Introduction
In this study, we evaluate the mid-term functional and radiological outcomes of Ceramic on Metal Total Hip Arthroplasty (CoM THA) THA. 66 CoM THAs were performed between 2008 and 2010. These were evaluated and followed up in 2017-18, at a mean follow-up of 9 years to record the Oxford Hip Score [OHS] and whole blood Cobalt and Chrome levels. Our all cause revision rate was 4.5% (3 out of 66). At mid-term follow up, patients with CoM THAs are mostly asymptomatic with reasonable functional outcomes, we have reported similar revision rates in conjunction with raised blood metal ion levels and frequency of radiolucent lines.
Keywords: Ceramic on metal, Cobalt levels, Chromium levels, Arthroplasty
1. Introduction
Ceramic on Metal (CoM) articulations are not common and its use is in rapid decline owing to reports of metal ion toxicity and poor survivorship with limited data on long term outcomes. The recent 2020 National Joint Registry (NJR) report showed that 2152 CoM THAs were carried out in England and Wales in 2020, which represents 0.2% of all primary THAs.1 Mid to long term outcome data on the performance and survivorship of CoM THAs is limited. This is the first study to report mid-term functional and radiological outcomes of CoM THAs including metal ion levels and implant survival.
2. Methods
A total of 96 CoM THAs were performed between 2008 and 2010 for primary hip osteoarthritis in 36 females and 44 males (80 THAs) with some patients having bilateral staged elective surgery giving a total of 96 THAs, with the only exclusion criteria being failing to consent to be part of the project. All operations were undertaken under spinal anaesthesia using the posterior approach and patients were mobilised on the day of surgery. Pinnacle acetabular components were used with Ultamet liners [DePuy, Leeds]. Corail Uncemented stems were used with 36 mm zirconium toughened aluminia ceramic heads [Biolox Delta; Ceramtec AG, Plochingen, Germany]. All patients in the sample had the same implants inserted; data was not collected on whether patients had additional cobalt or chromium containing implants. A review of all CoM THAs was carried out in 2017. There were 12 deaths in our cohort and 18 patients were lost to follow-up, which left a total of 66 CoM THAs that were assessed and included in the analysis at mean follow-up of 9 years. Functional outcomes were assessed pre-operatively, 6 months post-operatively, 1-year post operatively and at last follow up review using the Oxford Hip Score [OHS]. Standard anteroposterior [AP] pelvic and lateral hip radiographs were taken by the radiology department with marker balls to aid templating and two independent observers assessed for the presence and size of radiolucent lines [RLL] in Gruen Zones [1 to 14 of the femoral component] and DeLee and Charnley zones [1 to 3 for the acetabular component].2 Post-operative acetabular inclination and version were measured on plain radiographs using validated methods described by Liaw et. Al2 with the Traumacad application [Orthocrat, London, United Kingdom]. At the review, venous blood was taken to measure whole blood Cobalt and Chromium ion levels [ug/L], blood was taken using normal venepuncture equipment with the initial 10 ml sample discarded and a further sample taken in a lithium heparin tube with samples transported at room temperature to the regional assay centre for whole blood ion levels. The primary outcome measure was all cause revision surgery. Secondary outcome measures included presence of radiolucent lines, whole blood Cobalt and Chromium levels and Oxford hip scores. Fig. 1 demonstrates our methodology.
Fig. 1.
Flowchart demonstrating the methodology.
2.1. Statistical analysis
The Wilcoxon-Signed Rank Test was used to compare differences in pre-op, 6-month, 1 year and final follow up review OHS as well as differences in metal ion levels. Minimal clinically important difference [MCID] for the OHS was set at 5 points.3 Spearman's correlation coefficient was used to assess the relationship between inclination and version of the acetabular component and metal ions. Survival analysis was calculated using the Kaplan Meier method with the end point being any revision surgery; patients who passed away and those that did not undergo revision were censored. Statistical analysis was performed using Prism 8 [GraphPad Software Inc, La Jolla, California]. Statistical significance was set at p < 0.05.
3. Results
The median follow-up time up until most recent follow up review was 8 years [Range 1–10 years] with the average follow up time being 9 years. The mean age in our cohort was 59 years at time of procedure (42–70). During this time, 12 patients had passed away and 18 patients were lost to follow-up. As a result, 66 hips were assessed at final follow up.
