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Original Article
14 (
1
); 108-114
doi:
10.1016/j.jor.2016.10.023

601 metal-on-metal total hip replacements with 36mm heads a 5 minimum year follow up: Levels of ARMD remain low despite a comprehensive screening program

, , , , ,
a Orthopaedics, St Michael's Hospital, Toronto, Canada
b University of Toronto, Canada
c Royal National Orthopaedic Hospital, Stanmore, London HA7 4LP, UK
d Central Michigan University College of Medicine, CMED Building, 1280 S East Campus, MI 48859, USA
e Lower Limb Research Unit, Warwick Hospital, Lakin Road, Warwick CV34 5BW, UK

⁎Corresponding author: Amit Atrey. aatrey@me.com

Received: , Accepted: ,
Disclaimer:
This article was originally published by Reed Elsevier India Pvt. Ltd. and was migrated to Scientific Scholar after the change of Publisher.

Abstract

Abstract

We conducted a retrospective study to assess the clinical outcome, failure rate, and reason for failure of a large consecutive series of 36mm MoM Corail/Pinnacle total hip replacements (THRs).

Between 2006 and 2011, 601 consecutive 36mm MoM THRs were performed (585 patients). Patients were followed according to the UK Medicines and Healthcare Products Regulatory Agency (MHRA) guidelines. All patients were accounted for and 469 patients (78%) were clinically and radiographically assessed. 328 females and 141 males with a median age of 73 (range 36–94 years) and a median follow up of 7.2 years (range 5.2–9.7 years) were followed. Clinical data included blood cobalt and chromium, Oxford Hip Score (OHS), plain radiograph, ultrasound of hip and intra-operative findings in those patients who had revision surgery.

56 patients died of causes unrelated to their hip replacement. The mean survivorship of the implant was 92.8% (range 91.6–94%, 95% CI) at a median time to follow up of 84 months (62–113 months).

The functional outcome was good with a median OHS of 38 out of 48 (23–44). The dislocation rate was 0.99%, with all these 6 cases requiring revision.

476 patients had blood tests. 100 patients (21%) had elevated levels of either cobalt above MHRA guidelines of 7 parts per billion (120 and 135nmol/L respectively for cobalt and chromium). Cobalt was elevated independently of chromium in 75% of the cases (but never vice versa).

The mean cup inclination angle was 42°. Each incremental stem size increase resulted in a decrease in cobalt by 11nmol/L.

The most common reason for revision was adverse reaction to metal debris (ARMD) (12 cases).

This paper is the largest and longest follow up of 36mm MoM THRs. Using the MHRA guidelines for follow up, the revision rates of this cohort has remained low compared to other studies, but unacceptably higher than that of other bearing surfaces.

III.

Keywords

MoM
Revision
ARMD
Trunnionosis
Taper corrosion
Failure
Survivorship
Corail/Pinnacle
1

1 Introduction

The revision rate of the most popular MoM total hip replacement in the world, the Pinnacle MoM hip, varies widely according to the clinical unit. The joint registry with the largest collection of data is the UK NJR which at 10 years has shown revision rates of 15.69 (13.70–17.95 with a 95% confidence interval) and a cluster of high revision rates in the NE of England.27 This raw figure includes all head sizes and the XL head.

Reports in the literature have raised concerns about the adverse effects of metal on metal bearings in hip arthroplasty.2,9,10

Adverse soft tissue reactions causing local tissue damage,1 pain,2,9,11–16 metallosis and metal ions potentially being carcinogenic were the main concerns (ARMD).17–24 Implants such as the ASR system (DePuy Orthopaedics, Johnson & Johnson, Warsaw, IN, USA) experienced high failure rates and were recalled from the market.25–28

The same concerns were extended to all metal on metal bearing hip replacements/resurfacings and in 2012 The Medicines and Healthcare Products Regulatory Agency (MHRA) (UK) issued guidelines to assess the performance and safety of these implants.

The Corail/Pinnacle is the most popularly used THR in the UK with 80,842 implanted according to the National Joint Registry (NJR) of England and Wales. The aim of this study was to assess the clinical performance of the 36mm head Pinnacle MoM hip replacement. Our objectives were to quantify the clinical outcome, failure rate and cause of revion/failure.

2

2 Patients and methods

Between 2006 and 2011 a total of 601 (566 patients) MoM THRs were performed at a single center using a 36mm head. After concerns for MoM THRs were raised and the MHRA had set guidelines, a screening program was set up. Patients with a MoM hip replacement performed at this center were identified using the England and Wales NJR, the electronic hospital patient theatre list system and individual surgeons’ logbooks. To ensure inclusion of all patients, every theatre logbook since 2006 was checked for metal on metal hip replacements. This was crosschecked with the number of metal liners provided by the manufacturer (DePuy, Warsaw, IN, USA) to this Hospital Trust.

