Metal-on-metal (MoM) hip implants consist of a ball, stem, and shell, all made of metal materials, while hip resurfacing consists of a trimmed femoral head capped with a metal covering. This overview addresses the former; a previous American Academy of Orthopaedic Surgeons (AAOS) Technology Overview addressed the latter.1 Artificial hip systems have inherent risks associated with the procedure, such as reactions to anesthesia, wound infection, excessive bleeding, dislocation, and blood clots. MoM hip systems, in addition to the inherent risks, have additional unique risks. The metal ball and cup slide against each other when the patient is walking or running, and the ball and cup have the potential to release metal particles into the periprosthetic space and/or systemically. While precautions can be taken to optimize the positioning of the hip system, there is no way to fully prevent the production of metal debris.
There have been occurrences of patients experiencing less than optimal outcomes from MoM hip systems, sometimes termed “adverse local tissue reactions” (ALTRs) or “adverse reaction to metal debris” (ARMD). Patients react to metal particles in different ways, and it is not possible to predict which patients will experience an adverse event with MoM hip systems. These reactions have led to earlier failure rates for the affected patients.
In February 2011, the US Food and Drug Administration (FDA) issued a public health communication about hip replacement components that have both a metal ball and metal socket (MoM hip systems), and on May 6, 2011, issued an order for manufacturers to conduct postmarket surveillance on total MoM hip replacement devices to monitor adverse events.
The goals of this Technology Overview are to use the tools of evidencebased medicine to summarize information on the indications, effectiveness, prevalence of adverse events, and failure rates of modern MoM hip replacement technology. The findings will compare and contrast modern MoM hip replacement technology to the current generation of total hip technology using other bearing surfaces to highlight areas of overlap and inform the practicing orthopaedic surgeon concerning important differences.
This report was developed using the methods of a systematic review. We began by having a panel of physicians frame three key questions and next developed rules (inclusion criteria) for determining the information we would include. (The full list of criteria appears in Appendix I in the complete Technology Overview, available at http://www.aaos.org/research/overviews/Metal_On_Metal.pdf.) Finally, we conducted comprehensive literature searches (Appendix II in the complete Technology Overview) to ensure that the data we considered are not biased in favor of any particular point of view. Thereafter, we evaluated the quality of the relevant studies, including their methods of analysis; considered their results; and summarized this information in graphs and tables.
Our searches identified 3,038 hip replacement citations and data from 8 joint registries that were potentially relevant to this Overview and that could potentially meet our inclusion criteria. Of these, 19 articles2–20 and the data from 2 joint registry reports21,22 met these criteria. This information forms the dataset that we used to address the questions below.
We emphasize that this is a Technology Overview of MoM hip replacements and not of all hip replacement procedures. The THA outcomes that we consider are from only the most recent registry reports.21,22 We have chosen this strategy because the most recent reports provide direct statistical comparisons of MoM, hip resurfacing, and other bearing surface hip replacements.
Quality of the Literature
Assessing the quality of evidence is an important step in a systematic review. Readers can have more confidence in the results of high-quality studies than low-quality studies. To assess quality, we used a domainbased approach, which allows for the evaluation of studies of all designs. Design alone, however, does not adequately reflect the quality of a study. Therefore, we assessed study quality using a system that considers not only design but also several aspects of how well a study was conducted. Details about this system are provided in Appendix IV in the complete Technology Overview.
We also considered the quality of the statistical analyses provided in each registry report, and we discuss these considerations in the text of the main body of this overview.
The currently available studies do not allow us to determine whether one particular type of patient fares better than others. Subgroup analysis requires rigorous statistical analysis, including simultaneously accounting for the effects of all patient and device characteristics of interest and performing statistical tests for interactions between the relevant variables. To date, such analyses have not been conducted.
We included patient-oriented outcomes to assess clinical effectiveness of MoM hip replacements. Validated “paper and pencil” outcome measures (eg, Harris Hip Score) were used as subjective measures and revision rates as objective measures of effectiveness. These outcomes matter to patients (ie, patient-oriented) and indicate whether an intervention is effective. Serum metal ions may serve as potential surrogate markers for implant failure and thus were included as an outcome in this overview. Outcomes based on imaging of hip implants for positioning and imaging of periprosthetic tissues were also included in this overview. These surrogate markers were agreed upon by the physician task force a priori to literature retrieval and quality evaluation to minimize bias.
Key Questions and Supporting Evidence
What are the clinical outcomes in patients with MoM hip replacements in comparison with other bearing surface combinations?
