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Understanding the failure of Metal on Metal (MOM) implants

The role of the London Implant Retrieval Centre (LIRC) in shaping the future of hip surgery - Professor Alister Hart, consultant orthopaedic surgeon at the Royal National Orthopaedic Hospital NHS Trust, and chair of Clinical Orthopaedics at the UCL Institute of Musculoskeletal Sciences, writes for NHE about the LIRC.

The London Implant Retrieval Centre (LIRC), based at the Royal National Orthopaedic Hospital NHS Trust, is responsible for managing the first global retrieval program for medical implants.¹ Over the last five years we have received around 4,700 failed implant components, helping to lead the way in understanding how to improve clinical outcomes for patients undergoing hip replacements.

We have seen some major achievements and breakthroughs over a short period of time. This has enabled us to provide independent insight and guidance that has resulted in changes to UK Medicines and Healthcare Regulatory Agency (MHRA) standards and regulation. We have also developed new protocols for the retrieval and detection of faulty implants. Our experience has helped us to inform the health sector and implant manufacturing industries about best practice when it comes to the development of hip replacement implants and surgical procedures.

Setting up the LIRC

The idea for the centre was conceived in 2007 when my colleague John Skinner and I noticed that a number of patients who had received Metal on Metal (MOM) implants were experiencing unexplained pain, requiring them to need further surgery on their hips. We decided that this required further investigation and so we embarked upon drafting a proposal for a programme of international and interdisciplinary research. Our aim was to try to unravel the complicated relationship between surgical positioning, implant design and patient factors that may be responsible for the good or poor performance of hip implants.

In 2008, with funding from the British Orthopaedic Association, we successfully founded the London Implant Retrieval Centre (LIRC), combining the research expertise of a number of organisations including: Imperial College London (where I was senior lecturer at the time); the Royal National Orthopaedic Hospital NHS Trust (RNOH), where John Skinner worked; and University College London (UCL), where I worked closely with biomedical engineer Professor Gordon Blunn.

At this time we had many questions that needed answers; which patients are most at risk of MOM implant failure? And how can we improve the technique of implanting hip replacements?

To start to gather answers to these questions, we created a consortium of nine orthopaedic manufacturers² and worked closely with 20 orthopaedic hospitals to retrieve failed implants for analysis – sourcing 79 in our first year. We set about creating a standardised process for this retrieval, which included decontamination (soaking the implant in formalin 10% for a minimum of two weeks before rinsing and storing), detailed inspection, recording all reference numbers, associated data and the production of a receipt report.

From the outset, it was important to us to be collaborative and inclusive in our approach to ensure that the dissemination of information between orthopaedic consultants and specialists, the wider health sector, manufacturers and regulatory bodies would help to improve our understanding of these failures and affect change. To help with this process we opened up regular communication with the MHRA to keep them updated about our findings.

Key trends

Our research has enabled us to identify the factors that can impact on the effectiveness of MOM implants, including placement during surgery, understanding corrosion and wear rates, and patient factors. This has led us to new methods of monitoring and detecting failing implants, as well as the development of an award winning protocol and an MHRA action level for blood metal ions.

We discovered that patients experiencing corrosion or implant failure often showed traces of chromium in the tissue surrounding their implant. Fortunately, this was not the cancer-causing Chromium (VI), but Chromium (III), as shown by our experiments in the Diamond Light Source, the UK’s particle accelerator (pictured on facing page). Further tests suggested that this was caused by cobalt contained within the structure of the MOM component. This led us to create a new method of using blood metal ions to detect failure of MOM implants.

Patients can now be monitored using an MRI scan and blood test, helping orthopaedic specialists to assess the potential risk of corrosion and future failure of implants. These discoveries directly resulted in the creation of the MHRA action level for blood metal ions (cobalt and chromium) used to monitor all patients with metal MOM hip replacements, which currently amounts to around 100,000 in the UK and 1.5 million worldwide. 

This has also increased our understanding of the composition of the metal debris that appears as a result of hip replacements, allowing us to develop new tools to help us better understand the compatibility of medical devices within the body.

