Metal-on-metal hip resurfacing is an alternative surgical approach to THA, which generally is used for younger and more active patients [95, 143]. Unlike with a THA, the head of the femur is not completely removed in metal-on-metal hip resurfacing but is reshaped to accept a new metal head that fits a metal acetabular component (also referred to as metal-on-metal [MoM] implants). Still considered in the early stages of dissemination, with a limited number of metal-on-metal hip resurfacing studies now reaching 5 years of followup, emerging papers in the literature are a source of critically needed information to determine the degree and severity of adverse events.
Study heterogeneity, inconsistent outcome definitions, and unstandardized outcome measures challenge our ability to compare arthroplasty outcomes studies [124, 136, 143]. To date, there is limited evidence in the literature that compares adverse events across studies using a standardized metric. Without standardized metrics, it is not possible to make valid comparisons that account for differences in study sample sizes and followup times, which can have a substantial effect on the results. Therefore, it is critical to use a standardized metric such as person-years; however, to date this is not common practice in the arthroplasty literature. Additionally, it has not been common practice to analyze outcomes of medical devices according to market status. Our underlying assumption was that market status is a critical factor in assessing safety issues given that currently in-use metal-on-metal hip resurfacing and THA devices likely have greater efficacy and fewer adverse events. As a result, we organized our data analysis based on device market status.
The primary purpose of our systematic review was to compare studies of metal-on-metal hip resurfacing with THA among patients with hip osteoarthritis to determine rates of: (1) early revision or reoperation (within 5 years of surgery) and overall revisions; (2) revisions reported in four major joint replacement registries; (3) adverse events; and (4) postoperative component malalignment. In this review, we used the standardized metric, per 1000 person-years, to address gaps not previously addressed in the literature to compare outcomes between THA studies that had longer-term followups with metal-on-metal hip resurfacing studies with limited followup.
Materials and Methods
Search Strategy and Criteria
Our review protocol was based on well-established guidelines developed by the Centre for Reviews and Dissemination . The following electronic databases were searched: MEDLINE, PubMed, EMBASE, the Cochrane Library, BIOSIS Previews, and Web of Science from 1997 to 2011.
Using the PICO (patient problem or population [P], intervention [I)], comparison [C)], outcome(s) [O)]) framework to define our exclusion and inclusion criteria , we defined our population as adult patients (≥ 18 years) with primary osteoarthritis of the hip. Where study population included patients with hip and knee osteoarthritis, the study was included only if results were subdivided into hip and knee groups. The intervention was primary metal-on-metal hip resurfacing prosthesis and the comparator THA. Type of prosthesis used (ie, material components and prosthesis type) was recorded for comparative analysis. Study outcomes reported included adverse events, safety issues, or revision rates (Appendix 1. Supplemental material is available with the online version of CORR®).
Studies were excluded if they were: not English language; studies with fewer than 10 participants or populations younger than 17 years; hemiarthroplasty; preoperative or postoperative interventions for joint arthroplasty (eg, physiotherapy, rehabilitation, drug trials); management of osteoarthritis or related symptoms; variations on specific surgical techniques/procedures in THA or metal-on-metal hip resurfacing; focused on prosthesis modifications (except where studies looked at cement versus cementless prosthesis); or in vitro/in situ studies.
Primary outcomes were adverse events including revisions, reoperations, infections/sepsis, femoral neck fractures, other femoral fractures, dislocations, and mortality (all-cause, within 30 days of surgery). Revision is commonly defined as surgery where the patient underwent a subsequent surgery on their primary prosthesis where the component was replaced. Reoperations are commonly defined as a subsequent surgery on the primary prosthesis but the component was not replaced. These definitions are not standardized and sometimes the terms revision and reoperation are used interchangeably. We report definitions of adverse events as they were presented in the original studies. Rates of early failure outcomes included revisions/reoperations within 5 years after primary THA or metal-on-metal hip resurfacing. Postoperative component alignment data, for acetabular and stem device components, were extracted and included varus alignment, valgus alignment, and mean neck- and stem-shaft angles. Postoperative component alignment was important to include since clinical evidence of poor alignment as a predictor of device failure has been highlighted in the literature .
Initial searches revealed few relevant randomized controlled trials. Consequently, we included all quasiexperimental and observational study designs.
