Abstract
Introduction
Dual mobility (DM) use in total hip arthroplasty (THA) has increased, particularly for prevention and management of instability. However, a modular interface raises concern for metal ion generation. The purpose of this study was to determine the 1) serum cobalt and chromium levels; 2) prevalence of ion levels >1 mcg/L; and 3) effect of femoral head material on ion levels following THA using modular DM bearings.
Methods
We performed a systematic review (MEDLINE, Embase, Cochrane databases) for articles relating to metal ion levels and modular DM (MDM) THA. Eight studies (290 patients) met the inclusion criteria. We recorded post-operative ion levels at a minimum of 12 months, and compared levels with ceramic (n = 125) and metal femoral heads (n = 165). A meta-analysis could not be performed due to poor study quality and heterogeneity.
Results
At average follow-up of 30.4 months, mean cobalt level was 0.71 mcg/L, and mean chromium level was 0.66 mcg/L [22 patients (8%) had elevated ion levels above 1 mcg/L]. When compared to MDM with a ceramic head, metal head use had higher cobalt (1.26 vs. 0.42 mcg/L) and chromium levels (1.23 vs. 0.46 mcg/L). MDM with a metal head was 1.30 times more likely to have elevated ion levels >1 mcg/L. There was no effect of ion levels on outcome scores.
Conclusions
Measurable elevations of serum cobalt and chromium levels are present in patients with well-functioning MDM THAs. The impact and contributions of the additional metal liner interface are still unclear. The use of a ceramic head appears to mitigate ion release, while reducing other mechanisms of metallosis like taper corrosion. Higher quality studies are necessary to understand whether MDM bearings pose long term issues. Until then, the judicious use of MDM articulations is recommended.
Keywords: Dual mobility, Total hip arthroplasty, Systematic review, Metal ion, Cobalt, Chromium
1. Introduction
Instability following total hip arthroplasty (THA) remains a leading cause of revision.1, 2, 3 Despite improvements in materials and prosthetic design, leading to increased head size options and instrumentation for acetabular component positioning, the reported rate of instability in the Medicare population has remained largely unchanged over the past 15 years.2,3 Dual mobility (DM) bearings have been shown to decrease the risk of dislocation following both primary and revision THA.4,5 Because of this, the use of DM has increased in recent years, particularly for the management of patients at risk for instability following THA. The American Joint Replacement Registry (AJRR) reported significant increases in reported use of DM bearings in both primary and revision THA.6
Currently, there are several designs of DM articulations available worldwide. In the United States, a modular dual mobility (MDM), in which a cobalt-chrome metal liner is engaged into the acetabular shell is most commonly used. The effect and safety of another metal-on-metal interface remains an area of controversy and concern. Some authors have observed elevations in serum cobalt and chromium levels above the threshold of 1 mcg/L,7 while others have reported little or no significant increase in ion levels when compared to other implant bearings.8 However, most of these reports are smaller case series and lack power. Therefore, the purpose of this study is to perform a systematic review of the literature in order to determine 1) the serum cobalt and chromium levels; 2) the prevalence of cobalt and chromium levels greater than 1 mcg/L; and 3) the effect of femoral head material (ceramic vs. metal) on metal ion levels following hip reconstruction using MDM articulations.
2. Materials and methods
We performed a systematic query of the Medline, Embase, and Cochrane computerized literature databases for articles containing the keyword terms “dual mobility” and “total hip arthroplasty.” The search was performed on April 25, 2020, and all studies published in the English language prior to that date were considered. In addition, to this primary search, we performed a secondary search by scrutinizing all references cited in the articles retrieved from the primary search in order to identify additional studies of interest. Three evaluators independently reviewed all of the articles retrieved from the primary and secondary searches using the systematic strategy outlined below. The evaluators were blinded with regard to the determination of the other evaluators.
Studies were included in this systematic review if they met the following criteria: 1) THA performed using a modular dual mobility articulation; 2) reported serum cobalt and chromium metal ion levels at least 1 year following the procedure and 3) had a minimum enrollment of 10 patients. Review articles, case reports, technique descriptions, and editorials were excluded.
