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.

Predictive wear modeling of the articulating metal-on-metal hip replacements.

The lubrication regime in which artificial hip joints operate adds complexity to the prediction of wear, as the joint operates in both the full fluid film regime-specifically the elastohydrodynamic lubrication (EHL) regime-and the mixed or boundary lubrication regimes, where contact between the bearing surfaces results in wear. In this work, a wear model is developed which considers lubrication for the first time via a transient EHL model of metal-on-metal hip replacements. This is a framework to investigate how the change in film thickness influences the wear, which is important to further investigation of the complex wear procedure, including tribocorrosion, in the lubricated hip implants.

The composition of tribofilms produced on metal-on-metal hip bearings.

Following wear testing in a hip simulator, the bearing surfaces of 36 mm metal on metal total hip replacements showed the formation of tribochemical layers. These layers were investigated in a transmission electron microscope, and analysis was performed using electron energy loss spectroscopy, energy dispersive x-rays and selected area electron diffraction. The tribofilm formed at the edge of the wear scar was 100 s of nanometres thick and contained cobalt sulphide particles embedded within.

Bio-tribology.

It is now forty six years since the separate topics of friction, lubrication, wear and bearing design were integrated under the title 'Tribology' [Department of Education and Science, Lubrication (Tribology) Education and Research. A Report on the Present Position and Industry's Needs, HMSO, London, 1966]. Significant developments have been reported in many established and new aspects of tribology during this period.

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

The second generation Metal-on-Metal (MoM) hip replacements have been considered as an alternative to commonly used Polyethylene-on-Metal (PoM) joint prostheses due to polyethylene wear debris induced osteolysis. However, the role of corrosion and the biofilm formed under tribological contact are still not fully understood. Enhanced metal ion concentrations have been reported widely from hair, blood and urine samples of patients who received metal hip replacements and in isolated cases when abnormally high levels have caused adverse local tissue reactions.

Tribological characteristics of healthy tendon.

Tendons transfer muscular forces efficiently and painlessly, facilitating joint motion. Whilst the tribology of articular cartilage is constantly explored, a poorer understanding remains of tendon lubrication and friction. This study reports experimental data describing the tribological characteristics of tendon and its surrounding tissue, before presenting an arithmetic solution to facilitate numerical modelling.

The bio-tribological properties of anti-adhesive agents commonly used during tendon repair.

Frictional resistance to tendon gliding is minimized by surrounding loose areolar tissue. During periods of prolonged immobilization, for example, post-tendon-repair, adhesions can form between these two adjacent tissues, thereby limiting tendon function. Anti-adhesive agents can be applied during surgery to prevent adhesion formation, whilst reportedly providing some reduction in friction during in vitro tendon-bony pulley investigations.

The bio-tribological characteristics of synthetic tissue grafts.

The use of synthetic connective tissue grafts became popular in the mid-1980s, particularly for anterior cruciate ligament reconstruction; however, this trend was soon changed given the high failure rate due to abrasive wear. More than 20 years later, a vast range of grafts are available to the orthopaedic surgeon for augmenting connective tissue following rupture or tissue loss. While the biomechanical properties of these synthetic grafts become ever closer to the natural tissue, there have been no reports of their bio-tribological (i.

Electrochemical instrumentation of a hip simulator: a new tool for assessing the role of corrosion in metal-on-metal hip joints.

Polyethylene wear debris induced osteolysis has triggered investigations to find alternative material combinations to the well-established metal-on-polyethylene hip implants. Owing to some early successful clinical cases, metal-on-metal (MoM) hip replacements have been attracting more and more interest. There is, however, considerable concern about the propensity of MoM hip replacements to release metal ions and fine, nanometre-scale metallic wear debris.

Lubrication regime of the contact between fat and bone in bovine tissue.

Fat pads are masses of encapsulated adipose tissue located throughout the human body. Whilst a number of studies describe these soft tissues anatomically little is known about their biomechanics, and surgeons may excise them arthroscopically if they hinder visual inspection of the joint or bursa. By measuring the coefficient of friction between, and performing Sommerfeld analysis of, the surfaces approximating the in vivo conjuncture, this contact has been shown to have a coefficient of friction of the order of 0.

Head replacement, head rotation, and surface damage effects on metal-on-metal total hip replacements: a hip simulator study.

The possibility of replacing the femoral head alone, in either solid or articular surface replacement form, during revision operations on metal-on-metal total hip replacements remains an attractive feature of such implants. In the present investigation, laboratory simulator studies of the influence upon volumetric wear of inserting a new femoral head, of introducing some head rotation, and of damaging the femoral head by scratches have all been explored. New and rotated heads both involve an additional running-in period, but the experimental studies show that the volumetric wear associated with this process is less than the initial running-in wear.

Understanding the role of corrosion in the degradation of metal-on-metal implants.

In metal-on-metal joints the primary concerns in terms of long-term durability relate to corrosion, wear, and their joint (tribocorrosion) effects. The release of ions through corrosion processes and nanoscale debris from wear processes can seriously affect joint integrity and can lead to an adverse biological reaction by the host. In this paper an integrated study of corrosion-wear interactions in serum, Dulbecco's Modified Eagle's Medium and 0.

Tribological principles in metal-on-metal hip joint design.

