In-situ electrochemical study of interaction of tribology and corrosion in artificial hip prosthesis simulators

A mass balance analysis of the tribocorrosion process of titanium alloys using a single micro-asperity: Voltage and solution effects on plastic deformation, oxide repassivation, and ion dissolution

Within modular taper junctions of total hip implants (THA), nominally "smooth" metallic surfaces contain multiple micro-asperities that slide, are plastically deformed, have their oxide film surfaces disrupted and corrode during the fretting corrosion processes. In this work, a mass/volume balance approach is developed and used to assess the contribution of individual components of wear and corrosion to the entirety of the single-asperity tribocorrosion process for the popular THA alloy, Ti-6Al-4V. This analysis measures the total volume change (trough) in the surface due to low cycle single asperity fretting corrosion and compares it to the measured pileup volume which is comprised of plastic deformation, metal particles and oxide particles, plus the fretting current and the concentration of solution-bound species. A simple 17 μm spherical geometry diamond asperity was used and the trough volume, pileup volume, fretting currents and ion concentrations were measured to assess their contribution to the fretting corrosion process. The effects fretting in or out of solution (phosphate buffered saline), and the role of electrode potential, e.g., freely corroding or forced potential (-1.0 V, 0 V, and +1.0 V vs Ag/AgCl) were investigated. Under constant 30 mN loading, 100 cycles duration, 3 Hz cyclic frequency and 80 μm sliding amplitude, the volume abraded, fretting currents, ion release, and pileup volume were all recorded. Damage was analyzed and quantified using digital optical microscopy (DOM), atomic force microscopy (AFM), scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), and inductive coupled plasma mass spectrometry (ICP-MS). The results were analyzed with ANOVA statistics (α = 0.05). The extent of wear damage (asperity trough volume) is as follows: air = Ecorr, air > -1.0 V = 0 V = +1.0 V. As the amount of pileup volume decreased between conditions, visible oxide generation increased, with V > 0 V having more oxide debris generation and air fretting resulting in the least oxide (and most plastic deformation). Ions in solution were not significant, accounting for less than 1% of the damage. Volume analysis showed trough volumes and pileup volumes were very close to one another and were dominated by plastic deformation. Synergy between wear and corrosion were not observed in this work.

Electroplating of HAp-brushite coating on metallic bioimplants with advanced hemocompatibility and osteocompatibility properties

In cases of severe bone tissue injuries, the use of metallic bioimplants is quite widespread due to their high strength, high fracture toughness, hardness, and corrosion resistance. However, they lack adequate biocompatibility and show poor metal-tissue integration during the post-operative phase. To mitigate this drawback, it is beneficial to add a biocompatible polymer layer to ensure a quick growth of cell or tissue over the surface of metallic bioimplant material. Furthermore, this additional layer should possess good adherence with the underlying material and also accompany a rapid bonding between the tissue and the implant material, in order to reduce the recovery time for the patient. Therefore, in this work, we report a novel green electroplating route for growing porous hydroxyapatite-brushite coatings on a stainless steel surface. The malic acid used for the production of hydroxyapatite-brushite coatings has been obtained from an extract of locally available apple fruit (Malus domestica). We demonstrate the effect of electroplating parameters on the structural morphology of the electroplated composite layer via XRD, SEM with EDS, and FTIR characterization techniques and report an optimized set of electroplating parameters that will yield the best composite coating in terms of thickness, adherence to substrate and speed. The hemocompatibility and osteocompatibility studies on the electroplated composites coating show this technology's effectiveness and potential applicability in biomedical applications. Compared to other routes reported in the literature, this electroplating route is quicker and yields better composite coatings with faster bone tissue growth potential.

