Plastics in Dentistry

Dr David Brown

A lecture given to the PHS in May 1993 by Dr David Brown, Head of the Department of Dental Materials Science, U.M.D.S. Dental School, Guys Hospital, London. The following is an interpretation of the recorded content of the lecture, which was illustrated throughout by slides and fascinating models.

Dr Brown started by drawing attention to St Apolionia, patron saint of dental patients. to remind us of the lon2-established nature of toothache as an enervating illness. For many years dental decay was the major cause of ill health in the developing world, and caused by, it was alleged, the tooth worm. Whatever the cause the dentist, or in the earlydays the barber/surgeon, is required to remove the decay and take conservation action on the cavity. Today a high-speed drill, diamond or tungsten tipped, rotates at speeds of up to 500,000 rpm, removes the decay and the cavity is filled with dental amalgam, precious metals such as gold, or a modem polymeric composite.

The speaker gave us his guide to polymerisation chemistry and went on to discuss polymer-based fillings which have the great advantage that they are tooth coloured. The first polymers used in this application were polyacrylates such as polymethylmethacrylate. They were presented as a mixture of polymer/acrylate monomer and an initiator, but wear resistance of straight polymer mixtures was not satisfactory and fillers were added to remedy this. More recently, composites have been used successfully. They are a mixture of polymer, such as Bowens Resin, the reaction product of bisphenol A and glycidyl methacrylate, a filler which is generally inert glass particles, and an initiator, and curing is effected by combining a two-part pack of (a) resin / filler 1 activator, with(b) resin / filler 1 initiator, but the mixture must be in place before it sets. It is important to select the right filler and quartz particles, irregular in shape and in a range of particle sizes, are an attractive alternative to glass. The main disadvantage of these polymeric composites remains their tendency to shrink on polymerisation. The most recent development uses light-cured polymeric systems' In this case, a unidose ‘compule' containing a mixture of polymer paste, filler and initiator is used. The contents are extruded into cavity, and the composite is cured y exposure to blue light at a wavelength of 470 nanometres. Polymeric composites can also be used for the repair of damaged teeth, and the speaker illustrated a number of applications. In very difficult cases a polymeric composite can be used as a veneer on damaged teeth, in which case it is mechanically bonded by etching the dental enamel with phosphoric acid.

An interesting incidental application for polymeric materials is in plaque removal, where the medieval dental skills of scraping have been replaced by a polymeric fibre formed from a mixture of polypropylene and nylon and used by the patient himself.

In extreme conditions the removal of all the teeth leaves the patient requiring a denture, and polymers have a variety of roles in their preparation. The process starts by taking an impression of the patient's upper and lower jaws by requiring the patient to 'bite' onto a mass of silicone based polymer carried by a polymer-based tray, typically made of polymethylmethacrylate, self-curing or light-cured. From these impressions models are produced in plaster or dental stone, and wax rims are prepared from these models. The next stage is to insert polymethyl methacrylate teeth into the wax rim, using teeth of the required shape and shade to suit the patient. The wax rim, with the teeth in position, is then invested with dental plaster. The wax is then boiled away to leave a cavity into which the denture polymer may be introduced -the whole procedure being an example of the 'lostwax' moulding process. After the dental plaster matrix has been coated with a release agent an acrylic dough is introduced into the cavity. This dough-like composition contains, for example, 3parts of polymer and one part monomer. Pressure is applied to squeeze out excess dough and the denture base is then cured, generally in a waterbath at say 720 C for 16 hours, taking care to avoid excess temperatures which could cause monomer to boil off and form a porous moulding.

Historically, a range of materials has been used for denture bases, including stone, wood, shell, bone, horn, ivory and metal. The Romans used bone and ivory (from the hippopotamus) and included natural teeth in the dentures, and subsequently used ivory, and then porcelain teeth. In 1728 springs were invented to hold dentures in position, and even at the time of Waterloo scavengers were active on the battlefield recovering teeth for shipment to London for use in dentures. As a material for teeth ivory suffered from the time taken to shape the material by carving - six weeks work on a full set was not uncommon. ]'his led to ivory ‘plumpers’ being used to fill out the cheeks where a full set of dentures would have been too expensive. The samples shown at the lecture included a set of natural teeth on an ivory base (Fig 2), and a replica of a set of spring-loaded dentures made for the Duke of Wellington. By 1804 all-porcelain dentures, base and teeth had been developed but they proved very heavy in use. There was also the problem of retaining porcelain teeth in the base - modern acrylic denture bases flow successfully around acrylic teeth and form a physico-chemical bond with them.

