Markus A. Wimmer

The acting wear mechanisms on metal-on-metal hip joint bearings: in vitro results

Metal-on-metal (MOM) hip joint bearings are currently under discussion as alternatives to metal-on-polymer (MOP) bearings. Some criteria under scrutiny are the wear resistance, the influence of wear particles on the surrounding tissue, as well as the frictional torque. In order to understand and control the wear behavior of such a bearing a close correlation between the microstructures of the alloys used and the acting wear mechanisms has to be found. Thus, commercially available CoCrMo-balls were tested against self mating concave pins in a physiological fluid at 37°C under reciprocating sliding wear (1 Hz). The compressive load was 750 N (body weight). For 2×106 cycles tests were carried out continuously and with periodically occurring resting periods. On the basis of the observed wear appearances the acting wear mechanisms are defined and evaluated as to their contribution to the wear behavior. Due to the high local contact stresses surface fatigue prevails initially. Cr– and Mo–carbides are fractured and torn off the surfaces bringing about additional surface fatigue by indentations and initiating abrasion. The weight loss can be predominately attributed to these mechanically dominated wear mechanisms. In a parallel occurring tribochemical reaction layers are generated from denatured proteins. These adhere rigidly to the surfaces and cover parts of the contacting surfaces avoiding adhesion. Thus, the wear behavior is mainly influenced by the alternating balance between surface fatigue and abrasion on the one side and by tribochemical reactions on the other side.

Tribochemical reaction on metal-on-metal hip joint bearings: A comparison between in-vitro and in-vivo results

Abstract Metal-on-metal (MOM) hip joint bearings are considered one of the alternatives to the generally used metal-on-polyethylene bearings. In order to control and minimize wear of MOM bearings, an in-depth understanding of the acting wear mechanisms is essential. In a recent study it was suggested that layers of decomposed proteins are generated due to high pressures between contact spots of the cobalt–chromium alloy bearing. It was further suggested that these tribochemical reaction products greatly influence the wear behavior of the MOM articulation. Since these conclusions were limited to in-vitro test observations, the purpose of this study was to compare retrieved McKee–Farrar prostheses with the previously utilized in-vitro specimens to investigate tribochemical layer presence, composition and role in the overall wear process. Forty-two retrieved McKee–Farrar prostheses with a complete clinical record were compared to the in-vitro specimens. Ninety-three percent of the cups and 83% of the heads of the retrieval collection showed macroscopically and microscopically similar layers than the in-vitro bearings. SEM revealed a varying layer thickness with scratched or smeared sections. Combined SEM and EDS analysis suggested the presence of carbon and oxygen in most of the layers, while some layers showed traces of sodium, magnesium, calcium, nitrogen, sulfur, phosphorus and chlorine, too. These observations were quantitatively verified using XPS. By means of protein standards the organic origin of the layers was shown. Since the latter covered large areas of the contacting surfaces, adhesion is minimized and abrasion is reduced. Thus, the layers have a solid lubricating effect and the general wear behavior of the MOM bearing is affected by generation and delamination of the tribochemical reaction layers.

Metal-on-metal bearing surfaces.

Metal-on-metal bearing couples remain a popular option in total hip arthroplasty and are the only currently available option for surface replacement arthroplasty. In general, the intermediate-term clinical performance of metal-on-metal bearings has been favorable. There are, however, lingering concerns about the biologic consequences of metal release from these bearings in terms of both local tissue effects, including delayed-type hypersensitivity reactions in a subset of patients, and potential systemic effects as a consequence of chronic elevations in serum cobalt and chromium content. Advances in the understanding of the operant wear mechanisms in these bearings provide strategies for reducing the burden of metal released into the periprosthetic milieu, which in turn will mitigate the concerns about the biologic response to the metal debris. Continued surveillance of patients with these bearings is warranted to determine whether metal-on-metal bearing couples provide a long-term survivorship advantage over other bearing couple options and to evaluate whether chronic elevations in the body burden of cobalt and chromium is well tolerated over the long term.

Graphitic tribological layers in metal-on-metal hip replacements.

A graphitic layer is found to be the cause of low friction in metal-on-metal hip implants. Arthritis is a leading cause of disability, and when nonoperative methods have failed, a prosthetic implant is a cost-effective and clinically successful treatment. Metal-on-metal replacements are an attractive implant technology, a lower-wear alternative to metal-on-polyethylene devices. Relatively little is known about how sliding occurs in these implants, except that proteins play a critical role and that there is a tribological layer on the metal surface. We report evidence for graphitic material in the tribological layer in metal-on-metal hip replacements retrieved from patients. As graphite is a solid lubricant, its presence helps to explain why these components exhibit low wear and suggests methods of improving their performance; simultaneously, this raises the issue of the physiological effects of graphitic wear debris.

