1 Introduction

Metal on Metal (MoM) hip arthroplasty saw a new era of popularity with the development of its second-generation bearing surfaces, in the first decade of this century.¹ The main object of this shift was to avoid complications of polyethylene wear.² Very soon it became a desirable option, especially in younger individuals due to good functional outcomes.¹

However, by end of the decade, many reports emerged highlighting significantly high early failure rates of this articulation.³ Main reason attributed to early failures was attributed to adverse reactions to metal debris (ARMD).⁴,⁵ These concerns lead to a shift in choice from metal on metal bearing surfaces to alternatives like metal on ultra-high molecular weight polyethylene and ceramic on ceramic implants.⁶ Worldwide, government agencies (like MHRA in the UK) have recommended surveillance and follow-up of patients with MoM hip replacements.

We aimed to determine the survival of MoM hip replacement in the younger population. We also studied the rate of revision related to adverse reaction to metal debris (ARMD) along with reviewing the clinical and radiological progress of MoM hip arthroplasty in younger age (<55 yrs) group.

1.1

1.1 Materials & methods

This is a retrospective single centre cohort study of consecutive primary metal on metal total hip replacements performed on patients of 55 yrs or younger age, between 2003 and 2009. This series consists of 109 MoM total hip arthroplasty performed in 90 patients at our unit. Five patients died of unrelated reasons and three patients were lost to follow up (Fig. 1). We assumed that the patients, who were lost to follow up, are performing well. All patients received uncemented Corail (Depuy) femoral stems and uncemented Pinnacle (Depuy) metal cups with metal liners and metal heads.

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Fig. 1 Our cohort patient distribution at final follow-up.

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All patients were reviewed as per Medicines and Healthcare products Regulatory Agency (MHRA)⁷ guidelines and functional outcomes assessed using OHS. Clinical information was obtained from follow-up clinic notes. Assessments included history, clinical examination and patient-reported outcome measures (PROM) using Oxford Hip Score (OHS). OHS is scaled from 0 to 48. A higher number indicates better functioning joint.⁸

Plain radiographs were reviewed for measurement of acetabular cup inclination angles and to find any evidence of osteolysis. Serum metal ions levels (Cobalt and Chromium) were measured for all the patients on yearly basis. Patients with significant hip related symptoms (groin or thigh pain or symptoms of instability) were investigated with further imaging. Among asymptomatic patients, serum metal ion levels of 7 ug/l or above (MHRA acceptable threshold)⁷ was an indication for further imaging. Metal artefact reduction sequence magnetic resonance imaging (MARS-MRI) was done to assess ARMD (pathological hip effusion or pseudo-tumour formation).⁹ If investigations were reported normal, patients were followed up on yearly basis. Positive investigations lead to more frequent follow-ups. The decision for a revision was made if there were persistent hip symptoms with local tissue reaction to metal debris on MRI of hips, with increasing or substantially high serum metal ion levels. All the revisions were undertaken by a single surgeon (KB).

Patient list was populated from our hospital metal on metal (MoM) hip arthroplasty database. Initial data collected from the database included patient demographics and details related to primary surgery.

The primary outcome of this study was a revision for any cause. A revision was labelled when any component implanted at primary surgery was exchanged. We also studied evidence of ARMD in all symptomatic patients.

Statistical analyses were performed using SPSS version 24.0 (Mac). All tests were performed with a significance level of p-value <0.05. Kaplan Meier curve was used for survival analysis. Two-tailed Student’s t-test was used for continuous variables comparison.

2 Results

In our cohort of patients, mean age was 49.9 (SD 5.1) years (Table 1). Predominant indication for the primary procedure was osteoarthritis. Majority of patients (66 (81%)) had Medium (36 mm) size head, while, only 14 (17%) received small (28 mm) head. There was one patient each with large 40mm and 44mm heads.

At ten years mean follow up, twelve patients had revisions (Table 2). One patient was revised for symptoms of instability soon after the index procedure. The reason for instability was subsidence and in this case, a collarless stem was used. Head was revised to larger neck and patient had good relief of his symptoms and the hip is still well functional at the time of conclusion of this study. One patient had aseptic loosening of femoral stem at about 3 years post op, so required single-stage revision. Two patients had two stage revisions for deep infections at 2 and 6 years post operatively. One patient had a change of articulations for persistent hip symptoms and MR findings of trochanteric area cyst, although no significant evidence of metallosis was found at the time of surgery. She recovered well with good relief of symptoms.

Seven patients (all with 36 mm heads) had revisions for ARMD and findings were confirmed at time of surgery. All patients had debridement and change of articulations to other than metal on metal, with good results. The mean age for revision was 7 years.

Clinically, most of the patients are happy with their hip replacement, demonstrated by the reported objective outcome, measured with validated Oxford hip score (mean score 40.5) (Fig. 2). There are significant differences in the mean scores of revised and non-revised patients (25.4 and 43.2 respectively with p < 0.05). On enquiry, the majority of patients (66%) reported ‘forgotten hip’ as they did not have any restriction of daily living including work.

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Fig. 2 Functional outcome of MoM THR in younger patients – Oxford Hip Score – very good results with Mean 43.2 in non-revised patients.

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At final follow up (mean of 10 years), 13 patients had radiographic signs of osteolysis (a lucent zone devoid of trabecular bone with a sclerotic border, not visible on the immediate postoperative radiographs²). Out of these patients, 7 had revisions following further investigations.

