Metal-on-metal bearings surfaces: Materials, manufacture, design, optimization, and alternatives

Arterial embolization of metal-on-metal hip arthroplasty-related pseudotumor material causing acute limb ischemia

We have described a complication of a pseudotumor secondary to metal-on-metal hip arthroplasty. The patient experienced acute limb ischemia after pseudotumor material had invaded and stenosed the external iliac artery and embolized distally. Multiple vascular surgery procedures were performed to revascularize the limb. The present report highlights the importance of involving vascular surgeons early in a limb-threatening presentation of a pseudotumor to achieve limb salvage.

The risk of developing cancer following metal-on-metal hip replacement compared with non metal-on-metal hip bearings: Findings from a prospective national registry "The National Joint Registry of England, Wales, Northern Ireland and the Isle of Man"

Background and purpose

Over 1 million metal-on-metal hip replacements were implanted. Even well-functioning implants produce wear debris that can cause tissue damage, disseminate and cause DNA damage. We aimed to establish if there was an association between metal-on-metal hip replacement and the risk of subsequently developing cancer compared with alternative hip replacements.

Methods

We performed a population based prospective longitudinal cohort study using data from the National Joint Registry linked to Hospital Episode Statistics (n = 403,881 patients). We examined the incidence of a new diagnosis of cancer in patients who received a metal-on-metal bearing in comparison with those who received a non metal-on-metal bearing. Kaplan-Meier estimates of time to first cancer diagnosis were used with Cox proportional hazards regression models to assess the effect on the time to cancer diagnosis for all cancer types, haematological, malignant melanoma, urinary tract cancers or prostate cancer in men.

Results

The maximum follow up available was 11.8 years with 25% of patients followed up for more than 6.8 years (mean follow up 4.6 years; median 4.3; IQR 2.1–6.8; range 0.01–11.8). Analyses by gender that adjusted for age at primary and presence or absence of linked Welsh (PEDW) records showed no increase in the risk of developing cancer according to the bearing surface implanted for all cancers, haematological cancers, malignant melanoma, urinary tract cancers or prostate cancer in men. For patients receiving a second hip replacement, there was also no difference.

Conclusion

We have demonstrated that there is currently no evidence of an increase in the risk of cancer following primary hip replacement according to the type of bearing material used. Although the risk of revision in metal-on-metal bearing hip replacements is higher, it is reassuring that the risk of a new diagnosis of cancer is not currently increased. Despite the long term follow up available in this study, the latency period for some cancers is very long and therefore continued monitoring is required to ensure no new patterns emerge that may indicate need for universal screening.

Implications of Introducing New Technology: Comparative Survivorship Modeling of Metal-on-Metal Hip Replacements and Contemporary Alternatives in the National Joint Registry

BACKGROUND

New medical technologies are often used widely without adequate supporting data, a practice that can lead to widespread catastrophic failure such as occurred with metal-on-metal (MoM) hip replacements. We determined both how revision rates would have differed if, instead of receiving MoM hip replacements, patients had received existing alternatives and the subsequent cumulative re-revision rates of the patients who did receive MoM hip replacements compared with alternatives.

METHODS

This study is a population-based longitudinal cohort study of patient data recorded in the National Joint Registry (NJR) for England, Wales and Northern Ireland between April 2003 and December 2014. We ascertained implant failure rates separately among stemmed MoM total hip replacement (THR) and hip-resurfacing procedures and, using flexible parametric survival modeling, compared them with the failure rates that would have been expected had existing alternatives been used. We used Kaplan-Meier survivorship analysis to compare cumulative re-revision of patients who received stemmed MoM primary replacements that failed and of those who underwent hip resurfacing that failed with those whose non-MoM THRs had failed.

RESULTS

In all, 37,555 patients underwent MoM hip resurfacing, with a 10-year revision rate of 12.6% (95% confidence interval [CI]: 12.2% to 13.1%) compared with a predicted revision rate of 4.8% if alternative implants had been used. The 32,024 stemmed MoM THRs had a 19.8% (95% CI: 18.9% to 20.8%) 10-year failure rate compared with an expected rate of 3.9% if alternatives had been used. For every 100 MoM hip-resurfacing procedures, there were 7.8 excess revisions by 10 years, and for every 100 stemmed MoM THR procedures, there were 15.9, which equates to 8,021 excess first revisions. Seven-year re-revision rates were 14.9% (95% CI: 13.8% to 16.2%) for stemmed non-MoM THRs, 18.0% (95% CI: 15.7% to 20.7%) for MoM hip resurfacing, and 19.8% (95% CI: 17.0% to 23.0%) for stemmed MoM THRs.

