[Show abstract][Hide abstract] ABSTRACT: In this paper the performance of a magnesium based implant system is analyzed. A special emphasis is placed on the impact of stress on the corrosion behavior of the magnesium alloy. An implant system containing a plate and 4 corresponding screws is machined from Mg LAE442. Its corrosion behavior is tested in-vivo in New Zealand White Rabbits for 6 and 12 weeks of implantation. The plate is monocortically fixated on the medial tibia. At the interface between screw and plate increased corrosion is observed. This phenomenon is stronger on the caudal side of the screw. Parallel to the in-vivo test the influence of stress load on the corrosion rate is analyzed for LAE442 in in-vitro tests. Compressive load is applied on cylindrical specimens in axial direction and the corrosion rate is measured in 0.9 wt% NaCl solution by eudiometry and mass loss. Additionally rectangular samples are bent to apply tensile stress on the surface. A drop of 5 wt% NaCl is deposited on the surface and the corrosion is evaluated by microscopic images. It is shown that stress essentially influences the corrosion rate. While tensile stress decreases the corrosion, compressive stress leads to higher corrosion rates.
[Show abstract][Hide abstract] ABSTRACT: To date, bioceramics have not been applied successfully in total knee joint endoprostheses. Sintered bioceramics can be machined only by grinding and polishing processes. Due to high quality requirements, there are significant challenges with regard to these machining technologies. An automated precise economical process chain for the manufacturing of a new all-ceramic knee implant design was developed. It was assumed the geometrical accuracy and the shape of implant contact geometry specified during the manufacturing process has a substantial influence on the wear behavior of the prosthesis. The importance of the surface quality of the ceramic implant surface remains unclear and warrants future examination.
[Show abstract][Hide abstract] ABSTRACT: Ceramic 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.
A 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.
The 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.
It 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.
[Show abstract][Hide abstract] ABSTRACT: Throughout the world, more and more artificial knee prostheses are implanted due to the rise in cases of arthrosis. Inspired by all-ceramic hip joint implants, recent research in the field aims at enhancing the durability of the complex knee implants by using wear-resistant ceramics in a hard/hard joint pairing. Due to its kinematics and its complex geometry, the ceramic knee implant makes high demands on the manufacturing process if a high-quality level has to be assured. Firstly, the grinding process has to provide high surface qualities in order to reduce efforts in the polishing process; secondly, high contouring accuracy is required to avoid stress peaks in the ceramic joint pairings. Constant material removal in 5-axis grinding is assured by using toric grinding pins and accounting for the constantly changing contact conditions.In this paper, the contact conditions for a frontal grinding process with toric grinding pins are determined. Firstly, the contact length is calculated to detect the engagement zone in frontal grinding with toric tools. An angle range for constant material removal was found out. Furthermore, a model for the prediction of the resulting roughness of grinding processes with toric, ceramic bonded diamond grinding pins is developed and verified by means of grinding tests.
International Journal of Machine Tools and Manufacture 07/2010; 50(7):630-636. DOI:10.1016/j.ijmachtools.2010.03.008 · 3.04 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Material 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.
A 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.
The 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.
The 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.
[Show abstract][Hide abstract] ABSTRACT: Das fünfachsige Schleifen bietet eine Vielzahl an Möglichkeiten, komplexe Bauteile wirtschaftlich zu fertigen. Doch Werkzeug- und Strategiewahl sind entscheidend für eine erfolgreiche funktionsgerechte Bearbeitung
[Show abstract][Hide abstract] ABSTRACT: Currently about five million people are diseased by arthrosis. This painful inflammation is often attended by an artificial
Due to immune reactions resulting from the agglomeration of the polyethylene wear implant loosening appears. In the field
of hip endoprostheses the functionality of ceramic hip implants is well known since many years. To minimize the necessary
revision surgeries bioceramics will be also used for knee implants.
However, the use of bioceramic hard-hard-pairings for total knee arthroplasty results in high demands on the geometrical accuracy
as well as the surface topography of such complex implants. Since ceramics cannot cope with non-uniform loads, a new adapted
design for ceramic knee implants is required. Hence, the biomechanical requirements have been investigated. In a roll-gliding
wear simulator specimens of simplified geometries have been tested under movements and stresses similar to the knee joint.
Due to the fact that the complex implants will be machined in one clamping, the high geometrical requirements can be achievable.
After 5-axis-grinding with torical grinding pins the polishing process with flexible tools takes place in the same machine.
Two concepts for flexible polishing tools are traced. Here, a new belt apparatus for flexible polishing is presented in addition
to flexible rubber tools with bonded diamond grains.
[Show abstract][Hide abstract] ABSTRACT: INTRODUCTION Worldwide, the knee is the most medically treated joint. The complication rate amounts to about 26% on the whole. The main reason for the failure of the interconnection between bone and prosthesis is the agglomeration of wear debris of the polyethylene part . Inspired by the successfully employed ceramic hip joint prostheses, the trend in the develop-ment of implant material pairings goes towards the use of ceramics [2-3]. So the objective of this project within the scope of the Collaborative Research Center 599 sponsored by the DFG is to improve the durability of complex implants, like knee implants, by means of new machining technologies for ceramics . The objective is to realize automated high-precision machining processes using one single machine, so that the low-wear ceramic also gets the upper hand for other complex joint replacements. Two promising polishing methods using flexible tools with bound abrasives are presented. On the one hand, rubber bond dia-mond tools are applied with special focus on the modeling of the influencing parameters. On the other hand, a polishing belt apparatus is intro-duced including first experimental results. MATERIAL REMOVAL MECHANISMS In general, ceramic materials are machined us-ing abrasive processes with diamond particles. For controlling the surface properties, the in-volved material removal mechanisms have to be characterized. In the most commonly applied machining processes with geometrically non-defined cutting edges like grinding, honing, lapping and polishing, the material is removed by the penetration of separate diamond cutting edges into the working material. Zum Gahr  classified the mechanisms in-volved in the wear of the material. Two of these mechanisms are relevant for the polishing of ceramics using flexible tools with bound grains (FIG. 1). The penetration of the grains mostly causes plasticizing and a ductile removal of ceramic particles, but unbound grains, which are rolling between the flexible bonding and the workpiece, can cause micro-cracks and brittle outbursts.