Chapter 23 Polymers in prosthodontics
A denture is a removable dental prosthesis. It can be a full (sometimes also referred to as complete) denture, where it replaces all the teeth in an arch. It can also be a partial denture, replacing one or more teeth with some natural teeth remaining in the arch. This chapter discusses the resin polymers used in the construction of both types of denture and other associated materials.
Denture Base Resins
The denture base is the part of the denture which is in contact with the oral tissues and supports the teeth (Figure 23.1). The denture base may be constructed of a metal alloy (discussed in Chapter 21) or a ‘plastic’ such as an acrylic.
What is required of a denture base resin?
The material of which the denture base is constructed must have certain requirements to perform satisfactorily (Box 23.1). While many materials can achieve some of these requirements, no material currently available achieves all of them. The materials which fulfil most of these criteria are the resin polymers and more specifically polymethylmethacrylate (PMMA).
Box 23.1 Requirements of a denture base resin
PMMA is a polymer formed from the addition reaction of the monomer methylmethacrylate (MMA). Although PMMA is available as gel, sheet or blank, the powder/liquid presentation is most widely used. The powder is PMMA polymer and the liquid is MMA monomer.
The PMMA polymer powder is composed of small spheres called beads or pearls. It may also be in a fine granular form in some products. This enables the polymer to dissolve more readily in the monomer as its surface area to volume ratio is higher. The size of these particles is approximately 150 μm in those resins which are processed by heat, the most commonly used method. Other chemicals are added to the PMMA poly-mer to modify the final product (Table 23.1). For example, PMMA is inherently translucent and so pigments and opacifiers must be added to change its appearance unless a clear acrylic is desired.
|Constituent||Percentage (%)||Reason for inclusion|
|Titanium dioxide||Small amount||Increases opacity to match the translucency of the oral soft tissues|
|Inorganic pigments||1||Varies colour, respectively:|
|Dibutyl phthalate||Small amount*||Plasticizer|
|Dyed synthetic fibres – nylon or acrylic||Small amount||Simulate anatomical structures such as capillaries within the denture base material|
* Phthalates are now regarded as hazardous materials, which, when used in excessive concentrations, are potential carcinogens. Manufacturers are looking at alternatives to these chemicals, such as citrates and benzoate esters.
Benzoyl peroxide coats the surface of the polymer beads. It is important that these beads are not contaminated as only a very small amount of polymer is required to start the reaction. Contamination can potentially initiate a premature polymerization reaction. If kept in good condition, the powder is very stable and as such has a very long shelf-life.
The liquid monomer is mainly MMA. This is a volatile liquid whose boiling point is 100.3°C. It is toxic if inhaled for a prolonged period and it is also highly flammable. It should therefore be handled in a well-ventilated room or preferably a fume cupboard. As with the powder, additional chemicals are added to the liquid to modify the final product (Table 23.2). In some products higher methacrylate monomers such as ethyl and butyl are substituted for the methylmethacrylate because they are less irritant.
The polymerization reaction may be initiated prematurely either by ultraviolet light or by free radicals forming spontaneously within the liquid. In order to prevent this occurring, the monomer is supplied in a dark bottle and hydroquinone is added to the monomer to preferentially react with any random free radicals which may be produced. The action of the hydroquinone produces stabilized free radicals which are not able to initiate the polymerization process. In this respect the hydroquinone acts rather like a sponge, mopping up free radicals until it is saturated. During the polymerization process all the hydroquinone must be used up before the polymerization reaction may take place.
It is advantageous that the mechanical properties of the acrylic are improved to increase the wear and fracture resistance of the material and also its resistance to the action of organic solvents which may cause surface cracking or crazing. This is achieved by adding cross-linking polymers such as diethylene glycol dimethacrylate. These different monomer units (co-polymers) can react with another growing chain at each end of the molecule when the polymer chains are growing so linking the chains. They are present in relatively small amounts and they have little effect on the transverse strength or hardness of the denture base material.
Plasticizers are often added to acrylics to vary their mechanical properties. Chemicals such as dibutyl phthalate do not take part in the polymerization reaction but are distributed throughout the polymerizing mass. They prevent the interaction between polymer molecules. Since they are not part of the structure, the plasticizer leaches out slowly as the denture becomes saturated with water. This presents two problems:
Concerns have been voiced with respect to the biological effects of the phthalates and manufacturers are looking at alternative plasticizers for use in denture base resins. Other chemicals such as the esters octyl or butyl methacrylate can also be used for materials which are made intentionally softer such as soft denture linings. Since these methacrylates will polymerize as part of the overall polymerization process, they are much less likely to leach out with time. Failure to initiate polymerization of these materials will, however, lead to leaching of the monomer and subsequent hardening of the denture base over time.
