Denture Base Polymers	121

Fig. 13.11 Whitened denture base. This shows a section through a denture base which has been whitened by

the patient using an oxygenating cleanser in hot water (>90°C). The original colour of the denture can be seen in the middle of the section and we can see that the whitening effect has progressed inwards from the outside surface. The original pink colouration of the veins which are present in this material can also be seen.

Standard for denture bases contains a requirement for the upper limit of the residual monomer concentration (Table 13.2). The limits of 2.2% for types 1, 3, 4 and 5 materials and 4.5% for type 2 materials are quite generous and it is accepted that residual monomer levels of this magnitude could be problematical for some denture wearers. Another requirement of the standard therefore is to request manufacturers to recommend a curing cycle that can achieve a monomer content of less than 1%. Reduced monomer content very much depends upon the type of curing regime used. Short cures involving minimal or no terminal boil generally produce higher monomer concentrations (and inferior properties). Longer cures involving a terminal boil of 2–3 hours can reduce the residual monomer concentration to well below 1%. High residual monomer concentrations can be associated with a relatively high solubility. Type 2 materials with higher residual monomer levels tend to have greatest solubility (Table 13.3).

Acrylic resin should be treated with respect and handled with care by technicians involved in its manipulation. Levels of acrylic powder dust and MMA monomer in the atmosphere should both be kept to a minimum since both may be potentially harmful.

13.4Modified acrylic materials

The majority of acrylic resin is used in the unmodified form as discussed in Section 13.3. Some products have been developed, however, in which attempts have been made to improve impact strength, fatigue resistance or radiopacity.

The impact strength of acrylic polymers can be improved significantly by the incorporation of elastomers. The elastomer is able to absorb energy on impact and thus protect the acrylic resin from fracture. An alternative of the direct addition of elastomers is the use of acrylic–elastomer copolymers. These are, typically, methylmethacry- late–butadiene or methylmethacrylate–butadiene- styrene copolymers which are now available in certain commercial products. Despite the fact that impact strength can be increased almost tenfold in this way, these polymers are not widely used, mainly because of their greater cost.

One attempt to improve the fatigue resistance of acrylic denture polymers has involved the use of carbon fibre inserts. If the fibres are correctly positioned they may have a beneficial effect. They stiffen the denture, reducing the degree of flexing and the possibility of fatigue fracture. They also considerably increase the flexural strength. Carbon fibres can be used as loose strands or in woven mat form. Whichever form is used it is essential to get intimate contact between the reinforcing fibres and the matrix acrylic resin. In order to achieve this the fibres are often surface treated and formed into ‘prepregs’ by blending with resin to form a thin sheet which can then be incorporated into the denture base. The technique is not in widespread use however, for several reasons. In order to gain benefit from the fibres, the positioning is critical. They must be placed in that part of the denture which is under a tensile stress. Bonding between the fibres and the acrylic resin may be difficult to achieve and if bonding is not achieved the fibres may ‘weaken’ the denture. The technique adds a complicating factor to the denture construction process and, finally, the appearance of the denture is adversely affected because the

122 Chapter 13

Table 13.5 Radiopaque denture base materials.

Radiopaque additive	Comments

Metal inserts or powdered metals Inorganic salts such as barium sulphate

May weaken base and appearance is poor Insufficient radiopacity at low concentrations

Weaken base at high concentrations

Co-monomers containing heavy metals, e.g. barium acrylate	Polymer has poor mechanical properties
Halogen-containing co-monomers or additives, e.g.	Additives may act as plasticizers
tribromophenylmethacrylate	Co-monomers are expensive

carbon fibres are black. Other attempts at fibre reinforcement have involved the use of aramid, polyethylene and glass fibres. Aramid fibres are of particular interest. These fibres are manufactured as extremely fine filaments of polypara-phenylene terephthlalamide which can be woven to produce a commercial material (Kevlar) which is used to strengthen polymers used in the manufacture of bulletproof vests and fibre reinforced boat hulls. The aramid fibres have a tensile strength in excess of that of nylon and a modulus of elasticity at least twenty times greater than the common types of glass fibres. This phenomenal performance is thought to be partly due to the very thin filaments which have a lower probability of flaw inclusions. Also, aramid fibres are more readily wetted by methacrylate monomers and require no special surface treatment before incorporation in the denture base. Kevlar fibres are yellow in colour and this can detract from the appearance of dentures which are reinforced with this material. Glass fibre reinforcement is potentially the easiest method of strengthening but the process requires attention to detail, particularly regarding surface preparation of the glass and orientation of the fibres. An ineffective bond between glass fibre and matrix resin can result in a weakening rather than strengthening effect. Recently greater success has been achieved with glass fibre reinforcement by using polymer impregnated fibres which seem to improve the compatibility of the fibre reinforcement material and resin matrix.

