| Abstract
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2834 |
| Title |
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Flexible Polishing with Bounded Grains for Complex Ceramic Endoprostheses |
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Grinding, Polishing, and Lapping |
| Primary
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| Organization |
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| Content |
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Worldwide, the knee is the most medically treated joint. On the basis of this fact alone in Germany currently 80,000 knee prostheses are implanted per year. The complication rate on the whole amounts to about 26%. The reasons for revision operations are multiple: infections, dislocations, wear, fracture, and implant loosening are the most important causes. The main reason for the failure of the interconnection between bone and prosthesis is the agglomeration of wear debris of the polyethylene part. These wear fragments can activate bone decomposing osteoclasts via immune reactions. Besides functional losses strong pain is the consequence and soon a revision operation becomes necessary. Due to this the average patients age at first implantation relatively high with 65 years and the average prostheses durability amounts to only 10 years.
Inspired by the successfully employed ceramic hip joint prostheses, the trend for the development of implant material pairings goes towards the use of ceramics. Particularly advantageous are the lower friction coefficient, the increased biocompatibility and the reduced wear. So the objective of this project is to improve the durability for complex implants, like knee implants, via the utilization of ceramics.
Ceramics are featured by the ability to withstand great compressive stresses, yet they break in a brittle manner at relatively small tensile loadings. Furthermore, they exhibit very low fracture toughness. Depending on the fault state range and according to the fineness of the microstructure the strength properties are very scattered. Thus the finishing of ceramic components has a considerable influence on the durability. It is very important to realise a finishing process with high accuracy. Therefore tool and process engineering for free formed ceramic surfaces as well as 5-axes-manufacturing strategies for adapted grinding and polishing processes are essential. The intention is to realize automated high-precision machining processes in one machine, so that the low-wear ceramic also gets the upper hand for other complex joint replacements.
Targeted is to machine the macro geometry of the near net shape sintered ceramic component with high-precision via grinding by means of toric pins with five simultaneous controlled axes. Following, a 5-axes-polishing process with very small removal rate has to be realised. Only the roughness peaks generated by the grinding process will be levelled with a minimal influence on the already generated shape of the part.
Constantly changing contact conditions have to be considered. Here the polishing process with appropriate tools is focused. Flexible tools are applied which are able to adjust to the form variation of the component contour. Two promising polishing methods with bound abrasives are presented. On the one hand rubber bond diamond tools are applied with a closer look on the modelling of the influencing parameters. On the other hand a polishing band apparatus is introduced with first experimental results. |
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