CONCLUSIONS

This study highlights the consequences of widespread and poorly monitored adoption of a medical technology. Over 1 million MoM hip prostheses were implanted worldwide. The excess failure on a global scale will be enormous. This practice of adopting new technologies without adequate supporting data must not be repeated.

LEVEL OF EVIDENCE

Therapeutic Level III. See Instructions for Authors for a complete description of levels of evidence.

Combined Vascular and Orthopaedic Approach for a Pseudotumor Causing Deep Vein Thrombosis after Metal-on-Metal Hip Resurfacing Arthroplasty

Introduction. Metal-on-metal (MoM) hip resurfacings have been associated with a variety of complications resulting from adverse reaction to metal debris. Pseudotumors have rarely been reported to cause deep venous thrombosis (DVT). Study Design. A case report and a review of the literature. Case Presentation. A 75-year-old female who had left metal-on-metal hip resurfacing 6 years ago presented with left groin pain associated with unilateral lower limb edema and swelling. By duplex and MRI studies, our patient had an extensive soft tissue necrosis associated with a large pelvic mass causing extensive DVT of the lower limb secondary to mechanical compression of the left iliac vein. Results. Our case was initially treated for DVT followed by dual surgical approach. The pseudotumor was excised through a separate iliofemoral approach and revision of the hip implant was undertaken through a posterior approach in the same setting. An inferior vena cava (IVC) filter was inserted to minimise the perioperative risks of handling the iliac veins. Conclusion. A combined approach with vascular surgeons is required. Combined resection of the pseudotumor and revision of the metal bearing surfaces is essential, in order to achieve a good surgical outcome in this rare complication.

Novel artificial hip joint: A layer of alumina on Ti-6Al-4V alloy formed by micro-arc oxidation

In many hip replacement surgeries, monolithic alumina is used as a femoral head due to its high wear resistance. However, it is liable to fracture under load bearing operations in artificial joints. We propose a promising way to overcome this limitation by forming a dense alumina layer onto a relatively tough substrate such as Ti-6Al-4V alloy to obtain high wear resistance on a material that can sustain relatively high toughness. For this purpose, Al metal powders were deposited onto Ti-6Al-4V alloy by cold spraying in N2 atmosphere. Interfacial adhesion between Al and the Ti alloy was improved by the formation of a reaction layer of Al3Ti between them by heating at 640 °C for 1h in air. Subsequently, micro-arc oxidation treatment was performed to oxidize Al. The oxidized layer was composed of an outer porous layer of γ-alumina and inner-most dense layer of α-alumina. The α-alumina layer was almost fully densified and exhibited high Vickers hardness almost equal to that of alumina ceramics used as the femoral head. Thus, the newly developed dense alumina/Ti alloy can be potentially used to produce the reliable bearing surfaces of artificial hip joint.

On interlayer stability and high-cycle simulator performance of diamond-like carbon layers for articulating joint replacements

Diamond like carbon (DLC) coatings have been proven to be an excellent choice for wear reduction in many technical applications. However, for successful adaption to the orthopaedic field, layer performance, stability and adhesion in physiologically relevant setups are crucial and not consistently investigated. In vitro wear testing as well as adequate corrosion tests of interfaces and interlayers are of great importance to verify the long term stability of DLC coated load bearing implants in the human body. DLC coatings were deposited on articulating lumbar spinal disks made of CoCr28Mo6 biomedical implant alloy using a plasma-activated chemical vapor deposition (PACVD) process. As an adhesion promoting interlayer, tantalum films were deposited by magnetron sputtering. Wear tests of coated and uncoated implants were performed in physiological solution up to a maximum of 101 million articulation cycles with an amplitude of ±2° and −3/+6° in successive intervals at a preload of 1200 N. The implants were characterized by gravimetry, inductively coupled plasma optical emission spectrometry (ICP-OES) and cross section scanning electron microscopy (SEM) analysis. It is shown that DLC coated surfaces with uncontaminated tantalum interlayers perform very well and no corrosive or mechanical failure could be observed. This also holds true in tests featuring overload and third-body wear by cortical bone chips present in the bearing pairs. Regarding the interlayer tolerance towards interlayer contamination (oxygen), limits for initiation of potential failure modes were established. It was found that mechanical failure is the most critical aspect and this mode is hypothetically linked to the α-β tantalum phase switch induced by increasing oxygen levels as observed by X-ray diffraction (XRD). It is concluded that DLC coatings are a feasible candidate for near zero wear articulations on implants, potentially even surpassing the performance of ceramic vs. ceramic.