To ensure that 100% of patients were accounted for, we used the database from our own research department, the NJR of England and Wales, HES (Hospital Episodes Statistics for England and Wales) and the Office for National Statistics (Fig. 1).

Patient demographics.
Fig. 1 Patient demographics.
2.1

2.1 MoM follow-up clinic

All identified patients were sent a letter inviting them to attend a follow up clinic (led by the head surgeon). A standardized follow-up protocol, including the Oxford Hip Score (OHS),29,30 was used for each patient. Blood was taken for cobalt and chromium levels and a plain radiograph taken. Inclination angle was assessed by the method described by Reito et al.34

Patients were allocated into either “asymptomatic hip” or “symptomatic hip” by the reviewing clinician. Symptomatic hips were defined as those with pain around the hip joint and/or mechanical hip symptoms (clicking/clunking or giving way). If the clinician was unsure as to the source of the symptoms during the one-on-one consultation, the patient was allocated to the symptomatic group (Fig. 2).

Clinic treatment flow diagram
Fig. 2 Clinic treatment flow diagram
2.2

2.2 Whole blood metal ion levels

Blood for metal ion analysis was sampled with a 21-gauge needle and collected in trace element tubes containing sodium ethylenediaminetetracetic acid (EDTA). Samples were measured by inductively coupled plasma mass spectrometry for whole blood cobalt (Co) and chromium (Cr) levels.3 Normal ranges were given as 0–120nmol/L for Co, and 0–135nmol/L for Cr (equivalent to 0–7ppb) as set by the MHRA. The cost of a combined cobalt and chromium assay is $62 and the cost of individual cobalt or chromium is $47.

We highlighted those patients with cobalt and chromium levels greater than 4ppb (>69nmol/L and >78nmol/L respectively). These cut off levels are those postulated by Hart et al.39 above which there is an increased risk of revision due to ARMD.

2.3

2.3 Imaging

Patients are followed up on an annual basis with plain pelvic radiographs and the above blood tests. Symptomatic patients and those with elevated blood metal ions were referred for an ultrasound on the anterior and lateral aspects of the painful hip with a high frequency probe of 9–13MHz (Sonoline Antares – Siemens). A single radiologist with an interest and experience in ARMD performed this. Ultrasound has been shown to be a reliable diagnostic tool in the assessment of ARMD.49–51

2.4

2.4 Sources of funding

All MoM hip follow up clinics were paid for by DePuy-Johnson & Johnson.

2.5

2.5 Definition of ARMD

Joints were defined as having ARMD in the presence of1.Macroscopic evidence of metal debris in the synovium/surrounding tissue at the time of revision.2.The presence of a pseudo-tumor or pseudocapsule at the time of revision.3.Histological evidence of ALVAL as described by Amstutz.16If dislocation occurred in the absence of the above 3 it was counted as a true independent dislocation. Infection was ruled out with clear samples from at least 5 individual samples taken from the time of revision.

2.6

2.6 Revisions

All decisions to revise were made by and performed by the senior authors (SY, AJS). Clear indications were those patients that were symptomatic and had elevated metal ions and/or abnormal ultrasound findings (pseudotumour/pseudocapsule or soft tissue destruction). Stems were retained when well fixed with no signs of infection and with no macroscopic evidence of neck taper damage. Infection was eliminated by low C-reactive protein (CRP)/erythrocyte sedimentation rate (ESR) and white cell count (WCC). All cups were removed and sent to the London Retrieval Unit at Imperial College, London, for analysis (this does not form part of this study).

All soft tissue histological analysis was graded for ALVAL as set by Amstutz and co-workers.12

3

3 Results

Between January 2006 and August 2011, 601 sequential MoM hip replacement operations were performed in 585 patients (32 bilateral MoM THRs). The sizes of bearings were 36mm in all cases. The femoral implant was the Corail in four forms: 351 standard offset collared [KA], 135 standard offset collarless [KS], 77 varus neck implants [KLA] and 38 high offset [HO] (DePuy-Johnson & Johnson) stems.

There were 328 female and 141 male patients in this group (32 bilateral hips) [312 hips in females and 121 in men]. The average age (median) at follow up was 80 years and the average age of primary surgery was 73 years (36–94 years).

