Problems with Polymers in Dentistry Alan Grant

Despite the important contributions made by polymers in dental treatment procedures, many shortcomings of these materials are apparent. Rigid type denture base polymers (commonly P.M.M.A.) have low impact and abrasion resistance and lack radiopacity. Resilient polymers used as denture liners may bond poorly to the denture base and suffer changes in physical properties in use. Early problems of colour stability, high polymeri- sation and thermal contraction, low surface hardness and poor abrasion resistance of the first polymeric filling inaterials have been only partly overcome by the introduction of polynier/ceramic composites. In dental cement applications, present bonding systems include unfilled dirnethacrylates, which d o not attach to dentine. A system based on polyacrylic acid, or copolyrners thereof, is believed to bond chemically to the calcium in tooth structure, but the bond to dentine is poorer than that to enamel and also the hydrolytic stability is suspect.

1. INTRODUCTION

Certain difficulties exist in producing a satisfactory dental material or designing a technique that is useful for its applic- ation. For example, there are limitations to the design of an appliance or restoration ~ access t o the mouth is dif'fi- cult and only a certain amount of tooth structure can be removed.

Conditions i n the oral cavity seetii almost ideally suited to destruction. Biting stresses on restorations may be very high, temperature iiiay change almost instan taneously by sonie 65°C and the pH niay rapidly fluctuate from acidity to alkalinity. The warm and humid saliva rich environment is conducive t o corrosion. The dental pulp and soft tissues surrounding the tooth niay be readily injured by an irritant substance, and of course the possibility exists for toxic substances including breakdown products of materials t o be ingested.

Thus, in contemplating materials for use in the tiiouth, the basic chemical, physical and mechanical properties cannot be divorced from the bacteriological, physiological and pathological considerations.

By far the most widely used synthetic polymcrs in dciitistry are the acrylic materials, but even these when considered in ternis of industrial usage do not represent a large share of the plastics industry. McCabe and Wilson * noted that i n 1972, of 1,621,500 tons of thermosetting and thermoplastic materials produced in the UK, only 225 tons were used f o r dental purposes.

The purpose of this paper is t o give a broad review of inany of the polymers used in dental practice, highlighting those aspects which appear t o represetif problem areas t'rom the clinical point of view. Hopefully this may stimulate further interest in the application of polymers to dentistry.

Not that dentistry has been slow to consider possible applic- ations of a b,ewildering array of polymers, but perhaps because of the complexity of the conditions the mouth presents, many problems still remain. As a clinician, it will be easier for me to consider the poly- mers used in dentistry in relation to their use, and it will be necessary first to describe briefly sonie appliances and techniques commonly used in dentistry.

Depurttnenf of Prosthetic Dentistry, 7urtzcr Dental Sc l~)o l , University of'inunchester, Bridgeford Street, Munchestcr M I S 6b'tI.

Restorative dentistry includes prosthetic dentistry (the replacement of missing parts; basically complete and partial dentures) and conservative dentistry, which includes fillings, crowns and bridgework. There are many processes, e.g. impressions, which are common to both.

2. PROBLEMS IN PROSTHETIC DENTISTRY

2.1 Denture base materials

A denture consists of artificial teeth set in a denture base which retains the artificial teeth and rests on the soft tissues of the mouth.

The general method for producing a denture having a poly- meric base is as follows:

First a base plate of wax is produced on a gypsum model of the mouth and the artilicial teeth (or other inserts) are placed in position in the wax. The gypsum model with the wax base is thcn invested in ii denture flask (i.e. a sectional metal case) and a counter-die in gypsum is formed. After setting of the investment, the wax is reinoved leaving a mould space in which the denture base will be moulded.

2. I. I History The lirst non-nietnllic denture base material which was used with some success was vulcanite - an unsaturated polymer of isoprene supplied in the form of a plastic sheet impreg nated with some 32% of sulphur. This sheet was cut u p and packed in the mould space and polymerised under heat ( I 68°C) and pressure (620 kN/tn2). This material lasted in popularity for some 80 or more years, and was displaced as the front runner in this field by poly(methy1 methacrylate) which was introduced in the 1930s. With vulcanite the aesthetics were poor due to the opacity o f the rubber. I t absorbed saliva and could allow,bacterial proliferation and dimensional changes occurred during polymerisation (about 4% contraction).