The current state of bearing surfaces in total hip replacement

We reviewed the literature on the currently available choices of bearing surface in total hip replacement (THR). We present a detailed description of the properties of articulating surfaces review the understanding of the advantages and disadvantages of existing bearing couples. Recent technological developments in the field of polyethylene and ceramics have altered the risk of fracture and the rate of wear, although the use of metal-on-metal bearings has largely fallen out of favour, owing to concerns about reactions to metal debris. As expected, all bearing surface combinations have advantages and disadvantages. A patient-based approach is recommended, balancing the risks of different options against an individual’s functional demands.

Cite this article: Bone Joint J 2014;96-B:147–56.

On the matter of synovial fluid lubrication: implications for Metal-on-Metal hip tribology

Artificial articular joints present an interesting, and difficult, tribological problem. These bearing contacts undergo complex transient loading and multi axes kinematic cycles, over extremely long periods of time (>10 years). Despite extensive research, wear of the bearing surfaces, particularly metal-metal hips, remains a major problem. Comparatively little is known about the prevailing lubrication mechanism in artificial joints which is a serious gap in our knowledge as this determines film formation and hence wear. In this paper we review the accepted lubrication models for artificial hips and present a new concept to explain film formation with synovial fluid. This model, recently proposed by the authors, suggests that interfacial film formation is determined by rheological changes local to the contact and is driven by aggregation of synovial fluid proteins. The implications of this new mechanism for the tribological performance of new implant designs and the effect of patient synovial fluid properties are discussed.

Effect of head size on wear properties of metal-on-metal bearings of hip prostheses, and comparison with wear properties of metal-on-polyethylene bearings using hip simulator

The effects of articular head size on the wear losses of the metal insert and articular head for a metal-on-metal bearing were examined using a hip simulator manufactured to satisfy ISO 14242-1. The wear properties of metal-on-metal and metal-on-polyethylene bearings were also compared under the same conditions. The total wear losses of the metal insert and articular head decreased with increasing diameter of the metal insert in the range from 28 to 44mm. The total wear loss was greater for a diameter of 48mm than for a diameter of 44mm. When the articular metal insert diameter was smaller than 44mm, the wear loss was reduced because the contact surface pressure increased with increasing metal insert diameter. However, the increase in wear loss observed for the 48-mm-diameter insert might have been due to the considerable increase in the rotation moment with increasing insert diameter. The tendency of decreasing contact pressure calculated using the Hertzian contact stress equation nearly conformed to the change in wear loss. On the other hand, the wear loss of an artificial hip joint consisting of a cross-linked ultrahigh-molecular-weight polyethylene insert (UHMWPE) and a Co-Cr-Mo articular head was small.

Mortality rates at 10 years after metal-on-metal hip resurfacing compared with total hip replacement in England: retrospective cohort analysis of hospital episode statistics

OBJECTIVES

To compare 10 year mortality rates among patients undergoing metal-on-metal hip resurfacing and total hip replacement in England.

DESIGN

Retrospective cohort study.

SETTING

English hospital episode statistics database linked to mortality records from the Office for National Statistics.

POPULATION

All adults who underwent primary elective hip replacement for osteoarthritis from April 1999 to March 2012. The exposure of interest was prosthesis type: cemented total hip replacement, uncemented total hip replacement, and metal-on-metal hip resurfacing. Confounding variables included age, sex, Charlson comorbidity index, rurality, area deprivation, surgical volume, and year of operation.

