With concerns by patients, clinicians, and regulators about metal-on-metal (MoM) hip implants on the rise, the AAOS Board of Directors commissioned a special technology overview (TO) on the topic. The TO focused on the following three questions:
- What are the clinical outcomes in patients with MoM hip replacements?
- What are some predictive factors related to MoM hip replacement outcomes?
- What is the prevalence of adverse clinical problems related to MoM?
The TO was developed under the oversight of Kristy L. Weber, MD, chair of the Council on Research and Quality. The AAOS Guidelines Oversight and Evidence-Based Practice Committees provided input to the work group, which included Kevin J. Bozic, MD, MBA; James A. Browne, MD; Chris J. Dangles, MD; Paul A. Manner, MD, FRCSC; Adolf J. Yates Jr, MD; Charles Turkelson, PhD; Janet Wies, MPH; Kevin Boyer; Paul Zematis, MPH; and Anne Woznica. None of the work group members had any financial conflicts of interest related to this topic. The final document was approved by the AAOS Board of Directors during their December 2011 meeting and is now available on the AAOS website.
The MoM TO was prepared using systematic review methodology. It summarizes the findings of studies and international joint registry data published on modern MoM hip implants as of July 15, 2011. Although the document does not make recommendations for or against the use of MoM hip implants, it can serve as an evidence-based review of the available literature. Readers are encouraged to consider the information presented in this document and reach their own conclusions about MoM hip implants.
As with previous TOs, this specially commissioned MoM TO should be seen as an educational tool—not as an official position of the AAOS. Decisions about patient care and treatment should always be based on a clinician’s independent medical judgment, given the individual patient’s clinical circumstances.
The MoM TO dealt with the use of MoM implants both in hip arthroplasty and in hip resurfacing.
The first question addressed by the MoM TO concerned clinical outcomes in patients with MoM hip replacements. How did these outcomes compare to outcomes in patients with other bearing surface combinations?
According to the TO, analyses conducted on objective patient-oriented outcomes by two international joint registries indicate that, overall, patients who receive MoM hip implants are at greater risk for revision than patients who receive total hip arthroplasty (THA) using a different bearing surface combination.
Registries from Australia and the United Kingdom/Wales both indicated that individuals who receive MoM THA using larger femoral head components are at a greater risk of revision than individuals who receive smaller femoral head components (regardless of the type of bearing surface used).
Two high-strength studies comparing MoM THA with metal-on-polyethylene (MoP) THA found significant improvement in both groups, but no significant differences in functional outcomes at 5 years and at 10 years postsurgery.
Another study compared MoM THA with MoM hip resurfacing. This study found that MoM hip resurfacing patients had statistically significantly better functional outcomes at 1 and 2 years postsurgery, but the hip resurfacing patients were slightly younger than the MoM THA patients and had different postoperative protocols and a lower body mass index. Thus, there was no clinical relevance in the difference between the treatment arms.
Rigorous multivariate statistical analysis has not yet been conducted by the registries or in other peer-reviewed literature that would allow determination of whether one particular type of patient fares better than other patient types who receive MoM THA. Such an 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.
The second question sought to identify the patient, implant, and surgical factors that best predict successful or unsuccessful outcomes of MoM THA. Data from registries in Australia and the United Kingdom/Wales addressed patient age, gender, and implant head size as variables that are predictive of success or failure in MoM THA and hip resurfacing.
Data from both registries indicate that, after adjusting for age and gender, the larger femoral head components used in MoM THA have higher revision rates and a higher risk of revision than other implants. For example, based on data from the Australian registry, MoM THA using head sizes of 28 mm or less yielded the lowest number of revisions, while head sizes greater than 40 mm had the highest number of revisions per 100 observed years.
The United Kingdom/Wales registry reports that increased age is associated with increased revision risks of large-head MoM THA. The Australian registry also reported that patients older than 65 years with larger femoral head components had a greater revision risk than patients of the same age but with smaller femoral head components.
The third question sought to ascertain the prevalence of adverse clinical problems from MoM THA compared to other bearing surface combinations.
All MoM hip implants will wear and release metal ions. This wear and elevated ion levels are believed to cause soft tissue reactions in the periprosthetic space called “Adverse Reactions to Metal Debris (ARMD),” and/or “Adverse Local Tissue Reactions (ALTR).” Five high strength and 13 moderate strength studies were used to address the incidence/prevalence of complications, metal ions, and tissue reactions to metal.
Limited data exist comparing the prevalence of adverse clinical problems between 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 (“pseudotumors”). Several studies reported elevated serum metal ion (cobalt and chromium) concentrations in patients with MoM hip implants, especially in patients with malpositioned implants. The clinical significance of elevated serum metal ion concentrations remains unknown.
Registry data report on adverse clinical problems and reasons for revision surgery, but do not identify the specific bearing surface combination in all cases. The United Kingdom/Wales registry did begin gathering data on soft tissue reactions in July of 2009, but had too little data when the most recent report was published.
A comparison between hip resurfacing and MoM THA yielded statistically significantly better 1- and 2-year functional outcomes for hip resurfacing, although the difference in scores had little clinical relevance. The postoperative protocols and patient demographics between the treatment arms were different; hip resurfacing patients had a higher initial incidence of revision.
The United Kingdom/Wales registry reported an increased revision risk for hip resurfacing patients in all age groups, except males younger than 55 years of age, compared to patients who received cemented THA with an unspecified bearing surface. The Australian registry reported that hip resurfacing patients 65 years or older have the highest revision risk.
In contrast to results reported for MoM THA, head size and risk of revision for hip resurfacing procedures were inversely related to each other. Patients who received the smallest femoral head components had the greatest risk of revision. No gender differences were reported in hip resurfacing outcomes, but the implant size was associated with poorer outcomes when gender/implant size interaction was analyzed. Because these analyses were limited to one variable in Cox regression models, conclusions should be made cautiously. Further research employing rigorous multivariate analysis is needed to determine which variables best predict success or failure. Not including all variables that could potentially confound interpretations increases the chances of reaching spurious conclusions.
Kristy L. Weber, MD, chairs the AAOS Council on Research and Quality and is a member of the AAOS Now editorial board. Fareeha Shuttari-Khan, MPH, is the evidence-based medicine coordinator in the AAOS department of research & scientific affairs. The authors report no conflicts of interest.