Volumetric wear assessment of failed metal-on-metal hip resurfacing prostheses

Recent advancements in hip arthroplasty have allowed the operation to boast excellent results and high survivorship. However, failures do still occur and a major cause is complications arising from wear debris. It is essential therefore that debris is minimized by reducing wear at the bearing surface. One proposed method of achieving this wear reduction is through the use of metal-on-metal articulations. One of the latest manifestations of this biomaterial combination is in designs of hip resurfacing which are aimed at younger, more active patients who might wear out a conventional metal-on-polymer hip prosthesis. However, do these metal-on-metal hip resurfacings show less wear when implanted into patients?Using a co-ordinate measuring machine and a bespoke computer program, volumetric wear measurements for retrieved Articular Surface Replacements (ASR™, DePuy) metal-on-metal hip resurfacings were undertaken. Thirty-two femoral heads and twenty-two acetabular cups were measured. Acetabular cups exhibited mean volumetric wear of 29.00 mm³ (range 1.35-109.72 mm³) and a wear rate of 11.02 mm³/year (range 0.30-63.59 mm³/year). Femoral heads exhibited mean wear of 22.41 mm³ (range 0.72-134.22 mm³) and a wear rate of 8.72 mm³/year (range 0.21-31.91 mm³/year). In the 22 cases where both head and cup from the same prosthesis were available, mean total wear rates of 21.66 mm³/year (range 0.51-95.50 mm³/year) were observed. Compared with in many vitro tests, these are significantly higher than those expected in a well functioning metal-on-metal hip resurfacing prosthesis and are of concern.     

A novel approach to the estimation and application of the wear coefficient of metal-on-metal hip implants

A novel approach is proposed to estimate and model the wear of metal-on-metal hip implants. The approach is based on two distinct wear coefficients for the head and cup, derived from separate measurements on the two components. This is in contrast to the usual assumption that a single wear coefficient (k) is valid for both bodies. Actually, the head and cup do not wear equally; thus, assuming equal wear leads to predictive errors. Additionally, in most papers, k is chosen considering only implant materials while neglecting geometry and testing conditions. It is suggested that experimental procedures designed for hip implants should measure the head and cup volume losses separately and that wear maps should be provided to validate numerical models.     

Contact mechanics analysis of metal-on-metal hip resurfacing prostheses

Finite-element method was employed to study the contact mechanics in metal-on-metal hip resurfacing prostheses, with particular reference to the effects of bone quality, the fixation condition between the acetabular cup and bone, and the clearance between the femoral head and the acetabular cup. Simple finite-element bone models were developed to simulate the contact between the articulating surfaces of the femoral head and the acetabular cup. The stresses within the bone structure were also studied. It was shown that a decrease in the clearance between the acetabular cup and femoral head had the largest effect on reducing the predicted contact-pressure distribution among all the factors considered in this study. It was found that as the clearance was reduced, the influence of the underlying materials, such as bone and cement, became increasingly important. Stress shielding was determined to occur in the bone tissue surrounding the hip resurfacing prosthesis considered in this study. However, the stress-shielding effects predicted were less than those observed in conventional total hip replacements. Both the effects of bone quality (reduction in elastic modulus) and the fixation condition between the cup and the bone were found to have a negligible effect on the predicted contact mechanics at the bearing surface. The loading was found to have a relatively small effect on the predicted maximum contact pressure at the bearing surface; this was attributed to an increase in contact area as the load was increased.     

Characterisation of wear debris from UHMWPE on zirconia ceramic, metal-on-metal and alumina ceramic-on-ceramic hip prostheses generated in a physiological anatomical hip joint simulator

There is currently much interest in the characterisation of wear debris from different types of artificial hip joints. There have been numerous studies on the wear of UHMWPE in hip joint simulators, but relatively few studies on the wear of alternative materials such as metal-on-metal (MOM) and ceramic-on-ceramic (COC). The aim of this study was to compare the wear volumes and wear debris generated from zirconia ceramic-on-UHMWPE, MOM and COC hip joints under identical conditions in the same hip joint simulator.