The initial combined Medline, Embase, and Cochrane searches yielded 265 unique articles. The titles of these studies were independently reviewed by all authors. Studies that were clearly irrelevant to the topic in question based on their title, abstract, and/or primary review (n = 255) were eliminated. The remaining 10 studies were determined to meet the inclusion criteria by at least one author, so the corresponding full texts were independently reviewed by all authors. At every phase of the review, if one or more authors selected a study, that article moved on to the next phase. After full text review, one of these studies was eliminated because it failed to meet all of the inclusion criteria. This article used a nickel-chromium allow monoblock acetabular component. In addition, another publication was excluded because a subsequent publication used the same data set. Therefore, 8 articles were ultimately retained from the primary search. In the final phase of review (full text review) there was no disagreement over which study should ultimately be included. All references cited in the articles retrieved in the initial query were then compiled in our secondary search. These references were screened in the same manner as the articles from the primary search (title review, then abstract review, then full text review). No additional studies that met the inclusion criteria were retained from the secondary search. Therefore, 8 total studies were used for data retrieval (Fig. 1).
Fig. 1.
Diagram of search strategy for the systematic review of all studies reporting metal ion levels following dual mobility bearing articulation.
We considered patients to have elevated metal ion levels if either their serum cobalt or chromium level was greater than 1 mcg/L based upon prior studies on corrosion using this value.9 One study did not specifically report the number of patients with ion levels greater than 1 mcg/L and used 1.6 mcg/L as a cutoff for elevated ion levels.7 Due to the poor level of evidence, heterogeneity of femoral stem implants, consistency of metal ion testing, and mixture of primary and revision surgical indications, a meta-analysis could not be performed. However, there was sufficient data fidelity to allow the aggregation and report of serum cobalt and chromium ion levels and comparison between metal and ceramic inner femoral heads with DM bearings. We also performed a subgroup analysis comparing metal ion levels of DM versus conventional polyethylene bearings with studies that included a control group. We also compared the metal ion levels between primary and revision DM bearing patients. Weighted mean functional outcome scores were also reported at latest follow-up for the studies including these metrics. The statistical analysis was performed using Excel (Microsoft, Redmond, WA, version 15.38).
3. Results
The details of the 8 studies are included in Table 1. These 8 studies included 290 patients undergoing hip arthroplasty using modular DM articulations: 226 primaries and 70 revisions. The aggregated mean age was 61.1 years (range 53.6–66.8 years). The inner ball bearing was metal in 165 hips and ceramic in 125 hips. Seven studies utilized the Modular Dual Mobility bearing articulation (Stryker Corporation, Kalamazoo, MI). The remainder study reported the results of the modular Lima TT implant (Lima Corporation, San Daniele del Friuli, Italy).
Table 1.
Characteristics of each study included in the systematic review.
| Year | Journal | Author | Surgery | N | Study Design | Implant |
|---|---|---|---|---|---|---|
| 2019 | Bone and Joint Journal | Nam et al.10 | Primary | 43 | Prospective Cohort | Stryker MDM |
| 2017 | Journal of Arthroplasty | Barlow et al.11 | Primary | 20 | Prospective Cohort | Stryker MDM |
| 2019 | Hip International | Markel et al.12 | Primary | 28 | Retrospective | Stryker MDM |
| 2019 | Bone and Joint Journal | Chalmers et al.13 | Primary and Revision | 24 | Prospective Cohort | Stryker MDM |
| 2020 | Journal of Arthroplasty | Civinini et al.14 | Revision | 37 | Retrospective, Cross-sectional | Delta TT Lima |
| 2016 | Journal of Arthroplasty | Matsen Ko et al.7 | Primary | 100 | Retrospective | Stryker MDM |
| 2018 | Journal of Arthroplasty | Diamond et al.15 | Revision | 16 | Retrospective | Stryker MDM |
| 2019 | Bone and Joint Journal | Markel et al.16 | Primary | 22 | Prospective | Stryker MDM |
Overall, at an average follow-up of 30.4 months (range 16.6–61 months), the weighted mean serum cobalt level was 0.71 (range 0.16–1.99), while the mean serum chromium level was 0.66 (range 0.14–2.08) for all the aggregated patients (Table 2). Patients undergoing revision using MDM had a weighted mean serum cobalt and chromium levels of 1.51 (range 0.40–1.99) and 1.57 (range 0.42–2.08), respectively, compared to a weighted mean 0.58 cobalt (range 0.16–0.85) and 0.49 chromium (range 0.14–0.63) in primary THA patients.
