Robin Pourzal

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.

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.

Wear patterns of taper connections in retrieved large diameter metal-on-metal bearings

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.

New insights into hard phases of CoCrMo metal-on-metal hip replacements

The microstructural and mechanical properties of the hard phases in CoCrMo prosthetic alloys in both cast and wrought conditions were examined using transmission electron microscopy and nanoindentation. Besides the known carbides of M(23)C(6)-type (M=Cr, Mo, Co) and M(6)C-type which are formed by either eutectic solidification or precipitation, a new mixed-phase hard constituent has been found in the cast alloys, which is composed of ∼100 nm fine grains. The nanosized grains were identified to be mostly of M(23)C(6) type using nano-beam precession electron diffraction, and the chemical composition varied from grain to grain being either Cr- or Co-rich. In contrast, the carbides within the wrought alloy having the same M(23)C(6) structure were homogeneous, which can be attributed to the repeated heating and deformation steps. Nanoindentation measurements showed that the hardness of the hard phase mixture in the cast specimen was ∼15.7 GPa, while the M(23)C(6) carbides in the wrought alloy were twice as hard (∼30.7 GPa). The origin of the nanostructured hard phase mixture was found to be related to slow cooling during casting. Mixed hard phases were produced at a cooling rate of 0.2 °C/s, whereas single phase carbides were formed at a cooling rate of 50 °C/s. This is consistent with sluggish kinetics and rationalizes different and partly conflicting microstructural results in the literature, and could be a source of variations in the performance of prosthetic devices in-vivo.

Subsurface changes of a MoM hip implant below different contact zones

Metal-on-metal hip arthroplasties undergo distinct release of toxic metal particles and ions. Thus, it is necessary to minimize this. In order to evaluate the wear behaviour of metal-on-metal hip replacements it is essential to understand the micro-structural changes in the sub-surface region. Previous studies revealed that cobalt chromium metal-on-metal implants are able to alter their mechanical behaviour by adjusting the microstructure to load. The reason for this is the so-called mechanical mixing. This means that a nano-crystal layer is formed by rotating clusters of atoms that incorporate denatured proteins from the interfacial medium. This is followed by a layer of rhombic shaped nano-crystals in between sheared epsilon-martensite lathes, twins, and stacking faults. Although the primary wear zone has been well characterized, the sub-surface structure of the stripe wear and the non-contact zone of the hip ball have yet to be analysed. For this study a 28-mm cobalt base alloy femoral head and acetabular cup were analysed. The implant was simulator tested for 5 million cycles with the application of micro-separation resulting in a clearly visible stripe wear appearance. The TEM micrograph of the primary wear zone of the ball confirmed the presence of a sub-surface layer of nano-crystals. The thickness of this layer was approximately 200 nm and the average grain diameter ranged from 35 to 40 nm. Within the stripe wear zone the micrographs also revealed a nano-crystal layer but with a thickness of only 50 nm and an average grain diameter from 15 to 20 nm. The carbon and oxygen content was highest closest to the surface which proves the occurrence of mechanical mixing. The non-contact zone of the ball was analysed as well. When compared to the primary wear zone a nano-crystal layer with similar thickness but with an average grain diameter smaller than 15 nm was observed.

Tribolayer formation in a metal-on-metal (MoM) hip joint: an electrochemical investigation.