MAIN OUTCOME MEASURES

All cause mortality. Propensity score matching was used to minimise confounding by indication. Kaplan-Meier plots estimated the probability of survival up to 10 years after surgery. Multilevel Cox regression modelling, stratified on matched sets, described the association between prosthesis type and time to death, accounting for variation across hospital trusts.

RESULTS

7437 patients undergoing metal-on-metal hip resurfacing were matched to 22,311 undergoing cemented total hip replacement; 8101 patients undergoing metal-on-metal hip resurfacing were matched to 24,303 undergoing uncemented total hip replacement. 10 year rates of cumulative mortality were 271 (3.6%) for metal-on-metal hip resurfacing versus 1363 (6.1%) for cemented total hip replacement, and 239 (3.0%) for metal-on-metal hip resurfacing versus 999 (4.1%) for uncemented total hip replacement. Patients undergoing metal-on-metal hip resurfacing had an increased survival probability (hazard ratio 0.51 (95% confidence interval 0.45 to 0.59) for cemented hip replacement; 0.55 (0.47 to 0.65) for uncemented hip replacement). There was no evidence for an interaction with age or sex.

CONCLUSIONS

Patients with hip osteoarthritis undergoing metal-on-metal hip resurfacing have reduced mortality in the long term compared with those undergoing cemented or uncemented total hip replacement. This difference persisted after extensive adjustment for confounding factors available in our data. The study results can be applied to matched populations, which exclude patients who are very old and have had complex total hip replacements. Although residual confounding is possible, the observed effect size is large. These findings require validation in external cohorts and randomised clinical trials.

A prospective study of a ceramic-on-metal bearing in total hip arthroplasty

We prospectively assessed the efficacy of a ceramic-on-metal (CoM) hip bearing with uncemented acetabular and femoral components in which cobalt­–chrome acetabular liners and alumina ceramic heads were used.

The cohort comprised 94 total hip replacements (THRs) in 83 patients (38 women and 45 men) with a mean age of 58 years (42 to 70). Minimum follow-up was two years. All patients had pre- and post-operative assessment using the Western Ontario and McMaster Universities osteoarthritis index (WOMAC), Oxford hip score and Short-Form 12 scores. All showed a statistically significant improvement from three months post-operatively onwards (all p < 0.001).

After two years whole blood metal ion levels were measured and chromosomal analysis was performed. The levels of all metal ions were elevated except vanadium. Levels of chromium, cobalt, molybdenum and titanium were significantly higher in patients who underwent bilateral THR compared with those undergoing unilateral THR (p < 0.001). Chromosomal analysis demonstrated both structural and aneuploidy mutations. There were significantly more breaks and losses than in the normal population (p < 0.001). There was no significant difference in chromosomal aberration between those undergoing unilateral and bilateral procedures (all analyses p ≥ 0.62).

The use of a CoM THR is effective clinically in the short-term, with no concerns, but the significance of high metal ion levels and chromosomal aberrations in the long-term remains unclear.

Cite this article: Bone Joint J 2013;95-B:1040–44.

Prosthetic arthroplasty of the distal radioulnar joint: historical perspective and 24-year follow-up

This is a report of the first prosthetic hemiarthroplasty and full arthroplasty, designed and implanted for the distal radioulnar joint in 1988. Two case reports are presented, with follow-up of 24 years. Experience and problems in the design of both a hemiarthroplasty and total prosthetic arthroplasty are described, in the hope that future developments may avoid past failures.

Failure rates of metal-on-metal hip resurfacings: analysis of data from the National Joint Registry for England and Wales

BACKGROUND

Implant survival after conventional total hip replacement (THR) is often poor in younger patients, so alternatives such as hip resurfacing, with various sizes to fit over the femoral head, have been explored. We assessed the survival of different sizes of metal-on-metal resurfacing in men and women, and compared this survival with those for conventional stemmed THRs.

METHODS

We analysed the National Joint Registry for England and Wales (NJR) for primary THRs undertaken between 2003 and 2011. Our analysis involved multivariable flexible parametric survival models to estimate the covariate-adjusted cumulative incidence of revision adjusting for the competing risk of death.