Mechanical performance, corrosion and tribological evaluation of a Co-Cr-Mo alloy processed by MIM for biomedical applications

In this study, the processing parameters mechanical performance, corrosion and tribological evaluation of a low carbon content Co-Cr-Mo alloy are discussed. The production of parts using the Metal Injection Moulding (MIM) process is optimized, specifically concerning the rheological analysis of the prepared feedstocks, the optimum choice of the powder loading and the design of the debinding and sintering cycles. The mechanical properties as regards hardness, tensile strength and bending strength, as well as fatigue tests and wear characterization, are discussed for the full densified specimens obtained. Additionally, corrosion behaviour with the different methods and electrolytic solutions that simulate the biological environment has also been investigated. This approach allows us to confirm that the low-carbon cobalt alloy processed by MIM exhibits an adequate equilibrium between its mechanical and corrosion behaviour, with a notable performance during fatigue and wear tests. In the light of these findings, the use of this material for biomedical applications is discussed.

Tribocorrosion of a CoCrMo alloy sliding against articular cartilage and the impact of metal ion release on chondrocytes

Partial knee replacement and hemiarthroplasty are some of the orthopedic procedures resulting in a metal on cartilage interface. As metal implant material, CoCrMo based alloys are commonly used. The aim of the present study is to assess the role of biotribocorrosion on the CoCrMo-cartilage interface with an emphasis on metal release during sliding contact. The biotribocorrosion experiments were performed under controlled electrochemical conditions using a floating cell with a three electrode set up coupled to a microtribometer. Throughout the experiment the coefficient of friction and the open circuit potential were monitored. Analyses of the electrolyte after the experiment show that metal release can occur during sliding contact of CoCrMo alloy against articular cartilage despite the extraordinary low coefficient of friction measured. Metal release is attributed to changes in passive layer caused at the onset of sliding. The released metal was found to be forming compounds with potential cytotoxicity. Since the presence of metal ions in the cartilage matrix can potentially lead to cell apoptosis, the metabolic activity of human osteoarthritic chondrocytes (2D-cultures) was investigated in the presence of phosphate buffered saline containing metal ions using XTT-assay. The experiments indicate that critical concentrations of Co ions lead to a significant decrease in chondrocyte metabolic activity. Therefore, biotribocorrosion is a mechanism that can occur in partial replacements and lead to chondrocyte apoptosis thus playing a role in the observed accelerated degradation of the remaining cartilage tissue after the mentioned orthopedic procedures. STATEMENT OF SIGNIFICANCE: Partial replacements provide an alternative to total joint replacements. This procedure is less invasive, allows a faster rehabilitation and provides a better function of the joint. However, the remaining native cartilage experiences accelerated degradation when in contact with metallic implant components. This work investigates the role of tribocorrosion at the metal-cartilage interface during sliding. Tribocorrosion is a degradation process that can alter significantly the wear rates experienced by metallic implants and lead to the release of metal ions and particles. The released metal can form compounds with potential cytotoxicity on cartilage tissue. The knowledge gained in this work will serve to understand the mechanisms behind the failure of partial replacements and develop future biomaterials with an enhanced lifetime.

Improved Tribocorrosion Resistance of a CoCrMo Implant Material by Carburising

Tribocorrosion damage is a cause for the premature failure of hip implants made of cobalt-based alloys. Low-temperature carburising can be a plausible solution towards mitigating the tribocorrosion damage of articulating components. This diffusion treatment introduces a supersaturated carbon solid solution, termed S-phase, which hardens the CoCrMo alloy without detriment to the corrosion resistance. This work investigates and compares the tribocorrosion behaviour of untreated and carburised ASTM F1537 CoCrMo alloys tested in Ringer’s solution using a reciprocating sliding configuration against a polycrystalline alumina counterface under different electrochemical conditions. The research shows that whereas the carburised alloy suffered a slightly higher wear loss under a cathodic potential, it was able to reduce the material losses considerably when tested under both open circuit and anodic potential conditions. Under anodic conditions material losses by corrosion due to wear dominated. The better tribocorrosion resistance of the carburised layer was attributed to the better qualities of the passive film for the carburised sample coupled with an increased load support.

Systemic and local toxicity of metal debris released from hip prostheses: A review of experimental approaches

Despite the technological improvements in orthopedic joint replacement implants, wear and corrosion products associated with the metal components of these implants may result in adverse local tissue and perhaps systemic reactions and toxicities. The current review encompasses a literature review of the local and systemic toxicity studies concerning the effect of CoCrMo wear debris released from wear and corrosion of orthopedic implants and prostheses. Release of metallic debris is mainly in the form of micro- and nano-particles, ions of different valences, and oxides composed of Co and Cr. Though these substances alter human biology, their direct effects of these substances on specific tissue types remain poorly understood. This may partially be the consequence of the multivariate research methodologies employed, leading to inconsistent reports. This review proposes the importance of developing new and more appropriate in-vitro methodologies to study the cellular responses and toxicity mediated by joint replacement wear debris in-vivo.