Radiographic evaluation revealed mean cup inclination angle of 49° (SD 6.6). There was no significant correlation (p = 0.16) found between inclination angle and level of serum metal ions.

Thirty-three other patients had further imaging in the form of MARS MRI scans. Ten hips (10%) showed signs of ARMD. Out of these, seven went on to have a revision and other three remained under follow up with serial imaging and lab testing for metal ions.

Mean Chromium ion level was 82.9 nmol/L (4.3 μg/L) and Cobalt ion level was 114.9 nmol/L (6.8 μg/L). Serum metal ion levels were significantly high in revision group (p < 0.05). Patients with 36 mm femoral heads showed a significantly higher metal ion levels in comparison to 28 mm heads (p < 0.05). However, there was no statistically significant increase in metal ion levels in patients with bilateral MoM hip replacements (p = 0.208).

At 14 years, Kaplan-Meier survival analysis revealed the rate of 88.1% (95% CI, 82–95%), using revision for any reason as endpoint (Fig. 3). However, when revision for aseptic loosening was taken as an endpoint, the survival rate is 99% (95% CI, 97–100%) (Fig. 4). The cumulative incidence of ARMD related revision was 7% (95% CI, 2–12%) at 14 years.

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Fig. 3 Kaplan Meier Survival of MoM THR in younger patients (88.1%) – revision for all causes as endpoint.

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Fig. 4 Kaplan Meier Survival of MoM THR in younger patients (99%) – revision for aseptic loosening as endpoint.

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3 Discussion

In young and active patients implant of choice for a hip replacement has been uncemented implants, as it has shown good to excellent long-term survivorship.¹⁰ In these patients, MoM bearings were introduced to provide longevity by reducing risks of polyethylene wear. Some authors have reported excellent long-term survival of 99% for uncemented MoM hip replacements in younger and older population.¹¹,¹²

However, for Pinnacle MoM hip replacements, Langton et al. have reported unacceptably high revision rate (survival of 83.6% at 9 years).¹³ Using the same implant, Matharu et al. have reported a better outcome with a survival rate of 88.9% at 8 years.¹⁴ In a recent larger study of 601 hips with this implant, Atrey et al. reported survival of 92.8% at 7.2 years.¹⁵

Our cohort of younger patients with Corail Pinnacle MoM hip replacements, in comparison, has shown a good survival of 88.1% at 14 years, with revision for all causes. Further on, an excellent survival was noted (99%) with aseptic loosening as an endpoint. In our study, femoral osteolysis rate was 13%, which is comparable to the published rate for uncemented hip replacements.¹⁶

Typical complications associated with MoM bearings, like a high percentage of adverse reactions to metal debris resulting in implant failure, including metallosis, aseptic lymphocytic vasculitis associated lesions (ALVAL) and pseudotumor,¹⁷ were also seen in our cohort of patients. In some previous reports, almost all revisions for metal on metal articulations have been related to ARMD.¹⁸,¹⁹ With use of the same implant, Matharu et al. have reported ARMD related indications in 44% of their revisions. Our overall revision rate for this indication was 7% (58% of all revisions) in the younger population.

Up to 10% patients showed MRI signs of ARMD. One patient (1%) had a revision for symptomatic hip, without any significant signs of metallosis. This rate is comparable to previously reported cases,¹²,²⁰ and it has been associated to ALVAL, possibly secondary to a hypersensitivity reaction to metal debris.²¹ Although our cohort of patients showed good survival, results are below the expected outcome in the guideline for hip replacement.²² Moreover, a significantly higher proportion of revisions related to ARMD in our cohort and review of registry data²³ advises against the further use of this THR system.

Our cohort of patients reported a very good functional outcome, as the majority of non-revised hips are performing very well and these results are comparable to the reported outcome for younger patients.²⁴,²⁵ A majority of our patients also reported a much-desired outcome of ‘forgotten hip’. In spite of concerns about metal ion related complications and a need for regular monitoring, the majority of patients can be reassured about the future of their hip replacements.

About 18% of our non-revised hips have serum metal ion concentrations above the MHRA thresholds and about 3% had MRI findings of collections around hips. It remains unclear, which patients will proceed to have a revision. We will keep these patients under regular monitoring with repeat blood sampling and imaging, as it may help identify the development of ARMD.¹⁷ A decision for intervention should be on an individual case basis. It should involve a discussion with the patient about risks of further surveillance and surgery.²⁶

In our cohort, all hip revisions for ARMD have been changed to articulations other than metal-on-metal, with good functional results and a drop in metal ion levels. However, poor outcomes have been reported in ARMD related revision arthroplasty, so we will keep these patients under regular follow-up.²⁶

Our study has the limitation of being a retrospective. However, to our knowledge, this is the only study of younger patients for this THR system (Corail, Pinnacle – Depuy). Our patients are reviewed following the recall for this system.

4 Conclusion

MoM THR system has shown good functional outcome in non-revised hips with the majority of patients reporting ‘forgotten hip’. Survival of this system in our study is good but results are significantly below the expected results.²² We do not recommend continuous use of this implant due to risks of metal ion related complications, but patients with these implants can be reassured about relatively low need for intervention and a good functional outcome in surviving hips.