FINDINGS

The registry included 434,560 primary THRs, of which 31,932 were resurfacings. In women, resurfacing resulted in worse implant survival than did conventional THR irrespective of head size. Predicted 5-year revision rates in 55-year-old women were 8·3% (95% CI 7·2-9·7) with a 42 mm resurfacing head, 6·1% (5·3-7·0) with a 46 mm resurfacing head, and 1·5% (0·8-2·6) with a 28 mm cemented metal-on-polyethylene stemmed THR. In men with smaller femoral heads, resurfacing resulted in poor implant survival. Predicted 5-year revision rates in 55-year-old men were 4·1% (3·3-4·9) with a 46 mm resurfacing head, 2·6% (2·2-3·1) with a 54 mm resurfacing head, and 1·9% (1·5-2·4) with a 28 mm cemented metal-on-polyethylene stemmed THR. Of male resurfacing patients, only 23% (5085 of 22076) had head sizes of 54 mm or above.

INTERPRETATION

Hip resurfacing only resulted in similar implant survivorship to other surgical options in men with large femoral heads, and inferior implant survivorship in other patients, particularly women. We recommend that resurfacing is not undertaken in women and that preoperative measurement is used to assess suitability in men. Before further new implant technology is introduced we need to learn the lessons from resurfacing and metal-on-metal bearings.

FUNDING

National Joint Registry for England and Wales.

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. An understanding of the origin of metal ions is really important in order to design alloys for reduced ion release. Reciprocating pin-on-plate wear tester is a standard instrument to assess the interaction of corrosion and wear. However, more realistic hip simulator can provide a better understanding of tribocorrosion process for hip implants. It is very important to instrument the conventional hip simulator to enable electrochemical measurements. In this study, simple reciprocating pin-on-plate wear tests and hip simulator tests were compared. It was found that metal ions originated from two sources: (a) a depassivation of the contacting surfaces due to tribology (rubbing) and (b) corrosion of nano-sized wear particles generated from the contacting surfaces.

Edge loading in metal-on-metal hips: low clearance is a new risk factor

The revision rate of large head metal-on-metal and resurfacing hips are significantly higher than conventional total hip replacements. The revision of these components has been linked to high wear caused by edge loading; which occurs when the head–cup contact patch extends over the cup rim. There are two current explanations for this; first, there is loss of entrainment of synovial fluid resulting in breakdown of the lubricating film and second, edge loading results in a large local increase in contact pressure and consequent film thickness reduction at the cup rim, which causes an increase in wear.

This paper develops a method to calculate the distance between the joint reaction force vector and the cup rim – the contact patch centre to rim (CPCR) distance. However, the critical distance for the risk of edge loading is the distance from the contact patch edge to rim (CPER) distance. An analysis of explanted hip components, divided into edge worn and non-edge-worn components showed that there was no statistical difference in CPCR values, but the CPER value was significantly lower for edge worn hips.

Low clearance hips, which have a more conformal contact, have a larger diameter contact patch and thus are more at risk of edge loading for similarly positioned hips.

Deformation of 1-piece metal acetabular components

The success of metal bearings is dependent on several parameters. The effects of in vivo forces on the deformation of monoblock acetabular components have yet to be determined. The purpose of our study was to assess the amount of deformation with press-fit fixation of 1-piece metal acetabular components. Four manufacturers provided 1-piece metal acetabular components in each size (30 cups). Testing was conducted using a custom vise to simulate press-fit fixation, and measurements were performed with a Mitutoyo Test device (Aurora, Ill). Previously determined in vivo forces were used in the press-fit simulation. All components deformed under simulated in vivo applied loads. Component deformation ranged from 15 to 300 µm. Larger cups with thinner walls to accommodate larger heads had the greatest deformation and often exceeded the range of reported clearances from the manufacturers (76-227 µm).

A prospective metal ion study of large-head metal-on-metal bearing: a matched-pair analysis of hip resurfacing versus total hip replacement

The current study measured ion release among large-head metal-on-metal hip bearings. Twenty-six patients with a modular, Profemur® TL with A-Class® big femoral head total hip replacement were matched (gender, femoral size, BMI) with a group of 26 patients with the Conserve® Plus implant hip resurfacing. Compared with HR patients, THR patients had higher median serum cobalt ion levels at 6 months (3.26 vs 1.12 μg/L, P = .002) 1 year (4.51 vs 1.02, P = .002), and 2 years (3.77 vs 1.22, P<.001) following surgery. No differences in chromium ions were observed. Further research is required to determine the clinical significance of elevated serum cobalt ions.

