Adverse loading effects on tribocorrosive degradation of 28 mm metal-on-metal hip replacement bearings^*

Following the high clinical failure rates of metal-on-metal total hip replacements much work has been undertaken to investigate their poor performance. So called adverse loading scenarios such as acetabular inclination and microseparation have been attributed to indicators for failure of the implants. The ISO hip simulation standards (ISO 14242:1) still rely on gravimetric and ex situ analysis, considering only the total wear during articulation. Live in situ sensing can provide valuable insight into the degradation mechanisms of metallic interfaces under such scenarios. Clinical 28 mm diameter metal-on-metal components were articulated in a full-ISO hip simulator. The bearings were subjected to increasing angles of acetabular inclination and retroversion over short-term periods of articulation. Corrosive degradation was monitored during sliding by means of an in situ three-electrode cell. Changing acetabular inclination from 30° to 50° resulted in greater cathodic shifts in OCP upon the initiation of sliding; from −50 mV to as much as −150 mV. Under anodic polarisation (0 mV vs. Ag/AgCl) the resultant currents at the initiation of sliding also increased significantly with inclination; from approximately 4–10 µA to over 120 µA. Increased retroversion of 20° also resulted in increased anodic currents of 55–60 µA. Changing the nature of articulation demonstrated increased corrosive material loss compared to a standard ISO 14242 profile. The sole use of gravimetric assessment to determine a wear rate for hip replacement bearings under simulation can therefore neglect important degradation mechanisms, such as tribocorrosive loss in devices with metal sliding interfaces.</p

Highly lubricious SPMK-g-PEEK implant surfaces to facilitate rehydration of articular cartilage

To enable long lasting osteochondral defect repairs which preserve the native function of synovial joint counter-face, it is essential to develop surfaces which are optimised to support healthy cartilage function by providing a hydrated, low friction and compliant sliding interface. PEEK surfaces were modified using a biocompatible 3-sulfopropyl methacrylate potassium salt (SPMK) through UV photo-polymerisation, resulting in a ∼350 nm thick hydrophilic coating rich in hydrophilic anionic sulfonic acid groups. Characterisation was done through Fourier Transformed Infrared Spectroscopy, Focused Ion Beam Scanning Electron Microscopy, and Water Contact Angle measurements. Using a Bruker UMT TriboLab, bovine cartilage sliding tests were conducted with real-time strain and shear force measurements, comparing untreated PEEK, SPMK functionalised PEEK (SPMK-g-PEEK), and Cobalt Chrome Molybdenum alloy. Tribological tests over 2.5 h at physiological loads (0.75 MPa) revealed that SPMK-g-PEEK maintains low friction (μ &lt; 0.024) and minimises equilibrium strain, significantly reducing forces on the cartilage interface. Post-test analysis showed no notable damage to the cartilage interfacing against the SPMK functionalised surfaces. The application of a constitutive biphasic cartilage model to the experimental strain data reveals that SPMK surfaces increase the interfacial permeability of cartilage in sliding, facilitating fluid and strain recovery. Unlike previous demonstrations of sliding-induced tribological rehydration requiring specific hydrodynamic conditions, the SPMK-g-PEEK introduces a novel mode of tribological rehydration operating at low speeds and in a stationary contact area. SPMK-g-PEEK surfaces provide an enhanced cartilage counter-surface, which provides a highly hydrated and lubricious boundary layer along with supporting biphasic lubrication. Soft polymer surface functionalisation of orthopaedic implant surfaces are a promising approach for minimally invasive synovial joint repair with an enhanced bioinspired polyelectrolyte interface for sliding against cartilage. These hydrophilic surface coatings offer an enabling technology for the next generation of focal cartilage repair and hemiarthroplasty implant surfaces.</p

Brushing Up on Cartilage Lubrication:Polyelectrolyte-Enhanced Tribological Rehydration