Chromium oxide coatings with the potential for eliminating the risk of chromium ion release in orthopaedic implants

Chromium oxide coatings prepared by radiofrequency reactive magnetron sputtering on stainless steel substrates were exposed to Ringer's physiological solution and tested for their electrochemical corrosion stability using an open circuit potential measurement, potentiodynamic polarization, electrochemical impedance spectroscopy and Mott-Schottky analysis. The coatings were found to be predominantly Cr₂O₃, based on the observation of the dominance of [Formula: see text] and E_g symmetric modes in our Raman spectroscopic investigation and the E_u vibrational modes in our Fourier transform infrared spectroscopic measurements on the coatings. We investigated for the presence of chromium ions in Ringer's solution after all of the above electrochemical tests using atomic absorption spectroscopy, without finding a trace of chromium ions at the ppm level for coatings tested under open circuit and at the lower potentials implants are likely to experience in the human body. The coatings were further exposed to Ringer's solution for one month and tested for adhesion strength changes, and we found that they retained substantial adhesion to the substrates. We expect this finding to be significant for future orthopaedic implants where chromium ion release is still a major challenge.

Release of metal ions from nano CoCrMo wear debris generated from tribo-corrosion processes in artificial hip implants

CoCrMo alloys have been widely used in metal-on-metal (MoM) hip replacements due to their superior wear and corrosion resistance properties. However, metal ions like Co²⁺ and Cr³⁺, or even Cr⁶⁺ released from CoCrMo hip prostheses can induce macrophage apoptotic vs. necrotic mortality and damage the surrounding tissues. Simultaneously, osteolysis induced by the wear debris can be a cause of failure. Nano wear debris is more active than the bulk material, due to its small size. In this study, to accurately analyse the fresh wear debris retrieved from the hip simulator and the interaction between the particles and tribocorrosion of CoCrMo, wear debris was observed without protein digest, using a combined experimental approach involving the employment of TEM and ICP-MS. The results suggest that nanoscale wear debris generated from a hip simulator in bovine serum albumin (BSA) lubrication was Cr-rich, containing crystalline and amorphous structures; meanwhile, without any proteins, the wear particles mostly had an hcp-Co crystalline structure.

Evidence for the dissolution of molybdenum during tribocorrosion of CoCrMo hip implants in the presence of serum protein

We have characterized CoCrMo, Metal-on-Metal (MoM) implant, wear debris particles and their dissolution following cycling in a hip simulator, and have related the results to the tribocorrosion of synthetic wear debris produced by milling CoCrMo powders in solutions representative of environments in the human body. Importantly, we have employed a modified ICP-MS sample preparation procedure to measure the release of ions from CoCrMo alloys during wear simulation in different media; this involved use of nano-porous ultrafilters which allowed complete separation of particles from free ions and complexes in solution. As a result, we present a new perspective on the release of metal ions and formation of metal complexes from CoCrMo implants. The new methodology enables the mass balance of ions relative to complexes and particles during tribocorrosion in hip simulators to be determined. A much higher release of molybdenum ions relative to cobalt and chromium has been measured. The molybdenum dissolution was enhanced by the presence of bovine serum albumin (BSA), possibly due to the formation of metal-protein complexes. Overall, we believe that the results could have significant implications for the analysis and interpretation of metal ion levels in fluids extracted from hip arthroplasty patients; we suggest that metal levels, including molybdenum, be analysed in these fluids using the protocol described here.