An analysis of some 100 published and communicated findings on running-in volumetric wear and steady state wear rates from simulator tests carried out in eight laboratories in three countries has been undertaken. Powerful indications have emerged of the dominant role of mixed lubrication in current metal-on-metal hip replacements, with elastohydrodynamic film thickness controlling wear. The background to the calculation of film thickness in the elastic-isoviscous mode of lubrication has been outlined and graphs of representative film thickness and lambda ratio have been presented.

Metal-on-metal hip joint tribology.

The basic tribological features of metal-on-metal total hip replacements have been reviewed to facilitate an understanding of the engineering science underpinning the renaissance of these hard-on-hard joints. Metal-on-polymer hip replacements operate in the boundary lubrication regime, thus leading to the design guidance to reduce the femoral head diameter as much as is feasible to minimize frictional torque and volumetric wear. This explains why the gold-standard implant of this form from the past half-century had a diameter of only 22.

A hip joint simulator study of the performance of metal-on-metal joints: Part II: design.

The separate and combined roles of head diameter and clearance were studied experimentally in simulator tests and also theoretically. Head diameters ranging from 16-54 mm and various clearances were studied. Effective mixed-film lubrication achieved through careful design and manufacture greatly reduced wear.

A hip joint simulator study of the performance of metal-on-metal joints: Part I: the role of materials.

The use of metal-on-metal low-wearing bearings has promoted great interest in the factors determining the volume of wear debris generated in such joints, including the developing surface replacement alternatives. Twenty-six pairs of low- and high-carbon components in wrought or cast form of 36 mm nominal diameter and exhibiting similar clearances were studied over 5 million test cycles in a 10-station hip joint simulator. Low-carbon cast materials exhibited higher wear than high-carbon cast or wrought materials.

Demographic and occupational effects on the activity levels of normal subjects in the United Kingdom.

A study of activity levels, measured in steps per day, was made of 293 subjects from several parts of the United Kingdom. Each subject wore a pedometer adjacent to the hip for two weeks. The average number of steps taken each day over a two-week period was recorded together with additional details such as age and occupation.

New joints for the Millennium: wear control in total replacement hip joints.

Hip joint replacement is described as the greatest achievement in orthopaedic surgery in the twentieth century. The field has been dominated for some forty years by implants based upon metallic femoral heads and stems and polymeric acetabular cups. At the dawn of the new Millennium, many alternative materials and designs are now being proposed or evaluated.

The effect of activity levels of total hip arthroplasty patients on socket penetration.

Survivorship of total hip arthroplasties (THAs) has been linked to penetration of the femoral head into the polyethylene acetabular cup and to polyethylene wear. The activity level of patients with THAs is considered to be an important factor affecting wear, and the purpose of this study was to explore the relationship between activity as recorded by pedometers and cup penetration. The measurement of daily activity levels of normal subjects and THA patients of various ages are discussed in another article ([1]).

Comparative study of the activity of total hip arthroplasty patients and normal subjects.

The walking activity of normal subjects and total hip arthroplasty (THA) patients from the Wrightington Hospital for Joint Disease and The General Infirmary at Leeds was assessed by means of electronic pedometers. The principal objectives were to establish the extent to which joint arthroplasty patients recover their activity relative to normal subjects and to establish the number of loading cycles to which prostheses should be subjected in joint simulator studies of implant performance. A further objective was to establish an experimental procedure for the assessment of the role of activity in contributing to the well-known scatter in the measurements of femoral head penetration into acetabular cups in in vivo studies of implant performance.

The effect of femoral head diameter upon lubrication and wear of metal-on-metal total hip replacements.

It has been found that a remarkable reduction in the wear of metal-on-metal hip joints can be achieved by simply increasing the diameter of the joint. A tribological evaluation of metal-on-metal joints of 16, 22.225, 28 and 36 mm diameter was conducted in 25 per cent bovine serum using a hip joint simulator.

Prediction of lubricating film thickness in UHMWPE hip joint replacements.

An elastohydrodynamic lubrication model developed for a ball-in-socket configuration in a previous studies by the present authors (Jalali-Vahid et al., Thinning films and tribological interfaces, 26th Leeds-Lyon Symposium on Tribology, 2000, pp. 329-339) was applied to analyse the lubrication problem of a typical artificial hip joint replacement, consisting of an ultra-high molecular weight polyethylene (UHMWPE) acetabular cup against a metallic or ceramic femoral head.

A comparative joint simulator study of the wear of metal-on-metal and alternative material combinations in hip replacements.

While total hip replacement represents the major success story in orthopaedic surgery in the twentieth century, there is much interest in extending even further, early in the twenty first century, the life of implants. Osteolysis has been identified as a major factor limiting the life of prostheses, with indications that fine polyethylene wear debris, generated primarily at the interface between the femoral head and the acetabular cup, promotes the process. There is therefore considerable interest in the introduction of alternative wear resistant systems to limit the deleterious effects of wear.

Development of a ten-station, multi-axis hip joint simulator.

Joint simulators are now used extensively to evaluate the performance of materials and designs of total replacement hip and knee joints. In this Technical Note a new ten-station hip joint simulator with biaxial rotational articulation synchronized to a physiological loading cycle is described. The current simulator manufactured by ProSim Limited (Manchester, UK) is a development of a first generation machine designed and built in-house at DePuy International Limited (Leeds, UK).