This study presents new insights into the potential role of polyelectrolyte interfaces in regulating low friction and interstitial fluid pressurization of cartilage. Polymer brushes composed of hydrophilic 3-sulfopropyl methacrylate potassium salt (SPMK) tethered to a PEEK substrate (SPMK-g-PEEK) are a compelling biomimetic solution for interfacing with cartilage, inspired by the natural lubricating biopolyelectrolyte constituents of synovial fluid. These SPMK-g-PEEK surfaces exhibit a hydrated compliant layer approximately 5 μm thick, demonstrating the ability to maintain low friction coefficients (μ ∼ 0.01) across a wide speed range (0.1–200 mm/s) under physiological loads (0.75–1.2 MPa). A novel polyelectrolyte-enhanced tribological rehydration mechanism is elucidated, capable of recovering up to ∼12% cartilage strain and subsequently facilitating cartilage interstitial fluid recovery, under loads ranging from 0.25 to 2.21 MPa. This is attributed to the combined effects of fluid confinement within the contact gap and the enhanced elastohydrodynamic behavior of polymer brushes. Contrary to conventional theories that emphasize interstitial fluid pressurization in regulating cartilage lubrication, this work demonstrates that SPMK-g-PEEK’s frictional behavior with cartilage is independent of these factors and provides unabating aqueous lubrication. Polyelectrolyte-enhanced tribological rehydration can occur within a static contact area and operates independently of known mechanisms of cartilage interstitial fluid recovery established for converging or migrating cartilage contacts. These findings challenge existing paradigms, proposing a novel polyelectrolyte–cartilage tribological mechanism not exclusively reliant on interstitial fluid pressurization or cartilage contact geometry. The implications of this research extend to a broader understanding of synovial joint lubrication, offering insights into the development of joint replacement materials that more accurately replicate the natural functionality of cartilage

Degradation of metal hip implants

Regardless of their type, implanted metal devices will interact with the biological environment; thus, mechanical wear, corrosion, and their combined actions (i.e., tribocorrosion) are inevitable. The implications of these mechanisms have been observed in vivo and linked with early failure and revision of many types of hip implants. This chapter provides a basic overview of the principles of tribology, corrosion, tribocorrosion, and the major sources of material loss from metal prostheses. This is followed by a discussion of the methods used to quantify implant wear and map metal deposits in periprosthetic and systemic tissue, and the role of the degradation mechanism on the characteristics of the resultant debris.</p

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. &lt;/jats:p&gt

Biomedical devices

This chapter presents a broad overview of the fretting-corrosion mechanisms and observations pertaining to implantable biomedical devices. Fretting corrosion is a persistent issue faced by biomedical engineers and has been implicated in higher than acceptable revision rates owing to adverse biological reactions to metal debris and ions. Firstly, commonly used biomedical materials and the biological environment are introduced and discussed. The mechanisms of fretting corrosion in relation to the interfacial contact mechanics are discussed in detail. Finally, practical examples of fretting-corrosion and the implications of fretting corrosion from a biological perspective are discussed.</p

Engineering tribological rehydration of cartilage interfaces:Assessment of potential polyelectrolyte mechanisms

Articular cartilage, primarily composed of water and collagen, is vital for synovial joint function. Traditional hard biomaterials like ceramic or cobalt-chrome used in hemiarthroplasty often result in abnormal contact pressures and premature implant failure. This study investigates the tribological properties of polyelectrolyte functionalised PEEK (SPMK-g-PEEK) in contact with cartilage, proposing a solution to these issues by utilising tribological rehydration and effective aqueous lubrication. We demonstrate a new mode of polyelectrolyte enhanced tribological rehydration where SPMK-g-PEEK achieves low friction and promotes interstitial fluid recovery during sliding, independent of traditional hydrodynamic theories. This results in a rapid stabilisation of the coefficient of friction (CoF) to levels comparable to natural cartilage (CoF ∼ 0.01) and aids in approximately 8% cartilage strain recovery, indicating effective tribological rehydration even under cartilage degradation or altered osmotic conditions. Furthermore, we find that lubrication and rehydration against an SPMK-g-PEEK interface depend minimally on biphasic lubrication but significantly on the hydrophilic sulfonic acid groups of SPMK, which act as a fluid reservoir. Our findings suggest SPMK-g-PEEK as a promising biomaterial for cartilage interfacing implants that offer low friction and modulate cartilage interstitial fluid pressure. This study enhances our understanding of biotribological interactions and contributes to the development of joint replacement materials that support the natural function of cartilage.</p