BackgroundMaterial wear testing is an important technique in the development and evaluation of materials for use in implant for total knee arthroplasty. Since a knee joint induces a complex rolling-gliding movement, standardised material wear testing devices such as Pin-on-Disc or Ring-on-Disc testers are suitable to only a limited extent because they generate pure gliding motion only.MethodsA rolling-gliding wear simulator was thus designed, constructed and implemented, which simulates and reproduces the rolling-gliding movement and loading of the knee joint on specimens of simplified geometry. The technical concept was to run a base-plate, representing the tibia plateau, against a pivoted cylindrical counter-body, representing one femur condyle under an axial load. A rolling movement occurs as a result of the friction and pure gliding is induced by limiting the rotation of the cylindrical counter-body. The set up also enables simplified specimens handling and removal for gravimetrical wear measurements. Long-term wear tests and gravimetrical wear measurements were carried out on the well known material pairings: cobalt chrome-polyethylene, ceramic-polyethylene and ceramic-ceramic, over three million motion cycles to allow material comparisons to be made.ResultsThe observed differences in wear rates between cobalt-chrome on polyethylene and ceramic on polyethylene pairings were similar to the differences of published data for existing material-pairings. Test results on ceramic-ceramic pairings of different frontal-plane geometry and surface roughness displayed low wear rates and no fracture failures.ConclusionsThe presented set up is able to simulate the rolling-gliding movement of the knee joint, is easy to use, and requires a minimum of user intervention or monitoring. It is suitable for long-term testing, and therefore a useful tool for the investigation of new and promising materials which are of interest for application in knee joint replacement implants.

BackgroundCeramic materials are used in a growing proportion of hip joint prostheses due to their wear resistance and biocompatibility properties. However, ceramics have not been applied successfully in total knee joint endoprostheses to date. One reason for this is that with strict surface quality requirements, there are significant challenges with regard to machining. High-toughness bioceramics can only be machined by grinding and polishing processes. The aim of this study was to develop an automated process chain for the manufacturing of an all-ceramic knee implant.MethodsA five-axis machining process was developed for all-ceramic implant components. These components were used in an investigation of the influence of surface conformity on wear behavior under simplified knee joint motion.ResultsThe implant components showed considerably reduced wear compared to conventional material combinations. Contact area resulting from a variety of component surface shapes, with a variety of levels of surface conformity, greatly influenced wear rate.ConclusionsIt is possible to realize an all-ceramic knee endoprosthesis device, with a precise and affordable manufacturing process. The shape accuracy of the component surfaces, as specified by the design and achieved during the manufacturing process, has a substantial influence on the wear behavior of the prosthesis. This result, if corroborated by results with a greater sample size, is likely to influence the design parameters of such devices.

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Free form surfaces are used in various applications, such as in the aviation industiy, in the medicine, or for tool and die making. Compressor blades as well as knee prostheses and dies have complex curved surfaces. Five-axis grinding is a possibility to machine such curved swfaces in a high shape accuracy and surface quality. The use of this technology depends on a high degree of the operational background. Furthermore, the complexity of the tool path generation requires the use of computer-aided design/computer aided manufacturing {CAD/CAM) systems. This technical review gives an overview about state of the art of five-axis grinding and presents results, which can close some scientific lacks. Models were developed to predict the surface roughness and material removal dependent on the process parameters. Additionally, the relationship between tool geometry, shape accuracy as well as contact conditions is discussed. How to Grind With Five-Axis?Workpieces become more and more complex due to improving function, optic or haptic as well as downsizing of dimensions. These workpieces are often made of materials, which are difficult to machine. For example, complex shaped knee implants out of wear-resistant biocompatible oxide ceramics or forging dies for crankshafts reinforced by ceramic inlays [1,2]. Double curved or free formed surfaces can be ground with three-or five-axis kinematic. By applying three-axis kinematic, toric grinding wheels move line by line over the surface, whereby the tool axes are always parallel to machine tool axes [3]. However, profitability and precision of the three-axis manufacturing are limited. Changing workpiece curvatures leads to varying contact conditions and decreasing shape accuracy. In contrast to three-axis kinematic, five-axis machine tools have three translational axes (X, Y, and Z) and two additional rotational axis. These axes enable a simultaneous rotation and tilling of the tool relative to the surface of the workpiece [4,5]. As a result, the grinding wheel axis can always be oriented vertically to the normal vector of the surface and the variation of the contact conditions is reduced. Moreover, due to the high kinematic flexibility of five-axis manufacturing, undercuts as well as complex free formed surfaces are machined with high precision [6], Five-axes processes enable machining within

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Kurzfassung Am Institut für Fertigungstechnik und Werkzeugmaschinen (IFW) der Leibniz Universität Hannover entwickeln Forscher funktionsangepasste Fertigungsverfahren zur Herstellung medizinischer Implantate. Die in diesem Beitrag vorgestellten Projekte zeigen Ansätze, wie das spätere Einsatzverhalten der Implantate durch die Anpassung und Weiterentwicklung der Fertigungsprozesse an die spezifischen Implantatfunktionen verbessert oder auch neue Einsatzgebiete erschlossen werden können.