2.2 Poly (methyl methacrylate)

This compound for denture base application is supplied as a powder (P.M.M.A. beads or particles with the addition of a pigment and an initiator, benmyl peroxide 0.7-0.5%) and liquid (monomer with 0.006% hydro-quinone as a stabiliser, and often a cross-linking agent, e.g. ethylene glycol dimethacry

THE BRITISH POLYMER JOURNAL, VOLUME 10, DECEMBER 1978 241

Manipulation of the material is very simple, requiring the minimum of equipment to produce the required heat and pressure, and the excellent optical properties allow the appearance to be easily modified to resemble the mouth tissues. All the monomer must be converted to polymer as residual monomer is toxic and will act as a plasticiser. The low impact strength of acrylic resins predisposes the denture bases to accidental breakage. In addition the fdtigue resistance from repeated bending of the denture in service may cause f r a c t ~ r e , ~ since the tissue bed on which the denture is used is not of uniform consistency. Denture repairs cost the National Health Service in the United Kingdom in excess of &1M per year. Water absorption in service may be some 2% and drying out of the material is associated with shrinkage. Surface crazing may result from mechanical stresses induced from wetting and drying, or because of the difference in coeffici- ent of thermal expansion between the denture base and any inserts (teeth, clasps, etc.). Dimensional accuracy can be a problem - there is approxi- mately 7% volume shrinkage in polymerisation and the shrinkage on cooling of the material following the exother- mic polymerisation reaction may also cause dimensional changes. The abrasion resistance of P.M.M.A. is low, and this is a particular problem where artificial teeth of acrylic resin are used.6

2.3 Lack of radio-opacity Low strength values for P.M.M.A. and the consequent liability to fracture can have serious consequences as denture fragments may be swallowed or inhaled. Acrylic resin is radiolucent so that X-ray detection of fragments IS not possible. The addition of heavy metal compounds, principally of barium and bismuth, increases the X-ray absorbency, but also affects adversely the physical properties, including the appearance, of the basic material.’ Halogenated organic compounds have also been used, but their colour stability is suspect.

2.4 Other denture materials Other (stronger) materials have been suggested, and used, e.g. polycarbonates which have an impact strength greatly in excess (approximately ten times) of that of P.M.M.A. However, they are more difficult to mould, requiring a critical high temperature (335°C) injection technique.

Several other materials have found application in this field including polystyrene, vinyl copolymers and rubber-acrylic graft copolymers. However, the conventional acrylic resins continue for the present to be the most popular polymeric denture base materials.

2.5 Self-curing acrylic materials (R.T.V.) The liquid of these materials contains an activator such as dimethyl-p-toluidine and in other respects the composition is similar to that of the heat cured materials. These have a higher residual monomer content, lower transverse strength and poorer colour stability than heat cured acrylic.

2.6 Resilient materials As a clinical measure it may be necessary to provide a denture having a soft lining against that part which fits against the patient’s tissues. With these materials a low Tg is required so that the material will be soft and resilient at mouth temperature. They should be soft and resilient, non-toxic, tasteless, bond well to the denture base material, be dimensionally stable and unaffected by the oral environment. The materials currently used for these purposes are either acrylic type polymers or silicones.8 Higher methacrylates, e.g. polyethyl or polybutyl methacry- late combined with plasticisers are used, and leaching out of the plasticisers produces hardening and dimensional changes. The silicone materials have the best all-round resistance to the constituents of food, etc, and they retain their elastic properties for a long time. However, bonding to the denture base may cause difficulties and colonisation of the fungus candida albicans can occur. With both types of resilient lining material, the denture is likely to fracture more readily because the cross sectional area of the denture base must be thinner to allow for the lining. Hydrogels ( e g 2-hydroxyethylmethacrylate, cross linked with ethylene glycol dirnetha~rylate)~ have been consider- ed for use as soft liners for dentures, but problems related to dimensional stability have become apparent. At best, the available materials can only be considered as semi-permanent. Another class of soft materials - the tissue conditioners - (therapeutic linings on a denture) are intended for tempor- ary use only.I0 They consist of acrylic polymers such as poly(ethy1 methacrylate) or a copolymer of ethyl and butyl methacrylate. The liquid supplied may be a solution of plasticiser in ethanol. When mixed, a gel forms by a physical process. The softness is lost as the plasticiser is leached out and the material becomes hard. Few of these materials retain their softness for a sufficiently long period of time.