There have been many attempts to incorporate a degree of radiopacity into acrylic denture base materials. Table 13.5 summarizes the methods which have been suggested. Each method involves the incorporation of atoms of higher atomic number than the C, H and O atoms of which acrylic resin is comprised. One commercially

(a)

(b)

Fig. 13.12 Chest radiographs in which a segment of denture base has been placed over the lower right half of the chest: (a) for a radiolucent denture base material; (b) for a radiopaque denture base material. (From Journal of Dentistry (1976) vol. 4, p. 214, with permission.)

available product contained 8% barium sulphate. This did not produce sufficient levels of radiopacity. Increasing the barium sulphate content to a level of 20% gives sufficient radiopacity but unfortunately has a deleterious effect on the mechanical properties of the resin. The most

Denture Base Polymers	123

promising materials under development appear to be those in which bromine-containing additives or comonomers are used to give radiopacity. Figure 13.12 shows chest radiographs in which a segment of a denture has been placed over the lower right chest area. In the case of a conventional denture base material the denture segment is not visible on the radiograph. A material with a brominecontaining additive is clearly visible.

13.5 Alternative polymers

The major alternatives to acrylic polymers or modified acrylics are the polycarbonates and certain vinyl polymers. These may be considered when the patient has a proven allergy to acrylic resin or when greater impact strength is required. The polycarbonates and some of the vinyl polymers are processed by injection moulding and so can only be used when the specialist equipment is available.

Polycarbonates have Tg values around 150ºC and are generally moulded at temperatures well in excess of this. Consequently, the moulded bases

may have internal stresses after moulding and is likely to distort if placed in hot water. Some of the vinyl resins, on the other hand, have very low softening temperatures, as low as 60ºC in some cases. These must obviously be handled with care if distortions are to be avoided.

The other alternative to acrylic resin is vulcanite. The equipment required for processing a vulcanite denture is now a rarity, however, and the material can no longer be considered as a serious alternative.

13.6 Suggested further reading

Jagger, D.C., Harrison, A. & Jandt, K.D. (1999) The reinforcement of dentures. J. Oral. Rehabilitation, 25, 185–194.

Jorge, J.H., Giampaolo, E.T., Machado, A.L. & Vergani, C.E. (2003) Cytotoxicity of denture base acrylic resins: a literature review. J. Prosthet. Dent. 90, 190–193.

Robinson, J.G., McCabe J.F. & Storer, R. (1987) Denture bases: the effects of various treatments on clarity strength and structure. J. Dent. 15, 159.

Chapter 14

Denture Lining Materials

14.1 Introduction

Denture lining materials are of several types and are used for a variety of reasons. Occasionally, the fitting surface of an acrylic denture needs replacement in order to improve the fit of the denture. In this case, there are two options. Either the whole of the denture base can be replaced with fresh heat curing acrylic resin, or a lining of a self-curing resin may be applied to the fitting surface of the existing base.

Sometimes it is necessary to apply a very soft material to the fitting surface of a denture in order to act as a ‘cushion’ which will enable traumatized soft tissues to recover before recording an impression for a new denture.

Some patients are unable to tolerate a ‘hard’ denture base and must be provided with a ‘permanent’ soft cushion on the fitting surface of the denture.

The materials which satisfy the various requirements listed above can be classified into three groups:

(1)Hard reline materials;

(2)Tissue conditioners;

(3)Soft lining materials.

14.2 Hard reline materials

The materials discussed in this section are those products which are used to provide a chairside reline to the denture. The method should be distinguished from laboratory relining and rebasing techniques which involve replacing most of the denture base resin with fresh, heat cured polymer.

Composition: The materials are generally supplied as a powder and liquid which are mixed together.