STATEMENT OF SIGNIFICANCE

We have developed an important new protocol for the analysis of metal ion levels in fluids extracted from hip implant patients and also hip simulators. Using this procedure, we present a new perspective on the release of metal ions from CoCrMo alloy implants, revealing significantly lower levels of metal ion release during tribocorrosion in hip simulators than previously thought, combined with the release of much higher percentages of molybdenum ions relative to cobalt and chromium. This work is of relevance, both from the perspective of the fundamental science and study of metal-protein interactions, enabling understanding of the ongoing problem associated with the biotribocorrosion and the link to inflammation associated with Metal-on-Metal (MoM) hip implants made from CoCrMo alloys.

Incorporating corrosion measurement in hip wear simulators: An added complication or a necessity?

Corrosion is not routinely considered in the assessment of the degradation or the lifetime of total hip replacement bearing surfaces. Biomechanical simulations are becoming ever more complex and are taking into account motion cycles that represent activities beyond a simple walking gait at 1 Hz, marking a departure from the standard ISO BS 14242. However, the degradation is still very often referred to as wear, even though the material loss occurs due to a combination of tribological and corrosion processes and their interactions. This article evaluates how, by incorporating real-time corrosion measurements in total hip replacement simulations, pre-clinical evaluations and research studies can both yield much more information and accelerate the process towards improved implants.

Latham J, G. Dunlop D, Oreffo RO. Bone and metal: an orthopaedic perspective on osseointegration of metals

The area of implant osseointegration is of major importance, given the predicted significant rise in the number of orthopaedic procedures and an increasingly ageing population. Osseointegration is a complex process involving a number of distinct mechanisms affected by the implant bulk properties and surface characteristics. Our understanding and ability to modify these mechanisms through alterations in implant design is continuously expanding. The following review considers the main aspects of material and surface alterations in metal implants, and the extent of their subsequent influence on osseointegration. Clinically, osseointegration results in asymptomatic stable durable fixation of orthopaedic implants. The complexity of achieving this outcome through incorporation and balance of contributory factors is highlighted through a clinical case report.

Metal ion levels from well-functioning Birmingham Hip Resurfacings decline significantly at ten years

A retrospective study was conducted to investigate the changes in metal ion levels in a consecutive series of Birmingham Hip Resurfacings (BHRs) at a minimum ten-year follow-up. We reviewed 250 BHRs implanted in 232 patients between 1998 and 2001. Implant survival, clinical outcome (Harris hip score), radiographs and serum chromium (Cr) and cobalt (Co) ion levels were assessed. Of 232 patients, 18 were dead (five bilateral BHRs), 15 lost to follow-up and ten had been revised. The remaining 202 BHRs in 190 patients (136 men and 54 women; mean age at surgery 50.5 years (17 to 76)) were evaluated at a minimum follow-up of ten years (mean 10.8 years (10 to 13.6)). The overall implant survival at 13.2 years was 92.4% (95% confidence interval 90.8 to 94.0). The mean Harris hip score was 97.7 (median 100; 65 to 100). Median and mean ion levels were low for unilateral resurfacings (Cr: median 1.3 µg/l, mean 1.95 µg/l (< 0.5 to 16.2); Co: median 1.0 µg/l, mean 1.62 µg/l (< 0.5 to 17.3)) and bilateral resurfacings (Cr: median 3.2 µg/l, mean 3.46 µg/l (< 0.5 to 10.0); Co: median 2.3 µg/l, mean 2.66 µg/l (< 0.5 to 9.5)). In 80 unilateral BHRs with sequential ion measurements, Cr and Co levels were found to decrease significantly (p < 0.001) from the initial assessment at a median of six years (4 to 8) to the last assessment at a median of 11 years (9 to 13), with a mean reduction of 1.24 µg/l for Cr and 0.88 µg/l for Co. Three female patients had a > 2.5 µg/l increase of Co ions, associated with head sizes ≤ 50 mm, clinical symptoms and osteolysis. Overall, there was no significant difference in change of ion levels between genders (Cr, p = 0.845; Co, p = 0.310) or component sizes (Cr, p = 0.505; Co, p = 0.370). Higher acetabular component inclination angles correlated with greater change in ion levels (Cr, p = 0.013; Co, p = 0.002). Patients with increased ion levels had lower Harris hip scores (p = 0.038). In conclusion, in well-functioning BHRs the metal ion levels decreased significantly at ten years. An increase > 2.5 µg/l was associated with poor function.