The demand for total hip replacement (THR) surgery is increasing in the younger population due to faster rehabilitation and more complete restoration of function. Up to 2009, metal-on-metal (MoM) hip joint bearings were a popular choice due to their design flexibility, post-operative stability and relatively low wear rates. The main wear mechanisms that occur along the bearing surface of MoM joints are tribochemical reactions that deposit a mixture of wear debris, metal ions and organic matrix of decomposed proteins known as a tribolayer. No in-depth electrochemical studies have been reported on the structure and characteristics of this tribolayer or about the parameters involved in its formation. In this study, we conducted an electrochemical investigation of different surfaces (bulk-like: control, nano-crystalline: new implant and tribolayer surface: retrieved implant) made out of two commonly used hip CoCrMo alloys (high-carbon and low-carbon). As per ASTM standard, cyclic polarization tests and electrochemical impedance spectroscopy tests were conducted. The results obtained from electrochemical parameters for different surfaces clearly indicated a reduction in corrosion for the tribolayer surface (Icorr: 0.76μA/cm(2)). Further, polarization resistance (Rp:2.39±0.60MΩ/cm(2)) and capacitance (Cdl:15.20±0.75μF/cm(2)) indicated variation in corrosion kinetics for the tribolayer surface, that attributed to its structure and stability in a simulated body environment.

Dominant role of molybdenum in the electrochemical deposition of biological macromolecules on metallic surfaces.

The corrosion of CoCrMo, an alloy frequently used in orthopedic implants, was studied with an electrochemical quartz crystal microbalance (QCM) in three physiologically relevant solutions. Mass changes were measured during potentiodynamic tests, showing material deposition in protein solutions at potential levels that caused mass loss when the proteins were not present. X-ray photoelectron spectroscopy (XPS) data indicated that the deposited material was primarily organic and therefore was most likely derived from proteins in the electrolyte. Material deposition consistently occurred at a critical potential and was not dependent on the current density or total charge released into solution. Corrosion studies on pure Co, Cr, and Mo in protein solutions found material deposition only on Mo. We hypothesize that organic deposition results from the interaction of Mo(VI) with proteins in the surrounding solution. The organic layer is reminiscent of tribochemical reaction layers that form on the surface of CoCrMo hip bearings, suggesting that these types of layers can be formed by purely electrochemical means.

Microstructure of retrievals made from standard cast HC-CoCrMo alloys

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 Manuscript received April 26, 2012; accepted for publication August 27, 2012; published online March 20, 2013. Materials Science and Engineering, Univ. of Duisburg-Essen, Duisburg, Germany 47057. Dept. of Orthopedic Surgery, Rush Univ. Medical Center, Chicago, IL 60612, United States of America. Dept. of Materials Science and Engineering, Northwestern Univ., Evanston, IL 60208, United States of America. TU-Hamburg-Harburg, Institute of Biomechanics, Hamburg, 21073, Germany. Copyright VC 2013 by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. Metal-On-Metal Total Hip Replacement Devices STP 1560, 2013 Available online at www.astm.org DOI:10.1520/STP156020120033 phases: carbides and/or intermetallic phases. Some are fine ( 30 lm) types of mixed hard phases, which consist of carbides and intermetallic 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.

Nanoscale surface modification by anodic oxidation increased bone ingrowth and reduced fibrous tissue in the porous coating of titanium–alloy femoral hip arthroplasty implants

Hip arthroplasty femoral stems coated with Ti6Al4V beads were treated by anodic oxidation in H3 PO4 for enhanced bioactivity and were studied in a 6-month canine model to determine the effects of the treated surface on the ingrowth of bone and soft tissues. The area fractions of bone, marrow, and fibrous tissue in the porous coating of seven treated and seven untreated control implants were determined using histomorphological techniques. The area fraction of bone within the porous coating was greater for anodic oxide treated (23.6 ± 8.3%) compared to control implants (l2.7 ± 4.7%) (p = 0.013), and there was less fibrous tissue in the treated implants (18.0 ± 9.5%) compared to the controls (33.1 ± 7.9%) (p = 0.006). XPS, XRD, TEM, and SEM analyses of the treated implants revealed a 400 nm-thick titanium oxide layer of low crystallinity with an undulating surface, populated with more than 25 nm-size pores per square micrometer. There was no detectable increase in serum titanium or in generation of particulates locally compared to the control implants. Micro and nanoscale surface modification by anodic oxidation increased bone ingrowth and reduced fibrous tissue, which may extend the longevity of fixation, limiting pathways for particle migration, and impeding the progression of osteolysis and aseptic loosening of arthroplasty components.