A few manufacturers supply the denture base material in a gel form. Here all the components are mixed together and formed in a thick sheet. The nature of the material precludes the use of chemical initiators and storage of the unprocessed material is important. It is generally recommended that the sheets are stored in the refrigerator in the dark before heat processing in the normal way.
The reaction is initiated by an organic peroxide such as benzoyl peroxide, which produces free radicals either by heating (heat-cured acrylics) or reacting with a chemical accelerator such as an organic amine.
The reaction is highly exothermic and this temperature must be carefully controlled as the volatile monomer, whose boiling point is 101°C, may vaporize during processing. Since the processing is carried out in a plaster mould, this heat is slow to dissipate and there is a risk of gas bubbles being produced. These bubbles of gas can become entrapped in the denture base resin, leading to gaseous porosity (Figure 23.2).
Fortunately, a large proportion of the reacting chemicals is polymer (75%) so the amount of heat produced as a result of the chemical reaction is reduced. Various processing regimes have been established to reduce the heat production.
As the setting reaction is a polymerization reaction, shrinkage of the material occurs. This dimensional change must be compensated for during the manufacturing process otherwise the final denture or appliance will not fit. This shrinkage is of the order of 6% for the recommended 3:1 polymer:monomer mix. As a result of shrinkage, internal strains are set up within the material. Some of these strains will be relieved as the curing cycle is taken above the glass transition temperature of the denture base material allowing flow to occur.
During the processing of the denture base material, the addition of the monomer to the polymer powder leads first to the softening of the outer surface of the polymer beads and part dissolution of the outer structure. Since the monomer is primarily the same as the polymerized beads, the material becomes a coherent mass which will then set. It is often difficult to observe any difference between the original beads and the repolymerized PMMA under the microscope, although there may be some differentiation particularly where cross-linking agents are used to any extent. Cross-linking agents help to ensure that there is a network of molecules rather than long chains or branched chains that reduce the strength of the final material.
PMMA has low tensile strength and a low elastic modulus. This means that it will flex in use. This flexure due to cyclical loading will, over time, result in fracture of the denture base. This is known as fatigue fracture. The process takes some time and is the least likely reason for fracture to occur. It also depends on the degree of flexure and the load applied. For example, an upper denture will flex about the midline of the palate, particularly if there is a bony ridge in this region. The degree of loading is determined by the chewing force applied during mastication.
Fracture of the denture base (particularly impact fracture) is not an uncommon occurrence. Many patients attempt their own repair by using a cyanoacrylate adhesive such as Super Glue® (Loctite). This should be strongly discouraged as the cyanoacrylate cement can react with the PMMA, damaging the denture surface. The repair will also fail quite quickly as the cyanoacrylate will hydrolyse and breakdown. At this point it is very difficult to accurately position the fragments for further repair. If the PMMA has been contaminated, the acrylic must be cut back to remove all traces of the cement and then the denture repaired using cold-cure acrylic.
Its fracture toughness is also low and as it behaves as a brittle material on impact, a denture will frequently fracture if dropped onto a hard surface. To avoid this, patients should be advised to clean their denture over a bowl of water rather than directly over an empty hard ceramic bathroom basin (Figure 23.3). Dentures dropped on this type of surface will break very readily.
As would be expected from a resin, PMMA has a high coefficient of thermal expansion. It also has a relatively low glass transitional temperature. This does not present too much of a problem during normal function, as the variation of the temperature in the mouth is relatively low with a range of less than 50°C. However there can be problems where a polymeric denture base is used to support ceramic teeth. Due to the difference in coefficients of thermal expansion of these materials, the ceramic teeth may be lost due to variations in the contraction and expansion characteristics of the two materials during thermal cycling. PMMA has low thermal conductivity and diffusivity. This is problematic for two reasons:
• The low diffusivity means that the transfer of heat from one side of the denture to the other is slow, that is, it takes time for heat to permeate the denture base and reach the underlying soft tissues. This has two consequences:
The softening temperature of PMMA ranges from 75 to 95°C and within this range the denture may distort. Thus patients should be warned against cleaning their acrylic dentures in very hot water or placing them in very hot water. Failure to do so will result in distortion of the denture base which will then no longer fit snugly.
During use, PMMA absorbs fluid. This water sorption causes the denture base to expand by about 2%, which is considered to be high. The polymer is sufficiently accurate for the proposed applications, in other words it achieves a close fit to the denture-bearing tissues despite the various inaccuracies encountered during the processing procedures.