Analyses conducted on objective patient-oriented outcomes by two joint registries indicate that, overall, patients who receive MoM total hip arthroplasty (THA) and hip resurfacing are at greater risk for revision than are patients who receive THA using a different bearing surface combination. The Australian21 and UK/Wales22 registries both indicated that MoM THA using larger femoral head components places individuals at a greater revision risk than it does individuals with smaller femoral head components.
Studies evaluating validated, patientoriented outcomes included a highstrength study using the Harris Hip Score,Western Ontario and McMaster Universities (WOMAC) index, and the Medical Outcomes Study 12-Item Short Form (SF-12);12 a high-strength study evaluating the Harris Hip Score and Oxford Hip Score;20 and a moderate-strength study evaluating the WOMAC index.19
Two studies compared MoM THA with metal-on-polyethylene (MoP) THA,12,20 and one compared MoM THA with hip resurfacing.19 In each case of MoM THA versus MoP THA, both treatment arms improved significantly, but there were no significant differences between groups.
The comparison between hip resurfacing and MoM THA yielded statistically significantly better 1- and 2-year WOMAC scores for hip resurfacing, but there was no clinical relevance in the difference, postoperative protocols were different, and patient demographics differed between treatment arms.
Rigorous multivariate statistical analysis that would allow determination of whether one particular type of patient fares better than others with MoM THA has not yet been conducted by the registries or in other peer-reviewed literature. Such analysis must simultaneously account for the effects of all patient and device characteristics of interest and also take into account any interactions between relevant variables.
What are the patient, implant, and surgical factors that best predict successful/unsuccessful outcomes of MoM hip replacement?
Data from the Australian21 and UK/Wales22 registries addressed patient age and sex and implant head size as variables that are predictive of success/failure in MoM THA and hip resurfacing. Data from both registries reported that larger femoral head components used in MoM THA have higher revision rates and risk of revision after adjusting for age and sex.
The UK/Wales registry reports that increased age is associated with increased revision risks of large-head MoM THA.22 The Australian registry also reported that patients aged >65 years with larger femoral head components had a greater revision risk than did patients of the same age but with smaller femoral head components.21
The UK/Wales registry reported that hip resurfacing patients in all age groups, except males <55 years of age, were at an increased risk of revision compared with cemented THA with an unspecified bearing surface.22 The Australian registry reported hip resurfacing in a patient ≥65 years of age to have the highest revision risk.21
Head size and risk of revision for hip resurfacing procedures were inversely related to each other. Those patients receiving the smallest femoral head components had the greatest risk of revision. There were no sex differences reported in hip resurfacing outcomes, but the implant size was associated with poorer outcomes when sex/implant size interaction was analyzed.
The registry analyses were limited to one variable in Cox regression models; therefore, conclusions should be made cautiously. Further research employing rigorous multivariate analysis is needed to determine which variables best predict failure/success. Not including all variables that could potentially confound interpretations increases the chances of reaching spurious conclusions.
What is the prevalence of adverse clinical problems from MoM hip replacement compared to other bearing surface combinations?
Limited data exist comparing the prevalence of adverse clinical problems with MoM hip systems and other bearing surfaces. Several studies noted a correlation between suboptimal hip implant positioning and higher wear rates, local metal debris release, and consequent local tissue reactions to metal debris (eg, pseudotumors). Several studies reported elevated serum metal ion (cobalt and chromium) concentrations in patients with MoM hip articulations, especially in patients with malpositioned implants. The clinical significance of elevated serum metal ion concentrations remains unknown.
Registry reports present data on adverse clinical problems and reasons for revision surgery, but they do not identify the specific bearing surface combination in all cases. The UK/Wales registry did begin gathering data on soft-tissue reactions in July 2009, but it had too little data when the most recent report was published.22
This Technology Overview was prepared using systematic review methodology and summarizes the findings of studies on modern MoM hip implants. The overview addressed three questions. Regarding the clinical outcomes in patients with MoM hip replacements compared with those in patients with other bearing surface combinations, overall, patients who receive MoM THA and hip resurfacing are at greater risk for revision than are patients who receive THA using a different bearing surface combination. In regard to the patient, implant, and surgical factors that best predict successful/ unsuccessful outcomes of MoM hip replacement, data show that larger femoral head components have higher revision rates and risk of revision and that older age is associated with increased revision risks of largehead MoM THA. Regarding the prevalence of adverse clinical problems from MoM hip replacement compared with problems associated with other bearing surface combinations, data are limited. Still, several studies noted a correlation between suboptimal hip implant positioning and higher wear rates, local metal debris release, and consequent local tissue reactions to metal debris. In addition, several studies reported elevated serum metal ion concentrations in patients with MoM hip articulations. The clinical significance of these elevated ion concentrations is not known.
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