At 1 year the mean acetabular anteversion was 42.40 [310-610], mean version was 18.60 [50-300]. The mean pre-operative OHS was 14.04 [SD 6.6]. A significant improvement in OHS was seen at 6 months, mean 43.05 [SD 6.7], 1 year, mean 45.04 [SD 3.8] and at final follow up, mean 46.0 [SD 2.9] [p < 0.001]. The mean improvement in pre-operative and final follow up OHS was 31.96 points. In our cohort, 13 patients had metal ion levels >2 μg/L [19%], 6 of these had metal ion levels between 4 and 6.99 [9%] and 4 had values above MHRA thresholds >7 μg/L [6%]. In the latter group, 3 patients have undergone revision surgery.
From our data, the mean Cobalt level was 2.3 μg/l, the mean Chromium levels were also 2.3 μg/l]. There was a significant elevation of metal ions compared to normal levels of <0.5 μg/l in patients who had a CoM THA at the final follow up [p < 0.0001]. At final follow up, the Cobalt and Chromium levels were not significantly higher than the MHRA threshold of 7 parts per billion [ppb] which is equivalent to 7 μg/l for MoM hips [p < 0.0001]. 4 patients had metal ion levels above MHRA thresholds and were symptomatic with startup pain in the thigh and groin. All of those patients had a Metal Artefact Reduction Scan [MARS]. 3 patients underwent revision surgery for pain and elevated metal ion levels. Mean time to revision surgery was 6.2 years from the date of primary arthroplasty. Table 1 demonstrates reasons for revision surgery in the patients with metal ion levels above MHRA thresholds. The indication for revisions in these cases was for aseptic loosening of the femoral stem in 1 case and suggestion of asymptomatic lymphocyte-dominated vasculitis-associated lesion (ALVAL) on the MARS MRI in the remaining 2 cases. In this cohort there were no revisions due to infection, peri-prosthetic fracture or dislocation at the time of final follow up. There were no other cases of high metal ion levels other than those patients who were revised. 14 patients had radiolucent lines at follow-up. Table 2 demonstrates the oxford hip scores, metal ion levels and revision surgery in patients who had radiolucent lines at follow-up.
Table 1.
Patients with elevated metal ions above MHRA threshold of 7 μg/l including the reasons for revision surgery and presence of radiolucent lines [RLL].
| Patient | Cobalt [μg/l] | Chrome [μg/l] | RLL | MARS Findings | Revision |
|---|---|---|---|---|---|
| 1 | 14.6 | 12 | Zone 3 Cup | No Pseudotumour | Both Components |
| 2 | 16.7 | 7.1 | No | ALVAL | Both Components |
| 3 | 16.7 | 7.1 | No | No Pseudotumour | Monitoring |
| 4 | 35.2 | 9.2 | No | ALVAL | Liner Revision |
Table 2.
Radiolucent lines in Charnley & DeLee [cup].& gruen zones [stem], Oxford hip scores, mean metal ion levels & revision surgery.
| RLL Gruen Zone | RLL Charnley Zone | OHS | Cobalt [μg/l] | Chrome [μg/l] | Revision |
|---|---|---|---|---|---|
| Zone 1 | 40 | 0.65 | 1.88 | Monitoring | |
| Zone 1 | 47 | 0.53 | 1.77 | Monitoring | |
| Zone 1 | 48 | 0.30 | 1.20 | Monitoring | |
| Zone 1 | 48 | 0.65 | 1.82 | Monitoring | |
| Zone 3 | 32 | 16.60 | 10.20 | Yes | |
| Zone 1 to 7 | 45 | 1.20 | 1.22 | Yes | |
| Zone 1 and 7 | 47 | 0.71 | 2.03 | Monitoring | |
| Zone 1 | 47 | 0.71 | 2.03 | Monitoring | |
| Zone 1 | 48 | 0.83 | 1.50 | Monitoring | |
| Zone 1 | 48 | 0.65 | 1.61 | Monitoring | |
| Zone 3 | 44 | 0.62 | 0.66 | Monitoring | |
| Zone 1 | 47 | 0.12 | 1.20 | Monitoring | |
| Zone 1 | 47 | 0.95 | 1.61 | Monitoring | |
| Zone 1 | 43 | 0.59 | 2.23 | Monitoring |
There was no significant correlation between whole blood Cobalt levels and acetabular inclination [r = 0.18, p = 0.23], and Chromium levels and acetabular inclination [r = 0.13, p = 0.31].