2.7 Impression materials Not all of the impression materials used in dentistry are polymers. However, the elastomeric imprepion materials are a very important group of polymers. Needless to say, dimensional accuracy is of prime impor- tance in an impression material in addition to the usual requirements for materials to be used in the mouth. Impression materials are mixed outside the mouth, placed in position while soft, and removed after polymerisation has produced a rubber-like material capable of being withdrawn over undercut regions of the mouth.

The three main types of elastomers used are: l’olysulphides, Silicones, and Polyethers, and the chemistry and properties of these materials have been reviewed else- where. ]

The dimensions of impressions may be affected by the strains accompanying withdrawal from the mouth and the time and conditions of storage of the impression. The toughness of the elastomers is important so that tearing of the material does not occur. While the polyether mater- ials generally have good dimensional properties they have a higher modulus of elasticity than polysulphides and silicones.

242 THE BRITISH POLYMER JOURNAL, VOLUME 10, DECEMBER 1978

and they are consequently more difficult to remove from undercuts.

All three types have a high coefficient of thermal expansion, but the effect of this is reduced by efficient adhesion of the material to the impression tray in which they are carried to the mouth.

3. POLYMERIC FILLING MATERIALS

The first of the polymeric filling materials which was com- monly accepted was of poly(inethy1 methacrylate). Activa- tion and initiation was achieved at room temperature using either an amine-peroxide system, a mercaptan peroxide system or sulphinic acid. Polymerisation is accompanied b,y the evolution of heat and contraction of the material - both of these effects being potentially harmful. Clinical techniques involving these materials include procedures aimed at minimising the effects of contraction . I

The monomer is an irritant to the pulp of the tooth and the filling has poor mechanical properties relative t o that of the tooth. The high coefficient of thermal expansion allows percolation of fluids between the cavity in the tooth and the filling, and staining of cavity mar&' Tins occurs over a period of time.

T o overcome some or the liinitat>.ons of unfilled polymers, polymer-ceramic composites have been developed. These contain in general either acrylic copolymers or complex aromatic dimethacrylates with u p to 50% by volume of ceramic fillers (rods or beads, fused quartz, borosilicatc glass, barium containing glass, etc:.). The rather more complex aromatic dimethacrylate cotii- posites offer some advantages over the simpler acrylic ~na ter ia l s . '~ They consist in the main of bis-G.M.A. -~ ;I

reaction product of bisphenol A and glycyidil methacrylate (in the dental literature this niay be called Bowen's resin).I4 Co-monomers are generally present to reduce the viscosity of the inaterial and also the ceramic fillers are coatcd wilh a vinyl silane coupling agent. Benzoyl peroxide initiators and tertiary amine activators or sulphinic acid type initiators niay be used. One newly introduced material sets following exposure of- the prepolymer to high intcnsity visible light. This consists of urethane dimethacrylate and a difunctional acrylic co-mono me r . cu-diketone (excited by blue light) reacts with aminc giving 2 free radicals. This is an interesting development and instead of the dentist being limited in allowable manipul- ation time by a chemical reaction, a 'command setting'sit- uation exists. There is virtually an infinite working time available as the material will not harden until exposed to the high intensity light. Some U.V. light activated resins are also in use. These also have the 'command' setting facility but exposure t o U.V. radiation is necessary t o effect polymerisation.

While composites show higher strength values, lower thermal expansion, lower setting contraction and may be radiopaque, there are still a number of problems remaining. The different abrasion rate between the ceramic filler and the organic matrix produces a surface which is difficult to polish and wears t o a rough surface very prone t o staining.15 Some doubt has been expressed concerning the long term stability of the silane coupling agent.

3.1 Adhesion to calcified tissues

One desirable property of a filling material is that it should adhere t o the calcified tissues of the tooth. This would assist in the prevention of dental decay, eliminate leakage around the margins of fillings, permit ease of cementation of objects t o the teeth and preserve tooth structure by eltninating the need for the provision of mechanical reten- tion when preparing the tooth t o receive a filling.16 Two types of system are currently used:

(1) The attachment of unfilled dimethacrylate polymer mechanically bonded t o enamel which has been etched by the application of dilute acid.

(2) The adhesion of dental cements t o tooth substance. The acid etch system is currently used for the sealing of fissures and other developmental faults in the enamel. U.V. activated dimethacrylates have shown good results in this respect.

