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. 2020 Dec 31;23:123–127. doi: 10.1016/j.jor.2020.12.036

Metal on metal hip resurfacing arthroplasty: Where are we now?

EJ Clough a,∗, TM Clough b
PMCID: PMC7809170  PMID: 33488008

Abstract

Metal on metal hip resurfacing arthroplasty (HRA) was introduced in an attempt to address potential limitations regarding return to sport, return to high functional activity and premature wear in young patients requiring standard total hip replacement (THR). Around 12% patients undergoing hip replacement surgery are under 55 years of age. By more closely mimicking natural anatomy, and having a metal on metal (MoM) bearing, it was hoped that HRA would address the issues. However, concern has emerged about early failure, high revision rates from local adverse reactions to metallic wear debris and potential systemic consequences of metal ion cardiotoxicity.

In this article, we discuss the existing literature in the field, the current clinical evidence surrounding HRA, its indications, clinical outcomes, and risk factors for failure and conclude if it still has a role within orthopaedic hip surgery.

Keywords: Metal on metal, Hip resurfacing arthroplasty, Total hip replacement, Wear, Pseudotumours

1. Introduction

Standard total hip replacement (THR) designs provide excellent analgesic relief and predictable outcome, with a 93%–97% 10 year survival and a 60–80% 25 year implant survival.1,2 Such has been the phenomenal success of the hip replacement operation, that it has been described as the ‘operation of the century’.3 However THRs can impose significant limitations on return to high functional activity and are subject to wear.4 For young patients with hip arthritis, this is problematic.

Bone conserving hip resurfacing arthroplasty (HRA) was introduced as potential solution to these problems, by providing a theoretical option for the young high demand patient. By more closely mimicking natural anatomy, it was hoped to enable return to pre-osteoarthritic potential - particularly a return to sport. Having a metal on metal (MoM) bearing, as compared to a metal on polyethylene (MoP) bearing, as in a standard THR, it was also hoped to have greater resistance to wear, making it longer lasting. It has featured in recent headlines following high profile athletes opting specifically for HRA,5 with some even returning to the peak of their sport6 including high impact activities.7 However, concern has emerged about early failure, high revision rates from local adverse reactions to metallic wear debris and potential systemic consequences of metal ion cardiotoxicity. Lawsuits for various designs of the HRA implant are still ongoing both in the UK and the USA (where since 2002, hip manufacturers are reported to have paid out more than $7.5 billion in settlements and verdicts, and, as of July 2019, a further 14,000 cases are still pending).8

In light of the legal and financial climate surrounding the MoM HRA implant use, the aim of this article is to evaluate the current clinical evidence surrounding HRA, its indications, clinical outcomes, and risk factors for failure and conclude if it still has a role within orthopaedic hip surgery.

2. Total hip replacement (THR)

THR remains the mainstay of management of end stage hip arthrosis with excellent outcomes, low rates of revision,1 and high patient satisfaction.9 Whilst overall rates of primary THR surgery have, over the years, been steadily increasing worldwide, with incidences in the USA between 2005 and 2030, projected to increase by 174% - equivalent to 572,000 procedures per year,10 and incidences in the UK, projected to increase, from 170 per 100,000 population to 220 per 100,000 population,11 there remain three main concerns with this implant: firstly, ‘soft’ polyethylene and gradual wear (leading to young patients outliving the lifespan of the THR and needing difficult revision surgery), secondly, in most cases the inability for the patient to return back to high impact sports or very high functional demands, and finally, the increased incidence of dislocation risk (due to the relative small femoral head design of the implant). Although the average age of the patient undergoing hip replacement surgery in the UK is 69 years, around 12% of patients (approximately 12,000 patients per year in the UK) are <55 years of age.12 This led to the marketing and introduction of the MoM HRA.

3. Metal on metal hip resurfacing arthroplasty (MoM HRA)

In the late 1990s, Derek McMinn, introduced the Birmingham Hip Resurfacing (BHR, Smith and Nephew, Tennessee, USA) implant as a modern hip resurfacing arthroplasty (HRA). The postulated advantages of HRA were four-fold. Firstly, as it was a resurfacing implant rather than a replacement implant, and thus bone sparing, then potentially it would more closely mimic the anatomy of a natural hip joint, and thus potentially avoid the activity restrictions imposed by a standard THR (such as ability to perform high impact sport). Secondly, by being bone conserving as the patient's femoral head and neck did not need to be removed for implantation, it potentially would lead to easier revision options should it ever wear out. Thirdly, due to its inherent design of having a larger femoral head compared to a standard THR, it would reduce the risk of dislocation of the implant and offer increased stability when compared to the standard THR. Finally, as the bearing surface articulation with this implant was MoM, it conveyed a theoretical advantage of resistance to wear when compared to MoP.