Abstracts were screened by two independent reviewers (DAA, MCM), and each full-text article was screened by one of three reviewers (SH, DAA, MCM). Data were extracted by two independent reviewers (DAA, MCM) and extracted values were compared to identify discrepancies. Data were extracted as reported in the study. Data were quality-checked to reconcile any known discrepancies to the final approved digital data template.
Prosthesis device types were extracted from each article and sorted by market status: those currently in use and those discontinued. The term “in use” referred to metal-on-metal hip resurfacing and THA devices that were available for surgical use in North America when this research was conducted. Conversely, “discontinued” referred to devices not available for surgical use.
We identified 7421 abstracts, screened, and reviewed 384 full-text articles and included 236 (Fig. 1). Primary reasons for exclusion were: patients younger than 18 years, focus on surgical techniques, or adverse events not reported. None of the articles reviewed reported their findings using a standardized metric (per 1000 person-years). The most common study designs (Table 1) were prospective cohort studies at 46.6% (n = 110) followed by retrospective studies at 36% (n = 85).
We used the most recent registry reports available to extract revision data from four major joint replacement registries as a comparison for revision findings: the Australian Orthopaedic Association National Joint Replacement Registry, New Zealand National Joint Register, Swedish Hip Arthroplasty Register, and National Joint Registry for England and Wales [9, 47, 98, 102]. These registries were selected because of the homogeneity of available data (revision rates and followups) compared with other registries.
Data Analysis Methods
The observed counts of adverse events are assumed to have a Poisson distribution with a rate parameter for each study and/or adverse event given in units of events per 1000 person-years. Estimates and 95% CI for each rate parameter are made using the relationship between the chi-square and Poisson distributions as per Ulm . Comparisons between adverse event rates for metal-on-metal hip resurfacing and THA are made using a quasilikelihood generalized linear model with log link , with p values less than 0.05 considered statistically significant. The quasilikelihood generalized linear model accounts for the significant overdispersion of the data; the data are observed to be overdispersed in that the mean and variance of event counts for each adverse event are not near equal. Thus, we use the quasilikelihood model p values to determine statistical significance of comparisons, as this method accounts for the overdispersion of the data. We also included 95% CIs for reference.
Rates of Early Revisions/Reoperations (within 5 years of surgery) and Average Time to Revision
For all devices, revisions occurred earlier in patients treated with metal-on-metal hip resurfacing devices (3.0 years; 95% CI, 2.95-3.1) compared with THA (7.8 years; 95% CI, 7.2-8.3; p < 0.001) (Table 2). When discontinued devices were removed from the analysis, revisions still occurred earlier in metal-on-metal hip resurfacing devices (Table 2).
Early revisions/reoperations (within 5 years of surgery) were more frequent for metal-on-metal hip resurfacing (14.5 per 1000 person-years; 95% CI, 12.7-16.5) compared with THA (3.1 per 1000 person-years; 95% CI, 2.3-4.2; p = 0.003) for all devices (Table 2). When discontinued devices were removed from the analysis, early revisions/reoperations were still more frequent for metal-on-metal hip resurfacing compared with THA, with nonoverlapping CIs, but a nonsignificant p value (Table 2).
The average number of revisions per 1000 person-years was greater for all metal-on-metal hip resurfacing devices (10.7; 95% CI, 10.1-11.3) than for THA devices (7.1; 95% CI, 6.7-7.6; p = 0.068), with nonoverlapping CIs, but a nonsignificant p value (Table 2). For currently in-use devices, revisions per 1000 person-years were greater for THA (7.6; 95% CI, 6.5-8.8) than for metal-on-metal hip resurfacing (5.7; 95% CI, 5.2-6.2; p = 0.268). When discontinued devices were removed from the analysis, the average number of revisions for THA devices increased from 7.1 revisions per 1000 person-years (95% CI, 6.7-7.6) (Table 3), (Fig. 2. Supplemental material is available with the online version of CORR®) to 7.6 (95% CI, 6.5-8.8) (Table 4), (Fig. 3. Supplemental material is available with the online version of CORR®). For metal-on-metal hip resurfacing, removing discontinued devices decreased the average number of revisions from 10.7 per 1000 person-years (95% CI, 10.1-11.3) (Table 5), (Fig. 4. Supplemental material is available with the online version of CORR) to 5.7 (95% CI, 5.2-6.2) (Table 6), (Fig. 5. Supplemental material is available with the online version of CORR).
