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Report of the Expert Advisory Group looking at soft tissue reactions associated with metal-on-metal hip replacements**
It is estimated that at least 500,000 current generation metal-on-metal (MoM) hip replacements have been implanted worldwide over the last 15 years with excellent results from experienced surgeons. There are many thousands of patients with these MoM bearings who are functioning at high levels of activity without pain and who are thought to be at low risk of developing problems. However, over the last few years there have been increasing numbers of reports of revisions following unexplained hip pain, sometimes associated with soft tissue reactions which may be severe.
An Expert Advisory Group (EAG) involving members of the British Hip Society (BHS), the British Orthopaedic Association (BOA), the Medicines and Healthcare products Regulatory Agency (MHRA), and the National Joint Registry of England and Wales (NJR) was set up to:
- determine the incidence of this problem; and
- advise clinicians on any modifications to current practice which may be necessary tominimise the risk of this adverse reaction and optimise early detection.
Advice for use of MoM hip replacements
The Expert Advisory Group offers the following advice when the use of MoM hip implants is being considered:
(i) Not all components are the same and outcomes may differ.
(ii) Surgeons carrying out MoM hip resurfacing surgery should undergo adequate training and they should have the necessary experience to perform the operation.
(iii) When considering a resurfacing procedure, patient selection is important, with data suggesting that resurfacing performs best in male patients of less than 55 years of age. Resurfacing procedures should be used with caution in those over 65 years of age. Higher failure rates have been reported in females. While there have been successful pregnancies in the first two years following MoM hip replacements, women with MoM hip replacements should be advised to postpone pregnancy for at least two years post hip implantation.
(iv) Patients should be consented thoroughly by the surgeon. The consent process should cover the known risks and potential benefits of MoM hip replacements and specifically the risk of systemic and/or local soft tissue reactions, pain and elevated metal ion levels. Patients should be informed that metal ion levels may be raised for some time after the procedure and that the sequelae of the elevated metal ion levels are currently unknown.
(v) As there is uncertainty with regard to soft tissue reactions and their incidence and effects, all patients with MoM hip bearings should be followed up at least annually for five years post operatively and more frequently in the presence of symptoms. Beyond five years, follow up should be in accordance with locally agreed protocols.
(vi) The current role of screening for adverse soft tissue reactions is also unclear. Current evidence suggests that soft tissue reactions are extremely rare in the absence of pain and deteriorating function. Investigations that may be used include blood measurement of cobalt and chromium ions and cross sectional imaging using MARS (Metal Artefact Reduction Sequence) MRI and ultrasound or CT sca
(vii) It is thought that blood tests to measure cobalt and chromium ions should be performed if:
a) patients have pain or symptoms associated with MoM bearings; or b) there are radiological features associated with adverse outcomes including component position or small component size; or c) the patient or surgeon are concerned regarding the MoM bearing; or d) there is concern about a cohort of patients with higher than expected rates of failure.
(viii) If either metal ion levels are elevated above seven parts per billion (ppb) (119 nmol/L cobalt or 134.5 nmol/L chromium) in whole blood, then a second test should be performed three months after the first in order to identify patients who require closer surveillance, which may include cross sectional imaging.
(ix) If imaging reveals soft tissue reactions, fluid collections or tissue masses then consideration should be given to revision surgery.
It is recognised that these recommendations may require modification should new evidence become available.
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