Table 2.
Demographics, femoral head bearing surface, and mean cobalt and chromium ion levels at latest follow-up. CoCr = cobalt chromium; NR = not reported.
| Author | N | Age (years) | Inner Femoral Head | Follow-up (months) | Cobalt (mcg/L) | Chromium (mcg/L) | Patients with ion levels >1 mcg/L |
|---|---|---|---|---|---|---|---|
| Nam et al. (2019)10 | 43 | 52.6 | Ceramic + CoCr | 24 | 0.16 | 0.14 | 0 |
| Barlow et al. (2017)11 | 20 | 66.8 | Ceramic + CoCr | 16.6 | 0.85 | 0.61 | NR |
| Markel et al. (2019)16 | 28 | 62.4 | Ceramic | 24 | 0.57 | 0.5 | 2 |
| Chalmers et al. (2019)13 | 24 | 63 | Ceramic | 48 | 0.3 | 0.76 | 0 |
| Civinini et al. (2020)14 | 37 | 63.7 | CoCr | 61 | 1.99 | 2.08 | 5 |
| Matsen Ko et al. (2016)7 | 100 | Not Reported | Ceramic + CoCr | 27.6 | 0.7 | 0.6 | 9 |
| Diamond et al. (2018)15 | 16 | 65.5 | CoCr | 38.6 | 0.4 | 0.42 | 1 |
| Markel et al. (2019)16 | 22 | 61.7 | Ceramic | 24 | 0.63 | 0.63 | 5 |
| Weighted Average | 290 | 61.1 | 30.4 | 0.71 | 0.66 | 22 |
Collectively, there were 22 patients (8%) with elevated serum cobalt or chromium ion levels above the threshold of 1 mcg/L. 16 patients underwent primary THA, while 6 patients had revision THA using MDM implants. Eight patients undergoing primary/revision THA using MDM and an inner ceramic head had cobalt/chromium ion levels greater than 1 mcg/L. Of the studies that included a control group (total 37 patients), patients undergoing DM THA had comparable mean cobalt (0.65 vs. 0.64 mcg/L) and chromium (0.44 vs. 0.68) levels when compared to conventional polyethylene bearing articulations.
A DM bearing coupled with a metal head was associated with higher metal ion levels compared to DM with a ceramic ball head. The aggregated data included 165 inner metal heads and 141 inner ceramic heads (Table 3). On average, DM with a metal head had higher cobalt (1.26 vs. 0.42 mcg/L) and chromium (1.23 vs. 0.46 mcg/L) levels compared to DM with a ceramic head. Dual mobility patients with a metal inner head were more 1.30 times likely to have elevated cobalt and or chromium levels above 1 mcg/L compared to patients with a ceramic ball head (14/155 vs. 8/115). The weighted mean postoperative Harris Hip Score was 89.5 (range 85–91.2).
Table 3.