The development of alternative denture materials included cast tin dentures in 1820, and tortoiseshell dentures in 1850. Vulcanised gutta-percha proved to be unstable, various low melting point alloys and aluminium were ruled out by the problem of how to secure the teeth and it was vulcanite, a hard rubber composition patented by Nelson Goodyear in the period 1851-1858, that was the first successful polymeric material. Vulcanite, also known as ebonite, had a long period of use in dentistry, and examples of vulcanite dentures are still being produced by patients at Guys Hospital Dental School. The dentures were produced using the lost wax process and packing the cavity in stages with compound which was then cured under heat and presure, followed by polishing and trimming. Vulcanite was very difficult to break, but it did have problems since it relied upon mechanical retention of ‘teeth. It was difficult to repair and there were aesthetic disadvantages since it could never be made translucent. The Goodyear patents in the USA were controlled by Josiah Bacon with some tenacity and a chain of agents was employed to extract royalty payments from users. It was no surprise, therefore, when he was. found shot dead - by an angry dentist. The patents expired in 1881 and the use of the material for dentures developed more widely.

The Hyatt brothers in the US led the way in the development of celluloid dentures for a thirty-year period from 1869. The material was a tough, translucent, thermoplastic material that was light in weight, but the plasticiser, camphor, could leach out leaving an unpleasant taste in the mouth. Celluloid was also liable to change colour, and it was notably flammable. Confidence was lost in Celluloid when dentures were returned with lost teeth, discoloration and warping. Cellulose acetate was evaluated, and even a derivative, benzyl acetate, tried out, all without success. A Bakelite (phenolformaldehyde) denture resin sold as Walkerite was evaluated as a facing on vulcanite dentures, other partially cured resins were tried, and in the1930s a superior phenol formaldehyde resin Luxene was introduced. Many materials failed because of the difficulty of moulding one-off denture bases for each patient - the preformed semi-cured bases requiring heat and pressure to achieve the desired configuration were no improvement. Fig 4 summarises the polymers introduced as denture bases since 1851 and indicates the chemical nature of each material.

In the 1930s the range of available polymers increased and many of them were evaluated as denture bases but they all suffered from the common fault arising from the brittle nature of acrylic materials. (Fig 3) Modem acrylics still suffer from the ability to be accidentally damaged, but they can at least be repaired. Polymethyimethacrylate (PMMA) was commercially developed by ICI in 1931 and the first dental acrylic was Kallodent, an injection moulding grade of PMMA, and this was followed by a dough moulding compound from Kolzer of Germany Paladon. This achieved the required dough-like consistency by combining a powder form of the polymer with a liquid monomer. Amongst the polymers tried out were various forms of nylon but these absorb water and swell to make them unsuitable.

Resistance to fracture is most important for denture materials and the Bayer company introduced a polycarbonate Thermopont with a very high impact strength. This again has not been successful because of the need for it to be moulded at high temperatures, c30011 C, into an individual mould for each patient. This type of processing requires sophisticated facilities not generally available to dental laboratories. This is despite the availability of modem dental plasters, prepared eg by adding silicas to the normal gypsum compounds, that withstand high temperatures and pressures.

The most recent developments include the availability of acrylic sheet materials that can be cured by light sources, or even in the microwave. Attempts to overcome the brittleness of denture resins have included the addition of modern fibres, including high density polyethylene, carbon, Kevlar etc, but there is a necessity to keep fibres out of the fitting surface. Polymers modified with rubbers to provide high impact proerties have also been evaluated.

As an example of a typically ingenious modern development involving both the metallurgist and the polymer technologist the speaker showed a titanium implant that is screwed into the bone of the jaw, to carry a mechanically secured bar onto which an acrylic denture may be mounted - thus providing a denture for a patient otherwise incapable of retaining a conventional denture.

In answer to questions from the audience the speaker suggested that the reason that horn did not succeed as a denture. material was its ability to absorb water and swell, contributing to the retention of oral bacteria. Conventional dental amalgam continues to be used for fillings in major cavities because of its outstanding durability whilst modern composites are used for front teeth and small cavities. Residual monomer has not been a problem in dough moulding compounds, apart from some individual cases of allergy or sensitivity. Phenol formaldehyde resins were introduced as part of an attempt to reduce the cost of dentures to the population in general. Today’s ionomer cements have a very particular advantage since they have exceptional adhesion to both dentine and tooth enamel and are widely used for securing fillings where a big drilled cavity is not appropriate. They are formed by curing eg with blue light, a mixture of polyacrylic acid and glass, and additionally contain fluorides which are beneficial.

The speaker suggested that bonding techniques, eg teeth onto metal bridges, was an area of increasing complexity with polymer chemistry playing its part in the development of adhesive systems. Dr Brown confirmed that gutta percha is still the best material for the packing of cavities left inside tooth roots after the pulp has been removed. The material is gently warmed and packed into the cavity and sealed with a zinc oxide cement. There is a question mark of future supplies of gutta percha for this application.

Summarising prospects for the future Dr Brown emphasised the current rate of technological change, within a climate of improved dental hygiene. There is still a real need to replace dental amalgam as a filling material, and the technology of light- cured materials is in its infancy. Eventually, improvements in personal dental hygiene may completely eliminate the need for dentures.