Dimethacrylate based restorative composites are also used following acid etching to restore broken incisal edges of teeth, and also to provide esthetic coating for a tooth which may be badly stained. The method is not applicable t o dentine. The direct adhesion system contains polyacrylic acid or copolymers thereof, and these are believed t o bond t o the calcium of the tooth tissue. The materials concerned are the zinc poly-carboxylate cements which were developed in Manchester l8 and consist of a powder (ZnO with possibly some MgO) and a liquid, approximately 40% aqueous solution of polyacrylic acid. The tensile bond strength to clean dry enamel is 8.5 MN/m2 which is much greater than that of other dental cements. The bond t o dentine is poorer (1.7 MN/m2). These cements are very opaque because of the large amounts of unreacted ZnO remaining, and this is a disadvantage where a translucent restoration such as a procelain crown is t o be cemented. Class-ionoiner cements (Alumino Silicate Polyacrylic Acid = ASPA) consists of a powder of glass prepared froin fusing quartz and alumina in a fluorite/cryolite/aluniiniuiii phos- phate flux, i.e. similar t o dental silicate cement powder. The liquid is an aqueous solution of acrylic acid/itaconic acid co-polymer. The setting mehcanism is believed t o involve migration of protons into the glass displacing A1 3t and Ca2 cations which are accepted by polyacrylic acid to form chelates causing the material'to gel and set.I9 These cements have the adhesive properties of zinc poly- carboxylate cements. Apart from the poor adhesion t o dentine of the direct bonding cements, the hydrolytic stability of the polyacrylic acid cements may be suspect.

In conclusion, it must be made clear that the above does not comprise a complete review of the polymers used in dentistry, nor all the information available about them. The aim of this paper has been t o consider broadly some of the important polymers which find dental applications, and perhaps provoke interest in some of the problems associated with them from the point of view of a clinician.

References 1

2

Grant, A. A., Metals uscd in the Dental and Medical Profession, Metal Aus f . , 1969, 389-392, Dec.-Jan. McCabc, J . F. & Wilson, H. J . Polymers in Dentistry, J . Oral Reliah., 1976, 1, 335.

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Boucher, C. O., Hickey, J. C. & Zarb, G . A. ‘Prosthodontic Treatment for Edentulous Patients, 1975, 7th Edit., Saint Louis: C K M0sbe.y. Combe, I . C., ‘Notes on Dental Materials’, 3rd Ed., 1977, Edinburgh: Churchill Livingstoni: Smith, D. C., The acrylic denture base, Brit. Dent. J . , 1961, 110, 257. Bates, J. 1:. & Stafford, G. D., Polymeric Denture Base Specification, Biorned. Engiti., 1973, July, 288. Combe, E. C. & Grant, A. A., The selection and properties of materials for dental practice, 7 - polymeric denture materials, Brit. Dent. J., 1973, 134, 289. Storer, R., Resilicnt denture base materials, ibid. 1962, 113, 195 and 231. Wichteric, 0. & Lim, D., Biocompatible hydrophilic polymers, Natrrrc. 1969, 185, 117. Bradcn, M., Tissue conditioners, 1. Composition and structure, J. Dort. Res., 1970, 49, 145. Braden, M., ‘Scientijfc aspects of Dental Materials’, ed. J. A. vun I:raunhofer, 1975, Chapter 13, London: Butterworth. Deubert, L. W. & Jenkins, C. B. G., ‘Tooth Coloured Filling Materials in Clinical Practice’, 1972, Bristol: John Wright.

13 Hannah, C. McD. & Combe, E. C., Mechanical Properties of Composite Restorative Materials, Brit. Dent. J., 1976, 140, 167. Paffenbarger, G. C., ‘Dental Cements, Direct Filling Resins, Composite and Adhesive Restorative Materinls: A ResumP J. Biorned. Maf. Res. Symposium No. 2 (Part 2), 1972, p363, New York: John Wiley. Hannah, C. McD. & Smith, G. A., The Surface Finish of Composite Restorative Materials, Brit. Dent. J., 1973, 135,483. Retief, D. H. The Principles of Adhesion, J. Dent Assoc. Sth. Africa., 1970, 25, 285. Rock, W. P., Results obtained with Two Different Bis-GMA Type Sealants after One Year, Brit. Dent. J., 1973, 139, 193. Smith, D. C., A Review of the Zinc Polycarboxylate Cements, J. Canad. Dent. Assoc., 1971, 31, 22. Crisp, S., Fcrner, A. J., Lewis, B. G. & Wilson, A. D., Properties of Improved Glass-Ionomer Cement lormulation, J. Dent., 1974, 3, 125.

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