A comparison of metal ion levels at latest follow-up between patients receiving an inner ceramic and cobalt chromium femoral head.
|
Ceramic Femoral Head |
Cobalt Chromium Femoral Head |
|||||||
|---|---|---|---|---|---|---|---|---|
| Author | N | Cobalt (mcg/L) | Chromium (mcg/L) | Elevated Ion Levels (%) | N | Cobalt (mcg/L) | Chromium (mcg/L) | Elevated Ion Levels (%) |
| Nam et al. (2019)10 | 29 | 0.23 | 0.14 | 0 (0) | 14 | 0.08 | 0.14 | 0 (0) |
| Barlow et al. (2017)11 | 10 | NR | NR | NR | 10 | NR | NR | NR |
| Markel et al. (2019)16 | 28 | 0.57 | 0.50 | 2 (8) | 0 | 0 (0) | ||
| Chalmers et al. (2019)13 | 24 | 0.30 | 0.76 | 0 (0) | 0 | 0 (0) | ||
| Civinini et al. (2020)14 | 0 | 0 (0) | 37 | 2.08 | 1.99 | 5 (14) | ||
| Matsen Ko et al. (2016)7 | 12 | NR | NR | 1 (8) | 88 | NR | NR | 8 (9) |
| Diamond et al. (2018)15 | 0 | 0 (0) | 16 | 0.4 | 0.42 | 1 (6) | ||
| Markel et al. (2019)16 | 22 | 0.63 | 0.63 | 5 (22) | 0 | 0 (0) | ||
| Weighted Average | 125 | 0.42 | 0.46 | 8 (7) | 165 | 1.26 | 1.23 | 14 (9) |
Elevated ion level determined to be > 1 mcg/L.
4. Discussion
Dual mobility articulations have been shown to decrease postoperative instability compared to large head THA.4,5 With utilization increasing, there is concern and controversy on the effect and safety of a cobalt-chrome metal liner creating another metal-on-metal interface in modular dual mobility (MDM) constructs. While some studies have shown minimal increases in metal ion levels,8,12 some series have reported cobalt and chromium elevations above the threshold of 1mcg/L.7 Therefore, the purpose of this study is to perform a systematic review of the literature in order to determine 1) the serum cobalt and chromium levels; 2) the prevalence of cobalt and chromium levels greater than 1 mcg/L; and 3) the effect of ball head material (ceramic vs. metal) on metal ion levels following hip reconstruction using MDM articulations.
This study has several limitations. First, the level of evidence for the published studies was low, and therefore the overall quality of the data was poor. Heterogeneity of associated implants (femoral stems), lack of consistency in postoperative ion sampling, lack of controls, and a mixture of primary and revision surgeries included in some studies precluded our ability to perform a meta-analysis. This limited the ability to make conclusions beyond observations and associations rather than causation. Second, while all of the reports included only patients with well-functioning MDM implants, some studies did not exclude patients with other existing implants. Furthermore, other studies included patients undergoing revision hip surgery with prior metallosis, which can impact postoperative ion levels. However, because we chose the minimal interval of 12 months to aggregate the metal ions, elevations due to any other metallosis from a prior failed implant should have normalized.17 Third, the effective head size of a DM construct changes with increasing acetabular component diameters. Limited head options availability (i.e. metal heads only for 22 mm inner heads) coupled with smaller effective head sizes for smaller cup diameters may increase the risk of femoral neck impingement on the rim of the cup compared to MDM constructs with larger acetabular component sizes, potentially contributing to another source of metal debris. Finally, the majority of studies were performed by authors with significant financial conflicts related to one particular DM design. While there are no concerns about the integrity of these reports, the analysis of these results should be taken with caution. Additional studies controlling for diagnosis, cup position, effective femoral head size, and use of uniform preoperative and postoperative ion sampling are needed to define the true impact of the modular cobalt chromium interface in MDM implants.
Our analysis shows that, as a group, modular DM reconstructions used in both primary and revision THA are associated with small elevations in serum cobalt and chromium. The significance of this elevation and the source of the metal ions are both uncertain. Measurable levels of cobalt and chromium can be found in non-DM hip replacements. In a simulator study, Wyles et al. reported mean cobalt and chromium levels of 0.6 ng/mL and 4.3 ng/mL, respectively, after 1,000,000 cycles under ideal conditions using a ceramic-on-polyethylene bearing. In the same study, a metal-on-polyethylene articulation yielded significantly higher cobalt (2264 ng/mL) and chromium (217 ng/mL) levels.18 Additionally, Barlow reported no significant differences in metal ion levels across 4 bearing surfaces (metal-on-polyethylene, ceramic-on-polyethylene, ceramic-on-ceramic, and DM).8 Furthermore, when comparing these aggregated results to control patients, the levels of cobalt and chromium were similar [cobalt (0.65 vs. 0.64 mcg/L) and chromium (0.44 vs. 0.68)]. Revisions were associated with a higher level of metal ion levels compared to primary THA procedures. The causes of this observation are also unclear and multi-factorial possibly influenced by factors like small sample size, implant design, inclusion of revisions for prior metallosis, and/or use of modular femoral implants.