3.1. Mimic anatomy and avoid activity restrictions

Gerhardt et al.13 analysed the patterns of gait (speed, step length, stride time and range of hip motion during each gait cycle) and the mechanics of gait (maximum weight acceptance, midsupport, maximum push off force, and the impulse force) for HRA and THR against controls. Whilst no significant difference was noted in any parameter between HRA and THR at normal walking speeds (4 km/h), HRA patients achieved a higher top walking speed (7.3 km/h compared to 6.9 km/h for THR), returned to a more symmetrical weight acceptance (HRA maintained weight acceptance between the operated and un-operated limb, but for THR, operated limb only accepted 977N v 1106N for the un-operated side), and HRA limbs displayed a greater range of hip flexion with gait (8° more hip flexion) than THR hips.

Amstutz et al.14 reviewed the effect of sport on long term HRA survivorship (Conserve Plus, Wright Medical Technology, Tennessee, USA) in a young and active patient population, in a large cross-sectional study including 661 patients (806 hips) of mean age 51.9 years. Return to sport was assessed with standardised activity scores, impact scores and hip cycle scores to assess if different sets of activities (17 different sports) affected the implant survivorship. The paper detailed an acceptable implant survivorship rate of 95.3% at 10.1 years mean follow-up, despite the cohort returning back to a wide range of sporting activities, concluding that return to sport is safe after HRA.

3.2. Easier revision options

Revision THR is associated both with a significantly greater morbidity and a poorer function than primary THR.15 However, by contrast, in the event of HRA failure, by preserving the femoral neck at the time of the initial surgery, revision to a standard femoral stem is an option. Similarly, on the acetabular side, surgeons have the option of revising to insert a bipolar head of the right external dimensions, thus avoiding having to remove a well fixed cup with stress shielding behind it.16 Both of these options thereby theoretically make revision of failed HRA easier than that of failed THR.

3.3. Reduced rates of dislocation compared to THR

The prevalence of dislocation rates after standard THR ranges between 0.3% and 10%,17 with a reduction being shown with larger diameter heads.12 Data from the UK National Joint Registry (NJR) confirms that for HRA, revision for dislocation occurred at a frequency of 0.27 per 1000 prosthesis-years (eighth commonest cause for HRA revision), compared to 0.84 per 1000 prosthesis years for standard THR (second commonest cause for THR revision).12

3.4. Resistance to wear when compared to MoP

For ultra high molecular weight polyethylene (UHMWPE), the standard bearing within MoP THR, hip simulator data demonstrates volumetric wear rates of 23.2mm3/million cycles, equating to linear wear rates of the acetabular cup of approximately 0.06 mm/year.18 Because of these high wear rates with polyethylene, this led to an interest in MoM as a bearing surface, as volumetric wear is inversely proportional to the hardness of the softest bearing surface,19 as and such, it was postulated therefore that MoM would have very low wear rates. Indeed, subsequent hip simulator data demonstrated MoM bearings to have volumetric wear rates of around 2mm3/million cycles, and thus a tenth of that of MoP.20

4. Indications and relative contra-indications for MoM HRA

MoM HRA is typically now reserved for younger osteoarthritic patients, with higher functional demands (sport or manual labour) and with good femoral bone stock. Study results for MoM HRA have shown demonstrably poorer results in females, small femoral head sizes, if the acetabular cup component is inserted with an inclination angle >55°, pre-operative diagnosis of childhood hip disorders such as developmental dysplasia of the hip (DDH), certain specific implant types with poor radial clearances and in patients with poor bone stock (osteoporosis, patients with large bone defects/cysts or avascular necrosis).

4.1. Females

The UK NJR reported a cumulative revision rate for HRA of 10% at 5 years in females <60 years of age, which is at least three times the revision rate of cemented, uncemented or hybrid THR.21 The Australian NJR reported cumulative revision rates for HRA of 19.1% at 10 years in females <55 years, compared to 6.5% at 10 years for HRA in males <55 years.22 It is debated whether this is a specific gender issue in isolation, or rather due to the fact that women are more likely to have a combination of the poorer prognostic variables such as small femoral head sizes, decreased bone mineral density (placing them at higher risk of femoral neck fracture complications and thus failure of the prosthesis) and deformed acetabular sockets from congenital hip diseases (DDH).