Fig. 2 demonstrates the Kaplan Meier survival curve for our cohort with revision surgery being the end point and the red line demonstrating a 95% confidence interval. We report a revision rate of 4.5% (3 out of 66) for CoM THAs at a mean follow-up of 9 years.
Fig. 2.
Kaplan Meier survival curve for COM THA with revision surgery being the end point.
4. Discussion
This study has demonstrated that patients with CoM THAs have a reasonable functional outcome at mid-term follow-up. However, in our cohort of patients there is a significant increase in whole blood Cobalt and Chromium levels compared to normal values and a revision rate (4.5%) comparable with the literature at a mean 9-year follow-up.4, 5, 6, 7 Whilst there are some case series and reports demonstrating reasonable short term functional and radiological outcomes of CoM THAs; mid to long term outcome data is limited. Saracco and colleagues recently analysed 19 COM hips at 8 year follow-up and reported no revisions and excellent medium-term clinical and radiological follow-up.8
Patients with CoM THA continue to function well and majority are asymptomatic [97%]. Our series demonstrates significant improvement in OHS 6 months post operatively, 1 year post operatively and the final follow up as compared to pre-operative scores. Reported OHS in the literature vary from 19 through to 48 at short to mid-term follow-up4-6. We report a revision rate of 4.5% [3 out of 66] with a mean time to revision of 6.2 years from the date of index surgery [Fig. 2]. This is similar to the reported cumulative probability of revision of CoM THAs in the 15th NJR report, which is 4.86% at 5 years and 8.51% [95% CI 6.6%–10.9%] at 10 years. However, as acknowledged in the NJR report there are less than 250 cases at the 10-year follow-up mark. Our series appears to have revision rates similar to existing literature for this particular bearing combination [Table 3] and it approaches the revision rates of other bearing combination for total hip arthroplasty, 3.3–4.9%7. The largest series reporting outcomes of CoM THAs is by Hill and colleagues from Belfast who looked at 287 CoM THAs at a mean follow-up of 34 months.5 They reported higher than expected metal ion levels and presence of radiolucent lines with a 5-year revision rate of 3.1%5. A prospective randomised controlled trial comparing CoM [n = 36] to MoM [n = 31] demonstrated 2 revisions in the CoM group at five-year follow-up with a similar rise in whole blood Cobalt and Chromium in both bearing surfaces.9 Joyce et al. reported a revision rate of 10.7% in 56 CoM THAs at a mean follow-up of 1.5 years; most revisions were for pain and loosening with whole blood Cobalt Chromium levels of <2 μg/l10. A previous study from our unit of 94 CoM THAs by Kazi and colleagues reported a revision rate of 1% at a follow-up of 2 years.4 Table 3 summarises the reported revision rates and cobalt chrome levels in the literature for CoM THAs.4, 5, 6,8,10, 11, 12
Table 3.
| Study | CoM THA [n] | Mean Follow-Up [Years] | Revision Rate [%] | Cobalt [μg/l] | Chromium [μg/l] |
|---|---|---|---|---|---|
| Kazi et al.4 | 94 | 2 | 1 | 1.37 | 1.09 |
| Hill et al.5 | 287 | 2.8 | 3.1 | 0.83 | 0.78 |
| Schouten et al.6 | 36 | 5 | 5.5 | 1.16 | 1.05 |
| Saracco et al.8 | 19 | 8 | 0 | 1.08 | 0.69 |
| Joyce et al.10 | 56 | 1.5 | 10.7 | <2 | <2 |
| Maurer-Ertle et al.11 | 14 | 4.8 | 7.1 | 3.1 | 1.6 |
| Zheng et al.12 | 74 | 4.1 | Not reported | 2.82 | 2.41 |
In keeping with the literature, we found no correlation between acetabular inclination and version and raised metal ion levels in our series.5,6 Within our sample the average values for Cobalt, 2.3 μg/L and Chromium, 2.3 μg/L, were comparable to other studies [Table 3]. However, these are significantly elevated when compared to normal values in a healthy population [<0.5 μg/L] but in keeping with a well-functioning MoM THA [2 μg/L]5, It should be noted that raised ions levels are assumed to be from the CoM implant but this cannot be confirmed as the only source in patients who also have other Cobalt of Chromium containing implants. Our findings are similar to the study by Hill and colleagues.6 Evidence suggest that Cobalt and Chromium levels may rise over a prolonged period of time and long term follow up in ceramic on metal hips is warranted to monitor this which is supported by a recent prospective randomised control trial comparing MoM with CoM articulations, which found that there were significantly lower metal ion levels in CoM at 3 years but which later rose at the 5 year follow up