The polymerization shrinkage which occurs during curing partly compensates for this expansion improving the fit of the denture. For this reason, it is essential to keep the denture wet as drying out of the PMMA denture base will cause shrinkage and crazing. Long-term water sorption often causes the polymer to discolour and stain. This is particularly the case if the patient drinks a lot of tea, coffee or red wine. However, generally its colour stability is good.
The dental technician must be careful to use the correct proportions of the monomer and polymer. It is generally accepted that for a heat-cured prosthesis, the powder:monomer ratio should be in the region of 3:1. Failure to follow these proportions can result in the formed prosthesis having inferior properties. If the ratio of polymer powder to monomer is too high, there will be insufficient monomer to wet the polymer and a granular resin structure will be produced. Excess monomer leads to increased shrinkage. The pure monomer shrinks by 21% but with correct proportioning, a 7% shrinkage may be achieved, which correlates to a 2% linear shrinkage. If a good technique is used during processing, this may be reduced to 0.5%, although stresses will be incorporated within the material, which will need to be relieved during the curing process. Manufacturers provide vials to ensure that the correct proportions should be used. The optimum ratio is usually 3 or 3.5:1 by volume or 2.5:1 by weight.
• It (or the benzoic acid which can also leach out) is an irritant to oral soft tissues. Improper processing will compound this problem. Concern has been expressed that it may cause an allergic reaction but has largely been dismissed as the reaction of the oral soft tissues is irritancy to the monomer and not a true allergy
The polymer may creep under load over the long term and deform the denture base. This phenomenon is more marked with cold-cure acrylics and can create greater problems. The addition of cross-linking agents is an attempt to minimize this effect.
Prior to mixing the polymer and monomer, the containers should be thoroughly shaken to ensure an even distribution of the powder ingredients as some additives which are required to participate in the curing process may settle at the bottom of the container. This will also ensure that any pigments which are mixed with powder are evenly distributed.
As mentioned earlier, crazing is the presence of fine cracks on the surface of the acrylic. They represent localized areas of plastic deformation of the polymer caused by stress relief of internal strain. Tensile stresses will also cause rupture of the polymer chains leading to a weakened denture. These areas may be filled with microscopic voids and a crack may result if the crazed area can no longer support stress.
A crack is formed as a result of brittle fracture, as the walls of voids in the region are thin in the region of the fracture. The crack grows under externally applied load such as a patient biting, eventually leading to a continuous crack. This can cause fracture of the denture base. Crazes may be caused by heat, the action of organic solvents (for example alcohol or if the monomer comes into contact with resin during a denture repair) or differences in coefficients of thermal expansion around (dissimilar) inclusions that form stress concentrations. The most obvious example of this is ceramic denture teeth on a PMMA denture base or the inclusion of stainless steel clasps. The adhesion of PMMA to metal components and ceramic teeth is primarily by mechanical retention.
Crazing may also be seen if the denture is not kept moist at all times during its life. This is due to mechanical stresses set up within the denture base as it contracts and expands during the drying and wetting cycle. The cross-linked resins are less likely to craze as they contain fewer lines of weakness (Figure 23.4).
In order to prevent crazing, the polymer should not be allowed to dry out and so the finished denture should always be supplied to the dentist in water (Figure 23.5). The patient should then be advised to store the prosthesis in water when it is not in use. Wrapping in gauze or cotton wool is inadequate as the resin does not remain saturated.
Shade of denture base acrylics
Denture base acrylics are available in different shades to match the patient’s natural gingival tissues. This is more significant in patients whose mucosa is more pigmented. Some acrylics are available with small fibres of red acrylic in them which mimic small blood vessels and these are called veined materials (Figure 23.6). These fibre inclusions are frequently coated with a coupling agent, commonly triethoxysilane, to improve their wettability.
PMMA denture base resins are radiolucent. In case of accidental ingestion or inhalation of the denture or parts of it, it would be desirable if all components were radiopaque to facilitate identification by medical radiographic investigation. Radiopacity can be achieved by the addition of chemicals such as 10–15% bismuth or uranyl salts. Unfortunately their addition adversely affects the properties of the denture base material, increasing transverse deflection and water sorption, so making handling of the unset polymer more difficult.
Types of Acrylic
When the polymer and monomer are mixed they initially form a sand-like mixture. This changes to a tacky mass as the surface of the polymer beads start to dissolve in the monomer. Under normal conditions once the polymer and monomer have been mixed then the mixture is left undisturbed in the mixing container until the dough stage is reached (Figure 23.7).
The next stage is described as the dough stage. As the name implies, the mixture at this point is similar in appearance to the dough used in bread making. The mixture does not stick to the container walls and ‘snaps’ when pulled apart (Figure 23.8). Failure to pack the mixture into the mould at the dough stage will result in the material becoming too rubbery and stiff. It cannot then be moulded properly.