53 patients (53 hips) were contacted by telephone, as they were unable or unwilling to attend the clinic. Oxford Hip Scores and a symptomatic history were taken but no bloods were obtained. All of these reported themselves as being asymptomatic.

56 out of the 585 patients died more than 30 days after hip replacement and of causes unrelated to their hip replacement (Fig. 1) and no patients were lost to follow up. The mean survivorship of the implant was 92.8% (range 91.6–94%, 95% CI) at a median time to follow up of 7.2 years (range 5.2–9.7 years) (Fig. 3 and Table 1).

Kaplan–Meier survivorship curve.
Fig. 3 Kaplan–Meier survivorship curve.
Table 1 Kaplan–Meier survivorship with 95% CI.
Means and medians for survival time
Meana Median 95% confidence interval
Estimate Std. error Lower bound Upper bound
92.780 .607 91.591 93.969
a Estimation is limited to the largest survival time if it is censored.

In those patients still alive and unrevised at the time of follow up, the functional outcome was very good with a median OHS of 38 out of 48 (23–44). The dislocation rate was 0.99%, with all of these requiring a minimum of open reduction.

Of the 469 patients that had blood tests, 100 patients (21%) had elevated levels of either cobalt and/or chromium above the level described in the MHRA medical device alert on MoM52 hips of 7 parts per billion (120 and 135nmol/L respectively for cobalt and chromium). Cobalt was elevated independent of chromium levels in 75% of cases. Chromium was never independently elevated.

The mean cup inclination angle was 42°. Increasing stem size inversely correlated with blood cobalt levels (increase in 1 stem size results in a decrease of 11nmol/L).

The most common reason for revision was for ARMD (Table 2).

Table 2 All reasons of revisions.
Reason for revision Number revised
Dislocation 6
Infection 4
Patient request 1
Leg length discrepancy 2
ARMD 12
Aseptic loosening – normal bloods/US 4
3.1

3.1 Revision rates

20 patients had already been revised before the start of the MHRA follow up program (17 due to reasons other than adverse reaction to metal debris [ARMD] and 3 to direct ARMD). A further 9 patients with ARMD were picked up in the follow up program (Table 2).

56 patients had died for other reasons not associated with the hip replacement; these patients are included in the survivorship analysis only (Fig. 1).

The mean survivorship was 92.8% (range 91.6–94%, 95% CI) at maximum follow of 10 years (Table 1 and Fig. 3).

29 hips (25 patients) have been revised (Table 2). Of these 29 hips, 12 had been revised for adverse reaction to metal debris (ARMD) (Tables 3 and 4).

Table 3 All patients revised for ARMD.
Age Gender Symptoms US findings Co (mmol/L) Cr (mmol/L) Synovial thickening Osteolysis Trochanteric pseudotumour Iliopsoas pseudotumour ALVAL score
76 F Groin pain Cystic mass 5 5 Yes No No No 6
68 F “Lateral” hip pain Cystic mass 283 138 Yes Yes No No 5
69 M Groin pain/clunking Cystic mass 206 197 Yes Yes No Yes 5
84 F Groin pain Muscle loss 133 39 Yes No Yes Yes 9
66 F Clunking Nil 356 255 Yes No No Yes 1
87 F General pain Muscle loss 130 119 Yes Yes No Yes 8
75 M No pain Cystic mass 12 4 Yes No No No 5
80 M No pain Cystic mass 14 5 Yes No No No 5
82 F Groin pain Nil 22 6 Yes No No No 1
72 F General pain Nil 86 7 Yes Yes No No 5
71 F Groin pain Nil 152 32 Yes Yes No No 5
81 F “Clicking” Nil 131 46 Yes No No No 1
Table 4 US findings of those patients with AMRD.
Grade US
No pseudotumour 11
Thin walled fluid filled pseudocyst (class 1) 7
Fluid filled pseudocyst with thick walls (class 2a) 3
Pseudotumour with thick/irregular walls and atypical contents (2a) 1
Solid pseudotumour (class 3) 1
Total number of cases 12
3.2

3.2 Blood metal ions

Of the 469 patients seen in clinic, 100 had cobalt ions above MHRA guidelines (21%).

In total, 39 patients were symptomatic (8%). 6 patients were both symptomatic and had elevated ions. All of these were revised (see Fig. 4). The remaining 33 symptomatic patients had cobalt and chromium levels within recommended limits.

Symptomatic patients with elevated cobalt levels.
Fig. 4 Symptomatic patients with elevated cobalt levels.