All prostheses showed an initial higher ‘bedding in’ wear rate, which was followed by a lower steady state wear rate. The zirconia ceramic-on UHMWPE prostheses showed the highest wear rates (31±4.0 mm³/million cycles), followed by the MOM (1.23±0.5 mm/million cycles), with the COC prostheses showing significantly (P<0.01) lower wear rates at 0.05±0.02 mm³/million cycles. The mode (±95% confidence limits) of the size distribution of the UHMWPE wear debris was 300±200, 30±2.25 nm for the metal particles, and 9±0.5 nm for the ceramic wear particles. The UHMWPE particles were significantly larger (P<0.05) than the metal and ceramic wear particles, and the metal particles were significantly larger (P<0.05) than the ceramic wear particles. A variety of morphologies and sizes were observed for the UHMWPE wear particles, including submicrometer granules and large flakes in excess of 50 μm. However, the wear particles generated in both the MOM and COC articulations were very uniform in size and oval or round in shape.

This investigation has demonstrated substantial differences in volumetric wear. The in vitro wear rates for the zirconia-on-UHMWPE and MOM are comparable with clinical studies and the UHMWPE and metal wear particles were similar to the wear debris isolated from retrieved tissues. However, the alumina/alumina wear rate was lower than some clinical retrieval studies, and the severe wear patterns and micrometer-sized particles described in vivo were not reproduced here.

This study revealed significant differences in the wear volumes and particle sizes from the three different prostheses. In addition, this study has shown that the alternative bearing materials such as MOM and COC may offer a considerable advantage over the more traditional articulations which utilise UHMWPE as a bearing material, both in terms of wear volume and osteolytic potential.     

Influence of simulator kinematics on the wear of metal-on-metal hip prostheses

There is now considerable interest in metal-on-metal bearings for hip prostheses. Extremely low wear rates (0.1 mm3/10(6) cycles) have been reported in some simulator studies, while in vivo studies, although still very low, have shown wear rates of the order of 1 mm3/10(6) cycles. The aim of this study was to compare wear rates of metal-on-metal bearings in two hip simulators with different kinematic inputs. In the simulator with three independent input motions which produced an open elliptical wear path with a low level of eccentricity, the wear rates were very low as recorded previously in other simulators. In the simulator with two input motions which produced an open elliptical wear path with greater eccentricity the wear rate was at least ten times higher and closer to clinical values. The motion and kinematic conditions in the contact are critical determinants of wear in metal-on-metal bearings.     

The wear of metal-on-metal total hip prostheses measured in a hip simulator

New generation metal-on-metal prostheses have been introduced to try and overcome the problem of osteolysis often attributed to the wear particles of the polyethylene component of conventional metal-on-ultra-high molecular weight polyethylene (UHMWPE) joints. The wear rates of four metal-on-metal joints (two different clearances) were assessed along with that of a conventional metal-on-UHMWPE joint. Friction measurements of the metal-on-metal joints were taken before and after the wear test and compared. Two distinct wear phases were discernible for all the metal-on-metal joints: an initial wear phase up to 0.5 x 10(6) cycles and then a lower steady state wear phase. The steady state wear rate of the 22 microm radial clearance metal-on-metal joint was lower than that for the 40 microm radial clearance joint, although this difference was not found to be significant (p > 0.15). The wear rates for all the joints tested were consistent with other simulator studies. The friction factors produced by each joint were found to decrease significantly after wear testing (p < 0.05).     

A finite element method comparison of wear in two metal-on-metal total hip prostheses

The contact mechanics of two metal-on-metal (MOM) total hip prostheses was studied by means of the finite element method (FEM). The purpose of the work was to compare two total hip replacements (Durom and Metasul) with regard to the amount of wear debris released. Wear on the bearing surfaces was evaluated following Reye hypotheses from the pressure distribution, computed by means of three-dimensional FEM models; an approximate analytical model based on Hertz contact theory has also been developed and discussed. The results show that in the dry friction condition the Durom joint releases almost twice as much wear volume as produced by the Metasul joint. Therefore, while Durom implants can improve hip stability by increasing the prosthetic impingement-free range of motion (PIF-ROM), Metasul prostheses can be a valuable solution whenever wear represents a critical choice factor.     

Sliding metallic wear test with in-process wear measurement: A new approach to collecting and applying wear data

A wear test is described in which the edge of a hard wedge is loaded against the periphery of a rotating disc of softer specimen material. The applied normal load is kept approximately constant during a test. As the test progresses and the disc diameter is reduced by the wear taking place the wedge moves radially inward. By measuring this inward movement during a test it is shown how the wear can be continually monitored. Results are given and it is shown how these might be applied in practice, taking into account the influence of both surface roughness and lubrication.     

