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Item type:Publication, Access status: without files , Wear patterns of taper connections in retrieved large diameter metal-on-metal bearings(Wiley, 2013-02-25); ; ; ; Wear of the modular taper between head and shaft has been related to clinical failure resulting from adverse reactions to metallic debris. The problem has become pronounced in large metal-on-metal bearings, but the mechanism has not yet been fully understood. We analyzed retrieved components from five patients revised with various diagnoses. Two distinct wear patterns were observed for the head tapers. Three samples demonstrated "asymmetric" wear towards the inner end of the head taper. The other two showed "axisymmetric" radial wear (up to 65 μm) presenting the largest wear volumes (up to 20 mm3). Stem tapers demonstrated relatively little wear, and the fine thread on the stem taper surface was observed to be imprinted on the taper inside of the head. Our findings demonstrate that the cobalt-chrome head wears preferentially to the titanium stem taper. "asymmetric" wear suggests toggling due to the offset of the joint force vector from the taper. In contrast, samples with "axisymmetric" radial wear and a threaded imprint suggested that corrosion led to head subsidence onto the stem taper with gradual rotation. © 2013 Orthopaedic Research Society.Publicationtype: Journal Article67 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Microstructure of retrievals made from standard cast HC-CoCrMo alloys(ASTM International, 2013); ; ; ; During the past decade, self-mating metal bearings based on cobalt- chromium-molybdenum (CoCrMo) alloys have become very popular in total hip replacements and hip resurfacings. This led to a market share of more than 35 % for metal-on-metal (MoM) bearings in the United States before several cases of high wear with biologic consequences led to a sharp drop in popularity. In part, these failures are a result of a very shallow understanding of the wear mechanisms in MoM joints and their relation to the microstructure. In order to find such a relation, one has to keep in mind that the microstructures of metallic materials depend distinctly on the entire production sequence. In addition, they change markedly under tribological stresses. This paper does not discuss the wear of any specific retrieval or even try to relate that to the specific microstructure, because such a task would be impossible based on the unknown loading history of such retrievals. Thus, we depict only the possible range of microstructures from standardized high carbon (HC)-CoCrMo retrievals. These reveal different types of hard phases: carbides and/or intermetallic phases. Some are fine (<10μm) and homogeneously distributed, whereas others appear as thin (<1 μm) and brittle cord-shaped arrangements at grain or dendrite boundaries. Coarser (>30 μm) types of mixed hard phases, which consist of carbides and interme-tallic phases, often show microcracks already below the articulating surfaces. Such subsurface microcracks are known to destabilize the gradient below the surface and the balance between tribochemical reactions and surface fatigue. In this paper, the microstructures of retrievals manufactured from standard cast CoCrMo alloys are shown and evaluated.Publicationtype: Conference Paper45 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Micro-structural alterations within different areas of articulating surfaces of a metal-on-metal hip resurfacing systemMetal-on-metal hip arthroplasties have been successfully implanted for many years. During the last decade the metal-on-metal hip resurfacing has also experienced a renaissance. Both perform low wear rates due to nano-meter-scale wear particles. These particles represent a limitation causing harm to the human body. In order to investigate the emission of wear particles in ultra-low wear systems micro-structural changes in the subsurface-region have to be well understood. Previous studies of metal-on-metal total hip arthroplasties have revealed that an in situ formation of a nano-crystalline layer together with mechanical mixing enables the CoCrMo alloy to adjust to the current load situation and maintain a low wear rate. However, only little is known about the wear performance of hip surface replacements which are exposed to a different stress/strain situation. For this study a Durom hip resurfacing system (Zimmer, Warsaw, USA) with a 46-mm diameter was analyzed by means of transmission electron microscope (TEM) and electron energy loss spectroscopy (EELS) with energy filtered TEM (EFTEM). The results were compared to earlier findings for total hip arthroplasties. For the sampling locations three zones were chosen according to geometry and wear appearances: (1) primary articulating contact zone, (2) a zone which received edge loading and (3) inferior area of the cup. In the primary wear zone of the femoral component a nano-crystalline layer with a thickness of 250-400 nm and grain size of 25-40 nm occurred. This layer must be the origin of wear particle detachment. The EELS measurements showed no incorporation of carbon in-between the nano-crystals, whereas in THA samples it occurred down to a depth of 200 nm. This carbon is suspected to be of organic origin. In the second area a nano-crystalline layer of 100-150 nm exhibited a sharp border to another layer of orientated ε-martensite needles underneath. This paper will compare the different zones and relate the structure and chemistry to the differing load histories. The results will be compared to earlier findings for total hip arthroplasties.Publicationtype: Journal Article3 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Investigation on stick phenomena in metal-on-metal hip joints after resting periods(Sage Publications, 2006-02); ; ; ; Insufficient understanding of tribological behaviour in total joint arthroplasty is considered as one of the reasons for prosthesis failure. Contrary to the continuous motion input profiles of hip simulators, human locomotion contains motion interruptions. These occurring resting periods can cause stick phenomena in metal-on-metal hip joints. The aim of the present study was to investigate the tribological sensitivity of all-metal bearings to motion interruptions on in vitro test specimens and retrieved implants. Friction and wear with and without resting periods were quantified. Unlike the metal-on-polyethylene joints, the static friction of metal-on-metal joints increased up to micros = 0.3 with rest, while wear appeared to be unaffected. This effect is caused by the interlocking of firmly adhered carbon layers, which were generated from the protein-containing lubricant through tribochemical reactions. Since more than 80 per cent of the retrieved implants exhibited macroscopically visible carbon layers, the increase in friction presumably also occurs under physiological conditions, which is then transferred to the bone-implant interface. These recurrent tangential stress peaks should be considered for the design features of the cup-bone interface, in particular when larger-sized implant heads are used.Publicationtype: Journal ArticleScopus© Citations 31 328 632