Of the 469 patients seen in clinic, 430 patients were asymptomatic (92%). 94 of those had cobalt levels above the MHRA guidelines. All of these were offered further ultrasound imaging (which was unremarkable) (Fig. 5). The remaining 336 patients had normal cobalt and chromium levels.

Asymptomatic patients with elevated cobalt levels.
Fig. 5 Asymptomatic patients with elevated cobalt levels.

5 (1%) patients had an increased chromium level above MHRA suggested limit (>135nmol/L). The serum chromium level was never independently elevated without an associated cobalt elevation.

Regressional analysis showed no correlation between cup inclination and blood metal ion levels.

3.3

3.3 Radiological findings

All 100 patients with elevated metal ions had an ultrasound scan performed by a musculoskeletal radiologist. 5 had signs of pseudocyst/pseudocapsule and 2 with evidence of soft tissue destruction. These 7 were revised. See Table 4.

The mean cup inclination angle was 42°.41

3.4

3.4 Oxford Hip Scores

501 OHSs were available (including patients who declined a clinical appointment but were happy for telephone consultation). 462/501 hips (92%) were asymptomatic. 39 (7.8%) were symptomatic.

Sequential OHS were taken at 12 months and then annually after the start of the follow up program. The average OHS for the entire study group at 12 months was 42/48. The average OHS for the entire study group at 60 months was 38/48.

The average OHS of the symptomatic (29) and asymptomatic (42) groups was significantly different (p<0.001).

The OHS did not correlate with the serum cobalt (coefficient 0.05) or the serum chromium (coefficient 0.05). There was no significant difference in the OHS for asymptomatic patients with or without raised metal ions (41 vs. 40, p=0.62). There was no significant difference in the OHS for symptomatic patients with or without raised metal ions (32 vs. 27, p=0.19).

3.5

3.5 Stem size/shape and metal ion levels

Regressional analysis showed that the larger (and by inference the stiffer) the implant the less the cobalt released (every increase in stem size was associated with an decrease in serum cobalt by 11nmol/L). This is a significant difference (p=0.001) (Table 5).

Table 5 Regressional analysis of stem type, size and cobalt levels.
Model type Unstandardized coefficients Tsd coefficient t Sig. 95% CI for B
B Std. error Lower bound Upper bound
(Constant) 175.047 33.776 5.183 0.000 108.669 241.424
KLA −11.074 2.773 −.213 −3.993 .000 −16.524 −5.624
KA −9.173 14.395 −.031 −.637 .524 −37.463 19.118
KS 6.613 12.562 .026 .526 .599 −18.074 31.3000
KAR 12.323 22.962 .026 .537 .592 −32.803 57.449
KHO 6.064 19.327 .015 .314 .754 −31.918 44.046
Dependent variable: cobalt (mmol/L)
Model type Unstandardized coefficients Standardized coefficients, beta t Sig. 95% CI for B
B Std. error Lower bound Upper bound
(Constant) 165.874 33.452 4.959 0.000 100.133 231.616
KLA −11.074 2.773 −.213 −3.993 .000 −16.524 −5.624
KA −9.173 14.395 −0.44 −.637 .524 −19.118 37.463
KS 15.786 16.468 0.061 .961 .337 16.498 48.070
KAR 21.496 25.826 0.045 .832 .406 −29.259 72.251
KHO 15.237 22.338 .037 .682 .496 −28.662 59.136
Dependent variable: cobalt (mmol/L)

Regressional analysis showed no correlation between the revision rates and the type of stem used (either high off-set [HO], varus [KLA], KA [standard collared], KS [standard collarless]).

Similarly, no one stem type is associated with increased cobalt levels.

4

4 Conclusion

The aim of this follow up study was to assess the rates of ARMD, revision for any reason and the factors correlating to failure in a cohort of patients that had undergone a THR with the withdrawn MoM liner and head in an otherwise popular and successful hip replacement. The United Kingdom MHRA guidelines were used as a framework for follow up. We contrast the results by Lainiala et al.42 (Finland) who also used the MHRA guidelines for their study group of 378 hips and also Hug et al.43 (Duke University, USA) who reviewed arthroplasties or resurfacing with the withdrawn ASR implant (Table 6).