Three national registries (Australia, Sweden, and England and Wales) reported higher revision rates for metal-on-metal hip resurfacing devices compared with THA devices. The New Zealand registry was the only registry that showed a higher revision rate per 1000 person-years for THA (2.7; 95% CI, 2.6-2.8) compared with metal-on-metal hip resurfacing (2.4; 95% CI, 1.7-3.4) (Table 7). The registry revision rates were lower than those found in our systematic review for THA and metal-on-metal hip resurfacing devices, with the exception of THA and metal-on-metal hip resurfacing revision rates for the England and Wales registry (Table 7).
For all devices, reoperations were more frequent in metal-on-metal hip resurfacing (7.9 per 1000 person-years; 95% CI, 5.4-11.3) than THA devices (1.8 per 1000 person years; 95% CI, 1.3-2.2; p = 0.084) (Table 2). When discontinued devices were removed from the analysis, reoperations remained higher in metal-on-metal hip resurfacing devices (Table 2). The most common reasons reported for reoperation in metal-on-metal hip resurfacing were fracture (femoral neck, greater trochanter, subtrochanteric femur fracture), heterotopic ossification, and component mismatch. The most common reasons reported for reoperation for THAs were heterotopic ossification, osteolysis, excessive polyethylene wear, and infection.
Dislocations were more frequent for THA than metal-on-metal hip resurfacing devices for currently in-use devices (4.8; 95% CI, 3.5-6.5 versus 1.1, 95% CI, 0.8-1.6 per 1000 person-years; p = 0.01) and all devices (4.4; 95% CI, 4.2-4.6 versus 0.8, 95% CI, 0.6-1.2 per 1000 person-years; p = 0.008) (Table 2).
Other Adverse Events
For currently in-use devices, infections/sepsis per 1000 person-years were more frequent for THA (3.2 per 1000 person-years; 95% CI, 2.0-4.8) compared with metal-on-metal hip resurfacing (1.2 per 1000 person-years; 95% CI, 0.8-1.8; p = 0.18) (Table 2). When discontinued devices were removed from the analysis, infections/sepsis remained higher in THA devices (Table 2).
For all devices, femoral neck fractures were more frequent for THA devices (2.9 per 1000 person-years; 95% CI, 1.6-4.8) compared with metal-on-metal hip resurfacing (2.0 per 1000 person-years; 95% CI, 1.6-2.6; p = 0.654) (Table 2). For currently in-use devices, femoral neck fractures were more frequent for metal-on-metal hip resurfacing devices compared with THA (Table 2). Although femoral neck fractures are extremely rare with THA devices, they still were reported in studies (N = 2 for currently in-use THA devices and N = 7 for all THA devices) as adverse events.
There was a lack of comparable data to analyze other femoral fractures and mortality (all-cause, 30-day).
Rates of Postoperative Component Alignment
We were unable to draw conclusions for postoperative component alignment owing to limited comparable data in both market status groups.
In the United States, nearly 203,000 primary hip surgeries were performed in 2003 and this figure is projected to increase to more than 500,000 surgeries annually by 2030, and consequently the number of revisions will increase . The majority of individuals who require primary hip replacement undergo THA [9, 47, 98, 102]. Metal-on-metal hip resurfacing is used less often, targeted toward younger, more active patients as an alternative to THA, but remains controversial [65, 124, 127, 136, 143]. We believe there is limited evidence thus far in the literature that compares adverse events across studies of metal-on-metal hip resurfacing and THA using a standardized metric. Without standardized metrics, it is not possible to make valid comparisons that account for differences in study sample sizes and followup times, which can have a substantial effect on the results. In this review, we used the standardized metric, per 1000 person-years, to address gaps not previously addressed in the literature to compare outcomes between THA studies that had longer-term followups with metal-on-metal hip resurfacing studies with limited followup. We aimed to determine rates of (1) early revision or reoperation (within 5 years of surgery) and overall revisions; (2) revisions reported in four major joint replacement registries; (3) adverse events; and (4) postoperative component malalignment using standardized metrics and stratifying results by device market status.
