teeth are much heavier.

Porcelain is brittle and teeth constructed from this material are more likely to chip and fracture than acrylic teeth.

There is a vast difference in hardness between acrylic resin and porcelain. Acrylic teeth are more likely to suffer abrasion than porcelain teeth, although this does not constitute a serious problem in more than a few rare cases.

When excessive wear is evident on acrylic denture teeth, special environmental factors or patient habits are often responsible. For example, working in a dusty or gritty environment may accelerate wear if particles of grit are ground between teeth of an upper and lower denture. Likewise the use of a very abrasive material, such as pumice, for cleaning dentures may result in marked abrasion.

The extreme hardness of porcelain is a disadvantage when adjustments, requiring grinding of teeth, are necessary. With acrylic teeth, such adjustments are carried out quite simply. For porcelain teeth however, the process is difficult and results in the glaze being removed from the surface of the porcelain.

It is a clinical impression that porcelain teeth transmit higher forces to the supporting soft tissues than acrylic teeth. This is a function of the greater modulus of elasticity of porcelain.

The difficulties of grinding porcelain teeth combined with their tendency to transmit a greater force to the underlying tissues dictates the type of clinical case for which they can be used. They should not be used where there is diminished interocclusal distance or when there is poor ridge support.

One advantage of ceramic materials used in various aspects of medicine and dentistry is their perceived biocompatibility. This is related to their structure and lack of chemical reactivity under normal conditions. One aspect of the biocompatibility of ceramic denture teeth which has been a source of some controversy over the years is the occurrence of radioactive compounds of uranium238. An amendment to ISO 4824 was introduced to test for the presence of uranium-238 using neutron activation. The upper limit for the concentration of the radioactive isotope in the ceramic teeth is 0.2 Bq g−1.

The vast majority of artificial teeth used for denture construction are now of the acrylic type. Acceptable appearance coupled with convenient handling, greater toughness and compatibility with the acrylic denture base give the acrylic resins an advantage over the alternative porcelain materials.

The appearance of both porcelain and acrylic teeth can be customized using surface stains. Those for porcelain teeth are similar to the glaze stains used for porcelain crown and bridge materials. Their use necessitates the teeth being placed through a glazing cycle by a ceramic technician. The surface stains for acrylic are applied in the form of a surface lacquer. Both approaches can produce a dramatic increase in the realism of denture teeth. However, the effect of acrylic surface stains is rather short-lived as the lacquer is soft and hence worn away quite rapidly.

15.5 Suggested further reading

Woodforde J. (1968) The Strange Story of False Teeth.

Routledge & Kegan Paul, London.