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. Metal can also be released from other parts of the implant where two implant components connect. Metal release 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 some metal particles.

Over time, the metal particles around some implants can cause damage to bone and/or tissue surrounding the implant and joint. This is sometimes referred to as an “adverse local tissue reaction (ALTR)” or an “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 can cause soft-tissue damage and pain, which could lead to further surgery to replace the implant, the U.S. health regulator said, following several recalls of the artificial hip parts.

International regulatory agencies have 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 2012 with advice on the management and monitoring of patients with MoM hip systems.
• In May 2012 Health Canada issued a public health 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. 

Johnson and Johnson, the biggest manufacturer of all-metal devices, recalled its ASR hip implant in 2010 following safety problems. Smith and Nephew withdrew a component of one of its all-metal artificial hip systems last June, following higher level of patient problems with the device. Stryker Corp begun recalling some components of its implant in July due to risks associated with corrosion. Other hip implant makers include Zimmer Holdings Inc and Wright Medical Group.

At the current time, there is not enough evidence to support the routine need for checking metal ion levels in the blood or soft tissue imaging if patients with MoM hip implants have none of the signs or symptoms described above and the orthopaedic surgeon feels the hip is functioning properly. The FDA is recommending that asymptomatic patients with MoM hip implants continue to follow-up with their orthopaedic surgeon every 1 to 2 years to monitor for early signs of change in hip status.