Biological effects of clinically relevant wear particles from metal-on-metal hip prostheses

The problems of osteolysis and late aseptic loosening associated with ultra-high molecular weight polyethylene (UHMWPE) particles has lead to a renewed interest in metal-on-metal prostheses. Wear particles generated by modern Co-Cr-on-Co-Cr prostheses are nanometre in size (range: 10-120 nm; mean: about 40 nm), an order of magnitude smaller than the size of UHMWPE known to be critical for activation of osteolytic cytokines by macrophages. Co-Cr wear particles will induce osteolytic cytokine production by human macrophages, but only at high volumetric concentrations. Unlike UHMWPE, Co-Cr particles are not inert. Co-Cr particles have the potential to release metal ions; they may be toxic to cells, induce deoxyribonucleic acid damage or cause host hypersensitivity. The nanometre size range of Co-Cr wear particles means that they may be disseminated widely in the body. The potential for metal-on-metal bearings to induce adverse effects clinically will be dependent upon the rate of wear. What constitutes a safe wear rate for modern metal-on-metal bearings is unknown. However, the wear of metal-on-metal prostheses is critically dependent upon the design and, in particular, the carbon content of the alloy, the radial clearance, and the head diameter. Thus, the potential for adverse biological reactions associated with metal-on-metal bearings can be reduced by selection of appropriately designed implants.     

An in vitro study of the reduction in wear of metal-on-metal hip prostheses using surface-engineered femoral heads

Although the wear of existing metal-on-metal (MOM) hip prostheses (1 mm3/10(6) cycles) is much lower than the more widely used polyethylene-on-metal bearings, there are concerns about the toxicity of metal wear particles and elevated metal ion levels, both locally and systemically, in the human body. The aim of this study was to investigate the possibility of reducing the volume of wear, the concentration of metal debris and the level of metal ion release through using surface-engineered femoral heads. Three thick (8-12 microm) coatings (TiN, CrN and CrCN) and one thin (2 microm) coating (diamond-like carbon, DLC), were evaluated on the femoral heads when articulating against high carbon content cobalt-chromium alloy acetabular inserts (HC CoCrMo) and compared with a clinically used MOM cobalt-chromium alloy bearing couple using a physiological anatomical hip joint simulator (Leeds Mark II). This study showed that CrN, CrCN and DLC coatings produced substantially lower wear volumes for both the coated femoral heads and the HC CoCrMo inserts. The TiN coating itself had little wear, but it caused relatively high wear of the HC CoCrMo inserts compared with the other coatings. The majority of the wear debris for all half-coated couples comprised small, 30 nm or less, CoCrMo metal particles. The Co, Cr and Mo ion concentrations released from the bearing couples of CrN-, CrCN- and DLC-coated heads articulating against HC CoCrMo inserts were at least 7 times lower than those released from the clinical MOM prostheses. These surface-engineered femoral heads articulating on HC CoCrMo acetabular inserts produced significantly lower wear volumes and rates, and hence lower volumetric concentrations of wear particles, compared with the clinical MOM prosthesis. The substantially lower ion concentration released by these surface-engineered components provides important evidence to support the clinical application of this technology.     

Volumetric wear assessment of retrieved metal-on-metal hip prostheses and the impact of measurement uncertainty

In hip replacement the use of hard bearing materials, which have been shown to have lower wear rates than traditional metal-UHMWPE couples, has been encouraged in an effort to extend component life-in-service. With a view to further extending this period of joint activity metal-on-metal hip resurfacings have been developed which allow for implantation into younger and more active patients. However, these bearings have recently been the subject of a MHRA device alert due to unexplained hip pain and reported soft tissue reactions and indeed are under investigation having been shown to exhibit high failure rates. This has highlighted the need for a traceable metrological approach to the quantification and characterization of in vivo wear as currently there is no standard dedicated to the measurement of such wear.This study is the first to develop a comprehensive method for measuring the wear geometry of retrieved bearings whilst also assessing and quantifying both the magnitude and effect of the three-dimensional measurement uncertainty on the measurement process as a whole.This study shows that the magnitude of the expanded measurement uncertainty is an important, and until now overlooked, factor and that a statement of uncertainty is vital in measuring retrieved components as a means of comparison between studies and as a measure of confidence in measurement data.Based on the results of this study, it is recommended that care be taken when performing wear measurements on retrieved bearings. Factors such as point pitch and scan line separation can have a great effect on the likely uncertainty of the obtained measurement result and indeed the uncertainty of the measurement can be of the same order as the wear being measured.     

