All artificial hip implants carry risks including wear of the component material. Metal-on-metal (MoM) hip implants have unique risks in addition to the general risks of all hip implants.
In MoM hip implants, the metal ball and the metal cup slide against each other during walking or running which can cause the release of metal particles. Metal can also be released from other parts of the implant where two implant components connect. The sliding metal surfaces will cause some tiny metal particles to wear off of the device into the space around the implant. Wear and corrosion at the connection between the metal ball and taper of the stem may also occur. Some of the metal ions (e.g. cobalt and chromium) from the metal implant or from the metal particles will enter the bloodstream.
Orthopaedic surgeons take several precautions before and during hip replacement surgery to try to optimize the way in which the ball and socket rub against each other so that fewer wear particles are produced. However, there is no way to fully avoid the production of metal ions and some metal particles.
Different people will react to these metal ions and particles in different ways. At this time, it is not possible to predict who will experience a reaction, what type of reaction they might have, when the reaction will occur, or how severe the reaction will be.
Over time, the metal particles around some implants can cause damage to bone and/or tissue surrounding the implant and joint. This local response is sometimes referred to as an "adverse local tissue reaction (ALTR)"; the systemic and local responses are referred to as "adverse reaction to metal debris (ARMD)." Soft tissue damage may lead to pain, implant loosening, device failure, and the need for revision surgery (the old device is removed and replaced with another one). Patients with a progressing ALTR may be considered for earlier revision to prevent extensive damage to bone, muscle and nerves.
Metal on Metal Hip Implants Outside of the United States
Much of the available data regarding hip implants, including Metal-on-Metal (MoM) implants, involve registry data from countries outside the United States. Although the safety concerns for patients who have MoM hip implants are present throughout the world, there are differences both in device usage and device design of MoM hip implants in the United States and those outside the United States. When comparing U.S. patients to those in other countries, it is important to be aware of the differences. Since much of the available data are from countries outside the U.S., the data are not necessarily indicative of U.S. outcomes.
In June 2012, the FDA asked members of the FDA's Orthopaedic and Rehabilitation Devices Advisory Panel to discuss these differences and the following points were highlighted:
- Device Availability – There may be physical differences between a device marketed in the U.S. and a device marketed outside the U.S. under the same name. Devices used in other countries may not be available in the U.S., and among devices available around the world, device popularity and usage vary. There may also be substantial differences in the practice of "mixing and matching" implant components between the U.S. and other countries.
- Healthcare System – National guidelines regarding patient eligibility for hip implants can result in differing timeframes between diagnosis of a patient's underlying hip disorder and hip replacement surgery. For example, patients outside the U.S. may wait longer to receive a hip implant. This could mean patients are treated at a later stage of disease and/or later age, which will impact the device selected for treatment and the outcomes of hip surgery.
- Practice of Medicine – Surgeon experience and training programs differ between countries. In the U.S., there is greater variety in surgeon experience and hospital volume in comparison to some other countries, where hospitals are more focused on certain types of procedures. In the U.S., approximately half of all primary hip replacements are performed by surgeons who typically do fewer than 15 to 20 of these surgeries per year.
- Population – Factors including a patient's age, sex, weight, diagnosis and activity level influence the lifespan of a device. These factors may vary more in the U.S. population than in other countries, which may influence the outcomes of hip surgery including the revision rate or implant survivorship (need to remove implant and put another in its place).
- Patient Expectations – Patients may have different expectations for the longevity and performance of their MoM hip implants, which underscores the importance of discussion between the surgeon and patient prior to surgery.
In addition to the FDA's 2011 mandate that required postmarket surveillance ("522") studies of MoM total hip replacement systems marketed in the U.S. at the time and launch of the International Consortium of Orthopaedic Registries (ICOR), international regulatory agencies have also issued alerts and safety communications related to MoM hip implants.
- In April 2010, the United Kingdom's (U.K.) Medicines and Healthcare products Regulatory Agency (MHRA) issued a medical device alert that included specific follow-up recommendations for patients with MoM hip replacements. The recommendations included blood tests and imaging for patients with painful MoM hip implants. In February 2012, MHRA published a medical device alert and updated it in June 2017 with advice on the management and monitoring of patients with MoM hip systems.
- In May 2012 Health Canada issued a communication to orthopaedic surgeons and patients regarding MoM hip implants.
- The Therapeutic Goods Administration of Australia published their safety information for healthcare professionals on MoM hips in September 2012 which was updated in July 2017.
There are also reports in orthopaedic literature, data from international orthopaedic implant registries and presentations from professional/scientific meetings that have increasingly noted complications and potential problems of early failure of MoM hip systems, often requiring revision surgery. Much of the available data are from countries outside the U.S., and the data may not be directly applicable to the experience in the U.S. For example, some of the devices available outside of the U.S. are not marketed in the U.S. For this reason, recommendations from international regulatory agencies may not necessarily apply to U.S. patients with MoM hip systems.