period, again reinforcing the need for prolonged follow up for these patients.13 Debate exists in the literature regarding the cut off value of 7 μg/L and whether this is applicable to the CoM articulation5,. Given the gradual rise in metal ion levels in CoM THA and our lack of understanding of the systemic and local effects of elevated metal ion levels we would suggest life-long annual follow up of all CoM THAs as recommended in previous studies.6 As part of this prolonged follow up, it is key to ensure clinical and radiographic assessment of patients with CoM articulations; as supported by updated Medicine Health Regulatory Authority (MHRA) guidelines “MARS MRI scans or ultrasound scans should carry more weight in decision-making than isolated blood metal levels alone” and this is further supported by long term review of metal ions levels from MoM hips, suggesting that levels remain low even with 20 years of follow up.14 Given that our data and the literature suggest that CoM behave similar to MoM hips in the long term, we would suggest taking a similar follow up approach and assessment for these articulations as that described for MoM by MHRA.14
The presence of progressive radiolucent lines around the femoral stem and acetabular component are known to be associated with a higher rate of revision surgery.15 There is a lack of evidence regarding whether or not the rate of the progression of the lines correlates to the likelihood of failure, and it appears that non-progressive radiolucent lines are not associated with increased revision surgery.15 Our sample shows radiolucent lines in 14 patients, 5 in the cup and 9 in the stem; of the patients who underwent revision surgery only 1 had radiolucent lines present on their radiographs (Table 2). More CoM hips in our series had radiolucent lines (n = 14) in comparison to a recent study by Saracco et a (n = 2)8 at a mid-term follow-up. Although the Saracco and colleagues reported that 13/19 patients had proximal stress shielding at 8 year follow-up (62%).8
A limitation of this study is lack of data on mortality of patients in the cohort. Though it is known that their mortality was not directly linked to admission including their arthroplasty, the exact cause of their mortality is unknown. It is established that metal ions can lead to heart failure, cognitive impairment and autoimmune conditions which could contribute to mortality, and further analysis on this would be required to state if there was an effect of these on this patient cohort.14 Another limitation is there is a lack of data of other Cobalt or Chromium containing implants that might be present within the cohort group. The assumption has been made that the raised metal ion levels are contributed to directly from the CoM implant but this cannot be stated with certainty as other implants may also give rise to elevated metal ion levels.
5. Conclusions
At mid-term follow up, patients with a ceramic on metal hip replacement are mostly asymptomatic with reasonable functional outcomes and we have reported similar revision rates in conjunction with raised blood metal ion levels and frequency of radiolucent lines; this suggests that CoM hips do fail in a manner similar to MoM but at a prolonged rate. Due to the lack of long-term outcomes with this bearing combination, our suggestion is that patients with CoM hips have annual follow up which includes radiographs, blood metal ion levels and Oxford Hip Scores with the consideration of MARS MRI if there are abnormalities noted. This recommendation is based on recommendations for symptomatic and asymptomatic patients in high risk group from the MHRA 2017 updated guidelines.15
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Author statement
Nisarg Mehta: Conceptualization, Writing – original draft, Writing – review & editing, Dhawal Patel: Conceptualisation, Writing – original draft, Writing – review & editing, Justin Leong: Conceptualization, Writing – original draft, Writing – review & editing, Phil Brown: Conceptualisation, Writing – original draft, Writing – review & editing, Fintan Adrian Carroll: Conceptualization, Writing – original draft, Writing – review & editing
Acknowledgements
The authors would like to thank Mr H Kazi (Consultant Trauma & Orthopaedic Surgeon), Mr Peter Skellorn & Mr Joseph Alsousou for their help and support in data collection and analysis for this study.
Contributor Information
Nisarg Mehta, Email: nisargmehta@nhs.net.
Dhawal Patel, Email: dhawal.patel@nhs.net.
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