Whole blood metal ion concentrations in correlation with activity level in three different metal-on-metal bearings

Seventy consecutive osteoarthritis patients younger than 65 years undergoing a hip arthroplasty were included. Three different metal bearings were used (2 different resurfacing systems and one 28-mm metal bearing) and compared with a ceramic control group. Whole blood cobalt and chromium levels were analyzed preoperatively, at 3 months, 6 months, 1 year, and 2 years. Patients with steep cups (abduction >55°) were excluded. No significant differences in functional scores and activity level were found between either groups. Ion levels were significantly lower with one type of resurfacing. No correlation was found between ion and activity level. In conclusion, although the current second-generation metal bearings may show differences in ion release, more attention should be paid to the correlation between ion release and implant positioning.

Benefits of thin-shelled acetabular components for metal-on-metal hip resurfacing arthroplasty

The theoretical advantage of using thinner acetabular components in hip resurfacing has not yet been clinically verified. Our purpose was to test the hypothesis of bone conservation and assess the effects of using a thinner acetabular component on hip biomechanics and clinical outcome. We compared the bone conservation, biomechanical results, and functional outcomes between hips in 35 patients who received bilateral metal-on-metal resurfacing arthroplasties with acetabular components of 5 mm thickness on one side and 3.5 mm thickness on the other. Acetabular abduction angle and acetabular anteversion were measured using Ein-Bild-Röentgen-Analysis software. Medial acetabular wall thickness and position of the hip center of rotation were measured using Image J software. The change in position of the hip center of rotation was minimal and did not reach significance. Thin-shelled components showed greater bone conservation on the acetabular side measured by an increase in the medial acetabular wall thickness. Bone conservation on the femoral side was achieved as well with thin shells. Range of motion, pain scores, and complication rates were comparable. No appreciable difference was found in bone-cup radiographic appearance between the two types of components. These data suggest that patients can experience good clinical outcomes for resurfacing with either thin or thick-shelled acetabular components. However, thin-shelled components preserve acetabular bone stock and allow the use of a larger femoral component. The use of thinner acetabular components is an improvement in bone conservation for a hip resurfacing design.

Reduced articular surface of one-piece cups: a cause of runaway wear and early failure

BACKGROUND

Despite the clinical success of modern metal-on-metal articulations, concerns with wear-related release of metal ions persist. Evidence suggests metal ion release is related to the effective coverage of the head in the metal shell (the cup's functional articular arc). A recent study suggests a reduced functional articular arc is associated with increased ion release and the arc is a function of component design, size, and the abduction angle.

QUESTIONS/PURPOSES

The purposes of this study were to (1) measure the functional articular arc in different sizes of currently available one-piece metal shells from several different manufacturers; and (2) compare the functional articular arc of these one-piece metal shells with the 180 masculine arc of conventional hip arthroplasty acetabular components.

METHODS

We calculated the available articular surface arc for 33 one-piece metal cups using measurements of cup depth and internal cup radius.

RESULTS

The arc of the articular surface varied among manufacturers and generally decreased with decreasing shell diameter. The mean functional articular arc was 160.5 degrees +/- 3.6 degrees (range, 151.8 degrees -165.8 degrees), which was less than the 180 degrees arc of a conventional acetabular component.

CONCLUSIONS

Our data show certain cup designs are at higher risk for failure as a result of the decreased articular surface arc. This, along with analysis of abduction angles, supports the recent findings of bearing failure with vertically placed implants. Care must be taken when implanting these shells to ensure they are placed in less abduction to avoid edge loading and the potential for early bearing failure.

Metal-on-metal total hip arthroplasty

The effects of elevated levels of metal ions in patients who have undergone metal-on-metal total hip arthroplasty are not fully understood. The effects of femoral head size on serum metal-ion levels have been the subject of conflicting reports, and further investigation is needed to evaluate the impact of acetabular and femoral component alignment. The conduct of clinical trials of metal-on-metal total hip arthroplasties has been inadequate as few investigators have used a randomized controlled design to compare metal-on-metal bearings with other bearing surfaces. Additional clinical research needs to include appropriate validated patient-reported outcome measures, activity monitoring, and health economics.