Wear mechanisms in metal-on-metal bearings: The importance of tribochemical reaction layers

Metal‐on‐metal (MoM) bearings are at the forefront in hip resurfacing arthroplasty. Because of their good wear characteristics and design flexibility, MoM bearings are gaining wider acceptance with market share reaching nearly 10% worldwide. However, concerns remain regarding potential detrimental effects of metal particulates and ion release. Growing evidence is emerging that the local cell response is related to the amount of debris generated by these bearing couples. Thus, an urgent clinical need exists to delineate the mechanisms of debris generation to further reduce wear and its adverse effects. In this study, we investigated the microstructural and chemical composition of the tribochemical reaction layers forming at the contacting surfaces of metallic bearings during sliding motion. Using X‐ray photoelectron spectroscopy and transmission electron microscopy with coupled energy dispersive X‐ray and electron energy loss spectroscopy, we found that the tribolayers are nanocrystalline in structure, and that they incorporate organic material stemming from the synovial fluid. This process, which has been termed “mechanical mixing,” changes the bearing surface of the uppermost 50 to 200 nm from pure metallic to an organic composite material. It hinders direct metal contact (thus preventing adhesion) and limits wear. This novel finding of a mechanically mixed zone of nanocrystalline metal and organic constituents provides the basis for understanding particle release and may help in identifying new strategies to reduce MoM wear.

Osteolysis around total knee arthroplasty: a review of pathogenetic mechanisms.

Aseptic loosening and other wear-related complications are some of the most frequent late reasons for revision of total knee arthroplasty (TKA). Periprosthetic osteolysis (PPOL) pre-dates aseptic loosening in many cases, indicating the clinical significance of this pathogenic mechanism. A variety of implant-, surgery- and host-related factors have been delineated to explain the development of PPOL. These factors influence the development of PPOL because of changes in mechanical stresses within the vicinity of the prosthetic device, excessive wear of the polyethylene liner, and joint fluid pressure and flow acting on the peri-implant bone. The process of aseptic loosening is initially governed by factors such as implant/limb alignment, device fixation quality and muscle coordination/strength. Later, large numbers of wear particles detached from TKA trigger and perpetuate particle disease, as highlighted by progressive growth of inflammatory/granulomatous tissue around the joint cavity. An increased accumulation of osteoclasts at the bone-implant interface, impairment of osteoblast function, mechanical stresses and increased production of joint fluid contribute to bone resorption and subsequent loosening of the implant. In addition, hypersensitivity and adverse reactions to metal debris may contribute to aseptic TKA failure, but should be determined more precisely. Patient activity level appears to be the most important factor when the long-term development of PPOL is considered. Surgical technique, implant design and material factors are the most important preventative factors, because they influence both the generation of wear debris and excessive mechanical stresses. New generations of bearing surfaces and designs for TKA should carefully address these important issues in extensive preclinical studies. Currently, there is little evidence that PPOL can be prevented by pharmacological intervention.

TRACTIVE FORCES DURING ROLLING MOTION OF THE KNEE : IMPLICATIONS FOR WEAR IN TOTAL KNEE REPLACEMENT

Wear at the polyethylene tibial plateau in total knee arthroplasty (TKR) is one of the primary concerns with these devices. The artificial bearing of a TKR has to sustain large forces while allowing the mobility for normal motion, typically, rolling, gliding and rotation. The tractive forces during the rolling motion at the knee joint were analyzed to determine which factors cause these forces to increase in TKR. The implications of these tractive forces to polyethylene wear were considered. Traction forces were calculated using a model of the knee to evaluate the effect of variations in the coefficient of friction, gait characteristics, antagonistic muscle contraction and patellofemoral mechanics. The model was limited to the sagittal plane motion of the femur on the tibia. The input for the model was the shape of the articulating surface, coefficient of friction, contact path, muscle anatomy and gait kinetics common to patients with a total knee replacement. The generation of tractive forces on the tibial polyethylene plateau was highly dependent on the static and dynamic coefficient of friction between the femur and the tibia. A peak tractive force of approximately 0.4 body weight was calculated with a peak normal force of 3.3 body weight. Tractive rolling occurred during most of stance phase when the static coefficient was 0.2. Alterations in gait patterns had a substantial effect on the generation of tractive forces at the knee joint. When an abnormal gait pattern (often seen following TKR) was input to the model the posteriorly directed tractive force on the tibial surface was reduced. It was also found that variations in muscle contractions associated with antagonistic muscle activity as well as the angle of pull of the patellar tendon affected the magnitude of tractive forces. The results of the study suggest that there are feasible conditions following total knee replacement which can lead to tractive forces during rolling motion at the tibiofemoral articulation that should be considered in the analysis of factors leading to polyethylene damage in total knee replacement.

European multidisciplinary consensus statement on the use and monitoring of metal-on-metal bearings for total hip replacement and hip resurfacing.

