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Metal on Metal (MOM) Hip Replacements

There has been a lot of interest in the recent reports in the press about metal on metal (MOM) hip replacements.

The Medicines and Healthcare products Regulatory Agency (MHRA) issued an update on its previous advice regarding these implants at the end of June 2017. This advice is made available to all Orthopaedic surgeons to assist in the early detection of soft tissue reactions in those patients with metal on metal implants.

All moving surfaces wear and liberate wear particles. The wear particles liberated from metal on metal hip replacements include cobalt and chromium ions. Depending on the amount of wear particles liberated and the sensitivity of the patient to these wear particles; problems can arise both locally to the hip and throughout the body. This is often given the name: adverse reaction to metal debris (ARMD). The reaction around the hip joint can cause damage to the soft tissue (ligaments and muscle) or bone. This is an auto immune problem and is often called aseptic lymphocyte dominated vasculitis-associated lesions (ALVAL). This damage can occur in patients with no symptoms.

The need to follow up patients with these types of hip replacements has long been recognised and MHRA has given advice and continues to update the advice as our knowledge improves.

Blood cobalt and chromium levels are monitored, but in patients who are especially ‘sensitive’ to the metal ions, damage may occur at lower levels.

Imaging of the hip is recommended, either MRI or ultrasound. There are problems using MRI scanning as the large amounts of metal present within the hip replacements distorts the picture. Special ways of undertaking the MRI scan have been developed. These are called metal artefact reduction sequences (MARS). Ultrasound is very operator-dependent and therefore MRI is favoured by the majority of the clinicians.

The recent MHRA guidelines advise Doctors that the MRI scans should carry more weight in the decision making processes that metal ion levels alone.

Those patients with bone damage are of the most concern. A fluid collection (unless very large) around the hip joint in a symptomatic patient can be observed but the patient should undergo repeat scans and clinical review.

The recent update noted that MOM hips can cause problems that are seen in standard hip replacements and not related to the fact that they are MOM.

Many of the MOM hips implanted continue to work extremely well. Certain models (the ASR) did particularly badly but have since been revised (redone).

All of my patients who have a MOM hip replacement are having regular follow up. All have had initial ion levels undertaken and an MRI scan as well as a hip scoring questionnaire. This has given us a baseline for all of our patients. All patients continue to be followed up and any patient who is symptomatic, have high or riding ion levels or abnormal MRI scans are continuing to be followed up.

If you have no symptoms from your hip the probability is that it is working well. If you have any symptoms such as pain or swelling, it would be sensible to discuss this with your Orthopaedic surgeon.