Changes in whole blood metal ion levels following resurfacing: serial measurements in a multi-centre study

Seventy-seven patients implanted with unilateral resurfacing prosthesis were recruited from four centres. Serial whole blood samples were collected and ion levels were analysed. In most cases, the ion levels stabilized by 3 months. The 24 month median ion levels were 1.49ug/l for chromium and cobalt. In approximately 50% of patients the increase in chromium and cobalt level was less than 1ug/l. There were 6 patients with abnormally high metal ion levels. Of these 4 were significant outliers, had high ion levels that became apparent between 12 and 24 months after implantation, and had a high cup abduction angle. Not all patients with high cup abduction angles demonstrated high levels. There were differences in ion levels between the four centres that correlated with variation in acetabular component placement. Variability in ion levels was seen with the same prosthesis, underscoring the importance of surgical technique, longitudinal analysis, and multi-centre trials.

The influence of the size of the component on the outcome of resurfacing arthroplasty of the hip

The survivorship of contemporary resurfacing arthroplasty of the hip using metal-on-metal bearings is better than that of first generation designs, but short-term failures still occur. The most common reasons for failure are fracture of the femoral neck, loosening of the component, osteonecrosis of the femoral head, reaction to metal debris and malpositioning of the component. In 2008 the Australian National Joint Registry reported an inverse relationship between the size of the head component and the risk of revision in resurfacing hip arthroplasty. Hips with a femoral component size of ≤ 44 mm have a fivefold increased risk of revision than those with femoral components of ≥ 55 mm irrespective of gender. We have reviewed the literature to explore this observation and to identify possible reasons including the design of the implant, loading of the femoral neck, the orientation of the component, the production of wear debris and the effects of metal ions, penetration of cement and vascularity of the femoral head. Our conclusion is that although multifactorial, the most important contributors to failure in resurfacing arthroplasty of the hip are likely to be the design and geometry of the component and the orientation of the acetabular component.

A numerical study of failure mechanisms in the cemented resurfaced femur: effects of interface characteristics and bone remodelling

Failure mechanisms of the resurfaced femoral head include femoral neck fracture in the short-term and stress shielding and implant loosening in the long-term. In this study, finite element simulations of the resurfaced femur considering a debonded implant-cement interface, variable stem-bone interface conditions, and bone remodelling were used to study load transfer within the resurfaced femur and to investigate its relationship with known failure mechanisms. Realistic three-dimensional finite element models of an intact and resurfaced femur were used. Various conditions at the interface between the stem of the prosthesis and the bone were considered. Loading conditions included normal walking and stair climbing. For all stem-bone contact conditions, the tensile stresses in the cement mantle varied between 1 MPa and 5.4 MPa, except near the distal rim of the resurfacing component where they reached 5.4-7MPa. In the case of full stem-bone contact, high von Mises stresses (114-121MPa) were generated in the implant at the stem-cup junction. These stresses were considerably reduced (maximum von Mises stress, 76 MPa) where a gap was present at the stem-bone interface. Resurfacing led to strain shielding of the bone of the femoral head (20-75 per cent strain reductions) and periprosthetic bone resorption (50-80 per cent bone density reductions) for all interface stem-bone contact conditions. In the lateral femoral head and the proximal femoral shaft around the trochantric region, bone density reductions varied between 10 per cent and 50 per cent. Bone apposition was observed in the inferior-medial part of the femoral head and proximal femoral neck region. For full stem-bone contact, more load was transferred through the stem to the surrounding bone, exacerbating strain shielding. Although femoral hip resurfacing conserves bone stock at the primary operation, strain shielding and periprosthetic bone resorption might lead to eventual loosening over time. Post-operatively, the resurfacing procedure generated elevated strains (0.50-0.75 per cent strain) in the proximal femoral neck-component junction irrespective of the variation in interface conditions, indicating an initial risk of femoral neck fracture. Subsequent to bone remodelling, this strain concentration was considerably reduced (0.35-0.50 per cent strain), lowering the risk of neck fracture. In order to reduce the potential risk of neck fracture, patients should avoid activities which might induce high loading of the hip during the early post-operative period to allow the bone around the proximal femoral neck to remodel and heal.