INTRODUCTION There is an ongoing debate about the optimal use of metal-on-metal (MoM) bearings in total hip replacement, since there are uncertainties about local and systemic adverse effects due to wear and corrosion of these bearings. Despite various national recommendations, efforts to achieve international harmonization of specific evidence-based recommendations for best practice are still lacking. HYPOTHESIS An international consensus study group should be able to develop recommendations on the use and monitoring of MoM bearings, preferably at the European level, through a multidisciplinary approach, by integrating the perspectives of various stakeholders. MATERIALS AND METHODS Twenty-one experts representing three stakeholder groups and eight countries participated in this European consensus study, which consisted of a consensus meeting, subsequent structured discussion, and consensus voting. RESULTS The current statement defines first of all benefits, local and systemic risks, as well as uncertain issues related to MoM bearings. Safety assessment after implantation of MoM comprises all patients. A closer follow-up is recommended for large head MoM (≥36mm) and resurfacing. In these implants basic follow-up should consist of x-rays and metal ion measurement of cobalt in whole blood, performed with GF-AAS or ICP-MS. Clinical and/or radiographic abnormality as well as elevated ion levels needs additional imaging (ultrasound, CT-scan and/or MARS-MRI). Cobalt values less than 2 μg/L are probably devoid of clinical concern, the threshold value for clinical concern is expected to be within the range of 2-7 μg/L. DISCUSSION This is the first multinational, interdisciplinary, and multiprofessional approach for developing a recommendation for the use and monitoring of MoM bearings in total hip replacement. The current recommendations are in partial agreement with previous statements regarding the extent of follow-up and imaging techniques. They however differ from previous communications regarding measurement of metal ions and especially the investigated medium, technique, and eventual threshold levels. LEVEL OF EVIDENCE Level V, expert opinion/agreement conference.

Significance of Tribocorrosion in Biomedical Applications: Overview and Current Status

Recently, “tribocorrosion,” a research area combining the science of tribology and corrosion, has drawn attention from scientists and engineers belonging to a wide spectrum of research domains. This is due to its practical impact on daily life and also the accompanying economical burdens. It encompasses numerous applications including the offshore, space, and biomedical industry, for instance, in the case of artificial joints (Total Hip Replacement, THR) in orthopedic surgery, where implant metals are constantly exposed to tribological events (joint articulations) in the presence of corrosive solutions, that is, body fluids. Keeping the importance of this upcoming area of research in biomedical applications in mind, it was thought to consolidate the work in this area with some fundamental aspects so that a comprehensive picture of the current state of knowledge can be depicted. Complexity of tribocorrosion processes has been highlighted, as it is influenced by several parameters (mechanical and corrosion) and also due to the lack of an integrated/efficient test system. Finally a review of the recent work in the area of biotribocorrosion is provided, by focusing on orthopedic surgery and dentistry.

Bone mineral density in the proximal tibia varies as a function of static alignment and knee adduction angular momentum in individuals with medial knee osteoarthritis

Based on the premise that bone mass and bone geometry are related to load history and that subchondral bone may play a role in osteoarthritis (OA), we sought to determine if static and dynamic markers of knee joint loads explain variance in the medial-to-lateral ratio of proximal tibial bone mineral density (BMD) in subjects with mild and moderate medial knee OA. We utilized two surrogate markers of dynamic load, the peak knee adduction moment and the knee adduction angular momentum, the latter being the time integral of the frontal plane knee joint moment. BMD for medial and lateral regions of the proximal tibial plateau and one distal region in the tibial shaft was measured in 84 symptomatic subjects with Kellgren and Lawrence radiographic OA grades of 2 or 3. Utilizing gait analysis, the peak knee adduction moment (the external adduction moment of greatest magnitude) and the time integral of the frontal plane knee joint moment (the angular momentum) over the entire stance phase as well as for each of the four subdivisions of stance were calculated. The BMD ratio was not significantly different in grade 2 (1.32 +/- 0.27) and grade 3 knees (1.47 +/- 0.40) (P = 0.215). BMD of the tibial shaft was not correlated with any loading parameter or static alignment. Of all the surrogate gait markers of dynamic load, the knee adduction angular momentum in terminal stance explained the most variance (20%) in the medial-to-lateral BMD ratio (adjusted r(2) = 0.196, P < 0.001). The knee adduction angular momentum for the entire stance phase explained 18% of the variance in the BMD ratio (adjusted r(2) = 0.178, P < 0.001), 10% more variance than explained by the overall peak knee adduction moment (adjusted r(2) = 0.081, P < 0.001). 18% of the variance in the BMD ratio was also explained by the knee alignment angle (adjusted r(2) = 0.183, P < 0.001), and the total explanatory power was increased to 22% when the knee adduction angular momentum in terminal stance was added (change in r(2) = 0.041, P < 0.05, total adjusted r(2) = 0.215, P < 0.001). The BMD ratio and its relationship to dynamic and static markers of loading were independent of height, weight, and the body mass index, demonstrating that both dynamic markers of knee loading as well as knee alignment explained variance in the tibial BMD ratio independent of body size.