MHRA ANNOUNCES MOM HIPS SHOULD BE MONITORED ANNUALLY

Metal-on-metal (MOM) hip replacements consist of a femoral metal ball and acetabular socket, as opposed to the traditional metal ball and plastic socket replacement, and became popular in the 1990's.

The metal components of the MOM hips are made of a cobalt-chromium alloy; inert metals that are unlikely to cause any reaction within the body. These metal hips have been implanted into a million patients worldwide, but with the emerging problems coming to light over recent months, confidence in the products has fallen and the number of patients and orthopaedic surgeons using them has reduced to virtually zero today.

MHRA investigation

In September 2010, the Medicines and Healthcare products Regulatory Agency (MHRA) put out an alert regarding the safety of MOM hips, and those manufactured by DePuy were recalled following high failure rates and problems. The MHRA issued patient advice, recommending that those who had undergone a MOM hip replacement or whether they had symptoms of joint failure or not – should revisit their original orthopaedic surgeon to have tests on the implant and discuss the potential need for revision surgery. They go on to specify that MOM hips should be reviewed annually, and estimates suggest that 50% of those with a MOM hip replacement will need revision surgery within 5 years of implantation.

MOM hip replacement risks

Both components (ball and socket) of the hip replacement come into direct contact and rub together, releasing nanoparticles of cobalt-chromium into the blood stream in large quantities. This is often in excess of safe limits, and concerns focus around one of the agents being carcinogenic, and subsequent inflammatory problems in some patients.

There are three problems associated with MOM hip replacements:

  1. Inflammation due to metal particles: the release of cobalt-chromium nanoparticles stimulate an aggressive inflammatory response in some patients, resulting in tissue damage around the joint.
  2. Accelerated wearing: data indicates that in some patients, the MOM joints are more prone to accelerated wearing.
  3. Incorrect surgical positioning: again, data suggests that incorrect positioning of the metal socket component can have a significant negative impact on the failure rate Many patients experience "silent" symptoms – they feel well and have no joint pain. Despite this, recommendations are regular monitoring to ensure prompt identification of a potential failure.

Correcting the MOM hip replacement issues

The London Clinic leads the way in assessments and revision surgery with a multi-disciplinary expert team. Despite never implanting the recalled DePuy products, the Clinic offers a specialised and expert assessment of any patient with a MOM hip replacement. This includes measurement of the cobaltchromium nanoparticle levels within the blood, metal artefact reduction sequence (MARS) MRI to investigate soft tissue damage around the implant, and 3D CT scanning to determine the exact position of the hip. Based on the outcome of these tests, the Clinic offers a number of monitoring and treatment options, including detailed surgical planning of a revision operation if necessary.
 

Information provided by Professor Alister Hart FRCSG(Orth), Professor of Orthopaedic Surgery, an international clinical opinion leader and researcher for the assessment of patients with painful metal-on-metal hip replacements.