Metal-on-metal total hip arthroplasty

Although metal-on-metal total hip arthroplasty (MOM THA) has been used for over 3 decades, substantial improvements in manufacturing and design have led to improved durability with modern implants. Reported advantages of the use of MOM THA include very low wear and subsequent osteolysis, increased range of motion to impingement secondary to the availability of larger diameter femoral heads, and the potential to monitor implant performance by serial assessment of metal ion levels. Clinical results of both first-generation and second-generation MOM THA have revealed good survivorship and a low incidence of osteolysis. Although the advantages of low wear and increased range of motion have made MOM THA an attractive bearing surface option, more widespread use of MOM bearing surfaces has been tempered with concern for increased metal ion levels and hypersensitivity reactions.

Wear performance of large-diameter differential-hardness hip bearings

We hypothesized that differential-hardness hard-on-hard bearings would generate less wear debris compared with like-hardness metal-on-metal (M-o-M) bearings. We conducted wear testing on 3 types of large-diameter hard hip bearings: (1) contemporary cast-on-cast ("like" hardness) M-o-M; (2) differential-hardness M-o-M; and (3) differential-hardness ceramic-on-metal. A simulated gait profile ranging from 200 to 2000 N was applied to the bearings at a frequency of 1 Hz for 5 Mc. All bearings were tested in an anatomically inverted position in 90% alpha calf serum. Both differential-hardness bearing systems produced lower run-in wear rates (90%-97%), steady-state wear rate (45%-84%), and total metal wear (68%-88%) compared with the like-hardness bearing system. The ceramic-on-metal bearings exhibited the least wear followed by differential-hardness M-o-M bearings; like-hardness M-o-M bearings exhibited the greatest amount of wear. These findings support our hypothesis that differential-hardness hip bearing systems produce less metallic wear debris than those with like hardness and may result in lower metal ion release in vivo.

Development of computational wear simulation of metal-on-metal hip resurfacing replacements

As one of the alternatives to traditional metal-on-polyethylene total hip replacements, metal-on-metal hip resurfacing prostheses demonstrating lower wear have been introduced for younger and more active patients during the past decade. However, in vitro hip simulator testing for the predicted increased lifetime of these surface replacements is time-consuming and costly. Computational wear modelling based on the Archard wear equation and finite element contact analysis was developed in this study for artificial hip joints and particularly applied to metal-on-metal resurfacing bearings under simulator testing conditions to address this issue. Wear factors associated with the Archard wear equation were experimentally determined and based on the short-term hip simulator wear results. The computational wear simulation was further extended to a long-term evaluation up to 50 million cycles assuming that the wear rate stays constant. The prediction from the computational model shows good agreement with the corresponding simulator study in terms of volumetric wear and the wear geometry. The simulation shows the progression of linear wear penetrations, and the complexity of contact stress distribution on the worn bearing surfaces. After 50 million cycles, the maximum linear wear was predicted to be approximately 6 and 8 microm for the cup and head, respectively, and no edge contact was found.

Normalization of load and clearance effects in ball-in-socket-like replacements

A normalizing loading parameter useful in summarising the mechanical response of plane pin-in-plate-like contacts is extended to axisymmetric ball-in-socket-like contacts. An example addressing a compliant layered artificial hip joint is presented, and the usefulness of the normalizing loading parameter is evidenced.

Larger diameter bearings reduce wear in metal-on-metal hip implants

Metal-on-metal hip arthroplasty has the longest clinical history of all total arthroplasties. We asked whether large diameter femoral heads would result in less wear than those with small diameters. We also asked if there is a threshold diameter that ensures good wear behavior. We tested three batches of cast high-carbon cobalt-chromium-molybdenum hip implants (28 mm, 36 mm, and 54 mm diameters) in a hip simulator for 5 million cycles. We used bovine serum as lubricant and weighed the samples at regular intervals during testing. The 28-mm configuration had almost twice the wear of the 54-mm configuration, but we observed no difference between the 36-mm and the 54-mm configurations. The similarity in the wear performances of the larger configurations supports the presence of a threshold diameter that ensures good wear behavior.