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ePaper created 2016-10-04, 09:33:57 | version 1.38.0
Thematerialhasverygoodslidingpropertiesandpa- tients report that it is extremely comfortable to wear. There are two different methods of manufacture. One is injection moulding and the other is CAD/CAM milling. The minimum thickness of telescopes is 0.6 mm. The minimum thickness of frameworks and bars is distinctly higher, but varies depending on the design and the size of the telescopic prosthesis, as well as the number of available telescopes. Generally, a PEEK telescopic prosthesis will be a little thicker than a metal telescopic prosthesis. It is an absolute necessitythattheprimarycrownbemadeofzirconia, as abraded metal particles would otherwise collect under the secondary crown. Theveneerbondstrengthwastestedinastudyatthe University of Regensburg, Germany, in 2012. In order to pass the test, a value of 5 MPa had to be achieved. Of all the veneering systems tested, PEEK scored 10MPaandaboveandpassedallofthebondstrength tests. In other tests, such as discoloration and shear strength, it also achieved very positive results, con- firming the suitability of PEEK for use in the oral cavity. When subjected to load at fracture tests, a PEEK bridge achieved 2,354 N and was far superior to a ceramic bridge, with 1,702 N. Hence, PEEK can withstandhigherloadsintheoralcavitythancance- ramicmaterial,andsowide-spantelescopicdentures can be made of PEEK. It is necessary when handling telescopic dentures of PEEK to apply ceramic guidelines because the ma- terial could otherwise be weakened owing to crack propagation. In addition, the prosthetic design must followcertaincriteria.Forexample,aprosthesiswith- out a transverse bar must always include a backing plate in the secondary part to provide sufficient sta- bility.Dentaltechniciansrequiredtomakenon-metal telescopic prostheses should therefore receive suffi- cient training and instruction so that the required high-qualitylevelcanbemaintained.Thosewhowork with PEEK only rarely and who therefore lack experi- ence are advised to have telescopic prostheses of PEEK designed and cut in a specialised laboratory. Even in our laboratory, we have come across PEEK prostheses with cracks, but these have invariably been due to manufacturing mistakes. Prostheses made correctly exhibit no cracks. Cracks and spalls of theveneeringofPEEKdenturescanbefoundaboutas often as in telescopic prostheses of metal—that is, rather seldom. PEEK is extremely resistant to plaque and inert to acids and chemicals; therefore, the denture can be cleaned with a chemical dental cleaner. Friction is one of the most critical characteristics of telescopic prostheses. The friction of PEEK is very good and can be controlled excellently with expan- sion plaster. However, most important is that friction ispermanent.Wemadeourfirsttelescopicprostheses of PEEK about five years ago and we have not ob- served any loss of friction in that time (Figs. 7–13). Conclusion Our laboratory has the experience of having made over 300 non-metal telescopic prostheses over the course of 11 years. After initial problems and several tests, PEEK has finally proven a suitable material for telescopicdenturesinthelongterm.Non-metaltele- scopic prostheses are in no way inferior to metal telescopic dentures, provided they are made profes- sionally. On the contrary, the light weight, the high wear comfort and the absence of metal, in particular, arecompellingargumentsfordentaltechniciansand patients alike._ thermoplastic materials review | 19 CAD/CAM 3 2016 contact Claudia Herrmann is owner and CEO of Dental-Labor Herrmann in Bad Tölz in Germany. She can be contacted at abt@dl-herrmann.de. Fig. 12 Fig. 13 32016