The literature on joint arthroplasty is challenging because definitions of outcomes are not consistent (eg, revisions versus reoperations). Additionally, metrics are not standardized or comparable (eg, studies have varying followup periods). We were able to draw comparisons between heterogeneous studies by reporting averages per 1000 person-years. This is unique in the arthroplasty literature. Other limitations include underreporting of prosthesis type, some studies were not able to be grouped into market status categories and therefore were excluded from our analysis, and exclusion of non-English literature.
Furthermore, studies did not report outcomes consistently (eg, number of hips operated on versus number of patients). We standardized adverse event rates to the extent possible by using the number of participants in the study population and, if this was not available, the number of hips. Patients who receive metal-on-metal hip resurfacing devices tend to be male, younger, and more active compared with those who receive THA devices. Finally, when analyzing data from the literature, the inclusion of discontinued devices can dramatically skew findings. To date, it has not been common practice to analyze outcomes of medical devices according to market status. Our findings suggest that this type of analysis is important for continued implementation of medical devices or adoption of new devices. For example, we noted higher revision rates for all metal-on-metal hip resurfacing devices compared with currently in-use devices, which may reflect the inclusion of devices that have been discontinued. By removing discontinued metal-on-metal hip resurfacing devices, the average revisions per 1000 person-years was lower.
Revisions are more frequent (all devices market status group) and occur much earlier for all metal-on-metal hip resurfacing devices (in-use and discontinued market status groups). The average time to revision would be considered poor for metal-on-metal hip resurfacing and THA devices which supports the ongoing need for long-term followup studies on clinical outcomes. Four of five metal-on-metal hip resurfacing systematic reviews published since 2006 noted promising revision rates and pain relief [65, 124, 127, 136, 143]. Time to revision (in years) has not been reported in other reviews comparing metal-on-metal hip resurfacing with THA [65, 124, 127, 136]. According to data from the Canadian Joint Replacement Registry [20, 83], 88% of patients who received metal-on-metal hip resurfacing were younger than 65 years. Smith et al.  determined it is unclear how patient age might influence the incidence of adverse events when comparing THA and metal-on-metal hip resurfacing devices.
Lower revision rates noted among most registries, in comparison to our review, may reflect the larger sample size and higher participation rates in registry data. The International Society of Arthroplasty Registries requires national registries to have greater than 90% of procedures reported to obtain a full membership . Additionally, registries may define revisions differently than clinical studies and lack study protocols with controlled followup.
As anticipated as a result of the ball and socket design of THA devices, dislocation rates were higher than metal-on-metal hip resurfacing for both market status groups. This is consistent with other reviews of this literature [65, 124, 127, 136]. The number of femoral neck fractures was greater for metal-on-metal hip resurfacing devices (currently in-use devices) and these findings are consistent with the literature [65, 127].
Postoperative component alignment was not commonly reported in the literature, and thus no comparisons could be made. This finding is important because, even with the large number of studies included in our analysis, we still were unable to draw any conclusions regarding postoperative component malalignment in both market status groups. Methods of measuring component alignment are not standardized, further complicating comparisons across studies. Further studies assessing component alignment are needed in this area of research since the clinical evidence of poor alignment as a predictor of device failure has been highlighted in the literature .
The strengths of our study were threefold. First, we used averages per 1000 person-years to standardize findings and make valid comparisons. Second, we examined a large body of evidence, and third, we analyzed results by market status. The findings from our systematic review show that revisions and reoperations are more frequent and occur much earlier for metal-on-metal hip resurfacing, except when discontinued devices are removed from the analyses. Dislocations are more frequent with THA, even after removing discontinued devices from the analyses. We found that outcome definitions were reported inconsistently in the studies we identified, and that those studies rarely differentiated their findings regarding adverse events according to the market status of the device(s) in question. These deficiencies can result in clinicians drawing misleading conclusions and misinforming patients. We tried to mitigate the risk of this by using specific design elements in this study, including stratification by market status and standardization of event frequencies per 1000 person-years. Standardized comparative outcomes for metal-on-metal hip resurfacing and THA should be considered when selecting which device is most appropriate for individual patients.
We thank Denis Ako-Arrey DAA; MHA, MPH (McMaster University, Hamilton, Ontario, Canada), and Monica Cepoiu-Martin MCM; MD (University of Calgary, Calgary, Alberta, Canada) for their work reviewing abstracts and full-text articles for this systematic review. We also thank Gavin Steininger BSc, MASc, PhD (University of Victoria, Victoria, British Columbia, Canada) for biostatistics expertise and assistance.
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