Table 14.1 gives the composition of the two types of material in common use. The major difference between the two types is that the liquid in the type 1 material contains methylmethacrylate monomer, whilst the liquid of the type 2 material contains butylmethacrylate monomer. Both type 1 and type 2 materials may be classified as autopolymerizing resins and will readily polymerise at room temperature or mouth temperature.

Manipulation: The normal procedure is to ‘relieve’ the fitting surface of the denture by grinding away some of the hard acrylic denture base. The powder and liquid of the hard reline material are then mixed in the recommended proportions to give a fluid mix of material. This is applied to the fitting surface of the denture which is seated in the patient’s mouth whilst still fluid. The reline procedure must be undertaken using a ‘closed mouth’ technique in which the patient’s denture or dentures are inserted into the mouth and the patient is then asked to close into gentle contact. Care needs to be taken to ensure that the dentures maintain an appropriate relationship to the underlying alveolar ridge. Both of these steps are designed to prevent major positional or occlusal errors being produced in the relined dentures. The reline material soon becomes rubbery and the impression of the patient’s soft tissues is recorded. The denture is then removed from the patient’s mouth and allowed to bench cure. Setting may be accelerated by placing the denture in warm water or using a combination of warm water and pressure in an appropriately designed pressure vessel. The materials are not allowed to remain in the patient’s mouth throughout setting since the exothermic heat of reaction may cause an unbearably high temperature rise. The relined denture is normally ready for trimming and polishing within 30 minutes.

124

		Denture Lining Materials	125
Table 14.1 Composition of typical hard reline materials.
Type 1	Powder
	Polymer beads	Polymethylmethacrylate
	Initiator	Benzoyl peroxide
	Liquid
	Monomer	Methylmethacrylate
	Plasticizer	Di-n-butylphthalate
	Chemical activator	Tertiary amine
Type 2	Powder
	Polymer beads	Polyethylmethacrylate
	Initiator	Benzoyl peroxide
	Liquid
	Monomer	Butylmethacrylate
	Pigments	or isobutylmethacrylate
		or some other higher methacrylate
		monomer
	Cross-linking agent	Di-methacrylate
	Chemical activator	Tertiary amine

Properties: The major disadvantage of the type 1 materials is that they involve direct contact between the oral soft tissues and a fluid mixture of reline material containing methylmethacrylate monomer. The latter material is known to be irritant and may also sensitize patients who may then suffer allergic responses in the future. The advice offered by some manufacturers, to smear the soft tissues with petroleum jelly prior to recording the impression, is probably inadequate.

The type 2 materials contain butylmethacrylate monomer in the liquid component. This is known to be a far less irritant substance than methylmethacrylate.

Both type 1 and type 2 materials have low values of glass transition temperature (Tg). The reasons for this are the presence of plasticizer in type 1 materials and the use of higher methacrylates (ethyl and butyl) in the type 2 materials. This may lead to increased dimensional instability in the relined denture, particularly if the existing hard base has been significantly relieved in order to accommodate the lining.

The reline materials are often porous due to air inclusions during mixing of the powder and liquid. The initial fluidity of the mix, coupled with a relatively rapid increase in viscosity during setting at atmospheric pressure, ensure that it is difficult to eliminate voids. This is often considered unsightly and may affect patient acceptance. In addition

such a porous surface will be more likely to become contaminated with oral debris and be colonized by micro-organisms including Candida albicans, and will be more difficult to clean.

One criticism of the direct reline materials is that the dentist has little control over the thickness of the lining achieved and therefore over the ‘height’ of the denture. A reline to the fitting surface is usually undertaken to improve denture stability/retention, not to correct an occlusal error nor to modify the vertical relationship between the dentures. The ‘cushion’ of relining material can result in a marked increase in thickness of the denture base and infringement of the freeway space that is normally present between the dentures with the jaws at rest. Furthermore, there is no guarantee, even when using a closed mouth technique, that an antero-posterior positional error or lateral cant is not produced during this procedure, either as a result of poor operator technique or a greater bulk of lining material on one side of the mouth compared with the other. Such errors are highly undesirable.

A final problem is an increase in thickness of the ‘palate’ of an upper denture using this technique which patients often find unacceptable.

It follows that the direct reline materials should be considered as only a temporary or at best semipermanent solution to the problem of an ill-fitting denture.