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AAOS Now

Published 5/1/2012

AAOS Technology Overviews Focused on MoM

The AAOS has two technology overviews (TOs) focused on metal-on-metal (MoM) hip implants. The first, on hip resurfacing (HR), was released in December 2009. The second, on MoM implants used in both HR and total hip arthroplasty (THA), was released in December 2011. Both are available on the AAOS website, at www.aaos.org/technologyoverviews

TOs are not intended to convey any official position by the AAOS. Instead, the information is provided as a service to help members identify and evaluate the available published literature on this topic so that they can provide the best possible care to their patients.

Both documents were developed by physician workgroups under the direction of the AAOS Oversight Committee. Because TOs are considered educational tools that encourage readers to consider the information presented and reach their own conclusions, neither document makes recommendations for or against the use of HR or MoM THA.

Hip resurfacing
The HR TO focuses on four questions—the difference in revision rates for MoM HR and THA; the patient characteristics that are predictive of successful or unsuccessful outcomes in patients who undergo HR; the relative effectiveness of HR and THA; and the availability of evidence to support improved outcomes in HR through improvements in technique or patient selection.

According to the first TO, HR and THA are frequently offered to very different kinds of patients. For example, the TO notes that HR patients tend to be male and younger than THA patients. Data are not sufficient to identify patient predictors of outcomes or the effectiveness of HR and THA. However, low-quality studies suggest that improvement in technique and increased surgeon experience can improve HR outcomes.

Hip implants
The TO on hip implants focuses on three questions—clinical outcomes in patients with MoM hip replacements; predictive factors related to those outcomes; and the prevalence of adverse clinical outcomes related to MoM hip implants.

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 a THA using a different bearing surface combination. In a study that compared MoM THA with MoM HR, patients with HR had significantly better functional outcomes at 1 and 2 years postoperative, but were also slightly younger and had different postoperative protocols and a lower body mass index.

Additionally, registry data suggest that the larger femoral head components used in MOM THA have higher revision rates and a higher risk of revision than other implants. (See “The Bearing Dilemma in 2012: What Are We to Do?” for more on this issue.)

With regard to adverse clinical problems, the TO notes that all MoM hip implants will wear and release metal ions, which could lead to elevated metal ion levels within the body and result in adverse soft tissue reactions in the periprosthetic space. However, data are limited and the clinical significance of serum metal ion concentrations remains unknown. (See interview with Joshua J. Jacobs, MD, “Adverse Local Tissue Responses to MoM Hip Implants.”)

Additional Resources:
AAOS releases technology overview on hip resurfacing

AAOS releases TO on MoM Hip Implants