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.     

Three-body impact wear study on conventional and new P/M + HIPed wear resistant materials

A new hammer-mill type impact wear testing facility was built for impact wear testing and characterization. Tests with the hammer-mill impact wear device were carried out on conventional wear resistant materials such as Mn-steels of different compositions, white cast iron, and on new P/M+HIPed wear resistant materials. To verify the validity in using this laboratory wear testing apparatus, wear behavior and worn surfaces obtained on conventional and new Mn-steels generated from this device were compared with wear phenomena and worn surfaces developed in industrial applications, i.e. from certain types of rock crushers. The strain hardening effect in different Mn-steel grades was studied first. Second, the wear resistance of materials with different properties was studied using two different grades of abrasive. With silica sand (high hardness, low compressive strength), conventional Mn-steel and white cast iron perform in a manner comparable with the P/M+HIPed materials. With volcanite sand (low hardness, very high compressive strength), the P/M+HIPed wear resistant materials appear to have the best wear resistance.     

Quantification of self-polishing in vivo from explanted metal-on-metal total hip replacements

One of the claimed advantages of metal-on-metal total hip replacements is that they ‘self-polish’ in the body, however this assertion has not been supported by quantitative data. Two pairs of components, each consisting of a femoral head and acetabular cup, were obtained at revision surgery. They were subject to topographical analysis using a non-contacting profilometer. In the ‘worn’ regions a reduction in surface roughness was seen alongside a change in skewness values from positive to negative, while elastohydrodynamic theory suggested an improvement towards mild mixed lubrication during gait. Therefore self-polishing of metal-on-metal total hip replacements can occur in vivo.     

Steels’ wear resistance definition method by their standard mechanical characteristics

The basic regularities of abrasive outwearing for steels of different structural classes are considered. The reliable function bond of steels’ wear resistance with their mechanical characteristics is proved. The correlation of toughness and plasticity with wear resistance is revealed and the new method of steels’ wear resistance definition based on the use of standard mechanical characteristics is offered. The complex criterion for steels’ wear resistance estimation at mechanical outwearing—the product of ultimate strength on relative reduction of area—(σ_b×ψ) characterizes a nature of steels’ outwearing and is reliable enough for determination of wear resistance in conditions of sliding friction, rolling friction on an abrasive and at erosive outwearing. Using offered complex criterion of steels wear resistance, it is possible to determine or estimate the wear resistances of many steels without running their wear tests. The leading role of metal science in the decision of problem of wear resistance increase for equipment working under abrasive wear conditions is marked.     

Importance of head diameter, clearance, and cup wall thickness in elastohydrodynamic lubrication analysis of metal-on-metal hip resurfacing prostheses

The main design features of metal-on-metal (MOM) hip resurfacing prostheses in promoting elastohydrodynamic lubrication were investigated in the present study, including the femoral head diameter, the clearance, and the cup wall thickness. Simplified conceptual models were developed, based on equivalent uniform wall thicknesses for both the cup and the head as well as the support materials representing bone and cement, and subsequently used for elastohydrodynamic lubrication analysis. Both typical first- and second-generation MOM hip resurfacing prostheses with different clearances and cup wall thicknesses were considered with a fixed large bearing diameter of 50 mm, as well as a 28 mm diameter MOM total hip replacement bearing for the purpose of comparison. The importance of the head diameter and the clearance in promoting elastohydrodynamic lubrication was confirmed. Furthermore, it was also predicted that a relatively thin acetabular cup in the more recently introduced second-generation MOM hip resurfacing prostheses would be capable of improving elastohydrodynamic lubrication even further.     