4.2. Small femoral head sizes (femoral implant diameter <50 mm)

The Australian NJR reported significantly worse cumulative revision rates for small femoral head sizes, with 10 year revision rates of 17.6% in femoral head sizes <50 mm, compared to 10 year revision rates of 6.0% in femoral head sizes ≥50 mm.22 There are several reasons postulated for this. Firstly, smaller femoral head sizes result in poorer distribution of lubricating fluid between the articulating surfaces, and thus greater wear. Secondly, smaller femoral head sizes increase the risk of technical failures, mainly fracture of the femoral neck (as the overall size/dimension of the femoral component decreases, the femoral peg [whose size is constant for all implant head sizes] therefore contributes a greater relative proportion of the overall stiffness of the implant/femoral neck construct, resulting in stress shielding of the bone in the femoral neck, and therefore placing it at higher risk of subsequent fracture).23

4.3. Acetabular cup inclination angle >55°

Poor implant position is associated with increased risk of failure in any joint replacement. However, the HRA is a much less forgiving implant design24 regarding malposition, when compared to standard THR designs. The angle of inclination of the acetabular cup component is a critical determinant factor in wear rates of the MoM articulations as a steeply inclined acetabular cup (>55°) results in edge loading of the metal implant promoting accelerated wear with metal ion particle release and resultant pseudotumour formation.25 The optimal position of the acetabular cup has been reported as 20° anteversion and 45° inclination, with the risk of pseudotumour incidence being three times higher when >10° from this zone26,27

4.4. Pre-operative diagnosis of childhood hip disorders (DDH)

The Australian NJR reported significantly worse cumulative revision rates for patients with a pre-operative diagnosis of DDH, with 10 year revision rates of 20.3%, compared to 10 year revision rates of 9.3% for patients with a pre-operative diagnosis of osteoarthritis.28 There are several reasons postulated for this including the increased anteversion observed in these patients which may influence poor cup positioning and increased edge loading resulting in premature failure.

4.5. Implant designs with poor radial clearances

Whilst the BHR implant (Smith and Nephew, Tennessee, USA) reported good clinical results,29 the Articular Surface Replacement (ASR, DePuy Johnson & Johnson, New Jersey, USA), which to the naked eye looked almost identical, was more sub-hemispherical in design (for a 50 mm head, the arc of the articulating surface in the BHR was 162°, whereas 151° for the ASR). This resulted in poorer radial clearances between the femoral and acetabular components (BHR >100 μm clearance, whereas ASR <75 μm clearance), which resulted in poorer and limited fluid lubrication. This in turn meant that the articular contact patch was much closer to the rim of an ASR acetabular component, leading to high edge loading and uncontrolled ‘runaway’ metallic wear.30 This effect was exaggerated both in smaller size components and if the angle of inclination of the acetabular cup component was >55°. The Australian NJR reports 10 year revision rates of 6.8% for the BHR implant, compared to 31.7% 10 year revision rates for the ASR.22

4.6. Poor bone stock

Matharu et al.31 reported in their study of 447 hips at a mean follow-up of 10 years, best results for HRA in primary osteoarthritis and worst results in rheumatoid arthritis or avascular necrosis (AVN), with a 9.5% failure rate in patients with rheumatoid arthrosis, a 7.3% failure rate in AVN and a 3.0% failure rate in osteoarthrosis.

5. Clinical outcomes of MoM HRA

Large, single surgeon, series have reported long term outcomes for both the BHR (447 hips) and Conserve Plus (962 hips) HRA implants with a 96.3% 10 year survival31 and 98.7% 10 year survival respectively.32 Data from national joint registries report the 10-year overall survival rates for HRA of 88–92%, which is inferior to that of standard THR (93.7–96%).12,22 Sub-analysis of the data however has shown a variety of findings. Similar survival rates for HRA to THR have been found in males, in resurfacing head sizes above 50 mm and in patients below 55 years of age, with data from the Australian Joint Registry showing 10 year survival rates of BHR HRA in this subgroup to be 96.3%, compared to 94.2% 10 year survival in this subgroup for conventional THR.22,28 Certain implant designs however have much higher risks of failure than others; the BHR implant (Smith & Nephew, Tennessee, USA) has 10-year revision rates of between 5 and 8%, whilst one particular design, the ASR implant (DePuy Johnson & Johnson, New Jersey, USA) has revision rates of between 26 and 40%.12,22 These catastrophic revision rates ultimately led to the ASR implant being withdrawn from the market in 2010, but not before approximately 93,000 patients globally had undergone surgery using the implant. To date, 14 MoM hip resurfacing designs have been recalled by their manufacturers since 2010, due to high rates of early failure.