The bigger picture

We now have a strong network of collaborators and regularly receive hip implants from 192 UK surgeons as well as surgeons from 22 other countries. We have published in excess of 40 papers and have presented at more than 40 international meetings, including the American Academy of Orthopaedic Surgeons, and disseminated our research through meetings at academic institutions in Harvard, USA; Helsinki, Finland; Rome, Italy; Cambridge, UK; Oxford, UK; Exeter, UK; and Manchester, UK.

Through the dissemination of our independent research, reports and advice, we have been able to drive change in the health sector to help improve standards for patient care. In 2010, we produced a paper highlighting our findings.³ This was the first paper to emphasise patient factors in the failure of MOM hip implants.

The paper influenced the MHRA to issue a medical device alert concerning the safety of all types of MOM hips. They advised that every patient with this type of hip replacement (around 1 million had received these since 1997) should visit their surgeon for a check-up and those experiencing pain should be monitored for a five year period. The story received a great deal of national and international press coverage and prompted the health profession to question the use of MOM implants for future surgery.

Our ongoing collaborations with specialists have seen the continued development of our scanning and detection processes. Working closely with MRI physicist Dr Donald McRobbie, we have developed award winning⁴ MRI protocols to minimise heating and metal artefact around hip implants. This means it is easier to detect soft tissue inflammation and muscle damage that can occur around implants. It has also led to more effective clinical management and reduced cost of monitoring of patients with hip implants.

We have provided both manufacturers and the international health sector with a clear process to allow implant manufacturers to respond to concerns regarding the safety of one of their products. We are also able to supply them with independent clinical, laboratory and engineering analysis of failed MOM implants. Our award winning MRI protocol has enabled us to detect implant issues more easily, significantly improving the quality of life for patients. We have also led the way in the creation of information resources for public and patients and have encouraged a change in the regulation of all medical devices by the United States (new FDA device classification 2013), and the UK MHRA / British Orthopaedic Association initiative ‘Beyond Compliance’, initiated in May 2013.

Our goal for the LIRC is to continue to lead the way in the improvement of standards for hip surgery by maintaining our strong focus on research and reporting and continuing to keep our stakeholders informed and up to date about the latest developments. We are keen to extend our implant collections by offering to store and carry out wear measurements on all types of orthopaedic implants including spine, shoulder, knee and ankle implants. This could become a key part of the MHRA initiative ‘Beyond Compliance’ that is designed to provide early analysis of failures to determine whether newly introduced implants should continue.

Results

As of 2013, our research into patients with MOM hips has directly resulted in:

• Evidence that affects the 1.5 million patients with MOM hips worldwide

• One paper published per month for 36 months since 2010, all of which have been cited many times

• MHRA action level for blood metal ions (cobalt and chromium) used to monitor all patients with metal on metal (MOM) hip replacements: 90,000 in the UK and 1.5 million worldwide

• The first truly global (22 countries, all continents) retrieval program for medical implants: 4,700 components from 2,300 patients since 2008

• The improvement in quality of life after additional surgery for patients with MOM hips as a result of our award-winning MRI protocol to detect problems and our award-winning paper on the clinical outcome of redo surgery using our methods.

• The creation of information resources for public and patients: the Arthritis Research UK clinical advice

• A change in the regulation of all medical devices by the United States (new FDA device classification 2013) and the United Kingdom MHRA and British Orthopaedic Association initiative of ‘Beyond Compliance’, initiated in May 2013. 

References

 1. The LIRC retrieves Metal On Metal implants from over 22 countries across all continents.

2. The consortium of nine manufacturers include: Depuy, Zimmer, Smith & Nephew, Biomet, JRI, Finsbury, Corin, Mathys and Stryker. The contract allows for freedom to publish all results.

3. Hart, A. J., Quinn, P. D., Sampson, B., Sandison, A., Atkinson, K. D., Skinner, J. A., . . . Mosselmans, J. F. (2010). ‘The chemical form of metallic debris in tissues surrounding metal-on-metal hips with unexplained failure’. Acta Biomater, 6 (11), 4439-4446.

4. Award from the Radiological Society of North America, attended by 60,000 delegates.