Sliding wear behavior of stainless steel parts made by metal injection molding (MIM)

The wear resistance of stainless steel parts, which were fabricated by a metal injection molding (MIM) process, was studied under self-mating, dry sliding conditions, using a pin-on-disk type wear configuration. The materials used were an austenitic stainless steel 316L (of MIM and wrought steel), a precipitated stainless steel 17-4PH (of MIM and wrought steel) and a widely used ball-bearing steel (DIN 100Cr6). Two surface conditions, i.e. as sintered and as polished, were considered in the wear tests of the MIM samples. In summary, the wear resistance of 316L was lower than 17-4PH, and the latter was almost the same as 100Cr6. Further, the wrought materials showed a better wear resistance than the MIM materials. The mechanisms responsible for these tendencies were discussed by means of microscopical observations and chemical inspections of the worn surfaces and the microstructures of the samples.     

An automatic system based on vibratory analysis for cutting tool wear monitoring

The aim of this work is to develop a new, simple to use and reliable automatic method for detection and monitoring wear on the cutting tool. To achieve this purpose, the vibratory signatures produced during a turning process were measured by using a three-axis accelerometer. Then, the mean power analysis was proposed to extract an indicator parameter from the vibratory responses, to be able to describe the state of the cutting tool over its lifespan. Finally, an automatic detector was proposed to evaluate and monitor tool wear in real time. This detector is efficient, simple to operate in an industrial environment and does not require any protracted computing time.     

The relevance of wear-mechanism maps to mild-oxidational wear

The presence of oxygen in the environment in which a steel sliding system operates will promote a mild form of wear with wear debris consisting mainly of iron oxides. Of the oxidation-dominated mechanisms, mild-oxidational wear (the prefix describes the extent of oxidation and not the wear rate) has been most extensively investigated. In this paper, examples will be used to show that the wear-mechanism map for the unlubricated sliding of steels can adequately predict the occurrence of mild-oxidational wear and the trend of wear rates as well as describe the resultant features on the worn surfaces. It is also shown that this map is relevant to delamination wear and to test geometries other than the pin-on-disk configuration. It is suggested that the more-recently constructed wear maps for aluminium and magnesium alloys could similarly be used to predict the wear characteristics of these alloys during sliding.     

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. These alternatives include ceramic-on-ceramic and metal-on-metal configurations and the present paper is primarily concerned with the latter. Some six pairs of new metal-on-metal implants of 36 mm diameter and four pairs of existing metal-on-metal implants of 28 mm diameter were tested in a ten-station hip joint simulator in the presence of a 25 per cent bovine serum solution. The implants were tested in the anatomical position to 5 x 10(6) cycles. The new heads and cups were manufactured from CoCrMo alloy with careful attention being paid to sphericity and surface finish of both components. The wear performance of the new and existing metal-on-metal total hip replacements have been evaluated and compared. The overall wear rates have then been compared with previously reported wear rates for a zirconia-on-polyethylene prosthesis of 22 mm diameter tested on the same simulator. The comparison is taken further by recalling published penetration data for metal-on-polyethylene implants of 22 and 28 mm diameter and converting these to volumetric wear rates. It was found that the heads and cups in metal-on-metal joints wore by almost equal amounts and that the opposing surfaces converged to similar surface roughness as the testing time increased. Steady state wear rates were generally achieved after 1-2 x 10(6) cycles. The mean long-term wear rates for the metal-on-metal prostheses were very low, being 0.36 mm3/10(6) cycles and 0.45 mm3/10(6) cycles for the new implants of 36 mm diameter and established implants of 28 mm diameter respectively. These wear rates compare with 6.3 mm3/10(6) cycles for zirconia-on-ultra-high molecular weight polyethylene tested on the same simulator and representative clinical values for metal-on-polyethylene of 36 mm3/year for heads of 22 mm diameter and a reported range of 60-180 mm3/year for 28 mm heads. These values do not translate directly into numbers of particles, since the metallic debris from metal-on-metal joints is very fine. The number of metallic particles may exceed the number of polyethylene wear particles from an otherwise similar metal-on-polyethylene joint by a factor of 10(3). A detailed discussion of the size and morphology of wear debris and tissue reaction to various forms of debris is beyond the scope of this paper, but the biological response to polymeric, metallic and ceramic wear debris forms a major subject for further study. The present investigation nevertheless confirms the potential of carefully designed and manufactured metal-on-metal total replacement joints for the treatment of diseased and damaged hips.