6. Monitoring and surveillance of HRA

In the UK, the Medicines and Healthcare products Regulatory Agency (MHRA) have defined a cohort of patients as ‘high risk’ for HRA implant failure-that is, all females, males with femoral head diameter <48 mm, and all patients with the ASR device. It is recommended such patients have lifelong annual review with the Oxford Hip Score and cobalt and chromium blood ion level monitoring. MARS MRI or ultrasound is indicated in all symptomatic patients, as well as those with abnormal X-rays, patients with deteriorating Oxford Hip scores or rising blood metal ion levels. Asymptomatic ‘high risk’ patients with normal imaging and blood levels of <2 μg/L should be monitored, whilst symptomatic patients with progressive changes on the MRI require early revision. ‘Low risk’ patients – that is, males with femoral head diameter >50 mm and those with the BHR device - need less frequent monitoring.

Raised blood metal ion levels up to a certain level are a feature of all MoM bearings and do not necessarily represent underlying pathology. At present however, there is no overall international consensus on blood Cobalt/Chromium (Co/Cr) ion threshold levels for risk of adverse reaction to metal debris (ARMD), with the UK MHRA recommending 7 μg/L, but the USA identifying 10 μg/L. Matharu et al.33 investigated blood metal ion threshold levels as predictors for risk of ARMD finding metal ion levels were significantly higher in the ARMD group than the non ARMD group of patients (10.5 μg/L v 1.40 μg/L respectively; p < 0.001). When using implant specific Co/Cr thresholds, 2.8% of ARMD patients were missed, which was significantly less than when using the fixed regulatory authority recommended thresholds (UK > 7 μg/L 4.9% missed; p = 0.003: USA >10 μg/L 6.5% missed; p < 0.0001), highlighting that new implant specific metal ion thresholds are far more reliable as ARMD risk predictors than the older fixed thresholds proposed by regulatory bodies. Notably, the USA 10 μg/L recommendation missed more than double the number of patients with ARMD than the implant specific counterpart. Therefore, Matharu et al.33 recommend blood metal ion threshold levels be developed for all MoM prosthetic designs as a more accurate, early predictor of failure rather than fixed levels.

Whilst the Oxford Hip Score is used as an early screening tool for risk of ARMD and pseudotumour, it should not be used in isolation, as Bisschop et al.34 reported a series where only 27.5% of cases of pseudotumour following HRA presented symptomatically. As the size of the pseudotumour increased, however, so did the incidence of local clinical symptoms (groin pain and discomfort). Symptomatic pseudotumours were also of significantly greater volume on CT (53.3 cm3) compared to asymptomatic pseudotumours (16.3 cm3).

7. Local adverse reactions to metallic wear debris

For standard THRs with MoP bearings, the polyethylene wear debris is ingested by the body's macrophage system, which in turn, leads to an inflammatory cytokine cascade causing activation of osteoclasts, osteolysis and aseptic loosening (polyethylene debris particles <0.5 μm in diameter are reported to induce the strongest macrophage response).35 By contrast, metallic wear debris from MoM articulations are generally of a much smaller size-mean of 42 nm (0.042 μm), which stimulate a predominant lymphocyte dominated cellular response reaction.36 This can lead to severe soft tissue reactions adjacent to the prosthesis. Somewhat confusingly, there have been various terms, often used interchangeably for this, including metallosis, ALVAL (Aseptic Lymphocyte Dominated Vasculitis Associated Lesions), pseudotumours and adverse reaction to metal debris (ARMD).37

  • •

    Metallosis is defined as the macroscopic staining in periprosthetic soft tissues associated with abnormal wear debris.

  • •

    ALVAL refers to the microscopic histological appearances and process that is stimulated by the metallic wear products and debris.

  • •

    Pseudotumours describe a either cystic or solid mass, seen on magnetic resonance imaging (MRI), CT or at surgery, whose histology shows ALVAL, tissue necrosis and metallic debris inclusions.

