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ePaper created 2016-10-04, 09:33:57 | version 1.38.0
thermoplastic materials review | 17 CAD/CAM 3 2016 The good sliding properties and the high friction of the secondary crown particularly surprised us. When inserted, the secondary crown slides along the pri- mary crown and is retained partly by clamping and partlybysuction.Ourpatientsfoundthegoodsliding properties and the light weight comfortable. The modulus of elasticity of PA is very low, which lends flexibilitytothematerial.Thisgivesthepatientasen- sation of a readily adapting denture, rather than a foreign body, in his or her mouth (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 plastic 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 laboratory technicians. As a consequence, many dentures develop cracks and spalls in the bonds after several months. In addition, the large pores on the surface of the denture led to discoloration, particularly in patients with an altered acid–base balance. FPM A short while after PA, the industry launched a suc- cessor material with FPM. This thermoplastic fluoro- polymer offers some flexibility, but less than that of PA, however. The modulus of elasticity is marginally higher than that of PA, but distinctly lower than that ofmetal.Consequently,similarproblemsasthoseen- countered with telescopic dentures of PA occurred. PMMA We have obtained good results with PMMA (Poly- methylmethacrylate). This plastic material is very hard and inflexible. Finishable in different colours, it is used for complete dentures and occlusal splints, as well as for long-term temporary dentures, crowns andbridges.Thematerialisnotsusceptibletoplaque, and discoloration is very inconspicuous. The moduli of elasticity of bonding materials and PMMAaresimilar;thus,cracksandspallsofbondsdid not occur. Patients who had previously worn a tele- scopic prosthesis of PA or fluoropolymer, however, complainedthatthedentureofthenewmaterialwas uncomfortable to wear. PMMA’s lack of flexibility gave patients the sensation 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 material somewhat brittle. The greatest problem, Fig. 1 Fig. 2 Fig. 3 32016