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Dental TribuneAsia Pacific Edition | 1+2/2016 14 TRENDS&APPLICATIONS Thermoplastic materials have been usedinaviationandspaceengineer- ing for a long time. Owing to their high mechanical strength and low modulus of elasticity, they have be- gun to increasingly replace metal in manymanufacturingindustriestoo, particularly in those where metal has been the dominant choice until now. Implants for intervertebral discs, as well as hip and knee joints, are made of PEEK, a thermoplastic polymer.Four million implants have been fitted during the last 15 years with outstanding success. In recent years, thermoplastic materials have alsobeenusedindentaltechnology. This article discusses a number of commonplasticmaterials that have become alternatives for use in the manufacture of non-metal tele- scopic dentures. About 15 years ago, the first at- tempts were made, not without initial problems, to produce non- metal telescopic dentures. These dentures were made by injection moulding using a polyamide (PA) in the dental laboratory. A wax mould of the framework, bar and secondary crowns is made as an integral part, embedded in plaster in a flask and the wax boiled out. The plastic material, which is avail- able in the laboratory as granular material, is heated in the injection moulding device and injected into the mould. After a period of cool- ing, which should not be shorter than specified, the prosthesis is re- moved from the mould and fin- ished. Special milling cutters are needed because the material tends to become viscid when cut. Very importantly, absolutely no metal must be entrained. If the denture were to cut by a tool previously used for cutting metal, minute metal particles would be incorpo- rated into the thermoplastic mate- rial by the milling cutter. Friction would easily be controlled by ex- pansion plaster. The good sliding properties and the high friction of the secondary crown particularly surprised us. Wheninserted,thesecondarycrown slides along the primary crown and is retained partly by clamping and partly by suction. Our patients found the good sliding properties and the light weight comfortable. The modulus of elasticity of PA is very low, which lends flexibility to thematerial.Thisgivesthepatienta sensation of a readily adapting den- ture, rather than a foreign body, in hisorhermouth(Figs.1–3). The low modulus of elasticity, however, turned out to be the greatest drawback of the material. The moduli of elasticity of all plas- tic materials used for bonding are very high and two moduli as wide apart as these cannot be bonded reliably for a long time by any means available to dental labora- torytechnicians.Asaconsequence, many dentures develop cracks and spalls in the bonds after several months. In addition, the large pores onthe surface of the denture led to discoloration, particularly in patients with an altered acid–base balance. FPM A short while after PA, the indus- try launched a successor material with FPM. This thermoplastic fluo- ropolymer offers some flexibility, but less than that of PA, however. The modulus of elasticity is mar- ginally higher than that of PA, but distinctly lower than that of metal. Consequently, similar problems as those encountered with telescopic dentures of PA occurred. PMMA We have obtained good results with PMMA (Polymethylmeth- acrylate). This plastic material is veryhardandinflexible.Finishable in different colours, it is used for complete dentures and occlusal splints, as well as for long-term temporary dentures, crowns and bridges.Thematerialisnotsuscep- tibletoplaque,anddiscolorationis very inconspicuous. The moduli of elasticity of bond- ing materials and PMMA are simi- lar; thus, cracks and spalls of bonds did not occur. Patients who had previously worn a telescopic pros- thesisofPAorfluoropolymer,how- ever, complained that the denture of the new material was uncom- fortable to wear. PMMA’s lack of flexibility gave patients the sensa- tion of having a foreign body in their mouth (Figs.4–6). Unfortunately, denture breaks were reported after some time, particularly in free-end situations. Also, dentures not lined regularly and exposed to high force tended to break. We believe one reason for that is the fairly high modulus of elasticity, which makes the mate- rial somewhat brittle. The greatest problem, however, is that thermo- plastic materials cannot be re- paired.Thereisnowayofrepairing cracks or fractures. The only solu- tion is to make a new denture. PEEK PEEK (Polyetheretherketone) was first used for telescopic den- turesaboutsixyearsago.Ingeneral medicine, it has been used for hip, knee and intervertebral disc im- plants for almost 15 years. Accord- ing to German company Evonik Industries, as many as four million implants have been fitted and not a single case of proven allergy to that material has been reported. ThemodulusofelasticityofPEEKis similar to that of bone, with posi- tive consequences for integration. ThisisoneofthereasonsthatPEEK merits the attention of dental lab- oratory technicians. Finally, there is a material with a hardness simi- lar to that of bone, not as soft as PA or FPM plastics and not as hard as PMMA. These very rigid materi- als often cause dental technicians problems, for example with all- ceramic solutions for the upper jaw, where craniomandibular prob- lems frequently arise. PEEK is a very light-weight ma- terial with a long history of use in spaceflight.Non-conductive,ithas Thermoplasticmaterials indentaltechnology An overview in the case of non-metal telescopic dentures By Claudia Herrmann,Germany 1 1 2 3 6 4 5 7 8 9 13 12 11 10 DTAP0116_14-15_Hermann 12.02.16 13:00 Seite 1 123 45 789 DTAP0116_14-15_Hermann 12.02.1613:00 Seite 1