Table 6 Comparison of MoM cohort follow-ups.
Study group Number of implants Type of implant Mean time of follow up (years) Number of revisions Revisions directly due to ARMD Mean Co levels (nmol/L) Mean Cr levels (mmol/L)
This study group 585 36mm Corail-Pinnacle 7.5 29 (4.9%) 12 (2.0%) 49 (5–1396) 39 (5–680)
Lainiala et al. 203 36mm Corail-Pinnacle 6.3 34 (17%) 29 (14%) 27.8 (13–32) 20.13 (17–47)
Hug et al. 190 ASR resurfacing and THR 3.3 24 (13%) 14 (%) 250 (34–569) 115 (23–635)
4.1

4.1 Survivorship and ARMD

The Kaplan–Meier survivorship shows that the ‘mean survivorship’ at 10 years is 92.8% (range 91.6–94%, 95% CI) (Fig. 2). This figure is higher than other studies and it should be noted that the majority of revisions were due to issues not related to metallosis. At 7 and 10 years the NJR data27 shows a 9.05% and 15.69% failure rate respectively. In the long term we expect the number of revision attributable to ARMD to increase as shown by other studies and the NJR.

Even at this relatively early stage of follow up, the levels of revision for ARMD remain lower than other studies (12 cases out of 469 seen in clinic).42,43 Other studies of this implant indicate that serum cobalt levels are stabilized after 2 years. The results in this study reflect the literature on the Pinnacle-Corail MoM hip (including pooled data) which shows good survivorship at 5 years (99.4%) to 9-year survivorship at 99.4%.44 This conflicts with a study of the 36mm MoM by Lainiala et al.42 in which the 9-year survivorship was 86% (95% CI 82–90%) once the MHRA program had been started. Hug et al. had a 13% revision rate at a mean follow up of their ASR (THRs) at a mean of 45 months (Table 6).

4.2

4.2 Metal ion analysis

The serum metal ion levels were unhelpful in predicting who will have an undesired metal reaction and those that may need a revision. The serum cobalt levels showed no correlation in symptoms, OHS or pathology. Of the 12 patients that had a proven ARMD following revision in our study group, just three had raised cobalt levels (range 206–356nmol/L) and the others did not. The highest cobalt level (1396nmol/L) was found in an asymptomatic patient with a normal ultrasound and OHS of 48/48.

The chromium level showed no correlation in symptoms or pathology.39,40 Serum chromium levels were raised above MHRA guidelines in 5 of 469 (1%) patients. Only 1 of these hips was symptomatic hence did not correlate with the OHS. Chromium levels were never independently raised of cobalt. At our test center, not performing the additional chromium assay will save £14.6 per patient and has no bearing on the treatment algorithm. Our patients have had repeat serum metal ions levels and this would be an annual saving of £12,802 for the total cohort follow-up.

Hart et al.31 reported that cobalt (rather than chromium) is the more active ion that initiates an inflammatory reaction further adding to the argument that chromium assessment is not necessary.33

It is postulated that hard on hard bearing surfaces (MoM and CoC) allow less wettability and therefore greater friction. This in turn transfers increased torque on both the cup/acetabulum and also the neck-taper junctions.46

After comparing MoM THRs and resurfacings there is evidence that a singularly elevated cobalt level (without chromium) could be more attributable to corrosion at the head-neck trunnion.45

Whether the newly decreased taper size has any influence on the amount of cobalt released is not yet known.47

It has also been shown that the levels of cobalt ion release due to corrosion at the head-neck taper also decreases the stiffer and femoral implant.48 Our study confirmed that the larger and hence stiffer implants had lower cobalt release (each increasing stem size decreased serum cobalt by around 11nmol/L [p>0.01]).

4.3

4.3 OHS and symptomatology

The OHS was useful for finding which patients have a problem with their hip but it cannot predict which ones will have raised metal ions. Neither can it predict which patients will go on to develop an undesired metal reaction (such as ARMD).

4.4

4.4 The MHRA guideline

A comprehensive treatment algorithm for MoM hip replacements has already been suggested by the American Hip Society and published in the Journal of Bone and Joint Surgery (Br).38

All patients with elevated metal ion levels who expressed concern have been offered a revision operation. To date, only one asymptomatic patient (with normal USS) has requested this option [1/585 (0.17%)].

Despite these results, MoM bearing surfaces are no longer used in our center following the concerns of patients, industry and clinicians. We shall continue to closely follow this cohort of 36mm bearing MoM hip and report any further changes.

5

5 Discussion

The Corail-Pinnacle uncemented THR remains a popular and successful hip arthroplasty.37 MoM THRs have been withdrawn from the market. The MHRA program for monitoring MoM THRs was adopted and the results at a median follow up of 6.4 years show an overall revision rate of 5.4% (of which 2.2% were directly attributable to ARMD). While lower than other similar studies, this failure rate is still unacceptably high and we expect further failures as time progresses.

Conflicts of interest

The authors have none to declare.

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