tal on Metal-callaghan_f01.gif
Fig. 1 A, Anteroposterior radiograph of a metal-on-metal total hip arthroplasty obtained 6 weeks after surgery. B, Anteroposterior radiograph of the same patient 5 months later. The patient experienced onset of groin pain not present during the immediate postoperative period, thought to be due to a hypersensitivity reaction; however, continued follow-up for an additional 6 years has not shown progression of the osteolytic process. Reproduced from Callaghan JJ, Cuckler JM, Huddlleston JI, Galante JO: How have alternative bearings (such as metal-on-metal, highly cross-linked polyethylene, and ceramic-on-ceramic) affected the prevention and treatment of osteolysis? Supplement to J Am Acad Orthop Surg 2008; 16:S33-S38.


Published 6/1/2011
Jeanie Kennedy; Katherine Sale

FDA orders postmarket studies on MOM hip implants

On May 9, 2011, the U.S. Food and Drug Administration (FDA) issued orders for postmarket surveillance studies from several orthopaedic device manufacturers of metal-on-metal (MOM) total hip arthroplasty (THA) implant systems. The agency is seeking information on metal ion concentrations and their effects on long-term outcomes for THA patients.

Manufacturers must comply with the FDA order and submit a research plan within 30 days of receiving the FDA letter. The FDA then has 60 days to either approve the protocol or work with the manufacturer to amend the study design. The studies are to last at least 3 years.

Manufacturers will need the assistance of some orthopaedic surgeons to accomplish these mandated postmarket studies. Details of the studies are still being formulated and will likely vary among manufacturers.

Following the April 22, 2010, alert from the United Kingdom’s Medicines and Healthcare Products Regulatory Agency, the FDA formed an internal MOM hip working group to assess the worldwide data and attempt to understand problems associated with MOM hips.

Representatives from the AAOS, the American Association of Hip and Knee Surgeons, the Hip Society, the American Joint Replacement Registry (AJRR), the North American Spine Society, and the Orthopaedic Research Society began meeting with FDA officials in September 2010 to discuss issues relating to MOM implants and patient care.

Questioning the approval process
The debate in Washington, D.C., particularly for MOM hips, is largely focused on whether medical devices should be allowed to go through the premarket notification (also known as the 510(k) process) or whether they should be governed by the premarket approval (PMA) process. Applications for 510(k) devices claim equivalence to a device (other than a Class III device) that is already on the market and are supposed to be acted on by the FDA in 90 days. Class III devices, representing the highest level of risk, generally require a PMA to support their safety and effectiveness before they may be marketed.

PMA applications require 2 years of clinical data; devices that must go through the PMA process usually take at least 7 years to reach the U.S. market. Generally, the FDA approves approximately 40 PMA applications for devices each year, but clears more than 3,000 devices annually through the 510(k) pathway.

As early as 2009, the Government Accountability Office recommended that the FDA examine its processes for allowing device types classified as high risk to enter the market via the 510(k) premarket notification process.

The FDA is expected to move toward reclassifying Class III preamendments device types (devices that were on the market on or before May 28, 1976) or to require PMAs for these devices. This issue was discussed during the April 13, 2011, Senate Special Committee on Aging hearing “A Delicate Balance: FDA and the Reform of the Medical Device Approval Process.” As of the hearing date, the FDA had yet to take action on 26 remaining high-risk Class III preamendment devices.

The agency will likely require PMA applications for any future MOM THA implant systems, particularly in light of data from several foreign registries that show a higher-than-normal rate of revision for those devices.

Some consumer groups are calling for a requirement that all implantable devices go through PMAs. No data exist to support this policy recommendation. Notably, the FDA, which has the most scientific data of any entity in this nation, is not providing any credence for this argument.

Registries and standards
Even with premarket studies, the long-term consequences of drugs, devices, and biologics may not be known because these studies generally provide no more than 2 years of patient experience. Thus, many countries have developed registries that track device performance and patient outcomes.

On May 9, 2011, the FDA convened a meeting of the International Consortium of Orthopaedic Registries (ICOR), which was attended by representatives from the AJRR and more than 30 international and regional U.S. registries. The goals of ICOR are to leverage data from existing orthopaedic registries, improve methods of studying device performance and patient outcomes, enhance and harmonize worldwide registry data, and improve research collaboration.

Currently, different registries may provide conflicting data on the same research question. The development of ICOR may provide more useful data on MOM hip implants.

Key to any analysis of the impact of MOM hips are appropriate and consistent standards for metal ion testing. The American Society for Testing and Materials, International hosted a one-day workshop in Anaheim, Calif., to identify needs for characterization and standardized testing for MOM THA devices. The workshop was cochaired by William M. Mihalko, MD, PhD, who also chairs the AAOS Biomedical Engineering (BME) Committee; Jack Lemons, PhD, and A. Seth Greenwald, D. Phil (Oxon), who have served as consultants to the BME committee; and Steven Kurtz, PhD.

Experts on imaging, retrieval analysis, implant wear, and testing convened to begin establishing best testing practices in the following four areas:

  • characterization of adverse local tissue reactions
  • wear and corrosion: metallic product measurement in fluid and tissue samples
  • analyses of retrieved MOM implants
  • in vitro testing of MOM implants under adverse conditions

More than 100 scientists, industry representatives, and orthopaedic surgeons reviewed current evidence and discussed gaps in testing of MOM devices. AAOS Second Vice-President Joshua J. Jacobs, MD, delivered a keynote address. Key messages from the presentations are reflected in the five areas selected for standards development:

  • magnetic resonance imaging standards assessing wear-induced synovitis
  • standard methods for assessing metal allergy
  • standard histiological evaluation score for bearing materials
  • standard to measure ions in serum and correlation to wear
  • test methodologies to mimic clinical failure modes

Work began on these areas during subsequent subcommittee meetings and a 2012 symposium was proposed.

Turn to the AAOS for information
The AAOS is regularly in contact with FDA officials. When new information becomes available, AAOS will provide updates to fellows through AAOS Now, Headline News Now, Advocacy Now, and other member communications as needed. Fellows who have questions related to the MOM postmarket study issues should submit them to

Jeanie Kennedy is manager, regulatory and government affairs, in the AAOS office of government relations. Katherine Sale is manager, biomedical research and regulations, in the AAOS department of research and scientific affairs.

Additional Links:

The FDA postmarket surveillance page

The AAOS comments to the Senate Special Committee on Aging hearing