Despite relatively low ion levels, 8% of the patients in this study had serum cobalt and/or chromium levels greater than 1 mcg/L. Elevations above this threshold in painful hips has been associated with adverse local tissue reactions.9,17 This is higher the 1.1–3.2% prevalence for mechanically assisted crevice corrosion (MACC) at the head/taper junction raising questions about additional metallosis at the metal liner/cup junction.19 It is important to note that the clinical significance of these findings are uncertain, and other factors such as renal function can impact serum metal ion levels,10 but also, the threshold of 1 mcg/L was described only using metal-on-polyethylene THA.9 However, 50% of the patients in this series with elevated cobalt levels had ceramic heads as part of their DM articulation which can be a cause of concern. Additionally, these results may be an optimistic estimate of the incidence of significant metallosis, as all of these hips were reported to be well functioning. Intraprosthetic dissociation, incomplete seating, dissociation of the modular DM liner, and prosthetic dislocation may all contribute to increased metal ion levels. But the question remains: Is MDM safe for widespread use in both primary and revision settings? Barlow et al. modeled the cost effectiveness of DM use in primary THA and concluded that DM was only marginally cost effective provided the risk of bearing failure (e.g. intraprosthetic dissociation) did not exceed 0.29%. However, DM became significantly cost effective over conventional bearings if the probability of recurrent dislocation exceeded 16.2%.11 This would support its selective use only in primary and revision patients at high risk for instability following surgery. Consequently, surgeons must balance the risks of intraprosthetic dissociation and potential metallosis with the benefits of improved hip joint stability when choosing a DM bearing.
Finally, these results also suggest increased risk of elevated ion levels when MDM is combined with a metal head compared to a ceramic head. The causes of this observation are likely multifactorial and include metallosis at the femoral trunnion in combination with modular metal liner and acetabular component interfaces. Compared to metal-on-metal retrievals, fretting and corrosion at the backside of a modular cobalt chromium acetabular insert appears to be less severe and design specific.20 Additionally, Lombardo et al. showed that DM THA can be susceptible to the same fretting and corrosion damage observed in conventional THA systems.21 Metallosis at the head/neck junction can potentiate and exacerbate metallosis at the metal liner/acetabular shell junction and vice-versa.20 Finally, in certain systems, a metal head option (22 mm) is the only head option available for smaller MDM constructs. Impingement of the femoral neck against the acetabular component can be another source of metallosis. Consequently, when available, a ceramic head may be preferred in order to minimize the risk of metal ion elevation following MDM THA.14
In summary, while MDM constructs can reduce the risk of hip instability, these modular constructs can also be associated with elevations of serum cobalt and chromium levels beyond 12 months following surgery.13, 14, 15 In nearly 10% of patients, the elevation can be above 1mcg/L. The source and clinical significance of these elevations in well-functioning implants is unclear, but higher levels of evidence in future studies are necessary to define the impact of this added metal-on-metal interface. We advocate continued judicious use of modular DM in appropriate clinical settings.
Disclosures
AFK- Innomed (Royalties), Zimmer Biomet (Consulting), Depuy Synthes (Consulting), Signature Ortho (Research Support).
PMC- Zimmer Biomet (Consulting), Depuy Synthes (Consulting).
GCL- Stryker (Consulting), Corin (Consulting), Heron Pharmaceuticals (Consulting), Smith and Nephew (Research Support).
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