  • •

    ARMD is an ‘inclusive’ term for metallosis, ALVAL and pseudotumours.

The cumulative incidence of pseudotumours has been quoted as 0.3%–3.4% at a mean follow-up of 7 years, with an incidence of pseudotumour-related revision surgery of 1.8% reported after 1419 resurfacings.37 The incidence of asymptomatic pseudotumors has been estimated at 4%. As pseudotumour formation however is strongly related to rates of metallic wear, the incidence is therefore critically dependent on the factors discussed in section 4 above, in particular, implant design, with higher rates seen with the ASR design.

8. Systemic adverse reactions to elevated blood metallic ions

Elevated blood levels of metallic ions, most frequently chromium (Cr), cobalt (Co) and molybdenum (Mo) from the metal alloys of the prosthesis, have been noted in patients with MoM HRA. Long term studies have shown elevated levels of these ions persist throughout the period of implantation, but become grossly elevated when the implant becomes loose.37 Concern exists over potential autoimmune mediated effects, potential cardiotoxic and genotoxic effects of such chronically elevated blood ion levels.

8.1. Cobalt cardiomyopathy

Following publication of the ‘Quebec beer drinkers’ increase in rates of cardiomyopathy seen after drinking cobalt enriched beer (to improve the head) and sporadic publications (18 cases by 2016) reporting both cobalt cardiomyopathy and heart failure post MoM implantation,38,39 these inconsistent findings prompted a National Joint Registry review. 12-year consecutive data from the UK's NJR was analysed (study included 535,776 patients) and found no increased risk of severe heart failure within the first 7 years following MoM arthroplasty.40

8.2. Genotoxic effects

Chromium has been shown in tissue culture studies to cause DNA damage but results in animal studies have yielded negative results. Cobalt can cause lung cancer in animals, but a major epidemiological study and meta-analysis comprising 1,435,356 person years has shown no overall increase in cancers after joint replacement.37

8.3. AutoImmune effects

Reduced T and B lymphocyte counts have been reported in patients following MoM hip replacements.37 The exact clinical manifestation of this appears unclear, as only sporadic case reports exist potentially relating metal implants to systemic autoimmune pathology grouped as autoimmune/autoinflammatory syndrome induced by adjuvants (ASIA).41

9. Conclusions

The solution for the young patient with arthritic hip disease, and particularly for those patients with high functional demands, remains an enigma. Whilst the average age of the patient in the UK undergoing hip replacement surgery is 69 years, around 12% of patients undergoing hip replacement surgery are <55 years of age.

MoM HRA was developed as a potentially attractive concept, particularly for the younger patient likely to outlive their first implant and require revision surgery within their lifetime. Results of early HRA however demonstrated some unexpected complications such as risk of femoral neck fracture and avascular necrosis of the femoral head. The use of MoM HRA was set to explode, but NJR data quickly reported poorer than predicted results, with cumulative 10-year survival rates of 89.3%, worse than THR (95.4%). Results have been shown to be demonstrably poorer in females, small femoral head sizes, if the acetabular cup component is inserted with an inclination angle >55°, pre-operative diagnosis of childhood hip disorders (DDH), certain specific implant types with poor radial clearances and in patients with poor bone stock (osteoporosis, rheumatoid arthrosis and avascular necrosis). Current MHRA guidelines for surveillance have successfully risk stratified patients, but arguably, from the literature, could now be improved by adopting implant specific metal ion threshold levels. In the latest UK NJR published figures, only 636 HRA's were implanted, representing only 0.6% of the yearly total hip replacements, having dropped from a peak of 10.3% in 2006.

Whilst MoM HRA can, in very limited circumstances, with certain implant designs, provide a long-lasting, functionally improved hip that facilitates a return back to sporting activity (young adult male, suffering from osteoarthritis, who has appropriate anatomy with femoral head sizes of 50 mm or above), it has not been the surgical panacea that it was originally hoped or designed to be.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Author contributions

Clough EJ: Researched, wrote, edited and proofread the manuscript.

Clough TM: Researched, edited, and proofread the manuscript.

Declaration of competing interest

As a review article with no original data, the authors have no conflict of interest in writing this article.

Contributor Information

E.J. Clough, Email: emilyclough12@gmail.com.

T.M. Clough, Email: tim.clough@doctors.org.uk.

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